“32?;

. , ..
(:43:

.
5,).

:3

,.

N
a

. g...
:93. ‘

47.}. En. .
~ ’ . er-
13534
.. my. .
5‘ ‘ \
22.533

‘1‘
4-,;
25‘
~ .
'\

”$2.2“
"E!" '

9’.
I:

{3
*3 a":

.. w :55- “ 5,55”:
:- ‘ . - .
A 4.223;? $“5tfifi8‘ ‘ , ‘
"v§3;t,gc:§§a".u .5 ~~ ’3

'A

1.7::
mug.

a; “

4

155’
«v
.N.

‘ a:
1;:

‘13

.-.M 3
h‘lJb‘ \ I . I: ‘ K
37.“. “-c 1:1. vitfietc'm‘u “‘3 “J
" '- ' ' ‘ ":57. 1.35595)
. 3: 3"???“ .-.. ~ 3
,¢‘§5‘.‘§'§3‘“§““ ML; 6 . ~
:,fi¢§g§-.%xs;. . 7.35:» “:“i- :a
«3" M‘Anw...“ :3,
Jan- 4. .-
533,“ .
., ' ..‘ 3, » . - ... 4 "
2‘- ' .3. 1. .' ‘ .‘z a? -' ‘3
" . .... 52%? 3 ‘ . _ R“ _
.... .. ...... .. x
...W“. tfiwfi.fix “or
‘wfifififiw ..-3§-,9;“"i5‘a
. -... . ~ A,
- a"

, ‘
u{.-,_

.r a. . .. _
,- m; «an... ,
me... W .-.,
"w with: ->u:~Ԥ

1. ‘8'..."‘. _,. 33,-? -

‘21}; v"

M ‘32, 2mg»?

1 w. fivi’. ‘
WW}. mt:-
$153“; “Evie
<i¥"'-"~:~*‘:‘; ’9 - ..

.3) ’3’

...-‘1’}. &~
. u .
3‘ "as; 3* ‘" -
Ava. '

.‘1

 

'THESI'D

SITY R R ES

1 Itl'ii‘ii'z‘z‘li’i:iiii‘liiifix :miiiigmm

3 1293 01051 01 2

This is to certify that the

dissertation entitled

USE OF ND:YAG LASER FOR PROCESSING HIGH
‘ TEMPERATURE SUPERCONDUCTORS ’

presented by
Chung-Wen Chen

has been accepted towards fuifillment
of the requirements for

Ph. D. degree in Materials saw

0
Major profess

 

 

 

 

 

 

LIBRARY
M'Ch'gan State
University

 

 

 

PLACE ll RETURII BOX to roman this checkout from your need.

To AVOID FINES Mum on or before duo duo.

DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

i
i
I
l

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Jl I I

usu IaAn Affirmative Action/Equal Oppomnuy Imam

mm:

 

WW, *

USE OF ND:YAG LASER FOR PROCESSING HIGH
TERIPERATURE SUPERCONDUCTORS

By
Chung-Wen Chen

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department ofMaterials Science andMechanies
1993

ABSTRACT

USE OF NDzYAG LASER FOR PROCESSING HIGH
TEMPERATURE SUPERCONDUCTORS
By
Chung-Wen Chen

The problem associated with sluggish kinetics of formation of 2223 high TC phase
in Pb-doped Bi-Sr-Ca-Cu-O system was investigated A laser calcination process for
Pb-doped Bi—Sr-Ca-Cu-O (BSCCO) material was established with the consideration that
localized temperature surge, above the melting temperature, might enhance the formation
of the 110 K 2223 phase. The total processing time, to obtain a near single high Tc
phase, was reduced to half that for conventional processing (about 100 hours). High TC
phase was formed via a difl‘erent kinetic path in laser calcined sample compared with the
conventionally processed sample.

An ion beam assisted millisecond pulsed laser vapor deposition (IBPLD) process
was developed to fabricate YBa,Cu,Ox (YBCO) high Tc superconductor thin films.
Target overheating problem was greatly reduced by using a zigmg scanning pattern of the
target with an x-y target manipulator. High energy oxygen ion beam was used to
replenish oxygen and to blow vapor species onto the substrate. More than 25 mm
diameter, uniform coating of YBCO was successfully deposited As-deposited films are
composed of various compounds of Y103, 3210 and CuO as well as Y,BaCuO,. Due to
microscopic chemical homogeneity of films, films with majority 1-2-3 phase are formed
by just half an hour of post annealing at 850 'C. Silver buffer layer is found to effectively
minimize degradation caused by film/substrate reaction of (001)YSZ substrate during
post annealing.

A new process combining millisecond pulsed laser deposition (ms-PLD) and
vacuum thermal evaporation (VTE) techniques was deveIOped to produce multi-layer Ag
doped YBCO/Ag thick film. Flexible multi-layer thick films, with thickness about a few
tens to a few hundreds of microns, were made. This process starts with about 5 to 50 um
layer of silver deposited by VI'E on a fused silicate substrate at room temperature, then
followed by a 5 to 50 um layer of Ag doped YBCO using ms-PLD. This process is
repeated several times to achieve desired multi-layer number and thickness. When the
desired thickness is obtained, another layer of silver is thermally deposited above the Ag
doped YBCO before the multi-layer composite thick film is peeled from the substrate.

Post annealing is followed to recover superconductivity.

7c M pct/tam

mwdfclm

iv

ACKN OWLEDGIVIENT S

I wish to express my sincere gratitude to my advisor, Professor Kalinath
Mukherjee, for his valuable guidance and constant support throughout this study. My
appreciation is extended to the committee members, Professor Eldon Case, Professor
Carl Foiles and Professor K. N. Subramanian, for their helpful comments and
suggestions.

SpecialthanksareductoDr. P. A. AKhanformanyyearsofkindadvise and
assistance. I also would like to thank all faculty and staff in the Department of Materials
Science and Mechanics for their helpful support during the years.

Last but not least, I wish to express my sincere gratefulness to my family in
Taiwan and my wife Lina for their continuous support and encouragement.

TABLE OF CONTENTS

LIST OF TABLES
LIST OF FIGURES
1. INTRODUCTION
2. LITERATURE SURVEY
2.1 Superconductors and Theories of Superconductivity

2.1.1 Two-Fluid Model, London Equation and Non-local
Electrodynamics

2.1.2 Ginzburg-Landau Theory
2.1.3 BCS Theory

2.1.3.1 Phonon-Mediated Pairing of Electrons (Copper Pairs):

2.1.3.2 BCS Ground State and Energy Gap Near the Fermi
Ems)!

2.1.3.3 Critical Temperature and Isotope Effect
2.1.3.4 Probability Amplitude of Cooper Pairs

2.1.4 Type II Superconductors

2.1.5 Applicability of BCS Theory to Strongly Coupled
and High Tc Systems

2.2 High TC Oxide Superconductors
2.3 Crystal Structures of High TC Superconductors
2.3.1 YBa,Cu,OM (YBCO)
2.3.2 BSCCO system
2.4 High To Superconductor Thin Film Deposition Techniques

2.4.1 Pulsed Laser Deposition (PLD) of Superconductor
Thin Films

xii

\It

\O\O\IUI

13
13
19

24
25
26
28
30
36

37

2.4.1.1 Effect of Pulse-Width and Wavelength of the Laser
2.4.1.2 Nature of the Laser Induced Plume
2.4.1.3 Problem with High-temperature Post-Annealing

2.4.1.4 Methods to Improve Quality of Low-Temperature
Grown Films

2.4.1.5 Problem with Surface Particles

2.4.2 Pulsed Laser Ablation Using Millisecond Nd:YAG Laser
(ms-PLD)

2.4.3 Nd:YAG Solid State Laser vs. Excimer Gas Laser
2.4.3.1 Nd:YAG Solid State laser
2.4.3.2 Excimer Gas laser
3. EXPERIMENTAL PROCEDURE
3.1 Target and Bulk High Tc Superconductor Preparation
3.1.1 Pb Doped BSCCO 2223 Superconductor

3.1.1.1 Laser and Conventional Calcination of Pb
Doped BSCCO

3.1.1.2 Sintering of Pb-doped BSCCO
3.1.2 YBCO and Ag-doped YBCO Superconductors
3.2 Substrate Preparation and Silver Thin Film Deposition
3.3 Millisecond Pulsed Laser Deposition
3.3.1 Plasma assisted PLD Apparatus
3.3.1.1 Plasma Assisted PLD Chamber
3.3.1.2 Target and Target holder
3.3.1.3 Heated Substrate Holder
3.3.1.4 Oxygen Plasma Generation
3.3.2 Plasma Assisted Millisecond PLD Process (PPID)
3.3.3 Ion-Beam Assisted PLD Apparatus
3.3.3.1 Ion—Beam Assisted PID Chamber
3.3.3.2 Spatially Resolved Emission Spectrum Analyzer

vii

39
41
44

45

47
47
50
51
53
53
53

53
55
56
56
57
59
59
59
62
62
65
67
67
69

3.3.3.3 Oxygen Ion Gun
3.3.3.4 High Frequency High Voltage Power Supply
3.3.3.5 X-Y Target Manipulator and Heated Substrate Heater
3.3.4. Ion-Beam Assisted Millisecond PLD Process (IBPLD)
3.4 Post-annealing
3.5 Sample Characterintion Methods
3.5.1 Crystal structure and Phase Identification
3.5.2 Microstructures and Chemical Compositions

3.5.3 Electrical Resistance and Magnetic Susceptibility
Measurements

3.5.4 Four-Point Bending Test
4. RESULTS AND DISCUSSION
4.1 Factors Which Afl'ect Processing of Bulk YBCO Superconductors

4.2 Laser Calcination and Kinetics of Formation of
BSCCO - 2223 Phase

4.2.1 Kinetics of Formation of BSCCO - 2223 Phase
4.2.2 Microstructures andEDS Amlysis ofIaser Calcined BSCCO

4.2.3 Resistance and Susceptibility Measurements of Laser
Calcined BSCCO

4.3 MillisecondPulsedIaserVaporDeposition
4.3.1 Problems mthMiIhsecondPLD

4.3.1.1 Target Overheating Problem and Peak Power
D . 1' . 'on

4.3.1.2 Oxygen Depletion
4.3.2 Pressure Dependence ofLaser Induced Plume

4.3.3 Ion-Beam Assisted Millisecond PLD
4.3.3.1 Efi‘ect of X-Y Target Manipulator
4.3.3.2 Efi'ect of Oxygen Ion Gun
4.3.3.3 Spatial variation of film composition

73
77
79
81
8 1

S

85
91
93
93

93
93

103
105
105

105
106
106
109
109
109
110

4.3.3.4 X-ray Diffraction Analysis of IBPLD Films
4.3.3.5 Microstructure of IBPLD Films

4.3.4 VTE/ms-PLD Multi-Layer Ag/Ag-Doped YBCO Tapes
4.3.4.1 Effect of Ag Addition to YBCO Target

4.3.4.2 Adhesion between Ag Layer and Ag-Doped
YBCO Layer

4.3.4.3 Effect of Peak Power Density on the Composition
of Ag-Doped YBCO

4.3.4.4 X-ray Diffraction Analysis of Ag-Doped YBCO Layers
4.3.4.5 Critical Temperature of Ag/Ag-Doped YBCO Tapes
5. SUMMARY
5.1 Factors Which Affect Processing of Bulk YBCO Superconductors
5.2 Laser Calcination of Pb-Doped BSCCO
5.3 Ion-Beam Assisted Millisecond PID
5.4 VTE/ms-PLD Ag/Ag-Doped YBCO Tapes
APPENDD( Review of Selected Theories of Superconductivity
A. 1 Two-Fluid Model

AM Free Energy in Superconducting State

All Temperature Dependence of Superconducting
Electron Density

A. 1.3 Temperature Dependence of Thermodynamic Critical Field
A. 1.4 Electron Specific Heat

A.2 London Equation
A.2.1 London Penetration Depth

A.2.2 Exponentially Decay of the Magnetic Field B and
Supercurrent Density ]

A.2.3 Temperature Dependence of the London Penetration Depth
A.2.4 Magnetic Potential Field A and London Rigidity
A.3 Ginzburg-Iandau Theory (G.L. Theory)

ix

114
116
121
121

126

130
130
133
134
134
134
135
135
137
138
138

138
139
140
141
141

142
143
143
145

A.3.1 Ginzburg-Landau Order-Parameter

A.3.2 Ginzburg-Landau Free Energy

A33 Ginzburg-Landau Equation

A3.4 Equivalence of London Equation and Penetration Depth

A.3.5 Coherent Length

A.3.6 Upper Critical Field and Type II Superconductor
REFERENCES

145
145
147
147
149
151
155

LIST OF TABLES

Table Page
1. Various PLD processing parameters and properties of 1-2-3 films

associated with these processing parameters 38
2. Calcining conditions of Pb doped BSCCO 55
3. Important Processing Parameters for plasma assisted PID 65
4. Important Processing Parameters for IBPLD Films and

VTE / ms-PLD Tapes 82
5. Effect of the starting powder purity and powder compaction

on formation of YBCO 94
6. Typical compositions of YBCO films produced by IBPLD process 110
7. List of possible compounds which might produce a strong dimaction

peak near 30° of 26 for Cu-Ka radiation 114

LIST OF FIGURES

Figure

1.

10.
11.
12.
13.
14.
15.

16.

17.

Temperature dependence of normalized energy gap, calculated from
the BCS theory and experimental data

Current-voltage characteristic of a Josephson junction
Microscopic quantum interference experiment setup

Magnetization verses applied magnetic field for (a) type I and
(b) type II superconductors

A sketch of magnetic phase diagram of a type II superconductors

Temperature dependence of magnetization of a superconductor with
a irreversible temperature

Two different ways of drawing a ABO3 perovskite tmit cell

YBCO unit cell

2(1.a,CuO‘) unit cell

Unit cells of 2(I.a,CuO.) and BSCCO phases

Relation between crystal structures of La,CuO, and BSCCO - 2201
Relation between crystal structures of BSCCO - 2201 and -2212
Relation between crystal structures of BSCCO - 2212 and -2223
Typical pulsed laser vapor deposition setup

PLD of YBCO on SrTiO, substrate using fundamental (1064 nm),
2nd harmonic (532 nm) and 3rd harmonic (355 nm) of Nd:YAG
laser with 3.5 J/cm2 power density, 200 mTorr oxygen and 730 °C
substrate temperature

(a) Plasma-assisted PLD setup with 10“ Torr oxygen pressure during
deposition, and (b) De-biased PLD setup with 200 mTorr oxygen
pressure during deposition

Basic laser cavity

Page

12
18
18

20
23

23
27
29
31
32
33
34
35
40

42

46
48

18.
19.
20.
21.

23.

24.

25.

26.
27.
28.
29.
30.

31.
32.
33.

35.
36.
37.

38.

39.

41.
42.

Energy diagrams of (a) three-level laser and (b) four-level laser
Typical Nd: YAG laser cavity

Laser calcination apparatus

Laser beam delivery system

PLD chamber used for preliminary study

Specially designed mold, and YBCO target ring produced by
using this mold

Plasma assisted PLD setup (a) with a plasma ring and (b) with a
plasma tube

Plasma assisted PLD chamber (a) with the laser head and
(b) close-up view

A YBCO target ring after deposition process

The ion-beam assisted PLD chamber and the laser head

The ion-beam assisted PID chamber viewed from the right view port
Spatially resolved emission spectrum analysis setup

Spatially resolved emission spectrum analysis setup with
PLD chamber

Penning ion gun

Penning ion gun at work

Penningiongunmountedonthe frontportflange

Hish frequency high voltage power maply

(a) Low profile x-y target manipulator and (b) Heated substrate holder
Laser induced plume and oxygen ion-beam

Wiring diagram for both resistance and mutual inductance
measurements

Computerized resistance - temperature measurement setup

Cooling device for resistance - temperature measurement

Magnetic susceptibility measurement apparatus

Four-point bending setup

XRD patterns of conventionally processed Pb-doped BSCCO sample

49
50
54
5 8
60

61

63

68
70
71

72
74
75
76
78
80
83

86
88
89

92
96

43.

45.

47.

48.

49.

50.

51.

52.

53.

54.
55.

56.

57.

58.

59.

XRD patterns of 50 Watts laser-processed Pb-doped BSCCO sample
XRD patterns of 90 Watts laser-processed Pb-doped BSCCO sample

Scanning electron micrographs of (a) 90 Watts laser-processed
sample, sample A, and (b) conventionally processed sample, sample C

Scanning electron micrograph of a 50 Watts laser-processed sample

EDS spectrums collected from areas of the samples shown
in Figure 45 and 46

(a) Resistance vs. temperature curves and (b) Magnetic susceptibility
vs. temperature curves of conventionally processed and 90 Watts
laser-processed samples

Oxygen ion-beam and laser induced plume with 250 ps pulse-length
and 40 J/cm2 pulse-energy density at oxygen pressure of (a) 100 mtorr
and (b) 300 mtorr

Laser induced plume with 250 us pulse-length and 40 J/cm2 pulse-
energy density in one atmosphere air

Silver substrates in substrate heater after laser vapor deposition with
0.25 ms laser pulses, 40 J/cm pulse energy density and 100 mtorr of
O deposited (a) without ion-beam and (b) with ion-beam

Chemical composition distribution of as-depositcd IBPLD YBCO
film on fused silica substrate (quantitative EDS analysis)

Typical EDS spectrums from (a) a IBPLD YBCO film and
(b) a YBCO target

XRD patterns of YBCO films deposited on a silver substrate

XRD patterns of YBCO films deposited on a (001) YSZ substrate and
(001) YSZ substrate with an Ag buffer layer

Scanning electron micrograph of an as-deposited IBPLD film on
(001) YSZ (0.25 ms laser pulse with 40 .I/cm2 pulse-energy density)

Scanning electron micrograph of an as-deposited IBPLD film on
(001) YSZ with silver bufi’er layer (0.25 ms laser pulse with 40 J/cm

pulse-energy density)

Scanning electron micrograph of a 850 °C, 1 hour, annealed IBPLD
film on silver (0. 25 ms laser pulse with 40 J/cm pulse-energy density)

YBCOand23%Ag-dopedYBCOtargetsafter4or151aserscans

SEM micrographs taken from surfaces of (a) 23% Ag-doped YBCO
targetand(b)YBCOtargetafierfourlaserscans

97
98

100
101

102

104

107

108

111

112

113
115

117

118

119

120
122

123

61.

62.

63.

65.

67.

68.

Cross section SEM micrographs of (a) 23% Ag-doped YBCO and
(b) YBCO targets after four laser scans

Cross section SEM micrographs of (a) 23% Ag-dopcd YBCO and
(b) YBCO targets after fifieen laser scans

A 15 wt% Ag-doped target after 35 minutes of laser irradiation,
and two Ag/Ag-dopcd YBCO multi-layer thick tapes with
approximate 8 microns of Ag-doped YBCO top layers

SEM micrographs from (a) surface of an as deposited film, and
(b) fractured cross section of an Ag-doped YBCO/Ag film

Typical optical microstructure of an annealed, 15 wt% Ag-doped
YBCO film

EDS spectra from (a) 15% Ag -doped YBCO layer deposited with

2. 5 ms laser pulses, 270 chm energy density and about 0.2 MW/cm1
peak power density, (b) 15% Ag-doped YBCO layer deposited with
0.6 ms laser pulses, 270 J/cm1 energy density and about 0.9 MW/cm1

peak power density, and (c) 15% Ag-doped YBCO target

Typical x-ray diffraction patterns taken from as-deposited and
annealed 15% Ag-doped YBCO layers

Typical resistant-temperature curve of an annealed Ag/Ag-doped
YBCO tape

124

125

127

128

129

131

132

133

I. INTRODUCTION

Three most important high-TC (high critical temperature) oxide superconductor
systems are Y-Ba-Cu-O (yttrium-barium cuprates or rare earth-barium cuprates), Bi-(Pb)-
Sr-Ca-Cu—O (bismmh-lead-strontium-calcium cuprates), and Tl-Ba-Ca-Cu-O (thallium-
barium-calcium cuprates). All of these three systems have complicated crystal structures
with copper-oxygen planes, which are believed to be responsible for the superconducting
properties of these compounds. Among these cuprates Tl-Ba-Ca-Cu-O system has the
highest TC , about 125 K[1, 2], but the potential for industrial application is limited due to
severe health problem associated with the toxicity of thallium compounds.

Material processing utilizing high-power lasers, especially high-energy pulsed
lasers, have been widely adopted to produce high Tc superconductors. These techniques
include:

1. Pulsed laser vapor deposition (PLD) or ablation[3-23]
2. Thin film patterning
(a) by pulsed laser ablation[24-26]
(b) by direct laser writing of precursor films[27]
(c) by laser assisted etching[28]
3. Laser calcining of high Tc superconducting powder[29, 30]
4. Laser re-melting/texturing[31].
5. Laser annealing ofsuperconducting thin films[32].

Bi,_bexSr,Ca,Cu,O,r (2223 phase) of the Bi-(Pb)-Sr-Ca-Cu-O (BSCCO) system,
with Tcabout 110K, isagoodcandidate fortape applicationdueto itsthinplate shaped
grains that facilitate mechanical alignment of the grains to obtain higher critical current
density[33-34]. However, the BSCCO superconductor system has a number of phases

2
with difl'erent critical temperatures. The three major phases are referred to by their
cation ratios as 2201(Tc below 77 K), 2212 ('1C ~ 85 K), and 2223 (TC ~110 K)[35].
Since the discovery of superconductivity in this system[36], many efforts have
been made to maximize the amount of high Tc-2223 (110 K) phase. A major enhance-
ment in the formation of high TC-2223 phase was made by doping Bi-Sr-Ca—Cu-O
compound with Pb[37-41 ]. However, With Pb doping, about 200 hours of total
processingtimeisrequiredtoobtainasingleornearlyasinglehigthphase. Inthis
study, a new laser processing technique was developed with an objective to enhance the
kinetics of formation of the high Tc phase of a Pb-doped BSCCO superconductor. With
laser calcination process, near single-phase high Tc phase samples were obtained in
about 100 hours. The onset critical temperature of the laser calcined sample was found
tobeabout 110K However,thezeroresistancetemperaturewasabout 98K.
Thcdiscoveryoforddehightemperatmesuperconductorsopensmunemus
opportunities for applications that are previously impossrhle. At the present time,
potentialapplicationsofbulk oxide superconductorsareveryrestrictedbecauseoftheir
poor mechanical properties and low critical current densities.
Enormousefi‘onhasbeenmadewproducethinfilmswithconectstoichiomeuy,
high tramition temperature and high critical current density. YBa,Cu,O, (YBCO), with
Tcaround95K,isconsideredtobethemostfavorableoxidesuperconductorforthin
film applications, because compared with other systems, it has less complicated
chemistryandstructure, goodplnse stability, andhigh critical current density[3].
Amongthetechuesadaptedmgrowhigthmperconductorthmmthsed
laser vapor deposition is the most widely studied and a most successful one[3-6,9. 10,19].
ThekeyreasmsfmthemcessofthePLDprocessuehsmoichiomeuyconservauon
property, and the production of excited high energy vapor species[42-46].
ItiswellknowntlmthighqualityYBCOthinfilmscanbeobtainedbyusingan
excimerlaserwitha193,248,0r308nmwave1ength,nanosecondpulsewidthanda

3

few J/cm1 pulse energy density. However, the deposition rate is very low, typically a few
tenth ofnm per second Moreover, the deposited area coverage is very small, only a few
millimetersinwidth,duetothenarrowangulardismhmionsofthevapor,assharpas
cos' 0 to cos11 0[15.16,42]. On the other hand, high deposition rates up to 100 nm/s[15]
andalargetmiformdepositioncoverage,duetoabroader(cos0)angulardistributionof
the evaporated species can be obtained by using millisecond pulsed lasers. These
enhanced deposition rates make millisecond-PLD (ms-PLD) a practical process to
produce continuous coating of large substrates.

Adetailedstudyofthems-PLDprocesswascarriedoutusingamillisecond
pulsedneodymium: yttrium alrnninrnngarnetCthYAGHaserwith 1064nmwavelength.
Due to several orders of magnitude longer pulse-width compared with excimer lasers,
several hundred Joule/cm1 pulse energy density is required to achieve a peak power
densityclosetoMega Wattle-1,whichisstillmuchlowerthanthepeakpower density
achievedbyanmosecondhserprflseattypicallyafewlouldcmknergydemfiy.

Overheadngmdsevemlocalmelfingwhichresrfltedinmughflngofthetm‘get
andthuebymisdhecfingflnvapmpltme,ucmajmproblemsassociawdwithms-PID
process. Thisproblemisreducedbyusingazigzagscamingpaaemofthetargetwithan
x-y target manipulator. However, for prolonged deposition without fiequent resurfacing,
themflseenergydemityisresuicwdmabommJ/cm1matdsohmimmedeposifionmm
below 10 nm/s[47]. Because the ms-PLD process has a relatively high deposition rate
andlowpeakpowerdensity,apenningiongtmisusedtointroducehighenergyoxygen
iomtothelaserinducedplumeandthegrowingfilmtoreplenishoxygenandtoboostthe
energyofvaporspecies.

Mostpracficalengineedngapphcafionsrequinthesupemondmtmstobefabfi-
catedintodesiredshapessuchasthinfilm,wiresortapes. However,brittlenatureofthe
oxide ceramic superconductors prohibits the use of conventioml forming techniques, and
monofithicceramicmpercondmmrwireormpewinnmhaveenoughwughnessfm

4
normal handling. Both of these problems are expected to be reduced by forming

compodtes of superconductors and some ductile metals. However, compatibility is the
major problem in selecting a metal for this purpose[48,49]. Silver has been proven
not only to be compatible with both YBCO and BSCCO systems, but also to improve
superconducting properties of these compounds[50-53]. In addition, texture of the
superconducting layers is expected due to geometric constraint of layered structure.

