Photoluminescence measurements of high Sn-content Ge$_{\mathrm{1-y}}$Sn$_{\mathrm{y}}$ and Ge$_{\mathrm{1-x-y}}$Si$_{\mathrm{x}}$Sn$_{\mathrm{y}}$ grown on Ge-buffered Si

The optical properties of newly developed, high Sn-content Ge$_{\mathrm{1-y}}$Sn$_{\mathrm{y}}$ and Ge$_{\mathrm{1-x-y}}$Si$_{\mathrm{x}}$Sn$_{\mathrm{y}}$ thin films grown on Ge-buffered Si have been characterized using temperature-dependent and laser power-dependent photoluminescence (PL) measurements. The results show two distinct PL peaks related to both the direct ($\Gamma$) and indirect (L) bandgap transitions. Furthermore, the measured separation energy between the direct and indirect bandgap related PL peaks for Ge$_{\mathrm{0.948}}$Sn$_{\mathrm{0.052}}$ sample is only about 30 meV compared to the value of 140 meV for bulk Ge. This study shows a very encouraging result toward producing Ge- and Si-based direct bandgap semiconductors, whose predicted indirect-to-direct bandgap crossover could be near 6\% Sn. Clear competition between the two transitions is also observed as a function of temperature and laser power. Overall, this work represents an extensive PL characterization of Ge$_{\mathrm{1-y}}$Sn$_{\mathrm{y}}$ and Ge$_{\mathrm{1-x-y}}$Si$_{\mathrm{x}}$Sn$_{\mathrm{y}}$ materials over a wide compositional range and should be useful for the development of next-generation optoelectronic devices.