Temperature-Dependent Photoluminescence Studies of Ge_1−ySn_y (y = 4.3%–9.0%) Grown on Ge-Buffered Si: Evidence for a Direct Bandgap Cross-Over Point

The temperature (T)-dependent photoluminescence (PL) from Ge_1−ySn_y (y = 4.3%–9.0%) alloys grown on Ge-buffered Si substrates was studied as a function of the Sn content. The PL from Ge_1−ySn_y 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 (Γ-valley) to the valence band at all temperatures from 10 to 300 K. For the Ge_1−ySn_y 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 Ge_1−ySn_y 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 Ge_1−ySn_y should be about 6.4%–6.7% Sn content at room temperature. The E_D energies of the Ge_0.925Sn_0.075 alloys were estimated from the T-dependent photoreflectance spectra, and the E_D values was consistent with those obtained from PL spectra.