Uniform thick films can readily be obtained by vapor deposition techniques.
Among Me techniques, vacuum thermal evaporation is a simple yet effective way to
deposit thick films with simple composition such as pure silver. Oxide superconductors,
on the other hand, have a very complicated composition, so special techniques such as
thems-PIDarerequiredtopreservetheStoichiometryofthecompound Inorderto
fitrtherenlnncethedepositionrate, silverdopedYBCOwasstudiedastbetargetmaterial
fwthems-PIDprocesavviththehopetthubsmfaceoMeafingproblemcanbe
significantly reduced by increase density, thermal conductivity, and mechanical strength
ofthetarget[19]. AneWprocesscombiningtheabovetwodepositiontechniquesto
produce a multi-layer YBCO/silver thick film was developed

Theprhnaryobjectiveofthisresemehistodevelopnewprocessesofhigth
mpemondmtomwhichmkeadvmtageofmemiqmproperuesofamflhswondptused
Nd'YAG laser as the energy source.

2. LITERATURE SURVEY

2.1 Superconductors and Theories of Superconductivity“

The electrical resistivities of many materials, including elements, alloys,
ceramics, and organic compounds, vanish when a specimen of these materials is cooled
below some critical temperature, TC . This phenomenon, called superconductivity, was
first observed in mercury in 1911 by Heike Kammerlingh Onnes[54]. Subsequent
researches showed that superconductivity is fairly common among non-magnetic metallic
elements with the highest Tc about 9.5 K for niobium[55,56]. In 1933, Meissner and
Ochsenfeilf discovered that a superconductor completely expels magnetic flux lines from
its body when it is cooled through the critical temperature under a weak magnetic field
The Meissner effect, or perfect diamagnetism, is another essential property of
superconductivity, which cannot be obtained by the characteristic of zero resistivity, or
perfect conductor, alone. In a magnetic field, the Meissner efl‘ect implies the presence of
surface screening current that produces a magnetic field that exactly cancels the external
field Important parameters of superconductors such as magnetic penetration depth,
critical field and critical current density are related to the Meissner efi‘ect. Among
superconducting elements niobium, once again, has the highest critical field Hc about
0.2 T[ 55, 56]; however, it is too small for most practical applications such as high-field
superconductingmagnetsenvisionedbyOnnesinhis 1913 Nobel Price lecture.

2.1.1 Two-Fluid Model, London Equation and Non-local Electrodynamics

Following the discovery of Meissner effect, several important phenomenological
theories were developed to understand properties of superconductors[56]. Among these

‘ The COS-Gaussian units are used for formulas in this section.
A complete list of symbol definitions are included in the appendix.

5

6
is two-fluid model used by Gorter and Casimer (1934) to obtain a free energy expression
that in turn gives expressions for electronic specific heat and fraction of superconducting
electron density, nsln = 1 - (T/Tc)‘. Subsequently, the brothers F. and H London (1935),
based on the two-fluid model, formulated the London equation[ 55, 56, 58]

2 2
age _ _ n,e
C B or ]_ —A

ij =— mC

 

relating the supercurrent density 1 and the magnetic field strength B or the magnetic
potential field A. Combining with Maxwell's equation, the magnetic field and
supercurrent density have expressions in the same form[55,56].

v23: Mi ; v21: 10%

Theseequafionshavesolufionsthatdecayexponentaifly,fiomthesmfaceinma
nrpercon¢wtmbtflk,wimachamcwfisuclengmxhcauedmemagneficpenemuon
depth.

2.2 a mC1/4rtrr.,-e2

Non-local electrodynamics was applied to explain the strong dependence ofthe
penetration depth on alloying of elemental superconductors by Pippard (1953)[56]. It
wasthoughtthatthelocalmlafionbetweenjandAintheIcndonequafionmustbe
generalized to include the efi‘ect of field throughout a volume of radius 5 about the origin,

whereZisthemeanfieepathofnormalstateelecuonsdepcndentsuonglyonthe
alloying Thesuperconductingwavefimcfionshorfldhaveacharacteristiclength,the
wherencelengmwhichphysamlesimflmbtheelmnmeanfieepathinnormal
mealslhecoherencelengthcmbeesfimawdusingmelhcamtyPfimipleassmnmg
that only electrom within about k,,Tc of the Fermi energy (E,) are responsible for
superconductivity. Therefore, the importantelectronshave amomenturn uncertainty

7
AP is k,,Tc / V, , where V, is the velocity of electrons with the Fermi energy (E,). A

correspondingpositiontmcertaintyisAxe Il/APz h,/(k,T&. Thus, the coherence
lengthcanbedefinedas[56]

11V]:
:0 - akBTc

 

2.1.2 Ginflrurgolandau Theory

Ginzburg and Iandau (1950) proposed a phenomenological theory of
superconducting state intermsofa spatially variedorderparameter ‘1'(r)whichvanishes
at a second-order pinse transition temperature[55,56,58]. The order parameter is
hypothesizedtoberelatedtolocalsuperconductingcharge carrierdensityn,'by

more) = mm 2 = no) .

Mmyimponantchanctefisficsofnrperconductomcanbeunderstoodbyadopung
theparfideprobabilhymnpfimdemthecmterofmasswawfimeumoftheCmperpaim
fiomtheBCStheoryastheorderparameter. EvenwiththedevelomentoftheBCS
themy,theGinzbmg-Iandauthemyissfiflofmepreemimmimpmnmeespecifllyfm
thinfilmsandtypellsuperconductors. Thistheoryisessentiallytheonlywaytodeal

ThistheorystartsbyformulatingflreGinzbtrg-Iandaufieeenergydensity
difi'erencebetweenthesupercondwfingsmteandnormalstateas[55,56,58]

. 2 2
__ _ 2 _1_ 4 1 _ _ eA H
F3 - FN- GI‘YI +ZBI‘PI -l-2 , |( 311V C )‘I’l + 8!

Thefirfltwotemsalonerepresemflwfieeenergydemitydifi‘aememtheabsenceof
magneficfieldandspafialvafiafionofthemderpanmeter,whemaandfimepammem

8
to be determined The third term, arising from kinetic momentum —rhV and field
momentum -e'A/C, denotes the energy increase due to spatial variation of the order

parameter‘I’,wherem'ande'aremassandchargeofthechargecarrier,andAisthe
magnetic potential field defined by B = V x A. The H1/(8n) term gives the energy
increase due to magnetic flux expulsion, i.e. Meissner effect.
Inndonequationcanbederivedfromthefreeenergyexpression Minimizingthe
fieeenergywithrespecttothepotentialAyields

$.W- W)+m TT‘TA't—VXH: 0 .

whichiscalledtheGinzbmg-Iandauequation. ThefirsttermdropsduetotheLondon
rigidity, i.e. at ground state the superconducting wave function is essemially unchanged

by the application of the magnetic field. Combining with Maxwell's equation,
]= [C/(4rt)] V x H , yieldstheLondonequation

.2
- rash
Apenehationdepthsimflartothelcndonpeneuauondepthisdefincdas[55,56]
. - (are) "2
4xe’1a

MhfimizingthefieeenergyudthrespecttotheorderparameterVyields

               

 

(.0... BI‘PI1+— -mv— °—’A|1)st= o .

This is also called the Ginzburg-Landau equation that resembles a Schrédinger equation
for $755.56]. Comidering this equation in one-dimension and in the absence of a field,
yields

111 42
"21.73;:— - “‘1”

wherethenonlinearterm Bl‘l’i1isomittedfortemperatmesnear checausel‘I’l’ issmall.
This equation has a wave-like solution ‘P(x) = ‘I'oexp (—r'x / g). The coherence length,

g a [111/(2m’a)] 1/2 , marks the extent of coherence of the superconducting wave
function into the normal region[55,56]. The dimensionless ratio between the two
characteristic lengths A. and g, called Ginzburg-Landau parameter, is an important
parameter in the theory of superconductivity.

Simflartothenormalstateelecfionmeanfieepaththecohemncelengthandthe
penetrationdepthareverysensitivetopmityofthematerial. Asaresultthe
Ginzbmg-Iandwpanmeterxmmeasesmpidlyasthepmityofthemperwnducwr
decreases[56].

1C

lll
4" I?

2.1.3 BCS Theory

AcomprehensivequanumtheoryofsupereonducuvhywasproposedbyBardeen
Cooper,andSchriefi‘erin1957,almosthalfacennnyafierthediscoveryof
superconductivity[57,58]. ThisNobelPrizemasterpiece is fiequerrtlyreferredasthe
BCS theory. Main features of this theory include[55-58]:

2.1.3.1 Phonon-Mediated Pairing of Electrons (Cooper Pairs):

Cooperpairformsbyindirectelecuon-phonon-eleetronirueractionoftwo

e1echomudthoppositespimnearfiesphefichaminufacemndwaveveflmkand-L
respectively. Whenoneofflreelecuonsmovesdn'oughflrelatficemelauiceisdistmted

10
by means of a phonon such that the other electron adjusts itself to take advantage of the

lattice distortion to lower its energy. The so-called weak coupling assumption is used in
the pairing theory in which the electron-phonon interaction matrix elements U are

assumedtobeindependent ofkforstates withintheEpihcoD range, whercE,and
11m D are the Fermi energy and Debye energy, respectively. As a result, energy of the
paired electrons is lower than 2E, even if the wave-vector k and -k are slightly outside

theFermisplrm. ThebindingenergyofaCooperpairmaybewrittenas[55]

21100

«mi-ea)

..l’

whereN,isthedensityofnormal-stateelectronsattheFermienergy. Thispairingtheory
indicatesinstabilityoftheelectronsystemandleadstotheconstructionoftheBCS

groundstate.
2.1.3.2 BCS GroundStateandEnergy GapNeartheFermiEnergy

ThecrucialfeatureoftheBCSgromdstateisthepairoccupancyofthe
one-paruclembns(mates)becametheCmperpmrisfomedfiomapairofelecnons
Mthopposhespinsandwavevectorkand-k,ie.fimerevemedmomennnnstates.
Unlike electrons with half spin, a fermion obeying the Fermi-Dirac distribution of one
parficlepaqumhmstate,theCooperpaimhavezemspin,hencehavemmyamhmes
of bosons that follow the Bose-Einstein distribution and have no limit on the number of
identical non-interactingparticles perstate[59]. However, duetointeractionbetweenthe
prerpaim,meweak1ycoupledCooperpaimufisfyamixedmfisucsmwhichthe
Paufipfincipleholdsforpaimwiththesamekvalue,becausetherearemughlylO‘pairs
inthevolumeoccupiedbyagivenpair. Thisisalsothejustificationtousestatistical
“fieldapproximationtosolvemanybodyproblemsintheBCSwory.

1 1
Unlike the ground state of Fermi gas of non-interacting electrons, the filled Fermi

sea that allows arbitrarily small excitations, the BCS theory demonstrates a
superwnducfinggromdshtesepamtedbyamiformfimuenergygapfiofmeorder
k,Tc from its lowest excited state. Unlike energy gaps in semiconductors, the excitation
energies for quasi particles, i.e. the Cooper pairs, are positive for both above and below
the Fermi momentum. The energy E. is centered aromd the Fermi energy E,, however,
E,isusuallytakenasthezeroenergyscalebecausetypicalvalues ofE‘andE,areafew
meV and about 5 eV, respectively. The energy gap, which is closely related to the

bindingenergyofCooperpair,at0kis[56]

5,: ”0 z4hmDe-(fiJF-E) .
va

TempuahnedependemiesofnmmalizcdenergygapcdcrdfledfiomBCStborydong
withexperimental data from elemental superconductors are presented inFigure 1[56].
TheenergygapdecreasesasthetemperatmeincreasesanddisappearsatTc
Temperatuedependenceoftheenergygapmarchanbeapprordmatedbflfi]

E, re 3.06 no —'r/r.,.)"2 .

At lowtemperature, T/Tc<10, exponential temperaturedependenceof electron specific
heatcapadty,atypicdnmutusociawdwimexcflaumwossawnsmntemrgygap,is
obminedbecauseE‘isalmostconstantMthinthistempemunemnge,Figmel.
Mmeover,thecnucalfieldandmostoftheelecuomagneucproperfiesaremsultsofthe
energygap. However,inverymrecases,mperconducfivityisobservedwithomanactual

cncrsysap-

12

 

 

 

 

9’: 0.6
§ 1. — BCS
t 04- A Tin
4 ' o Tantalum
' + Lead
0.2” O Niobium
l—

 

0 J l J J l l J J

0 of 0.4 0.6 0.3 l.0
tlT/Tc

 

 

 

Figurel. Temperatmedependenceofnormalizedenergygap,calculated
fiomtheBCStheoryandexpenmentaldata.

 

13
2.1.3.3 Critical Temperature and Isotope Effect

The BCS theory predicts a second-order phase transition to a new electron state
with the critical temperature given by[55,56]

Tc: 1.14 061/ (N50) or

KBTC= 1.14 “(DOC-1,011 ) .

The isotope efl'ect, i.e. Tc proportional to inverse square root of the atomic mass of the
isompe,ismedimctmsrdtofatomicmassdependenceofmeDebyetemperatme0. An
WWmmWMWMMEMagMWmL
inthesenseofhigth,isabadconchrctoratroomtemperaunebecausethemom
umpemuneelecuicdresistivityisameasmeofelwuon-phmonmwmcumandcanbe
usedtoestimatetheelectron-phononinteractionmatrixelementsU. Sincethe
expressionforchisinthesameformasthatfortheenergygapE',theBCSthcory
predictstheparameterfieeratioEg/kBTCtobe3.52whichcanbeusedtogetanideaof
thevalidityofweaklycoupledpairingthcoryforagivensuperconductor.

2.1.3.4 Probability Amplitude of Cooper Pairs

Forabosongaswithalugenumberofbosominthesameorbital,theprobabflity
amphmdecmbeueatedasachssicdqumntycomisfingofanamplhudeandaphase;
therefore, both the amplitude and phase are meaningful and observable. For example, the
electric field intensity, with observable amplitude and phase, acts qualitatively as the
probabflityamplimdeforphotonswhenthetotalnumberofphotonsislarge. The
pmficleprobabilityamplimde‘P(r)forCooperpaim,thecenterofmasswavefunctionof

14
Cooper pairs or the Ginzburg-Landau order parameter, with net momentum can be

expressed as[56]
‘1’ = Your) = ‘l’oe’em = n’ in em’) and

‘11. = YOW‘(I) = Vac—190') = n13!” c—fe(r),

wherens'isthedensityofCooperpairs,and0(r)isthephase.
The London equation can be obtained using the particle probability density as

follows[55]:
From the Hamilton equation of mechanics, the velocity of particle is

v=,%(p-é-’A) = 1% “I“ ' 21:14) .

Theelectriccurrentdensity] ofthesupercurrentisgivenbytheparticle
charge e'timestheparticle flux ‘1”v‘1’.

j: e’W‘v‘P= e’7.l—,'(nv0 - %A)

Takingctn'lofbothsideandusingthefactthatctnlofagradientofa
scalarisidenticallyzeroandtherelationB=Vx A,theLondon

 

equationisobtained
..z
__ 03¢
ij— m’CB

ThisacwmmfmtheMaln'naefl'xtandthepeuarafioudmhasdescnhedmIcndon's
theory.

Thecoherencelengthcanbederivedbymodifyingthephasepartofthe
one-dimemional plane wave particle amplitude probability, w(x) = exp(r:kx) , to a
str'onglymodulatedwaveftmction[55]

15
«,0: 2-1/2(ci(k+q)r +crlu) .

Thus, the probability density, (Tow'X‘I’ow) = ns'(l + cos qx) , is modulated instead of a
uniform density, W = ns' , of the plane wave. Increase of energy due to the modulation
can be calculated fiom the kinetic energy difference, assuming q << k[55].

 

ldX¢‘(-2',‘:. :2)? -ldxv'(-5%i23)w

%{%[<*+4>2+k2]-k2} is

ThisemrgymcreasemaynmexceedmeenergygapE'mmemisethenmwnducuvity
willbedestroyed Thus,theintriudccolrerencelengtlréminterpretedasthemcan
spachgbetweenpahofelecfiomfmagivenCooperpahmanbedefinedbasedonthe
criticalvalueofthemodulatioanfi].

11sz 11v].-
§0 * 1’40 = r13: = 53';

wherek,isthehighestpossiblekvalue,andthecorrespondingvfistheelectronvelocity
attheFermisurface. AmorepreciseresultgivenbytheBCStheoryis[55]

§0=2UVFIKE8 .

Enquaufizafionisanimportantchmacterisucofsuperconducfivitythat
resuictsthemagneficfltnrtbroughaholeornon-superconducfingregionina
superconductor disk to integral multiples of a quantity O, called fluxoid or flux quantum.
Thismsfltcanbedefivedfiomtheexpressionfmsupercmrentdemflyjmmissecuon
HiingratedalmgaclosedpathCJeephanmemondmtmrhgfiheresuhiswo
becausethereisnosupercurrentintheinteriorofasuperconductor,i.e.[55,56]

16
_.c_’ 2 _°_’ . =
iA-d-m.ITI {C(hve CA)d1 0

or 11ch0- (11 = {T’ch-dl .

Integration of V0 along a path gives the phase change A0 from starting point to end point
which has to be integral multiple of Zn, i.e. 21m, for a closed path. Applying Stock's
theoremtoconvertthe integral ofA intoa surface integration over areaSboundedby C ,
yields[55,56]:

Iii. ve-dr = 21mh = flea-d! = ‘5 I. VxA-ds = ‘6 LB-ds = °E<I> .

Definetheflraor’dOo a 2nllC/e’. Thetotal fluxtbthroughthesuperconductor ring
becomes

6 =n(-2;fi) =nd>o .

Thevalueofe'wasdeterminedtobeZebyexperimentaflymeasmementofO. This
result ¢o=2nllC/(Ze) is in agreement with the electron-pairing theory. The flux
quamizationisagoodexampleoflong-rangequantmnefi‘ectthatthecoherenceofthe
superconductingstateextendsoveraringorsolenoid

Under suitable conditions superconducting electron pairs can move from a
supercondtwtmthroughammmulamrhyermtomomersupemonduamifthehyer
thicknessissmallerthanthecoherencelengthofthesuperconductors. Thisphenomenon
iscalledloseplrsorr srrperconductortrmnelr’ngflfl. Thisphenomenoncanbedescrrhed
using the phase difi’erence, A0=01-02=A00-2th/li , ofthe particle probability
ampfinrdesbetweenflntwosidesofthejmcfionwhereVandtarethedcvohage
appliedacrossthejtmctionandtime. Thesupercurrentdensityacrossthejtmctionis[55]

j = losin(A00 - 2eth 11)

17
Asaresult,thechoseplrsouefl'ecrreferstoadccmrentacrossthejtmctioninthe

absence ofany electric or magnetic field, and the ac Josephson eject refers to a rf
oscillating current across the junction with an applied dc voltage. The oscillation current
basaangularfreqwncyc0=2eV/1i ,e.g. 1 uVappliedvoltage producesafiequency of
483.6 MHz This can be interpreted as that a photon ofenergy 110) = 2eV is emitted or
absorbed when an electron pair crosses the biased junction. Besides the junction with
insulator middle layer, i.e. SIS junction, the dc Josephon effect is also observed on
junction with normal metallic middle layer, i.e. SNS junction. Josephson jtmctions,
whichcanbeusedashighspeedswitchingdevices,arethekeyelementof
superconducting electronic devices. This is due to the characteristic current-voltage
relationwithtwowell definedvoltage statesch, whicharerelatedtothe energy gap

by E.= e Vc , Figure 2[55]. A very important application ofthe Josephsonjunctions is
the superconductor quantum interference devices (SQUIDS) which are used as high
sensitivemagnetometers. TheoperationofSQUIDSisbasedonthemcroscopic
quantum interference that is another example of long-range quantum efl‘ect of
superconductivity. Similartothederivationoffiuxquantimtionfihetotalfltuttbpasses
throughtheloopinFigure 3[55]isrelatedtothephasedifi‘erence goingoncearoundthe
loop, A0, by A0=(2e/llC)d>. The phase difi‘erences caused by the magnetic flux at
jtmctionaandbare cc /(11C)and-ed>/(liC) , respectively. Total current density j is[55]

i=ja +11, =J'0[8in(490 + fl) + gin(M10 ‘ $11) J

. . co
= ZJomAOOcos-fé- .

 

l I Voltage

 

 

 

 

Figure 2. Current-voltage characteristic of a Josephson junction.

 

Insulatora

la
1 \

\

Insulatorb

 
 
        

/

/
//

—s

i
©B W

//
/

/

 

 

 

F1311" 3' “MC quantlnn interference experiment setup

19
2.1.4 Type II Superconductors

Most pure elemental superconductors are type I superconductor that show
complete Meissner efl‘ect up to the thermodynamic critical field Hc , the critical magnetic
field calculated from free energy balance, and becomes normal conductor in a field larger
tlnn He, Figure 4(a). On the other hand, the type II superconductors, Figure 4(b),
maintain complete Meissner effect up to the lower critical field He, and carries some
magnetic flux between the lower and upper critical field; however, the zero resistance
stateispreservedupto‘theuppercritical fieldI-IQ.

Using the Ginzburg-Landau equation, the upper critical field H0 is related to the
thermodynamic critical field Hc by[55]

Hc2= Edema/(21:9),

wherex=M§istheGinzbmg~Iandauparameten Forx<1/f2—,H°<Hcdefinesthe
typeIsuperconductorwhichshowscompleteMeissnerefl‘ect. Ontheotherhandfor

x>1lf2-, Ha>HcdefinesthetypeIIsuperconductorwhich shows completeMeissner
efi'ect inweakmagnetic field, i.e. belowHa, andallow some fluxto penetrate regions in

numalsmenmoundedbysuperwndmfingmauixmwongfieldiehetweenliamd
11°.ThetwocasesfmtypensupacomwmamcafledMeissnerstateandvonex
state,respectively. Thedimensionofavortexcordcanbededucedfromtheabove
equauonmbemtheorderofthecoheremelengthbecamethevorficesmepackedas
closeaspossibleatI-Ia.

20

 

 

 

 

 

 

 

 

 

 

 

 

 

(8)
HC -
Type I
2..
'T
Superconducting Normal
state 3% state
He
Applied magnetic field Ba —->
(b)
Hc ..
Type II
2.:
‘i
Meissner Normal
Vortex 8131: state
L
Hcl He Hc2

Applied magnetic field Ba —9

 

 

 

Figure 4. Magnetintionverses applied magnetic field for (a)type I and
(b)typeIIsupereonductors.

21
Lower critical field Hc1 also can be related to the thermodynamic critical field I-Ic by[55]

He] =11. / (flu) = so / (410.2).

From the above two equations the thermodynamic critical field is the geometric mean of
the upper and lower critical fields. For many type II superconductors, HQ is much larger
than1-1c,i.e.x>>1/f2—,soHaisverysmall.
Thepresenceofvortexstatecanbeunderstoodintermsofmterfacialenergy
betweennormalandsuperconductingregions. Thisinterfacialenergyispositiveifxis
smallandnegativeifxislarge. Thecrossoverfiompositivetonegativeinterfacial
energy occurs at x=llf2—. The term vortex state describes the circulation of
superchorfices,aromdthhmdsofmn—superconducfingmgionudthinthe
superconductoreachbearingasinglefltntoid Thisconditioncanbeunderstoodby
minimizationofmagneticenergyassociatedwiththevortices. Assumingasinglevortex
with n fluxoids, i.e. Bat n<bo, the magnetic energy associated with this vortex is
proportionalto(n00)2. Asarestfltthemagneticenergyisminimizedwhenthisvortex
splitsintonvorticeseachbearingasingleflturoid. Thisresultagreeswithexperimental
observations.
hthevmtéstatemorficesmpeleachotherdmbthelmentzforcebetweenthe
circulationctu'rentofavortexandthemagneticfieldofitsneighbor. Asaresult,the
repulsion energy is minimized when the vortices form a close-packed hexagonal array,
referred as the vortex lattice or flux lattice. However, vortex energy can be lowered by
mociafingflnvonexudthsomewucuualdefectsofmesupemondtwtmcanedthe
pinningcenter. Foragivenstructrualdefecttobeanefi‘ectivepinningcententhe
dimensionofthedefecthastobecomparabletothesizeofvortexcord,i.e.intheorder
ofthecoherencelength Foratypellsuperconduetorwithrandornlydistributedpinning
cemfls,flrevorficesmealsomndomlydisuibtuedmdmfenedasthewflexglmor

flux gloss.

