First-Principles Calculation of Spin-Relaxation Due to Alloy and Electron-Phonon Scattering in Strained GeSn

GeSn has emerged as a promising material for spintronics due to its long spin-lifetime, compatibility with silicon technology, high mobility and tunable electronic properties. Of particular interest is the transition from an indirect to a direct band gap with increasing Sn content, which enhances optical properties, electron transport and we find also affects spin transport behaviour, which is critical for spintronics applications. We use first-principles electronic-structure theory to determine the spin-flip electron-alloy scattering parameters in n-type GeSn alloys. We also calculate the previously undetermined intervalley electron spin-phonon scattering parameters between the L and Γ valleys. These parameters are used to determine the electron-alloy and electron-phonon scattering contributions to the n-type spin-relaxation of GeSn, as a function of alloy content and temperature. As in the case of phonon scattering, alloy scattering reduces the spin-relaxation time. However, switching the spin transport from the typical L valley of Ge to the Γ valley by sufficient addition of Sn, the relaxation time can be substantially increased. For unstrained, room temperature GeSn, we find a Sn concentration of at least $10%$ is required to achieve a spin-relaxation time greater than Ge, with $17%$ Sn needed to increase the spin-relaxation time from the nanosecond range to the microsecond range. At low temperatures (30K), adding $10%$ Sn can increase the spin-relaxation time from 10⁻⁷s to 0.1s. Applying biaxial tensile strain to GeSn further increases the spin-relaxation time and at a lower Sn content than in unstrained GeSn.