EXCITON LUMINESCENCE IN SI1-XGEX SI HETEROSTRUCTURES GROWN BY MOLECULAR-BEAM EPITAXY/

Coherent Si1-xGex alloys and multilayers synthesized by molecular beam epitaxy (MBE) on Si(100) substrates have been characterized by low-te mperature photoluminescence (PL) spectroscopy and transmission electro n microscopy (TEM). Phonon-resolved transitions originating from excit ons bound to shallow impurities were observed in addition to a broad b and of intense luminescence. The broad PL band was predominant when th e alloy layer thickness was greater than 40-100 angstrom, depending on x and the strain energy density. The strength of the broad PL band wa s correlated with the areal density (up to approximately 10(9) cm-2) o f strain perturbations (local lattice dilation approximately 15 angstr om in diameter) observed in plan-view TEM. Thinner alloy layers exhibi ted phonon-resolved PL spectra, similar to bulk material, but shifted in energy due to strain and hole quantum confinement. Photoluminescenc e excitation spectroscopy, external quantum efficiency, time-resolved PL decay, together with the power and temperature dependence of lumine scence intensity, have been used to characterize Si1-xGex/Si heterostr uctures exhibiting both types of PL spectra. The role of MBE growth pa rameters in determining optical properties was investigated by changin g the quantum well thickness and growth temperature. The transition fr om phonon-resolved, near-band-gap luminescence in thin layers to the b road PL band typical of thick layers is discussed in terms of a strain energy balance model which predicts a ''transition thickness'' which decreases with increase in x.