Preparation of high-quality Ge substrate for MBE

A high-quality Ge substrate suitable for molecular beam epitaxy (MBE) was prepared. First, native oxide on the Ge substrate was removed selectively using aqueous ammonia solution. X-ray photoelectron spectroscopy (XPS) data showed that the native oxide was completely removed after ammonia etching for 120 s. An atomic force microscope (AFM) image showed that the surface obtained after ammonia etching for 300 s has good uniformity. A highly ordered 2 × 1 reflection high-energy electron diffraction (RHEED) pattern appeared after desorption of the chemically formed protective oxide layer by annealing at 400°C for 30 min in ultrahigh vacuum.