Photoluminescence mechanisms in thin ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ quantum wells

Photoluminescence spectra were obtained from thin ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ quantum wells grown by molecular-beam epitaxy and rapid thermal chemical-vapor deposition. The effect of excitation power density is compared with recent results for thick quantum wells, in which a high-quantum-efficiency localized exciton luminescence band was observed under conditions of low excitation. The separation between the usual near-band-edge luminescence and the localized exciton feature is found here to decrease from 20 to ∼0 meV when the quantum-well thickness is decreased from 83 to 12 Å. In the very thin quantum wells (10–15 Å) the spectral line shape and position change very little with excitation density changes of over six orders of magnitude. However, the dependence of the luminescence intensity on excitation power and the very long decay time (∼750 μsec) at low excitation lead us to propose that a localized exciton process is also important in the very thin quantum wells grown by both techniques.