On the epitaxy of germanium telluride thin films on silicon substrates

The growth by molecular beam epitaxy of GeTe thin films on highly lattice-mismatched Si(111) substrates is reported. In situ reflection high-energy electron diffraction and quadrupole mass spectrometry were employed to monitor the growth process in real time and tune the deposition conditions. Epitaxy was achieved in a window of substrate temperatures between 220 and 270 degrees C, using a Ge/Te flux ratio of similar to 0.4. Extensive ex situ X-ray diffraction characterization showed that the epitaxial layers crystallize in the rhombohedrally distorted rocksalt structure a-GeTe, with orientation relationships to the substrate a-GeTe[0001]???Si[111] and a-GeTe<10${\bar {1}}$0>???Si<$11{\bar {2}}$>. ?-scans of the a-GeTe(000n) reflections (n?=?3, 6, 9) exhibit a full width at half maximum between 10 and 20?arcsec, indicating small mosaicity. However, a large twist is observed (similar to 14 degrees), pointing to the presence of rotational domains. In addition, the layer matrix is affected by twinning. The epitaxial films exhibit a slight tensile in-plane strain, which might be due to the difference in thermal expansion coefficients between a-GeTe and Si, and/or small deviation from stoichiometric composition, i.e., vacancies in the Ge sub-lattice.