22
Considering a current J flowing perpendicularly to the magnetic field B through a

vortex lattice, i.e. no pinning of the vortices, the Lorentz force acting on the vortices
pushes the vortices in the J xH direction. This motion of magnetic flux in turn induces an
electric field that opposes the current J, resulting in energy dissipation and appearing as
an electrical resistance! Thus, the dynamics of vortices is of technological importance.
Motion of the vortices can be classified into two major types, i.e. flux creep and
fluxflow. HumepreferstothecasethatthermalenergyandLorentzforeeare
smaller than the average pinning energy; therefore, vortex motion and electrical
resistance are tolerable for many applications. Flux creep is found immediately after the
change of external field or current and decays logarithmically. It can be detected by
mcasm'ingthetrappedmagnetic fieldortheelectrical resistance. Ontheotherhandflux
flowoccmswhenthethemalenergyandlcmntzfmeeamhrgeenoughmovercomethe
pinning force. Under this condition, referred as the vortex liquid, the vortices move
steadily, and the material is useless as a low-loss conductor. By measuring the transition
temperature fromvortex liquidtovortex glass T'under different magnetic field, aphase
boundarycanbeaddedtothephasediagramofatypellsuperconductor, Figure 5(56].
Forhigthsuperconductms,whicharetypeHsuperconductors,atmique
magneficpmpmtyisthattemperatmedependenceofthemagneficmomemeecomes
irreversibleatsometemperamre, calledtheirreversibletenqreratureTm, foragiven
magnetic field. Ifa Zero field cooled sample is warmed up in some given magnetic field
toatemperatrueabovethheMvs. temperatmecm'vewillnottakethereverseofthe
origiml warm-up path when the sample is cooled down in the applied field, Figure 6[56].
Simihrmthevonexhqmd-glassphasebomdary,minewrsibifiofinecanbedefimd
foragivenhigthsupereonductor. Thevortexliquid-glassphase boundaryandthe
ineversibflityhneforagivensupercondtwtmaresimflmbecmmebothofthemam
closelyrelatedtothepinningpotentialofthe superconductor.

23

 

 

 

 

 

T i.
— x Vortex ch '—
\x W V
E VTE state
F- Vortex glass \ .-
_ M x _
ii“... . :4
T00
Temperature (K)

 

 

 

Figure5. Asketchofmagneticphasediagramofatypellsuperconductors

 

 

 

 

 

 

 

 

 

8 cool in field
.g —* ‘

a

i

zero field cool
-1/41t.
Trap. To
Temperature

 

 

 

Figure 6. Temperature dependence ofmagnetization ofa superconductor
with a irreversible temperature

24
2.1.5 Applicability of BCS Theory to Strongly Coupled and High TC Systems

Despite the simplifying assumptions, i.e. 1: independent weak coupling and
spherical Fermi surface, the predication of essentially all known superconducting
properfiesbasedmtheBCStheorygeneraflyagreeweanexpefimenmlresulmfm
most conventioml low TC superconductors. However, certain deviations are observed
especiaflyforsocafledmadactors"suchasleadandmerctuywithverysuong
electron-phonon coupling such that Tc becomes a reasonable fraction of the Debye
Wo

The mean field approximation used to derive the BCS results becomes less
appropfiatewhenthenrmbaofpaim,whosecenterofmassfaflwithhthevoltme
occupiedbyagivenpair,approachtheorderofunityasinthestronglycoupledhigth

superconductors. Incontrasttoconventionalsuperconductorswithcoherencelength
between100nmtolx101nm,thelayeredhigth superconductorsystemshave
coherencelengthabout1.5to3.0nminthea-bplaneandassmallaso.3nminthec

direction. Nevertheless, the BCS theory continues to work sufliciently well on many
aspects of these strongly coupled systems.
Itshotddbemtedthatthephonon-mediatedpaifingmedintheBCStheorydoes
notrestricttheuseofthebasic formalisrninsystemswithotherpairingmechanism[58].
InfactthebasicequationsfortheBCScondensedstateapplynotonlytotheweakly
coupled Cooper pairs but also to strongly coupled electron pairs and other fermion pairs
that resulted in boson-like conderuiation. A typical example is the super-fluid transition
in He“, a fermion with a half nuclear spin. Below the transition temperature, liquid HeIn
atomsfomBCS-fikepaimwithaspinoflper-pairandporbhalangtdmmomenum,
p-wave,imteadofspinzeroands-waveforthe000perpairs.

25
2.2 High Tc Oxide Superconductors

The term high temperature or high TC .superconductor is generally referred to a
class of complex metal cuprates that have a defective perovskite structure with one or
moreCu-Oplane. TheCu-Oplaneistheessential featureofthisclassofmaterials
becauseitisbefiwedthattheCu-Oplaneisrespomiblefmthesupercondrmfing
properties. (Iauer)CuO,[60], discovered in 1986, with Tc about 38K is the first
compoundinthisclass,butitisnotthefirstoxide superconductor. Oxygen-deficient
SrTiO“ , found in 1965, is the first crude superconductor, while the highest Tc among
oxide superconductors without the Cu—O plane is about 13 K for Ba(Pb,_xBi,)O,[61] or
about 30 K for WiOJ62,63], before or after 1986. Although all these oxides
menfioncdmrelatedwflnpaovskitesnumme,hismtcleartthpmcmducuvitym
theseoxidesarerelatedtothehigthsuperconductingcuprates.

Three most important high-Tc (high critical temperature) oxide superconductor
systems are Y-Ba-Cu-O (yttrium-barium cuprates or rare earth-barium cuprates), Bi-(Pb)-
Sr-Ca-Cu-O (bismuth-lead-strontimn-calcimn cuprates), and Tl-Ba-Ca-Cu-O (thallium-
barium-calcium cuprates). These systems are often referred as YBCO, BSCCO and
TBCCOsystems. Allthesethreesystemshavecomplicatedcrystalstructmeswith
copper-oxygenplamwhicharebelievedtoberesponsrble forthesuperconducting
propertiosofthesecompounds. AmongthesecupratesT‘l—Ba—Ca-Cu-Osystemhasthe
highest T,D about 125 K[ 1.2].

YBa,Cu,Ox (1-2-3 compound), with Tc around 95 K, is considered to be the most
favorable oxide superconductor for thin film applicatiom because, in comparison with
othersystems,ithasalesscomplicatcdchenn'stryandstrucnue, goodphasestabilityand
a high critical current density[3]. On the other hand, Bi,‘,Pber,Ca,C.‘u,OY (2-2-2-3
compormd)oftheBi-(Pb)-Sr-Ca-Cu—O system,witthabout110K, isagoodcmdidate

26
fmtapeapphcafiondtwtonsthinplateshapedgrainsthmfacflimtemechamw

alignment of the grains to obtain higher transport critical current density[33-34].

2.3 Crystal Structures of High Tc Superconductors

AllthreeimportanthighTCsystemscanbevisualizedasstaekingofvarious
perovskitecellswithsomedefectsinthecellsorinstacking. Figure7showstwo
different ways of drawing a perovskite unit cell that has a ABO3 formula. Many
perovskite metal oxides, e.g. BaTiO, , are well known for their ferroelectricity. Two
distinctcationsitesexistinthestructme. Acationoccupiesthelargersitewith
coordinationnurnberoftwelve,ontheotherhandthesmallechationsitsintheoxygen
octahedron. Inhigthsupereonductingcuprates,copperionsoccupytheBcafionsites
whileotherlargercationsoccupytheAcationsites. Duetooxygendeficiencyorothor
Madonfiomthenandardperovskiwsmnuetheneuenoxygenionsofthecopper
cationisofienlessthansix. Infactcopper-oxygenplamwithfomoxygenionsper
coppercationarethekeyfeatureofhigthoxidesupereonductors.

ThereisanhnpoerehfionbetweenTcandschnneofmesemperconducung
cuprates,flntTcinaeasesasthemmberofadjaeemCu-Oplanes(n)incmases,atleast
upton=3. Meadjacentplanes,thatsandwichsparselyoccupiedY-atom orCa-atom
plane,areseparatedbyadistanceofabout3.2A. Thisdistanceisapproximatelythe‘
sameforaflhigthcupratesandissmaflerthanthestandudpaowkiteceusizem
latticeparametersaorbofthehigthcuprates. Tcofsupereonductingcupratesisless
than77Kforn=1,about85to95Kforn=2,andmorethan100Kforn=3.

 

 

 

        
 

 

c .u .

...ac.......

...........
... you a

 

 

     

 

...-c ..
......o...
... ....

 

 

 

 

 

o
......-

 

 

 

 

 

 

Figure 7. Two difi'erent ways ofdrawing a ABO, perovskite unit cell

 

23
2.3.1 motto“ (YBCO)

An YBCO unit cell consists of three oxygen deficient perovskite cells. Figure 8
shows the tmit cell of an orthorhombic YBCO with seven oxygen ions per tmit cell, i.e.
6=0,wherethetopandbottomCu-Oplanshavenooxygenions in ”a” direction,andthe
middle cell has no oxygen ions on the side edges. Two types of copper ions exist.
Copper ions in the middle ofthe unit cell form Cu-O plans, with four planar nearest
oxygen ions, while the copper ions at the top and bottom of the unit cell (also having four
planar nearest oxygen ions) form one-dimensional chains in the "b" direction The two
adjacent Cu-O planes are separated by a sparsely occupied Y-atom plane. The distance
betweentheseadjacentCu-Oplanesisabout 3.2Awhi1etheseparationbetweentwo sets
ofthese adjacent planes is > 8.2 A

At high temperatures, YBa,Cu,O.,,. loses additional oxygen (8 > 0) fiom the top
andbouomCu-Ophnes,accompamwbyadecreaseoftheoxygenordennginthese

planes. ThestructureonBCObecomestetragonalwhenSZ 0.7. Thechangeinoxygen
content, and the orthorhombic-tetragonal phase transition are reversible with transition

temperaturearound700 'C[64-70]. Thetetragonalphase is semiconducting, andthe
firflyoxygenfledmthmhombicphaseismehlficandwpacmdmfingaboveandbelow

Tc , respectively. The oxygen content in the orthorhombic phase is important because Tc
increaseswithincreasingoxygencontentandreachesthemardmmnwithfig 0.1.

29

 

 

 

 

 

 

 

. h; k

i
n
V

o it

 

v

1: ‘i
O

O
1|
0

ins;

‘. *
O

O

 

 

 

Figure 8. YBCO unit cell

 

30
2.3.2 BSCCO System

The high Tc Bi-Sr-Ca-Cu—O superconducting system has a number of phases with
different critical temperatures (T c,). The three major phases are referred to by their cation
ratiosas2201 ('I‘c below 77 K), 2212 (Tc~85K), and 2223 (Tc~ 110 K) phases[35].
The structures of these phases are very similar to the structure of (LaHer)CuO,. For
simplicity the following discussion is based on the un-doped La,CuO, (La-Cu-O).
Conventional unit cell of La-Cu-O contains two chemical formula units, i.e. 2(La,CuO,).
SimilartotheYBCO imitcell,thiscellisalsocomposedofthreeperovskitecells;
howeverthe top andbottom cells are displaced as shown in Figure 9. Similarity between
theBSCCOphasesandLa-Cu-OcanbeseeninFigure 10.

TheshrwhueonZOlphasecanbeobtainedbyinserfingtwopairsofBi-Oplanes
intotheIa-Cu-OmuctmeandreplacingthelaatomswiththeSratoms,Figme 11. The
La-Cu-Oand2201plnsehaven=landlowcriticaltemperatures.

The2212suucuneisobtainedbysphtdngthecopper-oxygenoctahechonsand
inserdngaCa-atomplaneinbetweenFigme 12. Itisclcarthatn=2forthisstructure.
Similmly,the2223phasecmbedenvedfiomthe2212phasebyimerungmaddifionfl
Cu-OplaneandaCa-atomplaneasshowninFigtne 13. Asaresultthe22238tructure
hasn=3anch>100K

TT-Ba-Ca-Cu-O systemalsohas2201,2212and2223phasesthathaveexactlythe
samesuucnnesasthecorrespondingBSCCOphasewithTTreplacingBiandBa
replacingSr.

31

 

 

 

 

 

 

Figure9. 2(La,Cuo,)unitoc11

 

32

 

 

BSCCO - 2223

BSCCO - 2212

3% 1
page»...

 

  

3......4
rm?

0
\

 

       

      

 

e
\

..lwihflm,

(Mag-a

 

  

      

BSCCO - 2201

51%499 .o a ... .sv...AP.,
A. efzaeznwre

    

 

 

5935331..

-.

    

.233.

903:1} ... s....0.

..4
c.2295 3..

WV.

    

 

 

          

 

...,4.
.47 .
....mwv

  

     

 

 

 

Figure 10. Unit cells of 2(1.a,CuOJ and BSCCO phases

 

BSCCO - 2201

33

 
 

 

La-Cu-O

.'
o

 

m.
u.

 

Figure 11. Relation between crystal structures of LaZCuO. and BSCCO - 2201

 

 

34

 

           

      

                      

 

       

           

a: $9104...-A at _’PC 4..

         

    

 

refinances...

 
 

           

i. r. .A

BSCCO - 2212

 

                 

      

BSCCO - 2201

 

.1 ,n 4 . . ......H
(«Vow W. MW" 4." m... $1.7.

4 so . - waved»
e»... e4.

   

     

         

     

       

        

  
      

.4

 

Figure 12. Relation between crystal structures ofBSCCO - 2201 and -2212

 

 

35

 

     

agree _ - . ($4 .
sneer... . .....1... he.
dfi?€£&¥ nave»... .

  

     

BSCCO - 2223

 

 

 
 
      

            

    

 

u o

n . a

O ... . 5. ... . . .
....eaeflgaez. ....

 

Figure 13. Relation between crystal structures of BSCCO - 2212 and - 2223

 

36
2.4 High To Superconductor Thin Film Deposition Techniques

Since the discovery of high Tc superconductors, many techniques, most of them
from semiconductor processing, have been adapted to fabricate superconductor thin and
thick films. Almost all high Tc superconductor thin film deposition techniques can be
categorized as physical or chemical vapor depositions. Examples of physical vapor
depositions are laser evaporation or ablation[3,5-23], plasma or ion beam
sputtering[71-74], electron beam or molecular beam epitaxy[75-77], and thermal
evaporation[78-80]. Examples of chemical vapor depositions is organometallic chemical
vapor deposition[81]. All vapor deposition processes involve three major steps which
are[82]:

1.Creationofvaporplnsespeciesbythermalenergy,photonenergyor

kinetic energy
2. Transportofthevaporspeciesfiomsourcetosubstrate: Dependenton
fliedepositiontechniqmusedthistransportcanocctuwithoutmuch
collisions between atoms and molecules, i.e. under molecular-flow
condifionorwithmanycollisionsinthevaporphasespecies,i.e.
higherpartialpressureofthevaporand/orgasspeciesorsomeionized
species.

3.Filmgrowthonthesubstrate:Dependentondepositionrateand
mobilityofatomsespeciallyinthesm'faoeandnear-stn'face,theas
depositedfilmcanbeamorphousorcrystalline. Increasingenergyof
thevaporspecies(e.g.producingspeciesinexcitedstateorincreasing
velocity of vapor species), increasing substrate temperature and/or
bombardmentofthegrowingfilmbyiominthevaporspecieswill
increasethemobilityofatomsonthegrowingfilm;thus,promote
nucleation and growth of crystalline films.

37

Deposition rate, stoichiometry and crystalline quality are the most important
parameters in depositing high quality superconductor thin films. The first parameter
mainlydependsonthefirsttwostepsofthevapordeposifionpmcess;whilethe
crystalfinequalitydependspfimmflyonthefilmgrowthstepoftheabovemodel.
Therefore, itwinbemucbeasiertooptimizetheseflneeparametersifthreemajorsteps
in the vapor deposition process can be independently controlled For all physical vapor
deposition techniques mentioned above, these three deposition steps can, more or less, be
independently controlled. On the other hand, in the chemical vapor deposition technique,
itisdificulttoseparatethesethreestepsbecausethedepositionspeciesarecreatedright
onthesurfaceofthe hot substrate, i.e. allthreemajorsteps ofvapor depositionoccurred
s-ultaneously. Nevertheless, if complicated substrate geometry is necessary, chemical
vapor deposition is the only non line-of-sight deposition method among all techniques
mentioned

2.4.1 Pulsed Laser Deposition (PID) of Superconductor Thin Fihns:

Amongthetechdqueswhichhavebeenadaptedmgrowhigthsuperconductor
Mfilmgptnsedhserabhfionmmusedhserdeposiumisthemomfidelystudiedmd
mostsuccessfulone. ThekeyreasonsforthesuccessofthePIDprocessareits
stoichiometry conservation property, and excited high energy vapor species, i.e. excited
andionized species,itproduced[42-46]. Becauseofthesetwoproperties, high quality
epimxialfilmofwmpficawdcomposifionsmhasYBCOsuperconductm,canbemade
insituatlowtemperatme(400-650°C)witharelativelylargedegreeoffieedomin
processing parameters[4,12.14]. A collection of various PLD processing parameters, and
properfiesofrenflfingfilmsuimethefimmdsuccessfiflPIDdeposifimofhigh
Testrperconductingthinfilms[4])isshownonnextpage,Table 1.

abusive-Eli's: Una-wounvochhu OIUpaa stun)! Iva-«lug! 1111er I. l

38

 

 

 

i l 2.1 Oz 8n 85.6.: §< 1 U< The. 83 Rodi. voo—

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

E .5... solace: a: 3. 8 0: n. 8‘ 85.33 S. n: 2.8. sac...—
e: at. a. I a: 8« on. 8.5:. 8» n.» 3....
E split-ices! 9 oz .2 8m 00: a a. v8.—
.2. 832:5: latencies I 3 Oz .3 93 3F”; a 8.. — ...: =3
... I 8 oz a. $4 an» on n. =8. «8
.2. s! 2.11.8?an all-J use...
In I... re! ...-a s... .l ... E... ....5 .25. Inwr any 8" an. 85:. 8' a n «a
E. :8 n. a an» 8" 8.. 3:»: 8' a a 3n
_=_ 8...! 3.5.1 29!... 8.8!»? a
is. at... s s... s1... s2 .....8. .. ...... 0.302. 3 an» o... 8" So 853 8' _ a man
.2. I no a: o 2 Sn 8. 8.5:. 8' n n n...»
E. I I 0: 8" 8c 35:. 8' n a n2
.... | 8 an» 8" can an» S ...n 8n
.3. in. .8! I... also:
:51. 8.51.1.1... 2:. l 8 E 8" 8m new 5 ed 8»
.... a: a k an» 8" one an» S ...n .8
.... 2:.de 8 an» 8" 8c 35.» S ...n .8
.2. ...: h 5559.... at. a." a a 832 83:. 3.533 8-3 .4 8n
5. ME%. 8 a 8" n2 area .. ... a:
an :2. a 8 E 8n on. Noise .. _ a:
.2. seats. :3 c 3-. at. e 8 a: 8" one 82.: I a:
.... ...-...... ...: c 2-. Salaam.— 8 E 8" 8. we. a:
.... at. .4 a an... 8" one we. .. a:
.— o1...- .. anon.
3 3.: 8. obs—S: .- ...-ol- oIial EeOod an» 8v 2... 82 i I a:
.... a: .u 8 a: 8" one 353 a: n.» a:
.8. aaamm it an» 2:. i i R.
E. 35.9.. 8 a 8“ en. 35:. 2. n.» a.
.— .- sl ass-allot...
853...: 8 >8». ...... 1:33.... ..l... «89:. 8 a a o By nor—Wasp. I1 n a.
use... 0. .852 so...
_ oooooo .0.. _ 8o.— «Eoiug aw- .aeoaoel. , tam—... annuals. uhgnnau «have Mun—.0.. He
he 0 a b 5n...- L 833-. SIR—ice.— Eo-oclx. nun-8.9!.

 

2825.3 9588.5 08... .23 e838»! nee—u n.~.— .«o 8:83.:— ue- E2052!- ufiaoook— Gan Bot; .— 03:.

39

AtypicalsetttpforprnlsedlaserdepositiomFigme l4,consistsofapulsedlaser
source, a vacuum chamber with optical windows, a rotating target holder, a heated
substrate holder, and a source for oxygen gas or activated oxygen atoms/ions. Due to the
fact that laser induced plume, i.e. the vapor species, ejected from the target surface
movesmadirecfionmmalwthetargetsurface,thesubsuawholderisposifioncd
parallel to the target and centered to the laser-target interaction spot

Substrate distance from the target, typically a few centimeters, is critical because
ofthemn-uflfomspatialdishibrfimoffievapmspeciesmddecflngenagyofvapm
species as they move away from target. For a given deposition condition, the substrate
distancealsoconu'olstheareacoverageofdepositedfilmonthesubstrate. Because
stoichiometry is usually preserved during pulsed laser ablation process, a single
stoichiometrybulktargetisnormallyused. Thetargetisusuallyrotatingatalowspeed
toexposedifl‘erethsofthetargettothelaserptflsemmderwprevemlocal
overheafinganderosionofthctargetlhisisbecauseoverheatmgwiflresuhm
composifimalvmiafionofthetargetbysegregafionmdevaporafiommdmn-phmr
targetsmfacewilldirecttheplmneawayfiomthesubsu'ate.

2.4.1.1 Effect of Pulse-Width and Wavelength of the Laser

InserptflsesusedforPLDprocesstypicaflylmveaptnse-widthofabomtenma
fewtemofnmoseconds,focusedwafewmiflimetersspotsizeonthetargeganda
musoemrgydensityofafewJ/cm’Mpmdtmapeakpowerdensitymthemduof
tenstohtmdredsofmillionWattspercm’. Duetoextremelyhighpeakpowerdensity
mdexuemelyshondmafiomthehsuptnsesonlyaflemaveryflfinsmfacehyerofthe
urgetandsponnncomlyconveflithnoavapormfiagmentswithomcomposifional
variationorsegregation. Suchvariationmightbeexpectedfi'omexceasivemeltingofthe
High ‘I‘c superconducting materials[4.83].

 

 

laser
eam

   
   

optiqglty/
w1ndow

 

 

 

'sufinfie

 

 

 

_____———_.

 

target

(

.J

D

 

 

—
l.—

J

0%

 

Figure 14. 'I‘ypicalpulsedlaservapordeposrtr’ 'onsetup

 

41
Although superconducting thin films have been successfully deposited with a
wide range of wavelengths, i.e. from infrared CO2 and Nd:YAG laser to ultraviolet
excimerlaser,highcstquahtymsimepitaxialfilmsamprodwedbyUVlasers. Thisis
because shorter wavelength lasers have higher photon energy and greater absorption by
the plasma plume resulting in a higher degree of decomposition, excitation and ionization
of the vapor species by photoionization and other related processes[16,43]; thus, the
qualityofdepositedfilmisenhancedbyincreasedmobilityofatomsnearthe surface of
thegrowingfilm,Figme15.Deposifionusingmfraredhserswinnsuhmhigher
absorptiondepthandhigberdepositionrate,butthequalityofdeposited frlmsare lower
than films deposited with UV lasers[9, 16,84].

2.4.1.2 Nature of the Laser Induced Plume

Oncevaporspeciesformedbyinitialpartofalaserpulse,theyarefitrther
decomposed,iodzedandheatedbyimemcfingwiththcremaininglaserpruse. Dueto
rapklheeungmdwtpandmgofthehsermducedphmaanmplmnecmsistsof
highenergyvaporspeciesejectedfi'omtbetargetwithveryhighvelocitiestowardthe
substrate[42-46].

Impomntmfomafionabomhserinducedpltme,stwhasvapmspecies,pfinciple
luminescent species, expansion velocities, and extent of ionization, lnve been
determined by many groups using emission specuoscopy[43,45,46], mass
spectrometry[44],andion probe[43]. 'I'heseparametersareveryimportamintermsof
mdenmndingthePIDprocessandhnprovingproperuesofhserdepositedfilms.

42

 

 

I (all an)

 

 

 

 

 

 

 

 

vvv'vviv'vvvavaI'vv
amp—Hou- 1_
—4-t31 .
4
-
n
1
1
1
-
a
a
"" c
v""
a"' ‘
0‘ ..-
"' ..................o 1
a"
"U"... 4
. AAIAAI Innnalnnnnl-nnnl
” 1“ 1” u 250 a

 

 

 

 

 

 

 

 

 

‘. I I 1
mum . °
a-l-532nm .

0.0 - "ll-355:!!!

‘ I
F . .
‘E at -
> ‘ '
E ’ A °
4 OJ '- ‘ .
I ..
u - . ‘
I
I
b I .
I
u 1 r “--.“
13 I5 fl
1mm (K)

 

 

 

' ”1.0.14.

91‘.

~
.¢

..w, -; .-

 

 

Figure 15. PID onBCO on SrTiO, substrate using

nm), 2nd harmonic (532 nm and 3rd harmonic (355 nm) of
Nd:YAG laser with 3.5 J/cm power density, 200 mTorr
oxygen and 730 °C substrate temperature

' fundamental (1064

 

 

 

 

43
Following characteristics of laser induced plume from high Tc superconductor

target have been reported by several groups under typical deposition conditions, i.e.
several Joule/cm’ energy density and a few tens of nanoseconds pulse-width

l. Angular distribution of vapor species consisted of two distinct
components. One of the components is non-stoichiometric and has a
cos 0 angular distribution which is characteristic of evaporation; the other
forward direction component is stoichiometric and has a cos'O to cosne
dism‘bution[15,16.42]. The forward component increases with respect to
the evaporation component as laser energy density increases[42].

2. The major luminescent species in the plume are neutral and positively
ionized atoms and dimers[43].

3. By observing the change in the emission intensity oflaser induced plume,
theplmneinitiallymdergoesaone—dimensional expansionforaboutone
spotdiameterandthenathree-dimemional expansion, inwhichthevapor
density rapidly falling ofi‘and the excitation diminishing[3,43].

4. The expansionvelocities ofthevapor species are about 10‘m/s, andthe
corresponding particle energies are in the 25-50 eV range. Relative
velocitiesforCuandYareformdtoconfirmtheM‘“dependence
predicted by vacuum fiee expansion model[13, 43, 45, 46].

