Synthesis of GeSn and GeSnSi by sputtering epitaxy

GeSn alloy has recently received significant attention due to the fact that their direct-bandgap behavior can be predicted when the Sn content is larger than about 0.1. Moreover, the bandgap of these alloys can be adjusted in the infrared range. This has the potential to extend Si-based materials to infrared application. However, the epitaxial growth of GeSn on Si substrates has several challenges. Firstly, the equilibrium solid solubility of Sn in Ge is just 0.5%, thus hampering the formation of high-Sn alloys. Secondly, there is a large lattice mismatch between SnGe and Si. Thirdly, Sn has comparatively lower surface energy than that of Ge, which causes Sn tend to segregate to surface during growth. Despite these difficulties, significant efforts have been made in growing GeSn alloys by molecular beam epitaxy (MBE) and ultrahigh vacuum chemical vapor deposition (UHV/CVD). In addition to progress in GeSn growth by MBE and CVD, there have further investigations into the use of alternative methods such as sputtering, solid phase epitaxy and metal-induced crystallization. Among these, sputtering appears to offer greater potential for the cost-effective mass manufacturing of GeSn and GeSnSi alloys.