Revisiting Effects of Nitrogen Incorporation and Graphitization on Conductivity of Ultra-nano-crystalline Diamond Films

Detailed structural and electrical properties of ultra-nano-crystalline diamond (UNCD) films grown in H$_\text{2}$/CH$_\text{4}$/N$_\text{2}$ plasma were systematically studied as a function of deposition temperature ($T_d$) and nitrogen content ($\%$ N$_2$) to thoroughly evaluate their effects on conductivity. $T_d$ was scanned from 1000 to 1300 K for N$_2$ fixed at 0, 5, 10 and 20 $\%$. It was found that even the films grown in the synthetic gas mixture with no nitrogen could be made as conductive as 1$-$10$^{-2}$ $\Omega$ cm with overall resistivity of all the films tuned over 4 orders of magnitude through varying growth parameters. On a set of 27 samples, Raman spectroscopy and scanning electron microscopy show a progressive and highly reproducible film material phase transformation, from ultra-nano-crystalline diamond to nano-crystalline graphite as deposition temperature increases. The rate of this transformation is heavily dependent on N$_2$ content. Addition of nitrogen greatly increases the amount of $sp^2$ bonded carbon in the films thus enhancing the physical connectivity in the GB network that have high electronic density of states. However, addition of nitrogen greatly slows down crystallization of $sp^2$ phase in the GBs. Therefore, proper balance between GB connectivity and crystallinity is the key in conductivity engineering of (N)UNCD.