PHOTOLUMINESCENCE MECHANISMS IN THIN SI1-XGEX QUANTUM-WELLS

Photoluminescence spectra were obtained from thin Si1-xGex quantum wel ls grown by molecular-beam epitaxy and rapid thermal chemical-vapor de position. The effect of excitation power density is compared with rece nt 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 lumine scence and the localized exciton feature is found here to decrease fro m 20 to approximately 0 meV when the quantum-well thickness is decreas ed from 83 to 12 angstrom. In the very thin quantum wells (10- 15 angs trom) the spectral line shape and position change very little with exc itation density changes of over six orders of magnitude. However, the dependence of the luminescence intensity on excitation power and the v ery long decay time (approximately 750 musec) at low excitation lead u s to propose that a localized exciton process is also important in the very thin quantum wells grown by both techniques.