Diamond has many superior electric and electronic properties over other materials, yet its application in electronic devices is severely limited due to difficulties in producing n-type conductivity in diamond as available dopants all have activation energy much larger than the thermal energy, kT, at room temperature. The substitutional doping of nitrogen in grain boundaries of granular graphitic-rich ultra-nanocrystalline diamond (UNCD) films provides for high conductivities even at room temperature with an apparent activation energy to be much lower than kT. Such low activation energy is a consequence of the formation of new electronic states associated with carbon and nitrogen near and above the Fermi level. However, the relative contribution of sp2 graphitic phase and incorporated nitrogen to the conduction of the films remains unclear.In the present work, structural and electrical properties of nitrogen-incorporated (N)UNCD films are studied as a function of deposition temperature and nitrogen concentration in the precursor synthesis gas mixture consisting of H2, CH4 and N2. Four sets of (N)UNCD films with 0%, 5%, 10% and 20% nitrogen concentration in the synthesis gas were produced by microwave assisted chemical vapor deposition on intrinsic (100)-oriented Si substrates.Resistance tunability over a range of about 4 orders of magnitude was achieved by varying the growth parameters. Raman spectroscopy and scanning electron microscopy (SEM) results show that the UNCD films undergo a progressive and highly reproducible material phase transformation, from ultra-nano-diamond to nanocrystalline graphite as deposition temperature increases. Addition of nitrogen increases the amount of sp2 bonded carbon in the films thus enhancing the physical connectivity in the grain boundary network that have high electronic state density and leads to improved conductivity. However, crystallinity of sp2 carbon phase remains another (previously underestimated) major factor in determining the conductivity of (N)UNCD films.
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