Comprehensive study on epitaxial growth of GeSn(111) layers with high Sn content on Si(111) featuring Ge buffer layer

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Spin MOSFETs are attractive for new functional devices having both switching and memory functionalities. We focused on germanium tin (GeSn), which is a group-IV alloy semiconductor, for channel materials because they can be direct transition semiconductor with a sufficiently high Sn content and strain [1]. Direct transition nature will lead larger spin diffusion length and spin lifetime than those of germanium (Ge) because inter-valley scattering of electron can be suppressed at the conduction band of L points [2]. The objective of this study is to realize (111) orientated direct transition GeSn toward the epitaxial growth of Co-based Heusler alloy thin films, which are promising ferromagnetic electrodes for realizing highly efficient spin injections [3]. Theoretical calculation suggests that GeSn(111) can show the direct transition nature even at a lower Sn content under a biaxial compressive strain [2]. However, the growth and characterization of direct transition GeSn(111) epitaxial layers have not been achieved yet.In this study, we examined the pseudomorphic growth of compressively strained GeSn epitaxial layer on a strain-relaxed Ge buffer prepared on Si(111) substrate. After the chemical and thermal cleaning of Si(111) wafer, a Ge buffer layer was grown by the two-step growth method [4] using molecular beam epitaxy (MBE) system; a 100 nm-thick Ge layer was firstly grown at a low temperature (LT-Ge) of 320 °C and then a 300–900 nm-thick Ge layer was grown at a high temperature of 400 °C (HT-Ge). Subsequently, a 100 nm-thick GeSn layer with a Sn content of 8–9% was grown at 150 °C.The crystalline structure of Ge and GeSn layers was characterized using X-ray diffraction two-dimensional reciprocal lattice space mapping. All samples exhibit the complete strain-relaxation of the Ge buffer layers and the partly strain relaxation of GeSn layers on the Ge layers; the compressive strain to GeSn with a Sn content of 8–9% was estimated to an approximately 0.9–1.0%. Considering the theoretical estimation [3], the prepared compressively strained GeSn layers with a high Sn content are likely to become a direct transition semiconductor.Furthermore, the 𝜔-rocking curve for the GeSn layers revealed that the full-widths of half maximum (FWHM) decreases with increasing the HT-Ge thickness and FWHM was saturated for the samples with a HT-Ge thickness of 500 nm or greater. However, the samples with a HT-Ge thickness of 700 nm or greater showed a slight increase in the intensity at the tail region of the diffraction peak which related to the existence of low-crystalline-quality region. Therefore, the thickness of 500 nm for HT-Ge buffer layer was the optimum condition to achieve with the smallest FWHM and tail intensity in this study. In the presentation, the photoluminescence properties of GeSn layers will be also discussed.