This project aims to build low-cost high-efficiency Si/III-V tandem cells by using the sputtered heteroepitaxial Ge/Si. The high quality sputtered heteroepitaxial Ge/Si will be used as a virtual Ge substrate or interconnecting layer for integrating silicon wafers with overlying III-V solar cells. For the latter, an atomically abrupt Si/Ge transition is used, where the lattice mismatch is taken up in a single atomic layer, which is thermodynamically feasible since it is a low energy configuration.