Evaluation of band gap narrowing of a tensile-strained Ge on InxGa1−xAs and its transfer onto glass substrate for solar cell applications

Photoluminescence measurement of a tensile-strained (TS) Ge thin film grown on In_xGa_1-xAs by molecular beam epitaxy has been carried out to verify the strain-induced band gap narrowing (SIBGN) of Ge, which is one of the most important properties of TS Ge for its solar cell application. The SIBGN of both the direct and indirect gaps of 0.56 % TS Ge have been observed, which well correspond to the theoretical prediction. After that, a “stressor-free” TS Ge/ ZnO/ glass substrate structure is demonstrated as a first step for the fabrication of TS Ge thin film solar cells. TS Ge films can maintain its initial tensile strain after eliminating In_xGa_1-xAs stressor layer due to the strain keeping ability of the ZnO layer. These achievements constitute significant steps toward the application of TS Ge to high-efficiency MJ solar cells.