5.Degreeofionizationoftheplmneiscstimatedtobe 1.4-4 %ofionized
speciesfiomintegratedionsignalandmassremovalperpulse,assming
atomicspeciesonlyandacoseangulardismbutionofthespecies
respectedtothetargetnormal[43]. This 1.4 -4 %ionization estimationis
likelytobemuchtmderestimatedbecausethesharpcos‘Gtocosne
distribution of the forward vapor component.

6. Plasmatemperatrneisestimateifromexpansionvelocityandlocal
thermodynamic equilibrium, to be in the range of 6400-13000 K[43].

44
2.4.1.3 Problem with High-temperature Post-Annealing

Correct crystallinity and oxygen content is essential for good superconducting
properties of the high Tc oxide superconductors. Low deposition rate in the order of
anyfiomperlaserpulse,a®quatesubsu'atetemperatmeabom600to750°c,and
suficient oxygen partial pressure of a few hundred millitorrs are important criteria for
obtaininginsituhigthsuperconductingfilms. lfthese criteriaarenotsatisfied,the
as-depositedfilmwiflmtbesuperconducfingorwiflnothavehigthandhithc
(critical current density). Therefore, a high temperature, above 850 °C, post- ' is
necessarytorestorethesuperconductivity. Carehastobetakentominimizethe
following undesirable properties associated with high temperature post-annealing
process.

l.Chemicalrcactionwiththesubstrateresultingintheformationofa
non-superconducting interface layer[5,85-87].

2.1moffilmstoichiomeu'yafieranncalingduetoswfaceevaporationand
film-substrate interaction[4,87,88]. Barium and/or copper deficiency has
beenobserved afterpost-amrcaling process in YBCO films.

3. Crackand/orroughingdmingthermalcyclingresultinginlowcritical
current densities[6].

2.4.1.4 Methods to Improve Quality of Low-Temperature Grown Films

Iowcdnsmmwmm.wmminsthcmmfinsmufiu.is
flrekeymgrowingmulu-layercomponethhdifi‘erentmatefialsmtomtegratehigth
variations of the basic PID process, i.e. plasma assisted PLD[10,46], dc-biased PLD[14],
andatonficoxygenassistedPID[13],havebeensuccessfiulyabletolowertheinsitu

45
deposition temperature to as low as 400 °C[10], Figure 16. Reduction of the deposition

temperature is made possible by creating reactive oxygen species such as atomic oxygen,
oxygen ion beam, or oxygen plasma and/or by enhancing mobility of surface atoms with
dc-bias or ion bombardment.

2.4.1.5 Problem with Surface Particles

OnetmiquefeanneofPLDdepositedfilmisthepresenceofsm'facepanicleson
otherwise smooth film surface. The formation ofthese surface particles has been
attributed to sub-surface micro explosion caused by overheating of the sub-surface layer
dminglaser-targetinteraction[3,89,90]. Korenetal fomdthatparticlesarefewerand
maflerasthewawlengmofthehserdecreasemwmhigherabsorpuonofhseremrgy
by the plasma plume and higher degree of fiagmentation in the vapor phase[16],
FigurelS.

Fmagivenhserwavelmgththesmfaceparficleproblemcanbeminfinizedby
Mgmeabsmpfioncoeffidemmmcmalmufityofthemgegwhichm
mesub-swfaceovaheafingandbyimprovmgmemmgthofmetugegwhichmcmses
theresistancetoexplosion[91]. Increaseddensityofthebulktargetbysilveraddition,
hserarfacemelfingmehprocesamhmisosmficpressmgaflPHsexpeaedmm
thesm'faceparticleformation. Recently,Eidellothetal.reportedamethodtoremove
surface particles by chemical polishing with diluted HF solution without degrading the
superconductingproperties ofthe films[92].

 

(a)

   

LIDIVL
DflllVlOI

 

 

 

 

 

 

 

 

 

Figure 16. (a)l’lasma-assistedPIDsetupvvith lO‘Torroxygenpressure
drmngdeposifiomandCb)l?c-biasedPIDsempwdth200
mTorroxygenpressuredm'mgdeposition

47
2.4.2 Pulsed Laser Ablation Using Millisecond NttYAG Laser (ms-PID)

Although high quality YBCO thin films can be obtained by using nanosecond UV
lasers,thedepositionrateisverylow,typicallyafewangstromsperlaserpulse,andthe
depositfimeacowmgeisverysmlhafewmminnddthduetothenmrowangmar
distributions as sharp as cos'O to cos‘2 6[15,16,42]. On the other hand, high deposition
ratesuptolOOnm/s[15]andalargetmiformdepositioncoverage,duetoabroadercose
angular distribution of the evaporated species can be obtained by using millisecond
pulsedlasers. 'I'hcsetmiquepropertiesmakemillisecondPLDapraeticalprocesstobe
usedtoproducecontinuouscoatingoftapes.

2.4.3 Nd:YAG Solid State Laser vs. Excimer Gas Laser

lasersmndsfmlighrwhficaionbysfimulaedauiafiouofmfiafiouthatis
prodwedbprgaflumescmgmatefidmaopficalcmdtytypicanywmistedoftwo
parallel mirrors facing each othcr[93], Figure 17. Usually both minors are coated to
increaserefiectanceforthewave-lengthofthelaser. Therearmirrorisalmost 100%
mflecfingandthefiomm,i.e.theouqnnminor,isparfiaflyuansmiuingwith
reflectanceR<1toextractlaseroutpm

Usually, the fluorescing medium is excited to higher energy state by optical
pumping for solid state laser or by electric arc discharge, electric glow discharge, or

microwave excitation for gas lasers.

48

 

 

reflectieficg outputmirror

rcarmirror

 

 

 

Figure 17. Basic laser cavity

'I'herearebasicallytwowaysbywhichatomsatexeitedstatescanradiate
energy[94]. Oneisthespontaneousemissionprocessinwhichexcitedatomschangeto
lowerenergystaterandomly. Fluorescentlightproducedthiswayisnotdirectionalnor
coherent,i.e.consistsofvariousfiequenciesandphases. Theotherprocessinvolves
stimulatingtheexcitedatomsbyanextermlelecu'omagneticfieldorlightfinthecaseof
laser)oftheproperfiequency.Thisiscafleds1inmlatedmb-ion. Thisprocess
prodtwescohemmhghtofflresamefiequeneyanddirecfionofpropagafionasthe
stimulatingsource. Sincethesfimulatedemissionindrwesmomemissiomthcresultis
amplification of the original light sigml.

Figure 18(a)showstheenergydiagramofatlnee-levelopticallypmnpedlaser
mdimegtriplyionizedchromimninarubylaser. Theactuallaseractioninvolves
transitionsfiomlevetholevellthatproducephotonsofenergyhv. Theatomsofthe
medittrnstr'onglyabsorbphotonenergy,hv,,ofthepurnplightandexcitetotheenergy
bandatthirdlevel. However,this1evelisneverpopulated,i.e.thefi'actionofatomsin
level 3 (n,) is approximately zero, due to nearly instantaneous non-radiative transition
fiomthisleveltolevelZ. 'I'hethirdlevelisrequiredtoachieveanecessaryconditionfor
ampfificafioncanedpopulaioniumsionthathefiacfionofatomsmleverz,n,,is

49
greaterthanthefi'actionofatominlevellmr Definingn=n,-n,,atgromdstate n,=
n,=0,n,=1andn=-l;thepopulationinversionisattainedwhenn>0.

Higher eficiency can be obtained with a four-level laser that has the ground level
is far below the lower laser level or the level 1, Figure 18 (b). In this case, the lower
laserlevelistmpopulated,i.e.n,=0,thus,anypopulationinleve12givesrisetoan
population inversion. Therefore, it is not necessary to pump a four-level laser fi'om n =
—1 to n = 0 before achieving gain. Moreover, the four-level laser medium never exhibits
absorptionofthelaserlightitselfbecausen,=0.

 

  
 

       
   

 

 

 

(a) (b)
,// / /,‘/_, ////// /,/ ///, /////////W///
hv'iia , // 1.71;" leve13 hm?” ZZ/C/y/W / ““13
rlevel 2 715W] 2
Imam 133“ hv-< 1”“
absoptitm em'nsion emision
k level 1
we 1mm

 

 

 

 

 

 

 

 

 

 

Figure 18. Energy diagrams of (a) three-level laser and (b) four-level laser

Iftheopticalcavityisconfigmedconectlyandthefluorescentmaterialis
homogeneous, multiple reflections are possible in the direction defined by the two cavity
mirrors. Lightampfificafionoccmsifthegainduewsfimmatedemissionisgreaterthm

50

theabsorptionlossandtheoutputthroughfi'ontmirror. Whenthisconditionoccursthe
laserissaidtooscillareorlasewithhighlydirectionalandcoherenthght

2.4.3.1 Nd:YAG Solid State Laser

TheNd:YAGlaserwasdiscoveredhl961,anditisnowthestandardindusuial
solid state laser. The active medium of Nd:YAG laser is triply ionized neodymium
atoms, a typical four-level laser medium, which replaces some of the yttrium atoms in the

host yttrium aluminum garnet, Y,Al,0n or YAG. Major components of a Nd:YAG laser
head are shown in Figure 19, which consist of a dual-elliptical pump cavity, adjustable
MandrearminomandtheopuonalQ-switchandnonlinear-crystalharmonic
generator. 'I'helaserrodandflashlampsareenclosedinfirsedquartzflow-tubesfor

 

directwatercooling
coolingwater
mm “he i. 1......

  
  

 

 

crosseddouble- \\ l l
ularrzer

 
  

 

 

\ \\t\\\\\\\\‘3\\\\\\\\\\\\\\\\‘li\\\\\\\\\\‘\\\\\\\\\\\\\\\\\\\\\\\\~‘I

 

.................................................................

......................................... » ...”...“un ..... ‘

 

x\\\\‘\\\\\\\\\V<\\\\\\\\\\\\\k\\\\\\\\\\‘\\\\\\\\\\\\\\\‘2\\\\‘Q‘b"».\§ ‘

 

 

 

 

 

 

 

 

 

Figure 19. Typical Nd:YAG laser cavity

51
In the long pulse mode, i.e. without Q-switch, the pulse-width is typically a few
hundred microseconds to a few milliseconds which corresponds to the fluorescent life
time of the laser medium and the duration of the excitation flash-light, respectively[95].
On the other hand, the Q-switched pulse-width is typically ten to a few tens of
nanoseconds which is closely related to the cavity life time determined by the distance
between mirrors and the reflectance of the output mirror[93].

2.4.3.2 Excimer Gas Laser

The term excimer stands for excited dimer, a molecule consisted of two identical
atomsandexistsonlyinanexcitedstate,suchasHe,andXe,. Nowadays,excimeris
usedtodescribeanydiatomicmoleculethatexistsonlyinexcitedstate,suchangBr,
ArF,KrF,XeClandotherrare-gashalide molecules. Thefactthatthesemolecules exist
mexchednatemakememgwdcandidatesaslasamediabecausemeydecomposesmm
mbondedabmsafiertmsifionfiomexcitedstateandmmaficaflydepopulatethe
lowerleveloftransition.

Excimerlaserswerediscoveredinmid-l970s,andverysoontheybecame
commercially available because existing transversely excited atmospheric-presstne
camondiotddeiasethntdmmwomsmsomhiyweuwimmenewiydewiopediaset
media. The major problem involved in this conversion is the halogen corrosion which
hasbeensolvedwithhalogenresistantinternalcoatiny96].

'I'helasermeditnnisamixttneof90-999ttofheliumorneonbufl‘ergasto
facilitateenergytransfer,l-9%ofAr,KrorXeraregastoformexcimermolecules,and
0.1 -0.2% of F3, Brz, C1,, HCl, HBrorHFashalogen donor. Thelife time, typicallyin
theorderoflO‘ahotsorafewhomsatZOOshots/s,ofexcimergasmixtureisveryshort.
'I'hisresultedintheproblemsofhighoperationcost,approximateSSOperchargeofgas
mixnne,mdhandlmgofhazardsdischargedgas[97].1heexcimer1asemaresfifl

52
relatively new, and mainly used in research laboratory. However, with the recent

development of large industrial excimer lasers in kilowatt range, the operation cost per
joule of laser energy is expected to be lower[98].

Excimer lasers used for PID process typically have a wave length of 193 nm
(ArF), 248 nm (KrF) or 308 nm (XeCl), a pulse-width of ten to a few tens nanoseconds, a
pulse-energy of a few hundreds mini-joules.

In this study, a millisecond pulsed Nd:YAG laser with 1064 nm wavelength was
used. The advantages of using an thYAG laser over an excimer laser are as follows:
First, high power Nd:YAG lasers can be easily operated at high repetition rate with high
pulse-energygthusalargerdepositionratecanbeexpected. Second,ithasawiderange
of possible output wavelength by harmonic generation, namely near-infrared 1064 nm
fundamental, green visible 532 nm 2nd harmonic, and ultraviolet 355 nm 3rd harmonic.
'l‘hird,therearechoicesofawiderangeofoperationmodes suchashighrepetitionrate
mode, long pulse (ms) mode and Q-switched (ns) mode. Finally, due to its solid state
design,itdoesnothavecomplicatedmovingcomponents, suchashighvacuurnpumps,
and,thereisnomdforexpensiveandcorrosivegases.

3. El

3.1T

3.1

3. EXPERIMENTAL PROCEDURE
3.1 Target and Bulk High TC Superconductor Preparation

Both BSCCO and YBCO superconductors were prepared from high purity, at
least 99.9% ptuity, single-metal oxides or carbonates. Initial grinding before calcination
and intermediate mindings between multiple calcinations and final sintering were
performed with a pestle and mortar. Afier initial crushing of large particles, methanol
was added to form a slurry to increase the minding efficiency. Methanol was chosen as
the minding medium to prevent possible degradation of the superconducting compound
due to moisture pickup[99, 100].

3.1.1 Pb-Doped BSCCO 2223 Superconductor

Starting materials of Bi,O,, PbO, SrCO,, CaC03, and CuO were weighed and
mixed to have Bi:Pb:Sr:Ca:Cu cation ratio of l.5:0.5:2:2:3. This starting composition is
chosenfiomthenponeddanfmmardmizingthecfificaltemperatmecomidedngmat
some lead loss may occur during laser calcination[38,40]. This powder was thoroughly
groundwithapestleandmortartoensurethroughmixing.

3.1.1.1 Laser and Conventional Calcination obe-doped BSCCO
The mixed fine powder was placed in a specially designed slow rotating pan and

irradiated with a defocused pulsed Nd:YAG laser beam at two different power levels of
50 and 90 Watts, Figure 20. .

53

54

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Beam
Lens
Shielding\
Gas ,
ozzle __W
7
o O
l I \ l \ [T
F J

 

 

Figure 20. Laser calcination apparatus

 

55

Dminglaserfiradiafiomthepowderwasconstanflysfinedbyastauonaryvane
attached to the rotating pan. Periodically, calcining powder was reground to ensure
uniform laser-powder interaction. The average linear speed of the rotating pan with
respecttolaserbeamwasabout25cm/s. A400WNd:YAGpulselaserwithapulserate
oflOOpulses-per-secondandapulsewidthof2.2mswasusedduringallthe
experiments. Thelaserbeamwasfocusedtoa4mmdiameterspot Thus,the
pulse-energy density was 1 or 1.8 J/cm’, and the peak-power density was about 450 or
800 W/cm’, Table 2.

chompafisomanidenficalbatchofpowdermixtm'ewasalsocalcined
conventionaflyinafinnacesetat830°Cfor7homs

Table 2. Calcining conditions of Pb doped BSCCO

 

 

 

 

 

 

 

 

laser power pulse-energy peak-power density time
(W) density (J/cnn’) (W/M’) (hr)
sample A 90 1.8 810 0.5
sample B 50 1 450 1.5
sample C Conventionally calcined at 830 °C for 7 hours

 

 

 

3.1.1.2 Sintering obe-doped BSCCO

mlasercalcinedandconvenfiomnycalcinedpowderswerepeuefizedudtha
0.5” diameter mold and a hydraulic press. One drop of amyl-acetate (CI-LCOOCJI“) per
gramofpowdermixtm'ewasmedasabinderforbettergreenstrength. Thescpellets
weresinteredat865 °Cinairfordifl‘erentdmatiom oflS, 50, 85,100,and240hours,
thenfiunacecooledtoroomtemperauneatarateabomIOO°Clh

56
3.1.2 YBCO and Ag-Doped YBCO Superconductors

YBCO targets with stoichiometric composition were prepared from high purity
Y,O,, BaCO,, and CuO using conventional solid state sintering process. Finely ground
powder mixture was firmly hand packed in a alumina pan for double calcination at
930 °C for 10 hours. It is important to pack the powder firmly because the calcination
and formation of YBCO will be greatly accelerated

The double-calcined sample was carefully ground and pelletized with molds of
different geometry and a hydraulic press. One drop of amyl-acetate per gram of powder
mixtm'ewasusedasabinderforbettergreenstrength Thesepelletsweresinteredat
950°C for36hours. OnlyYBa,Cu,0,phasewasdetectedinfinalsinteredYBCOtarget
by using a x-ray dim'action method.

SilverdopedYBCOtargetswerepreparedbycmshingYBCOpelletsandadding
16 wt% or 24.5 wt% of 99.9% purity Ag,0. Well-mixed powder was pelletized and
sinteredat 850 °C for 12hoursplus 930 °C for 36 hours plus960 °C for48 hours. Only
verysmallamormtofimpmityphasewasdetectedwithx-ray dimactionmethod

3.2 Substrate Preparation and Silver Thin Film Deposition

Substrates for ion-beam assisted millisecond PID were metallographically
polished polycrystalline silver (99.9% pure), commercial grade polycrystalline alumina
substratewithS umsilvercoating,optieallypolished(001)ytuimnstabilizedzirconia
(YSZ)and(001) YSZwith0.2 umsilverbuffer layer. Multi-layerAg-dopedYBCO/Ag
tapeswerestartedwithaStoSO umlayerofsilveroncommercialgradefirsedsilica
slides.

Silver bufl'er layers of the ceramic substrates and silver layers of the multi-layered
tapeswerepreparedbyconventionalvacmnnthermalevaporationM‘E). Atungsten

57
wire basket was used as heat source to evaporate silver chips of 99.9% purity onto room

temperature substrate. The base pressure of the evaporator was lxlO“ torr.

3.3 Millisecond Pulsed Laser Deposition

ARAYTHEON SS-500 SERIES 400 Watts pulsed Nd:YAG laserwasused This-
laser is operated at the fundamental line of the Nd:YAG crystal, i.e. wavelength of
1.064 um,andhasamaximum pulse-energy of 50 Joules. The pulse-rateiscontinuously
variable from 1 to 200 pulses per second The pulse-length, variable from 250 us to
7.2 ms, and pulse-shape, most often square wave or triangular wave, can be selected by
reconfiguration of the pulse-forming-network of the pumping flash lamp. The square
wavepulseshaveaminimumpulse—lengthofl.2ms,whilethetriangularwavepulses
have a maximum pulse-length of 1.2 ms.

Anadjustablebeamexpanderisusedtoexpmdthelaserbeamsizetoabom 10
mmdiamewr.1hisexpandedbeamisbentdownwardmthefocusingheadeqmpped
wimacoaxialmmoctflmmicroswpethatisahgnedwiththehsubeamandusesme
laserfocusinglensasobjectivelens. Thisconvenientmicroscopecanbeusedtoaimthe
targetandtoinsituobservethelaserirradiatedtarget. 4"or8”Plano-convexlensmade
ofUV-gradefirsedsilicaisusedtofocuslaserbeamontothetarget. Figure21isasketch
ofthelaserbeamdeliverysystem.

58

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Monocular
Safety Shutter
n == 1.06 um
Absorption
:MW Filter
Nd:YAG —’~.——-- l W
Mirror
Control
S Expander .... F .
Lens

 

 

Work Piece

 

Figure 21. Laserbeamdelivery system

 

59
3.3.1 Plasma assisted PLD Apparatus

3.3.1.1 Plasma Assisted PLD Chamber

The design and consti'uction of the PLD vacuum apparatus were a major
undertaking ofthis research project The preliminary results ofthis research were
gatheredwithavacuurnchamberthatisconvertedfi'omasectionofanoldHitachi
transmission electron microscope, Figure 22.

This chamber has an internal size of4" diameter by 4" high, one 3" diameter
fused silica optical window and three 3" diameter accessory ports on the side. Normal
O-ringsareusedthroughoutasvacuumseals.

Ahalfinchbrassplatewasmachinedtocoveroneoftheaccessoryportto
fadfihtewldefingmbmzingofgasifletandcoppermbefmelecuicfeed-mrough.
Oxygenisinuommmthechamberbyallvcoppermbmgthroughthebrassphw
withavacmnnsealformedbysoldering. Oxygenflowintothechamberiscontrolledby
aneedlevalveandmonitoredwithaflowmeter. Theelectricfeed—throughwasmadeby
embeddingsixcopperwiresinbakeliterodwithtwoO—ringgroves.

A3/4”almninmnplatewasmadetosealthebottomofthechamberandtomotmt
PLDparts. Anl.5"diameteropening,onthetopofthechamber,withanO-ringgroveis
adoptedtoanopticalwindowtodeliverhighenergylaserpulses.

3.3.1.2 Target and Target holder

Arotationtargetholderisdrivendirectlybyadcmotorinsidethechamber. A
speciaflydesignedfom-piecemoldwasusedmpmducebulkYBCOtargetfings,
Figure23. Targetringsinsteadofmoreconvenfionaltargetdiskswereusedtomaximize
targetstufaceutilizationandtopreventtmevenetchofthetarget.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure22. PLDchambermedforpreliminarystudy

 

61

 

 

 

 

 

Figure 23. Specially designed mold, and YBCO target ring produced by using this mold

62
The uneven etching of the target will result in misdirected plume, i.e. the
evaporated species, because laser induced plume always ejected perpendicularly to the
target surface. This concern is especially important for millisecond-PLD process due to
much higher target material removing rate compared to nanosecond-PID.

3.3.1.3 Heated Substrate Holder

Substrate holderwasmade ofstainless steeltoholdthe substrate of20mmx 10
mmsize.1hewbsuateheaterwasphcedmthemrbsuateholderdimcflybehindthe
substrate. ThisheaterwasmadebywindingFe—Cr-Alalloyheaterwireonagrooved
alumina blocktogetashighas650 °Csubstratetemperatm'e.

3.3.1.4 Oxygen Plasma Generation

Intenseoxygenplasmawascreatedbyapplyingmosciflafingdchighvoltagew
thesystem. Thehighvoltagepowerisgeneratedbyusingastep-downtransformerthat
converts110V60Hzacpowerinto6Vor12Voutputupt010A. Thisoutputis
convertedinto lZOHzoscillatingdccmrentbyafull—wave diode-bridge. Aautomobile
ignitioncoilisthenusedtostep-upthevoltagetoapproximatelylKV. Becausethis
Mgnexceedstheorigimldesigneapabilityofthehighvoltagecoil,itiscooledbya
copperwaterjacket.

Ibeiflfialdesignconsistedofasflverudre—lmpanode,mdmecathodecomisted
ofthetargetandsubstrateholders together, Figure24 (a). Figure 25 shows pictures of
thedepositionapparatusdmingadepositionprocess.

Ba 25.3 . 53 é a: 9E 8.3.. . as, 3 .58 3.. uses. Ea: .3 23:

 

63

 

 

 

 

 

 

 

 

’7.” . mi “......“

 

 

 

 

..m o
£85.33 33 3.8... a gown“ am
2:38 337.
Ntmaa
_ m
Emma ..me Emma .83.

A6 . any

 

 

 

 

 

 

 

 

 

 

 

Figure 25. Plasma assisted PID chamber (a) with the laser head and (b) close-up view

plasma
to the

3.3.2 1

MP5

for a

tilt:

65
lawnafitsedsihcambewiththreesflverwireloopelecuodeswasusedmstead

of the original single wire-loop anode, Figure 24 (b). This is to confine the oxygen
plasmatotheareabetweenthetargetandsubstrate, andtoreducethetarget heatingdue
to electron bombardment

3.3.2 Plasma Assisted Millisecond PLD Process (PPID)

Table 3 lists the important processing parameters. The chamber is pumped with a
two-stage mechanical pump to get a base of about 20 mtorr. The vacuum requirement
formillisecondPlDisnotasstrictasfortheconventionalnanosecondPIDbecausethe
totalprocessingtimeismuchshorterduetoordersofmagnitudehigherdepositionrate.

Ihechamberwasflushedwithptueoxygenseveralfimesafiermitialptmpmgto
diluteresidualair. Dmingdepositionprocessastreamofoxygenfloweddirectlytoward
thesubstrate,anda250mtorrofoxygenpressluewasmaintained

Table 3. Important Processing Parameters for plasma assisted PLD

 

 

 

 

 

 

 

 

 

 

 

 

PROCESS PARAMETERS
Wavelength 1064 nm
Pulse Width 2.2 ms
Repetition Rate 2 pulses / s
Spot Size on Target 15 mm2 elliptical spot
Pulse Energy Density 40 J / cm2
Peak Power Density 18.18KW/cm2
Substrate Temperature 500 °C
Target to Substrate Distance 3.5 cm
Chamber Pressure 250 mtorr of oxygen
Target Material Conventional sintered YBCO ring with
near stoichiometry composition
Post-Annealing $00,600,850 and 940 °C in air or oxygen

 

 

 

 

66
Laser pulses used had 6 joules per pulse pulse-energy, 2.2 ms pulse-width, and the
repetitionratewasZpulsespersecondwps). Theselaserpulseswerefocusedtoa 15
min2 elliptical spot on the target. This spot size was chosen to cover the entire width of
the ring to prevent uneven erosion of the target, Figure 26. However, the maximum peak
power density was restricted below 0.02 MW/cm’. The resulting pulse-energy density
and peak power density were 40 J/cm2 and 18.18 KW/cm’, respectively.

