Microstructural evolution of diamond films from CH_4/H_2/N_2 plasma and their enhanced electrical properties

摘要

The influence of N_2 concentration in CH_4/H_2/N_2 plasma on microstructural evolution and electrical properties of diamond films is systematically investigated. While the diamond films grown in CH_4/ H_2 plasma contain large diamond grains, for the diamond films grown using CH_4/H_2/(4%)N_2 plasma, the microstructure drastically changed, resulting in ultra-nanosized diamond grains with Fd3m structure and a_0 = 0.356 nm, along with the formation of n-diamond (n-D), a metastable form of diamond with space group Fm3m and a_0 = 0.356 nm, and i-carbon (i-C) clusters, the bcc structured carbon with a_0 = 0.432 nm. In addition, these films contain wide grain boundaries containing amorphous carbon (a-C). The electron field emission (EFE) studies show the best EFE behavior for 4% N_2 films among the CH_4/H_2/N_2 grown diamond films. They possess the lowest turn-on field value of 14.3 V/μm and the highest EFE current density value of 0.37 mA/cm~2 at an applied field of 25.4 V/μm. The optical emission spectroscopy studies confirm that CN species are the major criterion to judge the changes in the microstructure of the films. It seems that the grain boundaries can provide electron conduction networks to transport efficiently the electrons to emission sites for field emission, as long as they have sufficient thickness. Whether the matrix nano-sized grains are 3C-diamond, n-D or i-C is immaterial.