Electron field emission for ultrananocrystalline diamond films

Ultrananocrystalline diamond (UNCD) films 0.1-2.4 mum thick were conformall y deposited on sharp single Si microtip emitters, using microwave CH4-Ar pl asma-enhanced chemical vapor deposition in combination with a dielectrophor etic seeding process. Field-emission studies exhibited stable, extremely hi gh (60-100 muA/tip) emission current, with little variation in threshold fi elds as a function of film thickness or Si tip radius. The electron emissio n properties of high aspect ratio Si microtips, coated with diamond using t he hot filament chemical vapor deposition (HFCVD) process were found to be very different from those of the UNCD-coated tips. For the HFCVD process, t here is a strong dependence of the emission threshold on both the diamond c oating thickness and Si tip radius. Quantum photoyield measurements of the UNCD films revealed that these films have an enhanced density of states wit hin the bulk diamond band gap that is correlated with a reduction in the th reshold field for electron emission. In addition, scanning tunneling micros copy studies indicate that the emission sites from UNCD films are related t o minima or inflection points in the surface topography, and not to surface asperities. These data, in conjunction with tight binding pseudopotential calculations, indicate that grain boundaries play a critical role in the el ectron emission properties of UNCD films, such that these boundaries: (a) p rovide a conducting path from the substrate to the diamond-vacuum interface , (b) produce a geometric enhancement in the local electric field via inter nal structures, rather than surface topography, and (c) produce an enhancem ent in the local density of states within the bulk diamond band gap. (C) 20 01 American Institute of Physics.