Low temperature onset for thermionic emitters based on nitrogen incorporated UNCD films

Thermionic electron emission is a key phenomenon utilized in a variety of applications, from communications, space propulsion to direct energy conversion. Doped diamond films with a negative electron affinity (NEA) surface characteristic present a reduced effective work function for electron emission. Our prior research has indicated a work function of ∼ 1.5 eV for nitrogen doped diamond films prepared by microwave plasma CVD. The focus of this study is on the role of the interfacial or nucleation layer. A thermionic emitter was fabricated with a nitrogen incorporated ultra-nanocrystalline diamond (UNCD) nucleation layer and a nitrogen doped diamond surface layer with hydrogen termination. The film exhibited a resistance that decreased with temperature suggesting the role of the dopant, and thermionic electron emission was observed at temperatures < 250 °C. An analysis based on the Richardson-Dushman equation indicated an emission barrier of < 1.3 eV with a Richardson constant of ∼ 0.8 A/cm² K².