Photoluminescence spectroscopy of fully-strained GeSn layers on relaxed Ge buffers on Si substrates

GeSn alloys have attracted interest recently due to their unique optical properties and growth on Si substrates. Incorporation of Sn into Ge lowers the band gap of and enhances optical emission from the direct band gap transition. With sufficient Sn content (6-10%), the alloy becomes a direct band gap material, which has driven research to create lasers on Si using GeSn as the gain material. In this work, we studied the photoluminescence (PL) from GeSn layers with Sn content in the range 2-13%. These thin films (30-50 nm) were grown on relaxed Ge buffer layers on Si substrates, and the films are fully coherent to the Ge buffer. PL measurements show a red shift in both the direct and indirect band gap emission with increasing Sn content as the band gap is decreased. We used the results to calculate the bowing parameter for the two gaps, which can be used to calculate the band gap of unstrained GeSn as a function of Sn content. The results demonstrate that GeSn films that have not undergone relaxation are of high crystalline quality.