Photoluminescence of thin SI_1-xGE_x quantum wells

Well-resolved band edge photoluminescence spectra were obtained from SiGe quantum wells of various widths. In addition to the usual shallow bound exciton features, we observed a highly efficient deeper luminescence process, under conditions of low excitation density, in thick SiGe quantum wells. This luminescence band can be attributed to excitons localized by fluctuations in alloy concentration. The binding energy of the localized exciton feature is found to decrease with decreasing well width. In the thinnest quantum well samples only a single luminescence feature is observed at all power levels, while in several other thin quantum well samples having very sharp lines the localized exciton feature appears at higher energy than the bound exciton. Despite these changes in the spectra, the localized exciton luminescence could be identified in all cases by its characteristic intensity saturation at low excitation power density, as well as its slow decay time (approx. 1 ms). The mechanism behind the changes in the localized exciton luminescence may originate from limiting the exciton motion to two dimensions in thin wells, which at low temperatures would hinder migration to the lowest energy alloy fluctuation centers.