An insight into grain refinement mechanism of ultrananocrystalline diamond films obtained by direct current plasma-assisted chemical vapor deposition

Abstract

The grain-refinement mechanisms involved during the deposition of diamond films by direct-current plasma assisted chemical vapor deposition (DC-PACVD) were investigated as a function of the inter-electrode electric field (IEEF). As IEEF was increased from 260 to 940Vcm-1, the local electron temperatures near the growth front increased strongly; as a result, a strong grain refinement occurred ultimately yielding ultrananocrystalline diamond films (UNCD). Such observations were attributed to novel features of the DC-PACVD, including the electron-stimulated desorption (ESD) of the hydrogen-terminated moieties located at the surface, and the consequently enhanced generation of bi-radical sites at the growing diamond surface. The article reconciles the puzzling uniqueness of UNCD synthesis by direct current plasma-assisted CVD. UNCD films are obtained under relatively high pressure and positive substrate bias conditions using hydrogen-rich gas chemistry; similar conditions are not conducive to UNCD films when using microwave plasmas or hot filament CVDs. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.