Studies of heteroepitaxial growth of diamond

摘要

Large-scale heteroepitaxial growth of diamond depends critically on the development of a suitable lattice-matched substrate system. Oxide substrates, notably MgO and SrTiO{sub}3, on which thin epitaxial films of iridium serve as a nucleation layer for diamond have already shown considerable promise. We describe here improvements in the growth of single crystal diamond by low-pressure microwave plasma-enhanced CVD. Oxide substrates with flat, low-index surfaces form the initial basis for the process. Iridium was deposited on heated substrates in a UHV electron-beam evaporation system resulting in epitaxial films, typically 150-300 nm thick, with Ir (100) parallel to the surface of all substrates as confirmed by X-ray and electron backscattering diffraction. Following Ir deposition, the samples were transferred to a CVD reactor where a bias-enhanced nucleation step induced a dense condensate that completely covered the Ir surface. Uniform nucleation densities of order 10{sup}12 cm{sup}(-2) were observed. Interrupted growth studies, carried out at intervals from seconds to minutes subsequent to terminating the nucleation step, revealed a rapid coalescence of grains. One hour of growth resulted in a smooth, nearly featureless, (001) diamond film. For extended growth runs, slabs of diamond were grown with thickness as great as 38 μm and lateral dimensions near 4 mm. The crystals were transparent in visible light and cleaved on (111) planes along <110> directions, similar to natural diamond. Of particular significance is the successful use of sapphire as an underlying substrate. Its high crystalline perfection results in epitaxial Ir films with X-ray linewidths comparable to those grown on SrTiO{sub}3. However, Al{sub}2O{sub}3 possesses superior interfacial stability at high temperatures in vacuum or in a hydrogen plasma with a better thermal expansivity match to diamond. Since sapphire is available as relatively inexpensive large diameter substrates, these results suggest that wafer-scale growth of heteroepitaxial diamond should be feasible in the near future.