Temperature-Dependent Photoluminescence Studies of Ge1-ySny (y=4.3%-9.0%) Grown on Ge-Buffered Si: Evidence for a Direct Bandgap Cross-Over Point

摘要

The temperature (T)-dependent photoluminescence (PL) from Ge1-ySny (y = 4.3%-9.0%) alloys grown on Ge-buffered Si substrates was studied as a function of the Sn content. The PL from Ge1-ySny alloys with high Sn contents (>= 7.0%) exhibited the typical characteristics of direct bandgap semiconductors, such as an increase in the PL intensity with decreasing T and a single PL peak corresponding to a transition from the direct bandgap (Gamma-valley) to the valence band at all temperatures from 10 to 300 K. For the Ge1-ySny alloys with low Sn contents (<= 6.2%), the PL emission peaks corresponding to both the direct bandgap (E-D) and the indirect bandgap (E-ID) PL appeared at most temperatures and as T was increased, the integrated PL intensities of E-D initially increased, then decreased, and finally increased again. The unstrained E-D and E-ID energies estimated from the PL spectra at 75 and 125 K were plotted as functions of the Sn concentration, and the cross-over point for unstrained Ge1-ySny was found to be about 6.4%-6.7% Sn by using linear fits to the data in the range of Sn contents from 0% to 9.0%. Based on the results at 75 and 125 K, the cross-over Sn concentration of unstrained Ge1-ySny should be about 6.4%-6.7% Sn content at room temperature. The E-D energies of the Ge0.925Sn0.075 alloys were estimated from the T-dependent photoreflectance spectra, and the E-D values was consistent with those obtained from PL spectra.