Microstructure evolution and non-diamond carbon incorporation in CVD diamond thin films grown at low substrate temperatures

Abstract

We investigated the development of the microstructure and the incorporation of non-diamond carbon close to the low temperature border of the CVD diamond domain. Thin diamond films were deposited at low substrate temperatures (560°C–275°C) by microwave plasma-assisted CVD on silicon, varying only the substrate temperature. At elevated temperatures (560°C–430°C) the film mainly consists of nearly defect free near 112 oriented grains with smooth 111 facets, exhibiting steps and risers at the surface. Decreasing the substrate temperature an apparently sharp transition occurs, below which the film quality undergoes a rapid deterioration as evidenced by Raman spectroscopy, while crystalline faceted grains with a size of several microns and a growth texture of 〈100〉 remain. However, X-ray diffraction reveals a strongly decreasing crystal size (from about 1 μm to 10 nm) which can be attributed to an increased twin density within the macroscopic grains. High resolution transmission electron microscopy reveals that these twins consist of small twin lamellae with a spacing of only several atomic planes. Transmission electron microscopy of near surface areas evidences re-entrant corners at the grain surfaces formed by twin lamellae and the presence of steps and risers. Non-diamond carbon was detected in the form of amorphous inclusions at incoherent twin boundaries and probably at higher order twin boundaries. The observations will be discussed by means of two different competing nucleation mechanisms: above the low temperature limit the grains grow by lateral ledge motion and preferential nucleation at re-entrant corners. Approaching the low temperature limit, two-dimensional nucleation at growth 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.