 

 

 

 

 

Figure 26. A YBCO target ring alter deposition process

3.3.3 lt

Mb:

flmb

mcch

mach

3.3.3

6‘11

five-

55’

67
3.3.3 Ion-Beam Assisted PLD Apparatus

Due to the 0.02 MW/cm2 maximum peak power limitation, and the target
overheating problem, designing of a flexible material-processing chamber was initiated
Thegoalwastodesignanapparatusthatcouldsolvethepresentproblemsandwas
flexible enough to be adopted to solve future problems.

Tominimizethecostanddevelopmenttime,standardhighvacumnand
mechanical parts were used whenever probable, and all other parts were modified or

machined right in the laboratory.

3.3.3.1 Ion-Beam Assisted PLD Chamber

Thechamberbodyisastandudsminlesssteelhighvacmmfive-waycrosswith
6'mbediametermd8”standardwnflatflanges‘.1hedistamefiomthecemerofthe
five-waycrosstoeachoftheflangeisapproximately6.5",Figme27.

'I'hetopportandtheportontherightarecoveredwithstandardS"to4l/2"
nducingflangesthatacceptstandardpermanenflysededoptiealvieWponsor
customizedadapters for standard 3" diameterand 1/8" thickness optieal window blanks.
AllotherthreeportsarecoveredwithstandardS"blankflanges.

Iaserbeamisimroducedintothechamberthroughthetopponandasmaflhole
onthestainlesssteellining. Theliningisapieceofthinsheetthatcoverslefithrough
righttubesectionstoeatchdebrisfiomms—Plerocessandtolimitdebrisenteringthe
top, fiont and rear sections ofthe chamber.

" AllhighvaeuuncompormareobtainedfromtheKutJ. LaskarCompany,ClairtonPA

68

 

 

 

 

 

Figtue27.’l‘heion~beamassistedP1Dchamberandthelaserhead

‘0 oi
mov

lens

Slit,

69
The mechanical pump is connected to the chamber through a high gas

conductance brass right-angle high vacuum shutoff valve, with 1 1/2" bore, Viton O-ring
bonnet and seat seals and bellows shaft seal. A brass needle venting valve and a stainless
steel thermocouple gauge tube are located close to the chamber on the 2" brass tube
connecting the front port of the chamber and shutoff valve. The thermocouple gauge is
connected to a JC CONTROLS model lOOTC controller with one set point and a
bargraph display of vacuum from 1000 to 1 mtorr.

The right port with an optical window is used for process observation Figure 28
is a sketch of the chamber arrangement, which shows the penning ion gun, the heated
substrate holder and the x-y target manipulator, viewed from the right view port.

3.3.3.2 Spatially Resolved Emission Spectrum Analyzer

Figure 29 is a sketch of the spatially resolved emission spectrum analysis setup.
Alemisusedwimagethehsermdmedphmethroughtheobserwuonwindowmwme
entrance slit of a optieal multi-channel analyzer (OMA). This imaging lens is mounted
masmndardlemholderthatpermitseasychmgeoflenseswithdifi‘erentfocal lengths
toobtaindifi’erentfieldofviewandspatialresolution. Aprecisionlinearslideisusedto
movethelensmountforfinefocus. Acubebeamsplitterisloeatedbetweentheimaging
lensandtheOMAslittosendapproximatelyhalfoftheintermityupwardtothesingle
lensreflex(SLR)camera. Theopticalpathdistancesfi'omtheimaginglenstotheOMA
slit, andthe camerafilmplaneare settobethe same; therefore the plumeeanbefocused
throughthecameraviewfinderandrecordedonthefilmforfmtheranalysis. Alloptical
componentsexceptthecameraammadeofINgradefirsedsflica(veryofiencafled
"quartz”)topreservetheultraviolet spectrum. Theemissionspectrmnanalyzersetupis
placedonastablecartwithadjustablehigh,andtheoptiealcomponentsaremountedon
anoptical rail for precision alignment, Figure 30.

7O

 

 

 

 

optical window-q %
l

l
Nd:YAG: rue:- bum

— _-__1-

 

 

 

l
l
l
l
l
l
l
L
l
l
l
I
l
l
l
I
l
I

 

 

 

 

 

  

cathode
1
anode : substrate
| I ..heat er with
-—=- , ,I ran be radiation
III ' ' , : shield

gas i I

Panning Ion Gun

tpulator

 

 

 

 

Figure 28. The ion-beam assisted PID chamber viewed from the right view port

 

.Eaasiaafisesiaaeiaaeziasaga:

 

71

 

 

ampu__gm Emma
ease Needy:

 

:2.

 

 

cenm_nc0

.occmmo -
I.p_:E _
_au_uao

w\"/
<20 _\B_ R

 

 

 

 

 

 

 

 

 

wngmu EEmm

 

 

 

 

'72

 

 

 

 

 

Figure 30. Spatially resolved emission spectrum analysis setup with PLD chamber

3.3.3.3 (

15' die

draw

them

and th:

in the

feed‘

tll

73
3.3.3.3 Oxygen Ion Gun

AsketchofthehomemadepenningiongunisshowninFigure31. Asectionofa
1.5" diameterfirsedsilicatubeservesasthebodyof the iongun Major components of
thegunaremadeofbrass. Teflonandaluminaareusedaselectrical insulators. Vacuum
seals are formed using heat-resistant silicone O-rings. When high voltage is applied to
the gun under proper oxygen pressure, glow discharge occurs between the cathode plates
andthecylindricalanode. Theoxygenionbeamisextractedfromthegunthroughahole
inthefrontcathodeplate,Figme32.

Theiongunismomrtedinsidethefi'ontportwiththehighvoltagefeedthrough
andtheoxygengasfeed through mountedonthe flange, Figure 33. Thehigh voltage
feedthroughwich3/4”conflatflangehasfomcopperconductorsthatareeapableof
carrying75Aat5KV. 'I‘heoxygengasfeedthroughhasonell4”stainlesssteelmbeand
all/3"conflatflange. Aprecisionneedlevalveisusedtoconuolthegasflow. The
mdlevalveandtheiongmmeconnectedmthefeedthroughusingstandardbrass

compression fittings.

 

 

 

_-_;u

2 brass electric feed-through
6,11,18 teflon insulator
9,12 alumina insulator

10 optional magnet
16 bras protective ring

 

 

 

 

   
     

17 fused-silica ion gun body

1,4 teflontube
3,8,15 O-ring
5 brass end cap

7,14 cathode
13 anode

 

 

 

Figure 31. Penning ion gun

 

 

 

 

Figure 32. Penning Iongunatwork

 

76

 

 

 

 

Figure 33. Penning ion gun mounted on the front port flange

 

3.3.3.4 H

in safe

Figurei

tinge

Met
high VI

control
of a:
mid

Si

2'

77
3.3.3.4 High Frequency High Voltage Power Supply

A high fiequency high voltage is chosen to power the oxygen ion gun to increase
thesafetyoftheoperation. Thisisduetothereasonthathighfiequencycmrentis
confimd to a very thin surface layer ofa conductor.

The high frequency high voltage power supply is based on a TCL4 PLUS high
frequency high voltage supply parts kit furnished by INFORMATION UNLIMITED‘.
Figure34 isthecircuitdiagramof the modified high voltage power supply.

TheNPNuansistorQlissettooscillatebythefeed-backwindingofthehigh
voltageuansformer,andtheoscfllationfiequencyissettobe10t020Kl-Iz Output
wltageiscontofledbyavmiableresisththhroughaconfiol-tansistorQBanda
poweruansisth2thathmimthecmrentmroughthteandthepdmmywindingof
high voltage transformer. Two high power 2N3055 transistors and one T1P31 power
uansistorareusedtoreplacetheofiginalpoweruansistorsQlandQZ,andthe
control-transistor Q3toincreasethereliabilityandoutput of the circuit“. Performance
ofthecircuitisfin'therirnprovedbyfancoolingthetransistors. Themodifiedcircuit
providesapproximately40Wattstotheprimarywinding Thisgivesapproximatelyl
KVandafewtensofonutputatthehighvoltageside.

Tofiuthernrcreasetheoutpm,aseparatecirctntconsistedofa555TlMER
oscillatoratZOKHLanda120Wattspoweramplifiereanalsobeusedtopowerthe

‘ Normation Unlimited, Box 716, Amherst NH 03031
" Radio Shack Electronics Store

78

3&3 BEE ems—e» 52 85.62.. .35 .3 95mm

 

 

 

 

 

4|.
evoca o»

 

 

 

 

 

 

03033.9 on v0

  

   

 

 

 

 

 

 

 

 

 

 

 

 

u... r
«5 ma :8 n
«a u M
.1 e. "I ..
4.. e
M no _.- e
r. om . In ..wt: ...
3 '_r m momaom mmm M
F: L “ .532. b
" >3“; _
a. +. e“ m
V m
3 o 533%: u.
n 5.3.. Soiu

 

Q a»... on"

 

AHVHIHd

mummddnm mmzonm >I

 

 

3.3.3.5

Ihche:
mdal:

Figure

holder
With 2

by a b
Molt
feed-1

CNS-

00111!

“m2

ma

with

79
3.3.3.5 X-Y Target Manipulator and Heated Substrate Heater

Left port of the chamber is used for specimen exchange. The specimen stage, and
the heated substrate holder are mounted on a 8" flange that rides on rails for easy access
and alignment. A picture of this setup, with the substrate holder removed, is shown in
Figure 35 (a), and the heated substrate holder is shown in Figure 35 (b).

Anx-ytargetmaniprflatorisusedinsteadofamore commonrotatingtarget
holder,mgenerateazig2agareascammgthatfiulyufihzearectangulartarget surface
with any beam spot size. The x-y manipulator is designed to have a low profile, and
resembles a typical sample stage of a scanning electron microscope. Each axis is driven
by a bellows sealed linear motion feed through

Two stepper motors, SLO—SYN #MA61-FS-62019‘ with 200 steps per revolution
resolution,and6002-innormalholdingtorque,areusedtodrivethelinearmotion
feed-throughs through telescopic double universal joints, NORDEX CMX-Al-l and
CNS-Al-l“, to accommodate linear motion and axial mis-alignment.

The stepper motor controller is built around two single-chip stepper motor
controllers ( HURST MFG. CO. NO. 220001‘“ ), and the scanning pattern is generated
using a 556 DUAL-TIMER and six position switches.

Substrate heater is made of four layers of F e-Cr-Al strip heating element enclosed
maalmnmaboxwiththinalummasheembetweenthelayem.1hisheeteriscovemd
withsilverandhasfomstainless steelboltstoholdthesubstrates. Astainless steel
radiation shield is used to reduce chamber heating and to increase heating efficiency.

'6 and H Sales Co., 2176 E. Colorado Blvd, Pasadena CA 91107
" Nordex 00., 50 Newtown Rd, Danbury CT 06610-6216
”' Hurst MFG 00., Box 326, Princeton IN 47670

80

 

 

 

 

 

 

Figure 35. (a) Low profile x-y target manipulator and (b) Heated substrate holder

 

81
3.3.4. Ion-Beam Assisted Millisecond PLD Process (IBPLD)

An8"quartzlensfocusesthelaserbeamtoanellipticalspotabout1mm’insize
onthe target at 45° to the laser beam Laser induced plume is ejected perpendicularly
fromthetargettowardthe substrateatadistanceabout3to3.5cmfromthetarget.

Substrates were held at 650 to 750 °C for IBPLD during deposition by a
resistance heater. On the other hand, the deposition temperature for the multi-layer
Ag/YBCO composites were 200 to 250 °C. This is to prevent peel-off of the tape from
thesubstrateduetothermalmis-mathIOl],andyettoprovide enoughtemperatureto
achieve a good adhesion of YBCO film on the silver layer.

Dmingdeposiuomthechamberwaskeptat100to300mtorrofoxygenaftera
ptmpdowntoZOmmnbasepressme,andflushedwithoxygensevemlumesmfiiany.
Fmtheiombeamassifleddeposifiomastemofoxygeniomgenmfledbythepenfing
iongmopuatedataboleV,wasusedtosupplyacfivatedoxygentotheplumeand
growingfilm. Figure36showsthelaserinducedplumeandtheoxygenionbeam with
40 J/cm’ pulse energy density, 250 us pulse-widthand 300 mtorr oxygen pressure.

Important process parameter for IBPID films and VTE / ms-PLD multi-layer
tapesarelistedinTable4.

3.4 Post-annealing

Sineetheas-depositedPPIDandIBPlDfilmsandtheYBCO layersofthe
multi-layer tape were not superconducting, post-annealing was required

TheplasmaassistedPLDfilmswereannealedat500,600,850and940°Cinair
oroxygenfordifi‘erenttimes. FortthBPIDfilms,thepost-amlingwasmadeat
850°Cfor30t0120minutesinair. Inssoffilm,especiallyfiomthecomersandedges
ofsmples,wuoccesimallyobsewedThiscanbeprevMedbycmefingmefihthha

piece

Tab?

 

82
piece of YBCO sintered bulk during annealing. YBCO/Ag multi-layer tapes were
annealed in a YBCO box at 855 and/or 905 °C for 10 to 60 minutes in air.

Table 4. Important Processing Parameters for IBPLD Films and VTE / ms-PLD Tapes

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PROCESS IBPLD FILMS VTE / ms-PLD TAPES
PARAMETERS
SILVER LAYERS (VTE)
Heat Source I —- tungsten wire basket
Evaporator Base -- 1x10" Torr
Pressure
YBCO LAYERS
Wavelength 1064 run 1064 nm
Pulse Width 0.25 - 2.2 ms 0.6 -2.5 ms
RepetitionRate 2pulses/s lor2pulses/s
SpotSizeonTarget ~1mm’ ~1,3.5,or7mm2
PulseEnergyDensity 40-28OJ/em2 60-300J/cm2
PeakPowerDensity 0.12-0.32MW/cm2 0.05-1MW/cm2
Substrate Temperature 650 - 750 °C 200 - 250 °C
Target to Substrate 3.5 cm 3 cm
Distance
ChamberPressure 100to300 mtorrofoxygen 100to300 mtorrof
oxygen
GrowthRate 2.5nm/s 2-50nm/s
Target Material Conventionally sintered conventionally sintered
YBCO with near stoichio- YBCO, 15% Ag doped
metry composition YBCO, 23% Ag doped
YBCO
Post-Annealing 850 °C for 30 to 120 min. 855 and/or 905 °C for 10
in air to 60 min. in a YBCO box

 

 

 

 

83

 

 

 

 

Figure 36. Laser induced plume and oxygen ion-beam

 

3.5 Sample Characterization Methods

3.5.1 Crystal structure and Phase Identification

X-ray dim-action (XRD) method was used to study the crystal structures, to
identifyphasesandtocontrolthepost-annealingprocess. CuKaradiation, obtainedby
using a Ni filter or a double-crystal monochrometer, and a computer controlled
dimactometer (Scintac-XDS-ZOOO difl‘ractometer, Scintag Co.) were used As sintered
or as depodted surfaces were used most of the time; however, mechanically polished
surfaces were also examined occasionally to isolate any possible surface efl'ects such as

surface texture and surface composition variation

3.5.2 Microstructures and Chemical Compositions

Merosmwnuesweresmdiedusingbothopticalmieroscopeandscaming
electron microscope (SEM). Samples for optical microscopy were metallographically
pofishedfoflowingstandardprocedmesusmgemerypapermdaltmimpowderof
various particle sizes. Small samples were hot mounted with bakelite before polishing.
Methmolwasusedasthegrindingmedirmmpreventmoisuuepickupandposmble
degradationofthesuperconductors.

Scanning electron microscopy was performed directly on diamond-saw cut
smfacesmfiacuuesmfacesfmbulksamples,mdmpmnfacemfiacnuecross-secfion
for films and tapes. Samples were ultrasonically cleaned in methanol and mounted with
copperconductingtapeorsilverpaint Averythinconductingvactuunevaporated
coatingofPt-Pdwasusedformicrostructmeobservationofasdeposited
non-superconductingfilms.

85
A Link energy dispersive x-ray spectroscope (EDS) attached to the Hitachi

S-2500C scanning electron microscope was used for chemical analysis. EDS quantitative
analysisofthecafionswasmadepossibleusmgcahbmfionstandardsandtheZAF
software supplied with the system. The backgrormd noise calibration and processor gain
calibration were performed using Au Lal line at 9.711 KeV.

The standards have to cover all elements analyzed and have to be as close as
possible chemically to the unknown sample. For YBCO samples, a sintered
stoichiometric bulk was used as a standard due to non overlapping strong emission lines,
i.e. Y L-lines, Ba K-lines and Cu K-lines.

On the other hand, four different oxides, SrCuO,, Ca,PbO,, Bi,CuO4 and
CgCuvaemusedasstandmdsfmthePbdopedBSCCOdtwmoverlappedemission
lines. Achestandardswerepreparedfromhighpmitymetaloxidesorearbonatesby
whdsmtesimmngprocesssimflumtheprepamfimofbmkhigthmpemondmtom.
Allstandardswerecalcinedat600°Cfor15hand750°Cfor3 h,followedbysintering
at900°C for48hexceptforBi,CuO,whichwassinteredat800°Cfor2h Final
compositions of the standards were determined by ion coupled plasma (ICP) analysis.

3.5.3 Electrical Resistance and Magnetic Susceptibility Measurements

Resistancemeamuemmwemperformedusinganamo-balancingacbfidgewith
a lock-in amplifier (Linear ResearchLR-400) with a standard four-probe technique. This
bridgeiseqrfippedwithamrnualinducmnceopuonthatallowsthismsuumemm
firnctionasanauto—balancingmutualinductancebridge. Thus, the mutual inductance,
whichisproporfionflmtheumagneficsuscepubifityofmem-phasecompmemQGcm
bemeasureddirectly. Thisbridgehasanadjustableexcitationcmrentfiom 1 uAto3
mAandresoltuiomoflmicro-ohmandlmicro-hemyfmtheresismnceandmtuual
inductancemodesrespectively. Figure37isawiringdiagramforbothresistanceand
mutualinductaneemeasurements.

 

 

 

LR—4OO auto-balance 4-wire resistance
and mutual inductance bridge

 

 

precision voltage

[selectable constant J [ range selectable J
on sampler

current AC excitati

 

+—;
IIT
G

06)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

\/V\/\/\/\AW\/\/\/\/\/\/\/\/\/J

sample sample

 

 

 

 

 

 

mutual inductance setup resistance setup

 

Figure 37. Wiring diagram for both resistance and mutual inductance measurements

 

up

.mmhun

m

87
Figure 38 is a sketch of the computerized resistance versus temperature ('R-T)

measurement setup. Four-point contacts are formed by sandwiching four rectangular
gold-coated wire-wrap pins and the sample between two bakelite blocks and two
poly-vinyl chloride (PVC) blocks. The rigid bakelite outer blocks are used to insure
uniform clamping pressure on the four pins, and the deformable PVC inner blocks are
used to accommodate the thermocouple and slight variation of the pins and sample
thickness.

The sample is cooled by inserting the whole clamp setup into a half filled liquid
nitrogen dewar, Figure 39. A very stable cooling can be achieved in the liquid nitrogen
vapor to within 10 K ofthe liquid nitrogen boiling point The cooling rate ean be
increasedbydecreasingthedistancefiomthe liquidnitrogensm'faceorbyincreasingthe
heaterpowertocreetemorevapor. Amanual laboratoryjackorasteppermotor driven
linearmofionthreadedmdisusedtoconfiolthedistancebetweenthesample clampand
the liquid surface. For temperature close to 77 K, the sample clamp is completely
submergedinliqrridnitrogenandthenslowlyliftedoutofthe liquidnitrogen

Figme40isasketchofthemagneticsuscephbilitymeasmementdeficethat
consists of induction coils, specimen holder with thermocouple, and liquid nitrogen
deliverysystem ThecircuitdiagramisshowninFigure 37.

WhentheacexcitationcturentwasappliedtotheprimarycoilmadeofBOOO
turns of #32 AWG magnet wire, it produced a uniform magnetic field around the
secondarycoilsmadeof2000nnns#34AWGmagnetwireeach Intheabsenceofa
magneticmaterial,theacvoltageinducedbythesamplesecondarycoil exactlycancels
theacvoltageinducedbythereferencesecondarycofl.However,anon—zero signalis
proportional to the magnetic susceptibility ofthe sample ifa magnetic material is present
in the sampling coil.

88

 

 

 

 

computer
data acquisition
system

 

 

 

 

 

 

 

 

 

 

 

 

  
 
 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

mm four-wire Ac
. . resrstance
mes bridge
, w/ 001ng
‘ thermocouple
current probe(+) current WONG)
vow voltasc probe (-)
nylon
Au I high T083319]: bolt
probes PVC bloc;
Bakelite block
ILJJ

 

 

thermocouple IL...” IL.”

 

 

Figure 38. Computerized resistance - temperature measurement setup

89

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H “6"
D: motor
, . J
// s
L _] V
.— L H H...
F A -
V -porfit’p'robe
to the resistance liquid N;
bridge and data @/
l
0386! l'i H I“

 

 

 

 

 

 

 

 

 

Figure 39. Cooling device for resistance - temperature measurement

 

90

 

 

  

  
  

 
   

9 FINE;
5 i“

’ I U" ‘ ’
III/II/I/IIIIA ,0 ,WI/I/I/I/IA
'Illlllllllfl: :1. Fill/Illlllll

  

 

 

 

 

 

  

   

 

0
br 3?
pus ' ii nylon . .
51’ng g; push- liquid N;
Eu tube reservoir
1*
g}!
5ng
primary :5:
excrtatron a _
coil \

 

 

 

 

 

 

 

 

Figure 40. Magnetic susceptibility measurement apparatus

 

cooled b

control-i

sample

hacigro

3.5.4 Ft

91
To obtain the temperature dependence of magnetic susceptibility, the device is
cooled by liquid nitrogen After the temperature is stabilized, the liquid nitrogen level
control-valve is shut to let the temperature rise slowly. Temperature and mutual
inductance are continuously recorded via a computer. A reference curve, with an empty
sample holder, is collected before each sample curve as the background. The
background is deducted from the sample curve to get the final result.

3.5.4 Four-Point Bending Test

The mechanical strengths of YBCO and Ag-doped YBCO targets were compared
using a four-point bending test. Figure 41 shows the four-point bending device. The
dismebetweentheomersupponsa)is20mm,andmedisMncebetweentheimer
supports(a)i810mm. Rectangularbarspecimemof6mmwidth(b)and2.5mmdepth
(d)wereused ThemodulusofntptmeMORfistheflexmalsuengtniethefiacnue
strengthofamaterialtmderbendingload. Forfour-pointbendingMORequals
3P(L-a)/2bd’. Sets of five samples each fiom YBCO and 15%Ag-doped YBCO were
testedtodeterminestrength.

92

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 41. Four-point bending setup

 

4. RESULTS AND DISCUSSION

4.1 Factors Which Affect Processing of Bulk YBCO Superconductors

It was found that careful wet grinding and firm hand packing greatly accelerates
the calcination, and formation of the YBCO compound. Furthermore, higher purity of
starting oxides and carbonate, i.e. purity of 99.99% or better, also enhanced the formation
of the YBCO compormd over the 99.9% purity starting chemicals, Table 5.

It was also found that powder preparation before final sintering affects quality of
the sintered superconductor. Highest green density was obtained with well crystallized
YBCO powder, i.e. powder with shiny black particles.

4.2 Laser Calcination and Kinetics of Formation of BSCCO - 2223 Phase

Since the discovery of superconductivity in this compound[31], many efforts
have been made to maximize the amount of high Tc - 2223 (110 K) phase. A major
enhancementintheformationofhigth-2223phasewasmadebydoping
Bi-Sr-Ca-Cu-O compound with Pb[32-36]. However, with Pb doping, about 200 hours of
total processing time is required to obtain a single or nearly a single high TC phase.

4.2.1 Kinetics of Formation of BSCCO - 2223 Phase

It has been shown that addition of Pb enhances the formation of 110 K (2223)

phase in Bi-Sr-Ca-Cu-O system because of kinetic reasons[32]. It has also been shown
that formation ofthe high Tc phase can be enhanced by melt-quench method with Ca and

Cu rich starting compositions[ 3 7] .

93

94
Table 5. Effect of starting powder purity and powder compaction on formation of YBCO

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

99.9+% starting chemicals 9999+ starting
chemicals
loose jOWdCl' firmly hand packed power
color of bulk pale green dark green dark geen
bonding and loose powder very weakly bonded weakly bonded
930 °C shrinkage
7 hf X lineissn bulk no no no
effect wden no partially and very partially mixed of
weak strong and weak
color of bulk green very dark green dull black
bonding and loose powder weakly bonded and no strongly bonded,
930 °C shrinkage significant shrinkage shrunk significantly
7 hr X 2 but crushed powder is and crushed powder
notflufl‘yeasytopack iseasytopack
meissner bulk no weak weak
efl‘ect rowder no morethanhalfis morethanhalfis
“098 strong
color of bulk dark green dull black shiny black
bonding and very weakly strongly bonded and strongly bonded and
930 °C shrinkage bonded shrunk significantly very hard to crush
7 111' X 3meissner bulk no strong strong
efl'ect Powder partially and very majority strong all strong
weak
color of bulk very dark green shiny black —
bonding and weekly bonded strongly bonded and
930 °C shrinkage rand no significant well crystallized —
7 hr X 4 shrinkage
eissner bulk weak strong —
effect wder partially and all strong —
[PO weak
color of bulk dull black —— -——
bonding and bonded and
930 °C shrinkage shrunk —— -—-
7 hr X 5 sigificantly
eissner bulk fair — ——
effect [powder partiallystrogg -- ——
color of bulk shiny black —— —-
bonding and strongly bonded
930 °C shrinkage and well — —
7 hr X 6 cgstallized
eissner bulk stronL —-- —--
rlefiect [powder majority strong —- —

 

 

95
A similar melt-quench effect was obtained when Bi-Pb-Sr-Ca-Cu—O powder

mixture was exposed to high energy and short duration laser pulses. Coarse particles
were formed by localized melting. Due to a localized rapid heating and quenching,
chemical homogeneity of the well mixed starting powder was preserved Therefore, we
were able to greatly increase the rate of formation, and the volume fraction of the
high-Tc phase ('1'c ~ 110 K) without using any excess Ca and Cu in the starting powder.

