Direct bandgap cross-over point of Ge1-ySny grown on Si estimated through temperature-dependent photoluminescence studies

Epitaxial Ge1-ySny (y = 0%-7.5%) alloys grown on either Si or Ge-buffered Si substrates by chemical vapor deposition were studied as a function of Sn content using temperature-dependent photoluminescence (PL). PL emission peaks from both the direct bandgap (Gamma-valley) and the indirect bandgap (L-valley) to the valence band (denoted by E-D and E-ID, respectively) were clearly observed at 125 and 175K for most Ge1-ySny samples studied. At 300 K, however, all of the samples exhibited dominant E-D emission with either very weak or no measureable E-ID emission. At 10 K, E-D is dominant only for Ge1-ySny with y > 0.052. From the PL spectra taken at 125 and 175 K, the unstrained indirect and direct bandgap energies were calculated and are plotted as a function of Sn concentration, the results of which show that the indirect-to-direct bandgap transition occurs at similar to 6.7% Sn. It is believed that the true indirect-to-direct bandgap cross-over of unstrained Ge1-ySny might also take place at about the same Sn content at room temperature. This observation suggests that these Ge1-ySny alloys could become very promising direct bandgap semiconductor materials, which will be very useful for the development of various new novel Si-and Ge-based infrared optoelectronic devices that can be fully integrated with current technology on a single Si chip. Published by AIP Publishing.