Effect of the s p2 carbon phase on n -type conduction in nanodiamond films

Structural and electrical conduction properties of nitrogen-doped nanocrystalline diamond films are studied as a function of deposition temperature (TD) in a microwave Ar-rich/ CH4 plasma with 30% N2 addition. Hall- and Seebeck-effect measurements confirm n -type conduction for TD above 1100 K. For TD from 1100 and 1220 K, the electron concentration increases up to 1020 cm-3 and the electron mobility is in the range of 4-8 cm2 V-1 s-1. For TD above 1250 K, the mobility decreases to ∼1 cm2 V-1 s-1. Low conductivity films deposited at low TD exhibit semiconductorlike thermal activation in the Arrhenius plots, while high conductivity films deposited at high TD are almost temperature independent, indicative of quasimetallic conduction. The nitrogen concentration in the films is about 0.3 at. %, independent of TD. As TD is increased, the s p2 content and order increase. This is responsible for the appearance of midgap states, their delocalization, and the larger distance between diamond grains. The high conductivity at high TD is due to the amount and crystallinity of s p2 carbon, rather than the nitrogen concentration.