Heteroepitaxy of High-Mobility Germanium on Sapphire (0001) with Magnetron Sputtering

As an excellent semiconductor material with low band gap and high carrier mobility, germanium is widely used in the semiconductor industry in photoelectronic devices, high-frequency radio frequency (rf) circuits and devices, etc. Compared to bulk germanium wafer and silicon-based germanium on insulator (GOI) materials, the germanium on sapphire (GOS) substrate offers a highly cost-effective solution with several important advantages, including low thermal expansion coefficient mismatch, high insulator resistivity, lower rf losses, and superior crosstalk suppression. In this work, we present a method to grow a high-quality thin germanium epitaxial film on a sapphire (0001) substrate using direct-current (DC) sputtering at an elevated temperature of 600 °C. We demonstrate that thermal annealing at 850 °C reduced the germanium (111) twinning, increased the size of crystalline domains in the germanium film and thus further improved the crystalline quality. For a film with a thickness of 500 nm, the hole Hall mobility increased to 440 cm2/V·s after thermal annealing, which is about 10 times more than that of the as-sputtered film. For a 2.0 μm film, the hole Hall mobility increased to 824 cm2/V·s, which has not been reported in the literature for a GOS film. This method offers a simple but efficient way to fabricate a GOS substrate for optical electronic device applications.