Plasma Post‐Treatment Process‐Induced Grain Coalescence to Improve the Electron Field‐Emission Properties of Ultrananocrystalline Diamond Films

Abstract

Diamond films possess distinctive and superior properties such as high hardness, chemical inertness, high electron fieldemission (EFE) capacity, and semiconductivities (via doping), which have gained interest among researchers in recent years. [1][2][3] However, the characteristics of diamond films change markedly with their microstructure. The ultrananocrystalline diamond (UNCD) films consist of ultrasmall diamond grains with the size about 5 nm and sp 2 -bonded carbon in grain boundaries. Therefore, the UNCD films show better conductivity and superior EFE properties compared with the microcrystalline diamond films which contain large and faceted diamond grains. [4][5][6][7] However, the sp 2 -bonded carbon contained in the grain boundaries of the UNCD films is still inadequately conductive, which limits the EFE properties of these films. Fortunately, UNCD films possess a versatile granular structure, which can be altered easily to form nanographitic phase that leads to better conductivity and hence superior EFE properties. [8,9] It was reported that through the addition of N 2 in CH 4 /Ar plasma, a noticeable increase in the width of the grain boundaries resulted, which enhanced the n-type conductivity of UNCD films to a high level as σ ¼ 143 S cm À1 . [10][11][12] Arenal et al., [13] grew the films in a plasma containing a large proportion of N 2 species, that is, CH 4 /[80%Ar-20%N 2 ], at high substrate temperature (>700 °C), which resulted in the formation of needle-like diamond grains, exhibiting a high conductivity of σ ¼ 200 S cm À1 for UNCD films. Sankaran et al., [14] investigated the effects of substrate temperature on the granular structure of UNCD films grown in CH 4 /(100%N 2 ) plasma and observed that needle-like diamond grains were obtained only at a substrate with high temperature (700 °C). Moreover, the high conductivity in diamond films grown in CH 4 /(100%N 2 ) plasma (at 700 °C) was concluded due to the existence of nanographitic phase encapsulating the needle-like diamond grains. Furthermore, they observed that the C 2 and CN species (one carbon atom connected to one nitrogen atom) in the plasma were the main constituents, which resulted in the