Diamond-germanium composite films grown by microwave plasma CVD

We report on novel microcrystalline diamond-germanium composite films grown by microwave plasma-assisted chemical vapor deposition in CH4–H2-GeH4 mixtures on Si substrate. The structure of the films, characterized with scanning electron microscopy, X-ray diffraction, Raman and photoluminescence spectroscopy, is found to strongly depend on the deposition temperature: the Ge grains compete with diamond crystallites upon growth at 750–800 °C, but vanish at higher temperatures of 850–950 °C due to enhanced growth rate of diamond matrix. The Ge grains nucleated on the (100) oriented Si wafer at 800 °C are in an epitaxial relationship with the substrate, while Si–Ge alloy forms at intermediate temperatures 850–900 °C. As both components are transparent in the infrared spectrum, the composite shows certain optical transmission for wavelengths λ > 10 μm. The films revealed a bright optical emission of GeV color centers at 601 nm wavelength due to diamond doping with Ge. From the calculation of the effective coefficient of thermal expansion for the diamond-Ge composites, we show that it can be tuned to be close to that for SiC or Si single crystals by appropriate choice of the Ge volume fraction in the composite, facilitating the integration of the semiconductor wafers with the diamond-based film.