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 (Γ-valley) and the indirect bandgap (L-valley) to the valence band (denoted by ED and EID, respectively) were clearly observed at 125 and 175 K for most Ge1-ySny samples studied. At 300 K, however, all of the samples exhibited dominant ED emission with either very weak or no measureable EID emission. At 10 K, ED 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 ∼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.
Journal of Applied Physics
Thomas R. Harris, Mee-Yi Ryu, Yung Kee Yeo, Buguo Wang, C. L. Senaratne, John Kouvetakis; Direct bandgap cross-over point of Ge1-ySny grown on Si estimated through temperature-dependent photoluminescence studies. Journal of Applied Physics 28 August 2016; 120 (8): 085706. https://doi.org/10.1063/1.4961464