Improved Field Electron Emission Properties of Phosphorus and Nitrogen Co-Doped Nanocrystalline Diamond Films

Nanocrystalline diamond (NCD) field emitters have attracted significant interest for vacuum microelectronics applications. This work presents an approach to enhance the field electron emission (FEE) properties of NCD films by co-doping phosphorus (P) and nitrogen (N) using microwave plasma-enhanced chemical vapor deposition. While the methane (CH₄) and P concentrations are kept constant, the N₂ concentration is varied from 0.2% to 2% and supplemented by H₂. The composition of the gas mixture is tracked in situ by optical emission spectroscopy. Scanning electron microscopy, atomic force microscopy (AFM), transmission electron microscopy, and Raman spectroscopy are used to provide evidence of the changes in crystal morphology, surface roughness, microstructure, and crystalline quality of the different NCD samples. The FEE results display that the 2% N₂ concentration sample had the best FEE properties, viz. the lowest turn-on field value of 14.3 V/µm and the highest current value of 2.7 µA at an applied field of 73.0 V/µm. Conductive AFM studies reveal that the 2% N₂ concentration NCD sample showed more emission sites, both from the diamond grains and the grain boundaries surrounding them. While phosphorus doping increased the electrical conductivity of the diamond grains, the incorporation of N₂ during growth facilitated the formation of nano-graphitic grain boundary phases that provide conducting pathways for the electrons, thereby improving the FEE properties for the 2% N₂ concentrated NCD films.