Experimental evidence of different crystalline forms in Chemical Vapor Deposited films

Diamond films have been obtained on Ta polycrystalline substrates from mixtures of methane and hydrogen by the hot-filament chemical vapor deposition technique. The structural characteristics of the polycrystalline deposits have been investigated by reflection high energy electron diffraction (RHEED), while the surface morphologies have been observed by scanning electron microscopy or carbon replica transmission electron microscopy. For one of the films, the formation of thermal spikes during the deposition process yielded a structure giving a RHEED pattern with d spacings and intensities not corresponding to the already identified carbon and diamond phases. On the base of the RHEED pattern the observed phase has been identified as a face-centered-cubic lattice, belonging to the space group F (4) over bar 3m and ascribed to a so-called X-diamond polytype.