Evaluation of crystallinity of lattice-matched Ge/GeSiSn heterostructure by Raman spectroscopy

•Lattice-matched Ge/GeSiSn layers were examined by Raman spectroscopy•Crystallinity of the Ge/GeSiSn layers improves as the Ge layers become thicker•Crystallinity depends on the ratio of the growth times of Ge and GeSiSn layers Lattice-matched Ge/GeSiSn layers were formed on Ge substrates by sputter epitaxy, and the effects of the thicknesses of the Ge and GeSiSn layers on crystallinity of the Ge/GeSiSn layers were examined by Raman spectroscopy. First, a GeSiSn layer was formed on a Ge substrate, and then a Ge layer was formed on it. Raman spectra were obtained from the formed Ge/GeSiSn layers, and crystallinity was evaluated by using the full width at half maximum (FWHM) values of the Raman peaks. The growth temperature was about 300°C. The light wavelength of the Raman spectroscope used in this study was 532 nm. A single Raman peak was observed at the Raman spectra obtained from the Ge/GeSiSn layers, which was derived from the upper Ge layers due to the penetration length. From the Raman spectra, crystallinity of the Ge/GeSiSn layers depended on the thicknesses of the Ge and GeSiSn layers. Crystallinity decreased as the GeSiSn layer became thicker and increased as the Ge layers became thicker. We found that the normalization of the FWHM values by the ratio of the film thicknesses or growth times of the Ge and GeSiSn layers can be useful for evaluating the growth conditions.