Local Alloy Order in a Ge1−xSnx/Ge Epitaxial Layer

The local ordering of atoms in alloys directly has a strong impact on their electronic and optical properties. This is particularly relevant in nonrandom alloys, especially if they are deposited far from the equilibrium processes, as is the case for epitaxial ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ layers. In this work, we investigate the arrangement of $\mathrm{Ge}$ and $\mathrm{Sn}$ atoms in optoelectronic grade ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ epitaxial layers featuring a $\mathrm{Sn}$ content in the 5--14% range by using polarization-dependent Raman spectroscopy and density-functional-theory calculations. The thorough analysis of the polarization-dependent spectra in parallel and perpendicular configuration allowed us to properly tag all the observed vibrational modes, and to shed light on that associated to disorder-assisted Raman transitions. Indeed, with the help of large-scale atomistic simulations, we were able to highlight how the presence of $\mathrm{Sn}$ atoms, that modify the local environments of $\mathrm{Ge}$ atoms, gives rise to two spectral features at different Raman shifts, corresponding to distortions of the atomic bonds. This analysis provides a valuable framework for advancing the understanding of the vibrational properties in ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ alloys, particularly with regard to the impact of local ordering of the different atomic species.