MICROSTRUCTURE EVOLUTION AND NON-DIAMOND CARBON INCORPORATION IN CVD DIAMOND THIN-FILMS GROWN AT LOW SUBSTRATE TEMPERATURES

We investigated the development of the microstructure and the incorpor ation of non-diamond carbon close to the low temperature border of the CVD diamond domain. Thin diamond films were deposited at low substrat e temperatures (560 degrees C-275 degrees C) by microwave plasma-assis ted CVD on silicon, varying only the substrate temperature. At elevate d temperatures (560 degrees C-430 degrees C) the film mainly consists of nearly defect free near [112] oriented grains with smooth {111} fac ets, exhibiting steps and risers at the surface. Decreasing the substr ate temperature an apparently sharp transition occurs, below which the film quality undergoes a rapid deterioration as evidenced by Raman sp ectroscopy, while crystalline faceted grains with a size of several mi crons and a growth texture of [100] remain. However, X-ray diffraction reveals a strongly decreasing crystal size (from about 1 mu m to 10 n m) which can be attributed to an increased twin density within the mac roscopic grains. High resolution transmission electron microscopy reve als that these twins consist of small twin lamellae with a spacing of only several atomic planes. Transmission electron microscopy of near s urface areas evidences re-entrant corners at the grain surfaces formed by twin lamellae and the presence of steps and risers. Non-diamond ca rbon was detected in the form of amorphous inclusions at incoherent tw in boundaries and probably at higher order twin boundaries. The observ ations will be discussed by means of two different competing nucleatio n mechanisms: above the low temperature limit the grains grow by later al ledge motion and preferential nucleation at re-entrant corners. App roaching the low temperature limit, two-dimensional nucleation at grow th facets becomes an alternate nucleation mechanism, which introduces a high density of microtwins. If two-dimensional nuclei grow together, non-diamond carbon is incorporated during growth at this interface.