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

TL;DR

This study systematically investigates how nitrogen incorporation and graphitization influence the structural and electrical properties of ultra-nano-crystalline diamond films, revealing how growth parameters affect conductivity over a wide range.

Contribution

It provides a comprehensive analysis of the effects of nitrogen content and temperature on UNCD film phase transformation and conductivity, offering insights for optimizing electronic properties.

Findings

Conductivity of UNCD films can be tuned over 4 orders of magnitude.

Nitrogen increases sp^2 bonding and enhances grain boundary connectivity.

Higher deposition temperature promotes diamond to graphite transformation.

Abstract

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.