The mechanism of formation of high TC phase by laser and conventional
processesappeartobequite difl‘crentasindicatedbytheXRDdata. AsseeninFigure
42, the conventionally processed sample initially showed a mixture of low Tc - 2212
phase and Ca,PbO,. As the duration of sintering increased, the amount of Ca,PbO,
decreased, and a phase cycling behavior was observed[35]. The high TC - 2223 phase
was then formed by a reaction between the low Tc phase with Ca,PbO, and CazCuO,
[102,103].

Difi‘erent results were observed for two laser processed samples at two difl‘erent
laser powers, Figure 43 and44. At an early stage ofsintering, predominately a low Tc -
2212 phase was formed After prolonged sintering, the high Tc phase, Bi,Sr,Ca,Cu,Ox,
was formed via an unbalanced reaction ofthe following form[104]:

2Bi,Sr,CaCu,O, -> Bi,Sr,Ca,Cu,o, + 13i,Sr,Cuoz

Theinitialformationof2212phaseleavessomeexcessCaandCuionsofthe
starting stoichiometric 2223 composition The 2201 (Bi,Sr,CuO,) phase formed, then
combineswiththetmusedCaandCuionstoformtheloch-2212 phase. Therefore,
comparedtothehigth-2223phase,theamountofretained2201phaseinlaser
calcined sampleswasnotlarge.

Arbitrary Scale

96

 

    

 

 

 

 

 

sampleC .
Ohlgh Tc‘fl”)
ACaszq 0
*C32C003
#2201 DCUO .
e
o
e O 0 e
i C ‘ . ‘e
l . *
A . * 0 «a .,, 85b
e
e
' e
C . e
A . I
0 g» * 120h
O o
' o a
- °-l -‘ 24°"
:5 as as
29 (degree)

Figure42. mpatternsofconventionallyprowssede-dopedBSCCOmple

 

Arbitrary Scale

97

 

 

 

 

 

 

 

sample-B e .
Ohigh'lam .
Glow E0212) e
ACaszQ .
*C32C003 0 t.
“20‘ ' * 15h
. r I "1 " "‘ l
e
e 0 ‘
- at
A ‘ ° * * son
. o
o
e. e
C O O 0 e
A ‘ ' * * 85h
e
O. 0 O
.0 O
. o
* 120h
: a" as ' ' . ' air

 

  

29 (degree)

Figure43. XRDpatternsof50Watts haer-processede-dopedBSCCO sample

 

98

 

 

 

 

sampleA .
Ohlgh (2223) .
Clank 2th) . .
seazpbq,
#2201 - .
0040 O
9 e
‘ 15h
f. 1 e . e
e
o ' .0
o
O t D
‘ ° 0 50b
0
o .
3‘3 , o
E e
' e * O a
o ‘ 0 85h
. o
o
. 1 ° 2 .0 n
o O , 0 O ‘ 1%
23 36

13
29 (degree)
Figure 44. XRD patterns of90 Watts laser-promede-dopedBSCCO sample

 

99
A considerable amount of CazCuO, was also formed in sample B after prolonged

sintering. Due to the complexity of this sample, it is diflicult to determine how much
Ca,CuO3 was present. EDS analysis were performed on fiactured surfaces of this
sample. TheEDS andXRD data supportthe assumptionthat Cawas infact trapped in
the form of Ca,CuO, in the sample B[39]. Thus, a low volume fraction of high Tc phase
and the zero resistance temperature below 77 k were observed for this sample.

Duetothehighenergydensityofthelaserbeamsomeweightloss,intheform of
particulate ejection, of the starting powder is unavoidable during laser calcination
process. Thus, the final compositions of laser calcined and conventionally calcined
sample were analyzed by EDS rather than by a weight loss measurement. No significant
compositional difi‘erence was found through the chemical analysis.

4.2.2 Microstructures and EDS Analysis of Laser Calcined BSCCO

Scanning electron micrographs shown in Figures 45 (a) and (b) indicate a
characteristic plate-like grain structure. The 90 Watts laser-processed sample , fig 45 (a),
showedwell-grownplates 10-30 umwideandabout l umthick. Theplate size oflaser
processedsamplewasabomZ-3fimesashrgeastheconvenuomflyprocessedone.
Scanning electron micrograph of 50 Watts laser-processed sample shows plate-like grains
and some not very well defined phase, Figure 46. Figure 47 shows EDS spectra collected
fi'om areasofthe samples showninFigure 45 and46.

100

 

 

 

 

 

 

 

Figure 45 . Scanning electron micrographs of (a) 90 Watts laser-processed
sample, sample A, and (b) conventionally processed sample,
sample C

101

 

 

 

 

 

Figure 46. Scanning electron micrograph of a 50 Watts laser-processed sample

102

 

(a) ., ll : i sample A

 

 

 

 

 

 

Figure47. EDSspectrmnscollectedfromareasofthesamplesshownin
Figure45and46

103
4.2.3 Resistance and Susceptibility Measurements of Laser Calcined BSCCO

The measurements made on the resistance and the magnetic susceptibility of laser
and conventionally processed samples are plotted as a function of temperature in
Figure 48 (a) and (b). All these samples show superconducting onset temperature of
about 110 K. The zero resistance temperature of 90 Watts laser-processed sample, after
lOOhourssintering,isabout98K

The second inflections ofthe ac susceptibility curve were observed for sample A,
sintered for 100 hours, and sample C, sintered for 240 hours. The second inflection in
these two samples is likely due to Josephson-like weak coupling between the high Tc
grains[105]. A sharp drop in magnetic susceptibility for sample A, sintered for only 85
hours, can therefore be attributed to the absence of Josephson-like weak coupling
Although the laser and conventionally processed samples show similar onset transition
tempuatmesmdovmaflfiendsthemsmtsshowedthatdependmgontheprocessmg
conditions the laser processed samples can have a significantly large resistance transition
width.

104

 

SRKJ)
A _ _. Soaplr H
. 858 8|55°C ’— ... ’
-= ’ _._ See 0 "
r Infuse ”'
‘ _._. Soaper / .—-
V 2885 885°C / ’H'”
w3l.
U
z
c
'—
0')
CD
LU
Oi

 

 

 

 

01 I.
73 81 89 87 185 113 121 129
(b)

 

INDUCIHNCE ( Arbitrary Scale )

 

 

 

11111

73 81 89 97 185 113 121 129
TENPERHTURE (K)
Figure48. (a)Resistancevs. temwatmecurvesandw)Magnetic

susceptibility vs. temwaturecurvesofconventionally
prowssedand90Wattslaser-proceesedsamp1es

 

105
4.3 Millisecond Pulsed Laser Vapor Deposition

4.3.1 Problems with Millisecond PLD

4.3.1.1 Target Overheating Problem and Peak Power Density limitation

The greatest problem with the millisecond pulse deposition is overheating of the
target due to high pulse energy density input required to create a reasonable peak power
density on the order of a million W/cm2 for deposition

This is due to approximately five orders of magnitude longer pulse-width of the
millisecond laser pulse. Hundreds of Joule/cm2 pulse energy density is required to
achieve a peak power density close to a million Watt/cm’, which is still much lower than
thepeakpowerdensityproducedbyananosecondlaserpulseattypicallyafewJ/cm2
energy density.

With a rotating target, overheating and severe local melting are major problems.
Avery highpulse energyisrequiredtoobtaintherequiredpeakpowerdensitybecauseof
thelarge, 15 mm’elliptical, spotsizenecessarytocovertheentirewidthoftheringto
prevent uneven erosion of the target.

To avoid formation of excessive liquid phase, and severe segregation with a
mtafingdng-mrgetptuseenergyandptnseenergydensityamfimiwdm6lomesand
60 J/cm’. However, the maximum peak power density is restricted below 0.02 MW/cm’
with a 2.2 ms pulse-width.

106
4.3.1.2 Oxygen Depletion

As mentioned before oxygen stoichiometry is very critical in obtaining a good
YBCO superconductor, and the equilibrium oxygen content decreases at elevated
temperatures. Oxygen depletion on the deposited film is expected due to a combination
of high laser-induced plasma-temperature and a low oxygen pressure. Moreover, this
problem is expected to be aggravated by the high deposition rate of ms-PLD process.

4.3.2 Pressure Dependence of Laser Induced Plume

Itisfomdthattheshapeoftheplume,thustheangulardismbtnionofthevapor
spedes,isstonglydependeMonmepressmemthedeposiuonchamber,Figme49. The
difl'ennceisevenmoreevidentbycompafinglasermdwedpltmesmvacmnnandm
one atmosphere air, Figure 50. The angular dependence of various species and
dependenceoftheuspafialdismbufionmflnchamberpressmeandotherpamem
canbesmdiedusingaspatiallyresolvedemissionspecu'ometer.

107

 

 

 

 

 

 

Figure 49. Oxygen ion-beam, and laser induced plume with 250 us pulse-length
and 40 J/cm2 pulse-energy density at oxygen pressure of (a) 100 mtorr
and (b) 300 mtorr

 

108

 

 

 

 

Figures 50 Laser induced plume with 250 us pulse-length and 40 J/cm
pulse-energy density in one atmospherearr

 

4.3.3

4.3..

of
0.1

Sll

109
4.3.3 Ion-Beam Assisted Millisecond PLD

4.3.3.1 Effect of X-Y Target Manipulator

To ease the overheating problem and peak power limitation, a x-y target
manipulator was used to obtain a zigzag scanning pattern instead of a more commonly
used rotating target configuration. This allows the whole area of a rectangular target to
be scanned withamuch smaller laser spot size.

As a result, much higher peak power density can be obtained with a combination
of much shorter pulse and much smaller pulse-energy. Peek power density as high as
0.16 MW/cm2 was obtained with a 1 mm2 spot size, 0.4 J pulse energy and 250 um
pulse-width

Even with pulse-energy density as high as 0.16 MW/cm’, a relatively flat target
surfacecanbemaintained Thisisduetothefactthatasmalllaserspotandlow
ptflse-eneryaflowmorecoolingtimeoneachlocafionofthetargetmuface.

4.3.3.2 Effect of Oxygen Ion Gun

Averylargedifl‘erencewasfomdbetweenfilmsdepositedtmderidentical
conditions except with or without an oxygen ion-beam.

When deposited without an oxygen ion-beam, Figure 51 (a), only a very light
coating of a light brown color is deposited on the substrate which is about 0.5 cm
recessed in the heater. The brown color of the film indicates oxygen deficiency. The light
coatingmaybeexplainedbythepositionofthesubstratewhichisloeetedoutsidethe
plume, Figure 49 (a).

110
Under the same conditions, except with the oxygen ion-beam turned on, a

tmiform black coating is obtained on the substrate, Figure 51 (b). This observation
indicates that oxygen ion beam not only helps to replenish oxygen but also helps to drive
vapor species onto the substrate.

4.3.3.3 Spatial variation of film composition

An uniform coating of YBCO, with large area coverage more than 1" diameter,
has been successfully deposited on fused silica substrates by ion-beam assisted laser
vapor deposition, Figure 52. Typical energy dispersive spectra of YBCO film and YBCO
target are shown in Figure 53. Results of quantitative analysis using EDS are
summarized in Table 6. Compared to the stoichiometric composition, as-deposited film
is slightly high in barium content and low in yttrium and copper contents. However, an
increasehyttirmmdafiuthudecreasehcoppercontenEmeobsewedafiermepost
annealing treatment

Table 6. Typical compositions of YBCO films produced by IBPLD process

 

 

 

 

 

sample conditions yttritnn atomic % barium atomic 96 copper atomic
%
as deposited on silver substrate 14.6 37.1 48.3
850 °C 60 min annealed on sil- 18.8 36.6 44.6
ver substrate
850 °C 120 min. annealed on 18.4 34.1 45.8
Ag-coated (001) YSZ
850 °C 60 min. annealed on 21.2 34.1 44.7
Ag-coated alumina substrate

 

 

 

 

 

 

lll

 

 

 

 

 

Figure51. Silversubstratesinsubstrateheaterafierlaservapor
depositionwithO.25mslaserpulses,401/cm’pulseenergy
densityand100 mtorrofO deposited(a)withoution-beam
and(b)withion-beam

112

 

—+— r --e-- la -o- c-

 

 

 

0 .
0 8 16

Distance from Center (mm)

 

 

 

 

Figme52.ChemiealcompositiondisuibufionofasdepositedIBP1DYBCOfilm
onfirsedsilieasubstrate(quantitative-Sanalysis)

113

 

 

 

 

 

  

I V V

L

 

ACO

' romso “U

.2 _-:.r.+a=ieiw_ .
IFSII 32K: .EE: 15E iET ch 532= 3W0 cts
I

Figure53. TypicalEDSspectrmnsfiom(a)alBPIDYBCOfilmand(b)aYBCOtarget

 

 

4.3.3.4

broad
8:40 2

Ta

fi.fi 7' Fl. It?“

I

I

 

and

ind

114
4.3.3.4 X-ray Diffraction Analysis of IBPLD Films

Data on XRD study of as-deposited and annealed films on silver substrate are
summarized in Figure 54. Due to a high deposition rate of the IBPLD process, x-ray
dimaction patterns of as-deposited films only show peaks from the substrate plus a few
broad peaks around 30° of 20 angle which are attributed to various compounds of Y,O,,
BaO and CuO as well as YzBaCuO, , also known as 2-1-1 phase[106], Table 7.

Table 7. List of possible compormds which might produce a strong dim'action peak

 

 

 

 

 

 

 

 

 

 

 

 

near 30° of 20 for Cu K0. radiation
COMPOUND 2-theta I HKL 2-theta I HICL 2-theta I HKL
ZBaO.Y,O3 30.4 x 103 28.8 7 110 41.14 4 200
3BaO.2Y,O3 29.28 x l 10 30.09 9 112 42.6 4 207
4BaO.Y,O3 30.09 x - 29.09 9 -- 41.6 5 —
BaO.CuO 29.33 x 600 30.15 5 61 1 40.05 3 741
BaO.Y,O3 29.68 x 320 29.48 6 40 31.05 6 121
ZCuO.Y,O3 31.32 x 211 33.2 9 204 33.51 5 13
2Y203.32r02 29.7 x 211 29.42 4 3 34.35 3 122
BaO.ZrO2 30.14 x 110 43.14 4 200 53.52 4 211
2BaO.ZrO2 29.18 x 103 29.83 9 110 43.33 3 200
3BaO.ZZrO2 30.16 x 1 10 29.58 9 105 43.22 6 200
Y,BaCuO, 29.88 x — 30.51 7 - 53.52 4 211

 

 

 

 

 

 

 

 

 

 

 

 

Threestrongestpowderdiffiaetionlinesfi'omCuKalineradiation
column ”I": xfor strongestline (100%); othersx 10%

Justhalfanhourofpostannealingat850 °Cconvertsmajorityofthefilmtothe
1-2-3 phase which is believed to be influenced by microscopic chemical homogeneity of
thefilm. Fmtherameaflngremfltsmfiutherdecreaseofthemn-supercondmfingphases,
and development of a higher degree of c-axis preferential orientation which is clearly
indicatedbythe increasingratiobetween002 peaknear 15° and 103/110 peaknear 33°.

115

 

xY203'an0-2Cu0 8Y2 BuCuOSO

* YBC12CU3OX 0

YBCO on Ag Ag*

as deposited

    
   
    

YBCO on
850' 0.511 Ag

annealed o

YBCO on A9

850' 1b

Arbitrary Scale

 

 

YBCO target

 

 

10 20 30 40 50 60
20 (degree)

Figure54. XRDpattermonBOOfilmsdepoeitedonsihrersubstrates

1 16
As-deposited films on (001)YSZ and (001)YSZ with a 0.2 mm silver buffer layer

show similar results as those for the films on silver substrate. However, after post
annealing, a strong film/substrate reaction was observed on films deposited on (001)YSZ,
which is indicated by an additional peak of xBaOerO2 near 43°, Figure 55. It is found
that 0.2 mm of silver bufl‘er layer efi‘ectively minimizes degradation caused by
film/substrate reaction of (001)YSZ substrate, Figure 55.

4.3.3.5 Microstructure of IBPLD Films

Typical microstructure of YBCO as deposited film on (001) YSZ, produced by
ion-beam laser vapor deposition process with 0.25ms laser pulse and 40 J/cm2 energy
densityisshowninFigure56. Asimilarmicrostructurewasalsoobservedforthe
as-depositedonsilverzhowever,aquite difi‘erentmicrostructurewasformdfortheas
deposited film on (001) YSZ with silver bufi‘er layer, Figure 57. Figure 58 shows
microsfiuctmeonBCOfilmsdepositedonsflver,afierhighkmpemhneamalhg.

117

 

xY203-yBa0-2Cu0 8YzBaCu050
YBCO 0|: (001)YSZ Y802CU3OX.
8501: 1 ZrOZA
annealed A“
x BaO - y Zr021
3 YBCO on (001) YSZ with Ag buffer layer
E 850T 1"!
e annealed
g e
YBCO target

 

 

10 20 30 40 SO 60 70 80
20 (degree)

Figure55. XRD onBCOfilmsdepositedona(001)YSZsubstrateand
a(001 YSZsubstratewithanAgbufi‘erlayer

118

 

 

 

 

 

Figure 56. Scanning electron micrograph of an as-deposited lBPLD film
on (001) YSZ (0. 25 ms laser pulse with 40 J/cm2 pulse-energy
densitY)

119

 

 

 

 

 

Figure 57. Scanning electron micrograph of an as-deposited IBPLD film
on (001) YSZ with silver bufi‘er layer (0.25 ms laser pulse with
40 J/cm pulse-energy density)

120

 

 

 

 

 

Figure 58. Scanning electron micrograph of a 850 °C, 1 hour, annealed
IBPLD film on silver (0.25 ms laser pulse with 40 J/cm’
pulse-energy density)

121
4.3.4 VTFJms-PLD Multi-Layer Ag/Ag-Doped YBCO Tapes

4.3.4.1 Effect of Ag Addition to YBCO Target

In IBPLD process, a zigzag scanning pattern of the target with a x-y target
manipulator reduces target overheating problem and produces YBCO films with correct
compositions. However, for a prolong deposition without frequent re-surfacing, the pulse
energy density is restricted to about 60 J/cm2 which limits the deposition rate below
10 nm/s.

The target overheating and severe melting problems were completely resolved by the
combination of silver doped target and zigzag target scanning pattern. This improvement
is believed to be caused by higher densities, higher thermal conductivities and better
mechanical strengths ofthe silver doped YBCO targets.

The density of silver doped YBCO target increased from about 77% theoretical
density, for the pure YBCO target, to about 84% theoretical density, assuming the rule of
mixture, for both 15 wt% and 23 wt% Ag-doped YBCO.

The flexural strengths of YBCO and 15% Ag—doped YBCO targets, determined
by four-point bending, are 12.38 :t 2.39 and 30.51 :t: 0.75 MPa, respectively. Both the

strengthandthe scatteringofstrengthdataare greatly improvedbythe silver addition.

As a result, continuous deposition rate as high as 50 nm/s was attained with only
15% silver addition ‘

Picture of both YBCO and 23% silver doped YBCO targets after ms-PLD
processes, with a moderate pulse energy density, shows clear improvement in resistance
tosurfacerougheningbysilverdoping,Figure 59. Thedepositionparametersusedwere
2ppspulse-rate,1mm/ssean-rate,1mmfeedper-scan,and6OJ/cm’pulseenergy
density.

122

 

YBCO-+23% AG

2 cm

 

 

 

 

Figure 59. YBCO and 23% Ag-doped YBCO targets after 4 or 15 laser scans

Figure 60 shows seaming electron micrographs of target surfaces alter four laser
scans under same conditions. YBCO target shows severe cavitation and cracking while
23 wt% silver doped YBCO appears to be flat and smooth

Cross section micrographs of the same YBCO target, Figure 61 (b), shows an
almost delaminated resolidified layer of approximately 30 um thickness. On the other
hand, the 23 wt% silver doped target only shows a very thin and continuous resolidified
layer, Figure 61(a). These differences are even more evident after fifteen laser scans
under the same conditions, Figure 62.

123

 

 

 

 

 

Figure 60. SEMmierographstakenfrom surfacesofla) 23%Ag-doped
YBCOtargetand(b)YBCOtargetaflerfomlaserscans

124

 

 

 

 

 

 

 

Figure 61. Cross section SEM micrographs of (a) 23% Ag-doped YBCO
and(b) YBCO targets after fourlaser scans

125

 

 

 

 

 

 

 

Figure 62. Cross section SEM micrographs of (a) 23% Ag-doped YBCO
and (b) YBCO targets afier fifleen laser scans

126

Similar improvement in the resistance to high power laser pulses is also found
with only 15wt% of silver addition to YBCO target. Due to the superior properties of
silver doped YBCO targets, 15% silver doped YBCO targets were used for deposition

process.

4.3.4.2 Adhesion between Ag layer and Ag-Doped YBCO Layer

Figure 63 shows a target after 35 minutes of continuous deposition process and
two Ag/Ag-doped YBCO multi-layered thick tapes with approximately 8 microns of
Ag-dopedYBCOtoplayers. Oneofthefilmshasbeenbenttoshowgoodadhesion
betweentheAg-dopedYBCO layerandthesilver,nodelaminationwereobserved

The as-deposited Ag-doped YBCO layers appear to be nooth and dense with
goodadhesionthatcanbeseenfiomSEMmicrographstakenfiomsmfaceofan
as-deposited film, Figure 64 (a), and fiactured cross section ofa silver doped YBCO/Ag
film. Figure 64 (b)-

Figure 65 shows atypicalopticalmicrostructme ofanannealed 15wt%Ag-doped
YBCOfilmafierpolishing. 'I'hefilmappearstobeverydense,andmostofthesilver
content forms uniformly distributed micron size particles.

127

 

 

 

 

 

Figure63. A15wt%Ag-dopedtargetafler35 minutesoflaser
irradiation, and two AgIAg-doped YBCO multi-layered
thicktapeswithapproximate8micronsong-doped
YBCOtoplayers

128

 

 

 

 

 

 

 

Figure 64. SEM micrographs from (a) surface of an as deposited film, and
(b) fiactured cross section of an Ag-doped YBCO/Ag film

129

 

 

 

 

 

Figure 65. Typical optical microstructure of an annealed,
15 wt% Ag-doped YBCO film

130
4.3.4.3 Effect of Peak Power Density on the Composition of Ag-Doped YBCO Layers

EDS data collected from Ag-doped YBCO layers, deposited under different
conditions, are shown in Figure 66 along with an EDS spectrum from a silver doped
targetforcomparison. Peakpowderdensityisthemostimportantlaserparameterin
ms-PLD process. Samples produced with the same pulse energy density but different
pulse duration, hence difl‘erent peak power density, have quite difi‘erent compositions.
Samples deposited at peak power density of about 0.9 MW/cm2 have compositions very
close to that of the target. On the other hand, samples with lower peak powder densities,
e.g. about 0.2 MW/cm’, show yttrium and copper depletion.

4.3.4.4 X-ray Dimaction Analysis of Ag-Doped YBCO Layers

Typicaldataonmstudyofas-depositedandameeledAg-dopedYBCOlayers
aresummarizedinFigme67. TheXRDpatternofasdepositedAg-dopedYBCOlayers
showverybroadsilverpeaksandaverybroadpeaksarormd30°of2-Osimilarto
tie-deposited IBPLD films. The as-deposited layers are more amorphous than
as-deposideBPIDfilmsduemmmhlowersubsuawtempemuneandtheabsenceof
highenergyoxygenion-beam.

Just 20 minutes ofpost processing annealing at 855 °C, converts majority ofthe
layertol-2-3 phase. Thisisbelievedtobeeausedbymieroscopic chemical homogeneity
oflayers. Fmtherannealingat905°Cfor30minutesresultsinadecreaseofthe
non-superconducting phases. Higher degree of c-axis preferential orientation, indicated
bytheimreasmgintensflyoftheOOZpeaknear15°,isobmimdbyanaddifional30
mhanedhgaW5°Qhowwerthemdesirablesecondphasealwdewlopsdming
prolongannealing.

