Abstract
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.
Highlights
Diamond is a versatile material whose properties make it interesting for a wide range of applications, from abrasives to power electronics [1]
scanning electron microscopy (SEM) observations were carried out in order to analyze the changes in the grain morphology of the nanocrystalline diamond (NCD) films
The NCD sample grown at 2% N2, shown in Figure 1e, clearly exhibited small grains with many different facets in a clear cauliflower morphology that predicted larger grain boundary regions
Summary
Diamond is a versatile material whose properties make it interesting for a wide range of applications, from abrasives to power electronics [1] Among these applications, polycrystalline diamond-based field emitters have attracted significant interest in vacuum microelectronics due to their negative electron affinity for electron emission, strong bonding structure, extreme hardness to withstand ion bombardment, and good thermal and electrical conductivities to handle high currents [2,3]. Nitrogen addition in CH4/H2 plasma effectively reduces the size of the diamond grains from micron to nanosized, yielding nanocrystalline diamond (NCD) films It facilitates the formation of nanographitic phases in the grain boundaries, for which an improvement in the electrical conductivity and subsequent FEE properties have been demonstrated [5,6,7,8]. It is known that P forms a donor in both single as well as polycrystalline diamond, with the P atom mostly incorporating in substitutional positions in the grains [11,12]
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