Substrate-diamond interface considerations for enhanced thermionic electron emission from nitrogen doped diamond films

Thermionic electron emission from low work function doped diamond films can be related to materials' properties, which include donor states, surface electron affinity, and substrate-diamond interface properties. The focus of this study is on how the properties of the substrate material affect the emission. Two aspects are considered, the substrate electrical resistance and the substrate Richardson constant, and the effects of tungsten, molybdenum and rhenium substrates are explored. Low work function diamond films were deposited on the substrates, and the thermionic emission was measured to ∼530°C and described in terms of a fit to the Richardson-Dushman formalism. The results establish that all surfaces exhibit a similar work function but the Richardson constant and maximum emission current vary considerably. The rhenium based emitter displayed a low work function of 1.34eV, a significant Richardson constant of 53.1A/cm² K², and an emission current density of ∼44 mA/cm² at a temperature of 530°C. The results indicated that interface carbide formation could limit the emission presumably because of increased electrical resistance. For non-carbide forming substrates, an increased substrate Richardson constant corresponded to enhanced emission from the diamond based emitter.