131

 

 

 

ch 560= 517

 

 

 

Figue66. .Sapectrafi'om(a)15%Ag-dopedYBC01ayerdepositedwith2.5mslaser
pulses,27OJ/em’enerydensityandabom0.2MW/cm’peakpowerdenfity
(b)15%Ag-dopedYBCO layerdepositedwith0.6mslaserpulses,2701/cm;
energy density and about 0.9 MW/cm2 peak power density, and (c) 15%

WYBCOW

 

Arbitrary Scale

132

 

szanBaO'zCuO 8t YzBaCu05 O
YBazCuan 0

° A Aga

 

      
  

as deposited

D A

855° C/20m1% ‘

D I

85530/20min+°
905 C/30m1n A
U U 0 an A Cl 0
E]

.An

855°C/20min+
905°C/q9nin
[:1

DO

      

 

 

   
   

 

[6' I I IZIOI I I I3IOI I I I4|OI I I I5lOI I I I6|O
28 (degree)

Figure 67. Typical x-ray diffiaction patterns taken from as-deposited and
annealed 15% Ag-doped YBCO layers

133
4.3.4.5 Critical Temperature of Ag/Ag-doped YBCO Tapes

Critical temperature (Tc) ofthe annealedtapeswasmeastn'edusingastandard
four-probe technique with an auto-balance bridge. Figure 68 is a typical
resistant-temperature curve of Ag/Ag-doped YBCO tapes annealed at 855 °C for
20 minutes and 905 °C for 30 minutes respectively. A metallic behavior (positive
temperature coefiicient of resistivity) is observed in the normal state followed by a broad
transition, and a TC ofabout 77 K.

 

160

 

 

“120.. T

 

on
c:
I

up
<3
I

RESISTANCE (In-Ohm

 

 

 

0 l 1 1 1
'73 93 113 133 153 173

TEMPERATURE ( Kl
Figure 68. Typical resistant-temperatme curve of an annealed Ag/Ag-doped YBCO tape

 

5. SUMNIARY

5.1 Factors Which Afiect Processing of Bulk YBCO Superconductors

The preparation of bulk YBCO superconductors, through conventional solid-state
processes, has been systematically studied. Carefirl wet grinding and firm hand packing
were found to greatly accelerate the calcination and formation of the YBCO compound
Furthermore, impurity less than 0.1% in the starting materials can significantly retard the
formation of the YBCO compound.

High green density is important in obtaining high quality bulk superconductors.
High green density can be obtained using a well crystallized YBCO powder with a shiny
black appearance.

5 .2 Laser Calcination of Pb-Doped BSCCO

The problem associated with sluggish kinetics of formation of 2223 high Tc phase
in Pb-doped Bi-Sr-Ca-Cu—O system was investigated

A laser calcination process for Pb-doped Bi-Sr-Ca-Cu-O (BSCCO) material was
established that reduces total processing time of obtaining a near-single phase high Tc
material to about 100 hours. The high Tc phase was found to form via a different kinetic
path in laser calcined sample compared with the conventionally processed sample.

The onset critical temperature of the laser calcined sample was found to be about

110 K However, the zero resistance temperature was about 98 K.

134

135
5.3 Ion-Beam Assisted Millisecond PLD

An ion beam assisted millisecond pulsed laser vapor deposition process was
developed to fabricate YBa,Cu,Ox (YBCO) high Tc superconductor films.

Target overheating problem is greatly reduced by using a zigzag scanning pattern
of the target with a x-y target manipulator. With the x-y target manipulator, peak power
density up to 0.16 MW/cm2 can be attained to deposit YBCO film with a composition
close to the target composition. Large uniform YBCO film was successfully deposited
on fused silica substrate.

High energy oxygen ion beam used in this process is found not only to replenish
oxygen, but also to help propagating vapor species onto the substrate.

As-deposited films are composed of various compounds of Y,O,, BaO and CuO
as well as Y,BaCuO,. Due to microscopic chemical homogeneity of films, films with
majority of 1-2-3 phase are formed by just halfan hour of post annealing at 850 °C.

The silver buffer layer is found to efiecfively minimize degradation caused by
film/substrate reaction of (001)YSZ substrates during post annealing

5 .4 VTE/ms-PID MAS-DOM YBCO Tapes

A new process, combining ms-PLD and vacuum thermal evaporation (VTE)
techniques, was developed to produce a multi-layer Ag doped YBCO/Ag thick film.

The target overheating and severe melting problems were completely resolved by
thecombinationofsilverdopedtargetandazigzagtargetscanningpattem This
improvement is believed to be caused by higher densities, higher thermal conductivities
and better mechanical strengths ofthe silver doped YBCO targets.

136
As a result, continuous deposition rate as high as 50 nm/s was attained with only

15% silver addition Ag doped YBCO layer and the silver shows good adhesion, no
delamination were observed even after bending.

Peak power density is found to be the most important laser parameter in ms-PLD
process. Samples produced with high peak power density have compositions very close
to that of the target. On the other hand, samples produced with lower peak powder
densities show yttrium and copper depletion

The ms-PLD processed Ag-doped YBCO tapes show a metallic temperature
dependence of resistance in the normal state and a broad transition to a TC of about 77 K.

APPENDIX

APPENDIX

Addendum to the review of Selected Theories of Superconductivity

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

List of Symbols
H magnetic field strength E, Fermi energy
B magnetic flux density v, velocity of electrons at E'
A magnetic potential field F. superconducting state free
energy density
1 current density Fa normal state free energy density
T temperature 9 Debye temperature
Tc critical temperature 0n Debye frequency
e charge of an electron U electron-phonon interaction
matrix elements
m mass of an electron EI superconducting energy gap
C speed of light in vacuum 3. penetration depth
k, Boltzmann’s constant M London penetration depth
h Planck's constant g coherence length
ll h / 2n 5,0 intrinsic coherence length
e' charge of superconducting x Ginzburg-Landau parameter
charge carrier (1: = 7d: )
m' mass of superconducting (D total magnetic flint
charge carrier
n conduction electron density (Do fluxoid
ns superconducting electron Hc thermodynamic critical field
density
n', density of superconducting HeI lower critical field
charge carrier
N, normal state electron density at Hc2 upper critical field
the Fermi energy

 

137

 

138
Al Two-Fluid Model:

A. 1.1 Free Energy in Superconducting State

Letn=NNbethedensityofallconductionelectrons,whereNandVare the
total number of conduction electrons and volume of the sample. n, and n. denotes the
superconducting and normal-state electron densities, respectively. The two fluid model

assumes a free energy of the conduction electrons in superconducting-state as

 

been = xl/zrna)+(1—x)£sm [56].

 

where x = nn/n is the fraction of normal state electrons.
Freeenergyofnormal stateelectronsissettobe

fnm = —YT2/2

in order to get a linear electron specific heat ofnormal -state, i.e.

 

Cm: -T

5260')
m2 = 'yT.

Freeenergyofsuperwndiwfingelecuomissettobeequaltothesupemonducfing

condensationenergy-B,i.e.£r(‘r)= 43. Sinccx=0at0K,ie.allelectronsare
incondensedstate,F:(0.0)= —B

All Temperature Dependence of Superconducting Electron Density

The equilibrium fraction of normal-state and superconducting electron at
temperatmeTcanbeobtainedbysetting(6F/OX)T =0.

139

2 4 4
=.Y_T_ = l 2 -
xequg 1682 _ (Tc) ,where Tc — 4B/y. Therefore,

 

n,/n= l- (T/Tc)4 l;therefore,n,(Tc)= 0 and n,(0)= n .

A13 Temperature Dependence of Thermodynamic Critical Field

TheparametersBandycanbederivedusingthepropertythatthefreeenergy
difference between normal and superconducting states is equal to the magnetic energy at
the thermodynamic critical field, i.e.

2
Fn-F,=%:- anan=fn= -yT2/2

F" — F, = r.. - {XI/2f" +(1-x)f_,}
= {1- crrrc>2}(-v'r2/2) — {1— (mark—p)
.-. p{—3’§(r2 -— r4rr3) + 1- {my}
e{—(2/TZ:) (T2 - WI?) + 1— any}

2
B{l- (Hafiz: ’3‘;-

 

9

[Harm (81IB)"2{ 1 - any} = He(°){ 1 - arrc)2}|

 

 

 

140
A. 1.4 Electron Specific Heat

Consider the Helmholtz free energy, F(V,T) = E - TS, the following relations are

true for any reversible process:
dF=-PdV-S¢Il‘ ; (6F/5T)V=-S;
dS=dQRlT ;Cv=(dQ/d1),,=(dE/Jl')y;

(Cyril/Dy = (dQ/T)V= dS . Therefore,

lee = T(dS/dI‘) .-. T%(-6F/5I')y= awe/er?) V l

 

The electron specific heat in normal and superconducting states, Cmand Ces,can
be obtained by -r(a21=,,/ar2) and —T(02F,/er2) . respectively.

62F» a a 41" a
-T—2- = ‘Tfi[3f('T)] = “T5679 = YT

a... mm (ell-ail {1—(%)}‘<-m

_ r B) 4 _ 1 B 4 1 4
_——+—r---—+—r- =——r—
( 2T? r2 '3 I 2'r% «rs/mi} ‘3 41% '3

 

 

[cw 4:? =3vTc(%)3

 

 

(ca / em, = {swear/ref} Int.) = 3

141
A2 London Equation

The London equation is an important phenomenological theory based on the two
fluid model to try to understand the Meissner effect. Recall the result of the two fluid
model, the density of superconducting electrons is

2’f.—”—=1—c1‘rr.)“. 1130c): 0. and n.(0)= n ~

The current due to the superconducting electrons is j = -ev,ns where its is the

velocity of superconducting electrons which can be obtained from Newton's law
F = m(dv/dt) = -eE. Thus,

6I 6V 32:
bT=~cn;-;=l-%JE

substituting E above into Maxwell's equation, VxE = - gas/at) , yields

gtfiv x 1 + a): o [56].
The Londons showed that the Meissner effect can be account for by restricting the
solutions to those satisfying

B: TCVXJ whichiscalledtheLondon '
e n, i equation[55,56].

 

 

A2.1 London Penetration Depth

Another form of the London equation can be obtained by applying Maxwell's
equations, VxB=(4n/C)] and V-B=0, and the vector identity, V x (v x B)=
V(V . a) - v23 to the above equation that yields[55,56]

142

 

 

2
B=-%C-Vx(2%VxB) =_ ”C V213 or V23=B/xi J’

e n, 41tn_,e2

 

where 31. is the London penetration depth defined as

 

Lie—i a mC2 / 41tnse2I.

 

Yet, the other form of the London equation is obtained by taking curl of both sides of the
first form of London equation and applying the physical bormdary V 0 j = 0 , that
yields[56]

VxB = (%'—)j=—;¥2”iVxij=-§fi[V(V-j) —v21] or

|v2j=1/A§|.

 

A.2.2 Exponential Decay of the Magnetic Field B and Supercurrent Density 1

V23= BIL: and v21=y 1% indicate exponential decay ofthe magnetic field
andsupercturentfromthefieestn'face intoasuperconductorwithacharacteristic length
equal to the London penetration depth 3‘1.-

Forexample, letthefree surfaceofthe superconductortobeperpendiculartothe
x axis with positive x in the superconductor, and let the applied magnetic field parallel to
positive 2 direction and the supercurrent in the positive y direction Solutions for the
magnetic field and supercurrent density are[56]

 

 

[32 = Bz(O)exp(—x/ AL) land I” = jy(0)exp(-x/ KL) , ,respectively.

 

 

143
A.2.3 Temperature Dependence of the London Penetration Depth

Temperature dependence of the London penetration depth can be obtained by
adopting the temperature dependence of (us/n) from the two-fluid model[56].

mm = (mCZ/ 41m,c2) 1/2 = {(mC2/4rmc2)/ [1 _ (T/Tc)4]}1/2

 

[MID = xr(o>[1 - cr/ mfl’m I

 

A.2.4 Magnetic Potential Field A and London Rigidity

TheLondonequationcanalsobewrittenintermsofavectorpotentialA,which
willbeusedindiscussingGinzbtug-Iandautheory,suchthatB=VxA. Thefirstform
ofLondonequationcanberewriteintermsofMas

 

 

C C
j “C 4:1}. 41: x12 '1:

 

4x1 1.

 

SincejccA ,wehaveV-A=0andA,.=0,whereA,. iscomponentoprerpendicular
toextemalsrufaee,inordertosatisfyingphysicalbormdaryconditionV-j=Oandjn=0.

Thisequafioncanalsobeobtainedinamoreinmitivewaybyconsideringthe
canonical momentum P=mv+(—eA)/C. London proposed a quantum mechanical
theorytlntifthegrotmdstatewawfimefionismchangedbytheappficauonofthe

magnetic field, i.e. ‘I’ = To so-called London rigidity, the net moment P should be zero.
The London equation for the supercurrent density follows automatically[56, 58].

 

_ --E s; _-_nnei =_ C
I"’“’”” m(P+C)0’ tnCA 4,,sz
L

144
1/2
2.1,(0): (mC2/41tnc2) or in the SI units
“2
11(0) = (eomc2/ nez)

107/41tC2 Coul.2/N-m2 -9.110xio-31(kg)-02 1112/52
( )( ) ( )

n(m-3)-(1.602x10-19)2(Cou1.2)

 

= tr“2 . 5.315 x106(m)= tr"2 - 5.315 x1015(nm)

For metallic elements the conducting electron concentration, n, is in the order of
1028~1029m'3[55]. The corresponding 2., in the order of 101~ 102 nm.
Experimentally measured penetration depth in elemental superconductors tend to be
largerthanthe calculated 11(0) [56].

Fmsuperconductorthinfilmsinmagneficfieldsparaflelwthefilm,themagnefic
field will penetrate the whole film resulted in incomplete Meissner efl‘ect ifthe thickness
offilmismuchsmallthanthepenetrationdepth. Veryhighcriticalfield,HOis
expectedmthissnuafionbecausetheindwedfieldismuehlessthantheapphed

magnetic field, and the free energy expression,

2
a

- 81:

F" -F, = , no longer holds.

rte.
w

145
A3 Ginzburg - Landau Theory (G.L. theory)

All Ginzburg-Landau Order-Parameter

Ginzburg and Landau (1950) proposed a phenomenology theory of
amerwndtwfingsmtemmmsofaspafialwfiedorderparamewr‘l’(r)WMchvammesm
a second-order phase transition temperature[55,56.58]. ‘I’(r) is related to local
superconducting charge carrier density, n,’ .

 

‘I”(r)‘I’(r) = l‘1’(r)l2 =ne’(r) f

 

A.3.2 Ginzburg - landau Free Energy

In the absence of magnetic field and spatial variation of order-parameter, the
difi‘erenceinfieeenergydensityisexpressedintypicallandaufomfi].

F, 43. = -aI‘I’I2 + -;-Bl‘~l'l4 = —otn..’ + %Bn,;2

ParametersaanchavepropertiesthatB>0foranysecond-orderphase
transition,a>0fortemperaturebelochanda<0fortemperatureaboveTc. This
energydensitydifi‘erenceexhrbitsastableminimmnat ‘I’=0forT>Tc,i.e.thenormal
state. Inthesuperconductingstate,i.e.T<Tc,adoublewellpotentialappearswith
minimaat‘I’=¢‘I’,[56]. ‘I’,thegoundstateorderparameter,where‘l’,’definedasthe
m1 value in the interior ofthe sample without spatial variation of‘I’and magnetic field,
canbeobtainedbysettingaF,—F,.)/bn,’=0[56].

 

‘1’3 " (fiber-Fallen; - o = “’9

 

146
The equilibrium difference in free energy density can also be expressed in terms

of the thermodynamic critical field H:

F,-F.. = —ot(o/p) + %B(a/B)2 = -o2/(2t3) = —H§/(8n)

An expression of the thermodynamic critical He in terms of a and B is obtained.

II-Ic = (4m2/B) 1/2J

Taking into account the spatial variation of ‘I’ and magnetic field yields[55,56, 5 8]

 

 

2
[FM—F" aI‘I’I ”pm +—2 ,|( rllv —C )WI 0 MdBal,

 

where m' and e' are mass and charge of the charge carrier. (1/2m’)|(—iliV - e’A/C)‘I’|2
tammpresentsemrgymcmasecausedbyspafialvmiafionof‘l’,afismgfiomkinefic
momentum -illV and field momentum -e’A/ C. The trailing term is the energy increase
due to magnetic flux expulsion, i.e. Meissner effect, where B,| = H is the applied field
UsingtherelationB= H+ 4xMandB=0, duetocompleteMeissnereffect, yields

e. -_Ba at =3Laz.
_Io M-dBa- IO —4x-dBa 8x- 8x '

147
A33 Ginzburg - Landau Equation
Minimizing F s with respect to the order-parameter ‘1” yields

6F;__ 6
appear”

 

{Fn - a‘l’T‘ + #3512?“
+ ~2-ée((—ihV - 53A)? ' (ihV " %A)‘¥I) + 151-E}

= —a‘I’ + eras" +fi{(-ihv- gays. (av- {34)} = 0

 

2 A; -2’. 2) _
K—awm +2m,| rhv CAI suol

 

This is called the Ginzburg - Landau equation that resembles a schrodinger equation for
‘I’ with an energy eigenvalue 0155,56 ].

A3.4 Equivalence of London Equation and Penetration Depth

MinimizingF,withrespecttothe potential A yields

3%": = e—K-m "flew in") l "i"
A a

2
_ _ Zerqro ille’ c _ e e_’ c 2 1 6(VxA)
‘2m'aA{” + (:(I W 1” )A+(c) ‘P TA }+8—1t 6A

 

. , r2
=lingf‘Ecr'W-WW‘)+;%E§W‘WA+ 3‘;Vx(VxA)=0l-

 

This is another form of the Ginzburg-Landau equation Combining this result with
Maxwell's equation) = (C / 41:)V x H , yields

__ ie’ll e e 51 e
j,(r)—-2—m-,-(‘I’ V‘P—‘I’V‘I’ )1th WA

148
This equation can be reformulated by using the expression ‘1’ = ‘I’oeiem from the
BCS theory where ‘1', and 0 are magnitude and phase of ‘1’, respectively.
. , , . . . 92
j,(r) = —%{‘Poe"9V(‘Poe‘e) - Toe-‘9V(‘I’oe”e)} — fifmza

js(r) = §|r|2(hve — °E’a) 2 e’l‘I’IZVs

In the London gauge 0 is constant[56], yields

 

 

 

52 ’2
1,. mm A— m,C‘I’OA

 

Thisisinthesame formastheLondonequation
1 = -{C/(4u2tfi) }A .
A penetration depth 2. similar to the London penetration depth can be defined[55,56].
1/2
A: A = (m’CZBJM
4134??) 4xe’2a

aandpcanbeexpressedintermsofkandchsingthisequationand

 

 

 

H, = (4m2/B) 1/2 .

2 ,2
a = .11; - .e_H§;,2
4N5 m’c2

 

 

149
Using temperature dependencies from two-fluid model, i.e. t = T/TC, ‘I’o’ 0: ns/n
«(14‘)ch °C(1-t)’, temperature dependencies ofh’, a and B are obtained[56].

xzoc(1-t4)-lz(1—t)‘1
aoc(l—t2)/(l—t4)zt-l
Bcc(l—t2)/(l-t4)2zl

A3.5 Coherence Length

Consider the Ginzburg - Landau equation in one-dimension and in the absence of
field, i.e. A = 0.

33.-“3% = (a— WWW)?

For temperature near Tc,i.e. I‘I’l2 is small, the nonlinear term BI‘I’I2 can be neglected

 

 

 

This equation has a wavelike solution [3110: ‘I’oexp (-iX / Q] where

lg e I h2/(2m’a)] 1’ 2I is the coherence length[55,56].

An interesting special solution of the one-dimensional Ginzburg-Landau equation
considering the nonlinear term [SI‘PI2 can be expressed in terms of: defined above[55].

 

 

rot) = (welmtanh {x/(fz' 6.) }

Thissoltnioncanbevefifiedbydirectsubsuuuionbacktotheofiginalequafion
Knowing-é-tanhx=sech2x,%secx=-sechx-tanhx and tanh2x+sech2x=1.

150

--:§.-;—:{eaman(7;_§)} = -(g) “(319% “°h2l'tia)}(%)

=M()(%){-2ml-afmlezl} medals»

= aIl — tanh2(x #2:) I‘I’(x) = a‘I’(x) — Bl‘l’lz‘fix)

 

Deep inside the superconductor, i.e. large x, ‘I’ = To = (tr/B)“. The characteristic
length E, marks the extent of coherence of the superconducting wave-function into the
normal region

The dimensionless ratio between the two characteristic lengths A. and 5,, called
Ginzburg-Landauparameter,definedasrcE K/§,isanimportantparameterinthe
theory of superconductivity.

* -L_s1_’(_3 B 1/2
I‘=§-e’li ‘27) I

 

 

15 1
A3.6 Upper Critical Field and Type II Superconductor

Let the interface of the superconductor parallel to Y-Z plane and the magnetic
field B parallel to Z axis, i.e. B = B zk ; therefore, the supercurrent and the potential field
A will be in the Y direction, i.e. A =ij.

_ _ _ 6A, aAy) 6A,, 5A, (6A,, 6A,)
B‘sz-VXA-(w’w ”(w-7h + “ax—“'3; k

Bz=6Ay/6x or sz =Ay or xB=A

At fieldnearthe uppercritical field H62 ,because PM2 is small, theGinzburg-Iandau
equationcanbelinearized

%(—iliv — g-A) 2‘? = 03’

.(-h2V2 + 2%v - A + ($1) 2)?

 

 

 

 

This is ofthe same form as the schrodinger equation ofa free particle in a magnetic
field[55]. We should look for a solution in the form ‘I’z 0(x)epri(kyy + kzz)I .

Substituting this back to the equation yields[55]

152

ZJJI-hsz + (9%" ' m‘yIzI‘l’ = (01- ":fIc
{—h 2m 4(52 +— :222) +— 2m 47% 332+) Zille —Bxaay +(-e— Bx) 2:II¢(X)ci(kyY+kzz)

= —£{ei(kywkzz) '%¢ + 1:320" wkzz) (DI
dx

 

..
...{H . ...-(w) . 4%..) wet) .I

= ei(kyy+k’z)f;—,Idx -b2— 22+l12k2+(—x- hky) 2I<l> .

By shifting the origin from x=0tox=xo=—likyC/(e’B) , the x coordinate becomes
X = x - x,. Substituting to the above equation yields[55]

 

e’B 2 nzki
m-éazz-d’m +— (2)2 X @= ((1- 2111’ )0 ,

Thisequationisintheformoftheschrédingerequationforthemagnitudepart,u(x), of
an linear harmonic oscillator[107].

--“—- du+«1-kx2u=Eu

Undertheboundary condition u—)0asx-+oo,the energy eigenvalueE canonlybe in
the form En =(n+1/2)hv=(n+ U2)“, W60=Wm)m is the angular frequency of
the oscillator and n is any non-negative integer.

( are

 

153
The maximum magnetic Bmax =Hc2 is corresponding to n = 0[55]. Because the
supercurrent andA is in the Y direction, IE2 is zero.

H02 23m = 2am’C/(e’h)

1/2
This can be related to the thermodynamic critical field He = (41tt12/B) by using the

Ginzbug-Iandauparameterx:%f(%)m.

Hc2 = J5me

At onset of vortex state, i.e. H 2 Hcl , the cross-section area of a vortex is in the order of
1:)? because the magnetic field decays exponentially to zero over a distance in the order

of penetration depth M55].

He] sz¢OI n12)
Similarexpress forch canbeobtainedintermsofthe coherence length; [55].

He: = W = (”C- (ZnfiB '1 = eo/(mz)

e’h e’

 

Actual expression for Hcl can be derived by considering the crossover point 1: = 1/ J5
betweentypelandtypellsuperconductor,atwhicth, H01 andch shouldbeequal.

 

Relation between Hcl and ch with the thermodynamic critical field He can be
obtained.

154

 

Pa = <Do/(Zfiliz) = 5&ch

 

 

 

Hcl = oo/(mmz) = —-°°—- - H62 — He 1

2x21r§21c2 — 2:2 - (5‘)]

 

Hcl'HCZ =H3 and Hal/HCZ = ”(21(2)

For many type II superconductors, ch >>Hc or 1: >> 1/J'2' , so He] is very small.

REFERENCES

10.

ll.

12.

REFERENCES

R Beyers et al., ”Crystallography and Microstructure of Tl-Ca-Ba-Cu-O
Superconducting Oxides," Appl. Phys. Lett., 53 (5) (1988) 432-434.

W. Y. Lee et al., "Low-temperature Formation of Epitaxial Tl,Ca,Ba,Cu,Om Thin
Films in Reduced 02 Pressure," Appl. Phys. Lett, 60 (6) (1992) 774-774.

R K. Singh and J. Narayan, "The Pulsed-Laser Deposition of Superconducting Thin
Films," JOM, no. 3 (1991) 13-20.

D. Dijkkamp and T. Venkatesan, "Preparation of Y-Ba-Cu Oxide Superconductor
Thin Films Using Pulsed Laser Evaporation from High Tc Bulk Materials," Appl.
Phys. Lett. 51 (8) (1987) 619-621.

X D. Wu et al, "Epitaxial Ordering of Oxide Superconductor Thin Films on
(100)SrTiO Prepared by Pulsed Laser Evaporation," Appl. Phys. Lett. 51 (l 1)
(1987) 8613863.

X. D. Wu, A. Inam, T. Venkatesan et al., "Low-temperature Preparation of High Tc
Superconducting Thin Films," Appl. Phys. Lett. 52 (9) (1988) 754-756.

S. Miura and T. Yoshitake, "Structure and Superconducting Properties of
Y,Ba,Cu:O.,4 Films Prepared by Transversely Excited Atmospheric Pressure CO2
Pulsed Laser Evaporation,” Appl. Phys. Lett. 52 (12) (1988) 1008-1010.

H S. Kwok, P. Mattocks, L. Shi, X W. Wang, S. Witanachchi, Q. Y. Ying, J. P.
Zheng, and D. T. Shaw, "Laser Evaporation Deposition of Superconducting and
Dielectric Thin Films," Appl. Phys. Lett. 52 (21) (1988) 1825-1827.

Osamu Eryn, Kouichi Murakami, Koki Takita, Kohzoh Masuda, Hiromoto Uwe,
Hiroshi Kudo, and Tsunetaro Sakudo, " Y-Ba-Cu Oxide Films Formed with
Pulsed-Laser Induced Fragments," Jpn. J. Appl. Phys. 27 (4) (1988) L628-L631.

S. Witanachchi, H. S. Kwok, X. W. Wang, and D. T. Shaw, "Deposition of
Superconducting Y-Ba-Cu-O Films at 400 °C without Post-annealing" Appl. Phys.
Lett. 53 (3) (1988) 234-236.

C. C. Chang, X. D. Wu, A Inam, D. M. Hwang, T. Venkatesan, P. Barboux, and J.
M. Tarascon, ”Smooth High Tc Y,Ba,Cu,Ox Films by Laser Deposition at 650 °C,”
Appl. Phys. Lett. 53 (6) (1988) 517-519.

B. Roas, L. Schultz, and G. Endres, "Epitaxial Growth of YBa,Cu,O,_x Thin Films
by a Laser Evaporation Process," Appl. Phys. Lett. 53 (16) (1988) 1557-1559.

155

13.

14.

15.

l6.

l7.

l8.

19.

20.

21.

22.

23.

24.

25.

26.

156

G. Koren, A Gupta, and R J. Baseman, "Role of Atomic Oxygen in
Low-temperature Growth of YBa,Cu,Ox_d Thin Films by Laser Ablation
Deposition," Appl. Phys. Lett. 54 (19) (1989) 1920-1922.

R. K. Singh, J. Narayan, A. K. Singh, and J. Krishnaswamy, "In situ Processing of
Epitaxial Y-Ba-Cu-O High TC Superconducting Films on (lOO)SrTiO3 and
512(;O)YS-Zr02 Substrates at 500—650 °C," Appl. Phys. Lett. 54 (22) (1989)

1-2273.

M Balooch, D. R. Olander, and R E. Russo, "Y-Ba-Cu-O Superconducting Films
Pgo7du3ed by Long-pulsed Laser Vaporization," Appl. Phys. Lett. 55 (2) (1989)
l -1 9.

G. Koren, A. Gupta, R. J. Baseman, M. I. Lutwyche, and R. B. Iaibowitz, "Laser
Wavelength Dependent Properties of YBa,Cu,Ox4 Thin Films Deposited by Laser
Ablation," Appl. Phys. Lett. 55 (23) (1989) 2450-2452.

B. H Moeckly, S. E. Russek, D. K. Lathrop, R A Buhrman, Jian Li, and J. W.
Mayer, ”Growth of YBazCusO, Thin Films on MgO: The Effect of Substrate
Preparation," Appl. Phys. Lett. 57 (16) (1990) 1687-1689.

J. P. Zheng, S. Y. Dong, and H S. Kwok, ”Texturing of Epitaxial in situ
fY-(B-ag-gu-O Thin Films on Crystalline Substrates," Appl. Phys. Lett. 58 (5) (1991)
4 .

R K. Singh and D. Bhattacharya, "Improvement in the Properties of High Ta Films
Fabricated in situ by Laser Ablation of YBa,Cu,O,-Ag Targets,” Appl. Phys. Lett.
60 (2) (1992) 255-257.

Y. Iijima, N. Tanabe, O. Kohno, and Y. Ikeno, ”In-plane Aligned YBa,Cu,Ou Thin
51991193366??? On Polycrystalline Metallic Substrates," Appl. Phys. Lett. 60 (6)

D. K. Fork and K. Nashimoto, "Epitaxial YBa,Cu,OH on GaAs(OOl) Using Buffer
Layers," Appl. Phys. Lett 60 (13) (1992) 1621-1623.

J. A Alarco,G. Brorsson, Z. G. Ivanov, P.-A, Nilsson, and E. Olsson, "Effect of
Substrate Temperature on The Microstructure of YBa,Cu,O.M Films Grown on
(001)Y-ZrO2 Substrates,” Appl. Phys. Lett. 61 (6) (1992) 723-725.

S. Komuro, Y. Aoyagi, T. Morikam and S. Namba, "Preparation of High-'1‘c
Superconducting Films by Q-switched YAG Laser Sputtering,” Jpn J. Appl. Phys.
27 (l) (1988) 1.34-1.36.

Arunlnam andX D. Wu, "PulsedLaserEtching oingh Tc Superconducting
Films," Appl. Phys. Lett 51 (14) (1987) 1112-1114.

J. Mannhart, M. Scheuermann, C. C. Tsuei, M M Oprysko, C. C. Chi, C. P.
Umbach, R H Koch and C. Miller, "Micropatterning of High Tc Films with an
Excimer Laser," Appl. Phys. Lett. 52 (15) (1988) 1271-1273.

K G. Humphreys,J. s. Satchell, N. G. Chew, and J. A Edwards, "Narrow Tracks in
3553360707 Thin Films Defined by Laser Ablation,” Appl. Phys. Lett 54 (1) (1989)

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

157

Arunava Gupta and Gad Koren, "Direct Laser Writing of Superconducting Patterns
of Y,Ba,Cu,Ou,” Appl. Phys. Lett. 52 (8) (1988) 665-666.

B. W. Hussey and A. Gupta, "Laser-assisted Etch of YBa,Cu,OH," Appl. Phys. Lett.
54 (13) (1989) 1272-1274.

G. E. Jang, K. Mukherjee, P. A A. Khan and M Tayal, "Manufacturing of High
Temperature Superconductor by Using A Nd:Yag Laser," Laser Materials
Processing III (Warrendale, PA: The Minerals, Metals & Materials Society, 1988),
159-169.

C. W. Chen, P. A. A. Khan and K. Mukherjee, "Laser Fabrication of Pb Doped
Bi-Sr-Ca-Cu-O Superconductor," J. Mat'ls Sci. 27 (1992) 3221-3224.

M Levinson, S. S. P. Shah, and D. Y. Wang, "Laser Zone-melted Bi-Sr-Ca-Cu-O
Thick Films," Appl. Phys. Lett. 55 (16) (1989) 1683-1685.

Naoaki Aizaki, Koichi Terashima, J un-rchr Fujita and Sinji Matsui, ”YBa2CuJO
Superconducting Thin Film Obtained by Laser Annealing," Jpn J. Appl. Phys. 27
(2) (1988) 1231-233.

P. Haldar, J. G. Hoehn, Jr., and J. A. Rice, "Enhancement in Critical Current
Density of Bi-Pb-Sr-Ca-Cu-O Tapes by Thermomechanical Processing: Cold
Rolling versus Uniaxial Pressing," Appl. Phys. Lett, 60 (4) (1992) 495-497.

S. Jin, G. W. Kammlott, T. H. Tiefel and S. K. Chen, "Formation of Layered
Microstructure in the Y-Ba-Cu-O and Bi-Sr-Ca-Cu—O Superconductors," Physica C
198 (1992) 333-340.

J. M Tarascon, W. R McKinnon, P. Barboux, D. M Hwang, B. G. Bagley, L. H
Greene, G. Hull, Y. LePage, N. Stofl'el, and M Giroud, "Preparation, Structure, and
Properties of the Superconducting Compound Series Bi,Sr,CaMCu_O with n= 1,
2, and 3, " Phys. Rev. 38 (13) (1988) 8885-8892.

H Maeda, Y. Tanaka, M Fukutomi, and T. Asano, ”A New High-Tc Oxide
182er without a Rare Earth Element," J. Appl. Phys. 27 (2) (1988)

D. Shi, M S. Boley, J. G. Chen, M Xu, K. Vandervoort, Y. X Liao, A Zangvil, J.
Akujieze, and C. Segre, ”Origin of Enhanced Growth of the 110 K
Superconducting Phase by Pb Doping in the Bi-Sr-Ca-Cu-O System," Appl. Phys.
Lett. 55 (7) (1989) 699-701.

Y. Yamada and S. Murase, "Pb Introduction to the High-TC Superconductor
Bi-Sr-Ca-Cu-O," Jpn. J. Appl. Phys. 27 (6) (1988) L996-L998.

M Takano, J. Takada, K. Oda, H Kitaguchi, Y. Miura, Y.lkeda, Y. Tomii, and H.
mazaki, "High-Tc Phase Promoted and Stabilized in the Bi, Pb-Sr-Ca-Cu-O
System," Jpn J. Appl. Phys. 27 (6) (1988) L1041-L1043.

MMizuno, HEndo, J. Tsuchiya,N. Kijima,A Sumiyama,andY. Oguri,
"Superconductivity of Br,Sr,Ca,Cu,Pbe ( F0. 2, 0. 4, 0. 6),” Jpn J. Appl. Phys. 27
(7) (1988) L1225-L1227.

41.

42.

43.

45.

47.

48.

49.

50.

51.

52.

158

S. A Sunshine, T. Siegrist, L F. Schneemeyer, D. W. Murphy, R J. Cava, B.
Batlogg, R B. van Dover, R M Fleming, S. H Glarum, S. Nakahara, R Farrow, J.
J. Krajewski, S. M Zahurak, J. V. Waszcmk, J. H Marshall, P. March, L. W. Rupp,
Jr. ,and W. F. Feck, "Structure and Physical properties of Single Crystals of the
84-K Superconductor Bi,_,Sr2CamCu,OMI " Phys Rev. B38 (1988) 893.

T. Venkatesan, X D. Wu,A Inam, andJ. B. Wachtman, "ObservationofTwo
Distinct Components During Pulsed Laser Deposition oingh Tc Superconducting
Films," Appl. Phys. Lett. 52 (14) (1988) 1193-1195.

P. E. Dyer, R D. Greenough, A Issa, and P. H Key, ”Spectroscopic and Ion Probe
Measurements of KrF Laser Ablated Y-Ba-Cu-O Bulk Samples," Appl. Phys. Lett.
53(6) ( 1988) 534-536.

T. Venkatesanetal., "Nature of'l‘hePulsedIaserProcessfor'l'heDeposition of
High TC Superconducting Thin Films,” AppL Phys. Lett. 53 (15) (1988) 1431-1433.

J. P. Zheng, Z. Q. Huang, D. T. Shaw, and S. Kwok, ”Generation of High-energy
Atomic Beam in Laser-Superconducting Target Interactions," Appl. Phys. Lett 54
(3) (1989) 280-282.

J. P. Zheng, Q. Y. Ying, S. Witanachchi, Z. Q. Huang, D. T. Shaw, and S. Kwok,
"Role of The Oxygen Atomic Beam in Low-temperature Growth of
35W Films by Laser Deposition," Appl. Phys. Lett. 54 (10) (1989)

6.

C. W. ChenandKMukherjee, 'ReviewofProgressinPulsedlaserDepositionand
UsingNd:YAGLaserinProcessingoingth Superconductors, Proceedingof
BeamProcessingofAdvancedMaterialsInternational Conference (Warrendale,
PA: TheMrnerals Metals&MaterialsSociety, 1992) 301-321.

Shin-ya Aoki, Taichi Yamaguchi, Yasuhiro Iijima, Akira Kagawa, Osamu Kohno,
Shigeo Nagaya and Toshio Inoue, “High Jc Y-Ba-(hr-O Thin Film on Metal
Substrates Prepared by Chemical Vapor Deposition,” Jpn J. Appl. Phys. 31 part 2
(5A) (1992) L547-L549.

Naotaka Minami, Nobuyuki Koura and Hiromasa Shoji, "Preparation of
YBa,Cu,O,W Superconductor Coating on Austenitic Steel by Electrophoretic
Deposition Method-Study onBufl'ering Layers,” Jpn. J. Appl. Phys. 31 part 2 (6B)
(1992) 170n84-L786.

P. N. Peters,RC. SiskandE. W. Urban, ”ObservationofEnhancedPropertiesin

Saml fSilverOxide " LP Lett.521990
206613;?) DopedYBazCIhOm App hys ( )

D. LeeandK. Salm'EnhamementsinCrmentDensity andMechanical

Propertieson-Ba-Cu-O/AgComposites,” Jpn J. AppL Phys. 29 (1990)
L2017-L2019.

G.Kozlowski,1. Maartense,R spyker,R leeseandC.E. Oberly, ”CriticalCurrent
Density Enhancement in YBa,Cu,O.,¢-silver Composit Superconductor," Physica
C 173 (1991) 195-200.

53.

54.

55.

56.

57.

58.

59.

60.

61.

62.

63.

65.

159

Sungho Jin, "Processing Techniques for Bulk High-TC Superconductors," JOM
March (1991) 7-12.

C. J. Gorter, "Some Remarks on Superconductivity of the Second Kind," Rev. Mod.
Phys. 36 (l) (1964) 27-31.

Charles Kittle, Introduction to Solid State Physics, 6th edition, (SINGAPORE: John
Wiley & Sons, Inc., 1986) 133, 319-354, 631-636.

Gerald Burns, High-Temperature Superconductivity, An Introduction, (San Diego,
CA: Academic Press, Inc., 1992) 8-53.

J. Bardee, L. N. Cooper, and J. R Schrieffer, "Microscopic Theory of
Superconductivity," Phys. Rev., 106 (1957) 162-163; "Theory of
Superconductivity," 108 (1957) 1175-1204.

J. R Schrieffer, "The Pairing Theory - Its Physical Basis and Its Consequences",
Physics of Hi gh-Tempcrature Superconductors, edited by S. Maekawa and M. Sato,
(Berlin Heidelberg, Germany: Spring-Verlag, 1992) 3-17.

W. G. V. Rosser, An Introduction to Statistical Physics, (Chichester, England: Ellis
Horwood Limited, 1982) 274-278.

J. G. Bednorz and K. A Muller, "Possible High TC Superconductivity in the
Ba-La-Cu-O System," Z Phys. B64 (1986) 189-193.

A W. Sleight, J. L. Gillson andP. E. Bierstedt, "High-Temperature
Superconductivity 1n the Ban1 xBixO System," Sol. State Comm. 17 (1975)
27-28.

L. F. Mattheiss, E. M Gyorgy and D. W. Johson, Jr., "Superconductivity above 20
K in the Ba-K-Bi-O System," Phys. Rev. B37 (1988) 3745-3746.

R J. Cava, B. Batlogg, J. J. Krajewski, R Farrow, L. W. Rupp Jr. , A E. White, K.
Short, W. F. Peck and T. Kometani, "Superconductivity near 30 K without Copper:
the BauKMBiO, Perovskite," Nature 332 (1988) 814-816.

R Beyers, G. Lim, E. M Engler, R J. Savoy, T. M Shaw, T. R Dinger, W. J.
GallagherO and R L. Sandstrom, "Crystallography and Microstmcmre of

a perovskite-based Superconducting Oxide," Appl. Phys. Lett. 50
(2%)?1987)’1’31s-1920.

Xiaoping Jiang, Huafeng Yu, Ze Zhang, Naiping Zhu, Hongbo Qi, Guoyue Shu,
Yunchuan Tian, Dexing Pang, Xiaobiao Zeng and Zhongjin Yang, "Effect of
Crystal Structure on Superconductivity of Y-Ba-Cu-O System Compounds," Appl.
Phys. Lett. 51 (8) (1987) 625-627.

Xiaobiao Zeng, Xiaoping Jiang, Hongbo Qi, Dexing Pang, Naiping Zhu and Ze
Zhang, "Relationship betweenS Superconductivity and Lattice Distortion in
Y-Ba-Cu-O Compounds" Appl. Phys. Lett. 51 (9) (1987) 692-693.

67.

68.

69.

70.

71.

72.

73.

74.

75.

76.

77.

78.

79.

160

Shigemitsu Nakanishi, Mineo Kogachi, Hiroyuki Sasakura, Nobuo Fukuoka,
Shinnosuke Minamigawa, Kiyotaka Nakahigashi and Akira Yanase, "The
Orthorhombic to Tetragonal Transition Under Controlled Oxygen Content in
High-TC Superconductor YBazCuaOy," Jpn J. Appl. Phys. 27 (3) (1988) 1329-332.

W. Lo, Tong B. Tang and Chaorui Li, "Thermodynamic Studies of the
Orthorhombic-tetragonal Transition in Ba,YCu,O,_,," Appl. Phys. Lett. 53 (26)
(1988) 2710-2712.

J. C. Phillips, "Physics of High-Tc Superconductors", (San Diego, CA: Academic
Press, Inc, 1989) 85-86.

S. Lapinskas, A. Resengren and E. E. Tornau, "Oxygen Ordering and
Superconducting Temperature of YBa,Cu,O‘,x", Physica C 199 (1992) 91-94.

D. W. Face et al., "Preparation of Superconducting Thin Films of Bismuth Calcium
Copper Oxides by Reactive Sputtering," Appl. Phys. Lett. 53 (3) (1988) 246-248.

R L. Sandstrom, W. J. Gallagher, T. R Dinger, R. H Koch, R B. Laibowitz, A W.
Kleinsasser, R J. Gambino, B. Bumble, and M. F. Chisholm, "Reliable
Single-target Sputtering Process for High-temperature Superconducting Films and
Devices," Appl. Phys. Lett. 53 (5) (1988) 444-446.

A Hohlder, D. Guggi, H Neeb, and C. Heiden, "Fully Textured Growth of
Y B 03(6) Films by Sputtering on LiNbO, Substrates," Appl. Phys. Lett. 54 (11)
(198 1 63,1067.

IanD. Raistrick, M Hawley, J. G. Beery, F. H Garzon, and R J. Houlton,
"Microstructure and Growth Mechanism of Thin Sputtered Films of YBa,Cu,O., on
MgO Substrates," Appl. Phys. Lett. 59 (24) (1991) 3177-3179.

B. Oh et al., "Critical Current Densities and Transport in Superconducting
YBa,Cu,O7 Films Made by Electron Beam Coevaporation," Appl. Phys. Lett. 51
(11) (1987)1152-854.

X K. Wang, K. C. Sheng, S. J. Lee, Y. H Shen, S. N. Song, D. X. Li, R P. H.
Chang, and J. B. Ketterson, "Oriented Thin Films of YBaCu(F)O with high TC and
J Pre by Electron Beam Multilayer Evaporation," Appl. Phys. Lett. 54 (16)
(T989) 1573-1575.

N. G. Chew, S. W. Goodyear, J. A Edwards, J. S. Satchell, S. E. Blenkinsop, and
R G. Humphreys, "Effect of Small Changes in Composition on the Electrical and
Structural Properties of YBa,Cu,O., Thin Films," Appl. Phys. Lett. 57 (19) (1990)
2016-2018.

P. Berberich, J. Tate, W. Dietsche and H Kinder, "Low-temperature Preparation of
Superconducting YBa,Cu,O.,4 Films on Si, MgO, and SrTiO3 by Thermal
Coevaporation," Appl. Phys. Lett. 53 (10) (1988) 925-926.

M. Bee and R W. Vook, "Y-Ba-Cu-O Thin Films Prepared by Flash Evaporation,"
Appl. Phys. Lett. 54 (26) (1989) 2722-2724.

80.

81.

82.

83.

84.

85.

86.

87.

88.

89.

90.

91.

92.

93.

94.

161

Yukio Yasuda, Yasuo Koide, Shigeaki Zaima, and Naoki Sano, "Y-Ba-Cu-O
Superconducting Thin Films Prepared by Plasma-assisted Flash Evaporation," Appl.
Phys. Lett. 55 (3) (1989) 307-309.

A D. Berry, D. K. Gaskill, R. T. Holm, E. J. Cukauskas, R. Kaplan, and R. L.
Henry, "Formation of High TC Superconducting Films by Organometallic Chemical
Vapor Deposition," Appl. Phys. Lett. 52 (20) (1988) 1743-1745.

Rointan F. Bunshalr, ed., Deposition Technologies for Films and Coatings:
Developments and Applications (Park Ridge, NJ: Noyes Publications, 1982), 3-9.

0. Auciello, A. R. Krauss, J. Santiago-Aviles, A F. Schreiner, and D. M. Gruen,
"Surface Compositional and Topographical Changes resulting from Excimer Laser
Impacting on YBa2C113O7 Single Phase Superconductors," Appl. Phys. Lett. 52 (3)
(1988) 239-241.

Osamu Eryu, k Murakami, K. Masuda, K. Shihoyama, and T. Mochizuki, "Strong
Wavelength Dependence of Laser Ablation Fragments of Superconductor
YBa2Cu,Oy," Jpn J. Appl. Phys. 31 (2A) (1992) L86-L88.

M. Gurvitch and A T. Fiory, "Preparation and Substrate Reaction of
Superconducting Y-Ba-Cu-O Films," Appl. Phys. Lett. 51 (13) ( 1987) 1027-1029.

M Aslam, R E. Soltis, E. M. Logothetis, R. Ager, M. Mikkor, W. Win, J. T. Chen
and L. E. Wenger, "Rapid Thermal Annealing Of YBaCuO Films on Si and 8102
Substrates," Appl. Phys. Lett, 53 (2) (1988) 153-155.

M J. Cima, J. S. Schneider, and S. C. Peterson, "Reaction of Ba,YCu,O Films
with Yttria-stabilized Zirconia Substrates," Appl. Phys. Lett. 53(8) (1988) 710-712.

S. I. Shah, "Annealing Studies of YBa,Cu,O.,,, Thin Films," Appl. Phys. Lett. 53 (7)
(1988) 612-614.

A Zherikhin, V. Bagratashvili, V. Burimov, E. Sobol, G. Shubnii and A. Sviridov,
"The Action of Powerful Laser Radiation on 1-2-3 Superconducting Thin Films and
Bulk Materials," Physica C 198 (1992) 341-348.

Shigeru Otsubo, T. Minamikawa, Y. Yonezawa, A. Morimoto, and T. Shimizu,
"Thermal Analysis of Target Surface in the Ba-Y-Cu-O Film Preparation by Laser

Ablation Method," Jpn J. Appl. Phys. 29 (1) (1990) L73-L76.

Rajiv K. Singh, D. Bhattacharya, and J. Narayan, "Control of Surface Particle
Density in Pulsed Laser Deposition of Superconducting YBa,Cu,O7 and
Diamondlike Carbon Thin Films," Appl. Phys. Lett. 61 (4) (1992) 483-485.

W. Eidelloth, R L. Sandstrom and M M. Plechaty, "Polishing of Highly Oriented
YBa,Cu,O.H Thin Films," Physica C 197 (1992) 389-393.

M Young, Optics and Lasers, 3rd edition, (Berlin, Germany: Springer-Verlag,
1986) 145-171.

Allan Lytel and Lawrence Buckmaster, abc's of Lasers and Masers, (New York,
NY: Howard W. Sams & Co., Inc.; The Bobbs-Merrill Co., Inc., 1972) 7-20.

95.

96.

97,.

98.

100.

101.

102.

103.

104.

105.

106.

162

Jeff Hecht, "Neodymium lasers Prove Versatile Over Three Decades," Laser Focus
World, 28(4) (1992) 77-94.

Daniel W. Trainor, "Military Excimer-Laser Technology Seeks Real-World Uses,"
Laser Focus World, 29(6) (1993) 143-149.

Jeff Hecht, "Excimer Laser Produce Powerful Ultraviolet Pulses," Laser Focus
World, 28 (6) (1992) 63-72.

Marc Stehle, "Will Kilowatt Excimer Lasers Become Future Industrial Tools,"
Laser Focus World, 29 (6) (1993) 135-139.

M. F. Yan, R L. Barns, H. M. O'Bryan, Jr., P. K. Gallagher, R. C. Sherwood, and S.
J in, "Water Interaction with the Superconducting YBa,Cu,O, Phase," Appl. Phys.
Lett. 51 (7) (1987) 532-534.

R L. Barns and R A Laudise, "Stability of Superconducting YBa,Cu,O7 in the
Presence of Water," Appl. Phys. Lett. 51 (17) (1987) 1373-1375.

Takuya Hashimoto, Kazuo Fueki, Akira Kishi, Tadahiko Azumi and Hideomi
Koinuma, "Thermal Expansion Coefficients of High-TC Superconductors," Jpn J.
Appl. Phys. 27 (2) (1988) L214-L216.

T. Uzumaki, K. Yamanaka, N. Kamehara, and K. Niwa, "The Effect of 4
Addition on Superconductivity in a Bi-Sr- Cu-O System," Jpn J. Appl. Phys. 28 (1)
(1989) L75-L77.

H Endo, J. Tsuchiya, N. Kijima, A. Sumiyama, M Mizuno, and Y. Oguri,
"Thermal Stability of the High-Tc Superconductor in the Bi-Sr-Ca-Cu-O System,"
Jpn J. Appl. Phys. 27 (10) (1988)L1906-L1909.

N. Kijima, H Endo, J. Tsuchiya, A Sumiyama, M Mizuno, and Y. Oguri,
"Reaction Mechanism of Forming the High-Tc Superconductor in the
Pb-Bi-Sr-Ca-Cu-O System," Jpn J. Appl. Phys. 27 (10) (1988) L1852-L1855.

A. Sumiyama, H. Endo, J. Tsuchiya, N. Kijima, M Mizuno, and Y. Oguri, "A C.
Susceptibility of the Superconducting Bi-(le)-Sr-Ca-Cu-O System," Jpn. J. Appl.
Phys. 28 (3) (1989) L373-L376.

Powder Diffraction File (Philadelphia, PA: American Society for Testing and
Materials, 1988).

 

107. William Band, Introduction to Mathematical Physics, (Princeton, NJ: D. Van

Nostrand Co., Inc.) 294-295.

HICHIGAN $1an UNIV. LIBRARIES
1|lWIWMlWI'llWll“IHIWINIHIWHIH1|
31293010510182