DIRECT OBSERVATION OF BAND-EDGE LUMINESCENCE AND ALLOY LUMINESCENCE FROM ULTRAMETASTABLE SILICON-GERMANIUM ALLOY LAYERS

Ultrametastable silicon-germanium (Si1-xGex) layers with a Ge content x in the range from about 20% to 27% were grown by Si-MBE at temperatures far below 550-degrees-C (325 - 450-degrees-C). The thicknesses of the layers (up to 500 nm) exceed the equilibrium thickness by a factor of up to 50. We observe in the as-grown samples without any annealing both the excitonic Si1-xGex band-edge luminescence and a broad alloy luminescence of unknown origin. The two peaks have an energy difference of almost-equal-to 144 meV and shift linearly with the Ge content. The alloy band luminescence disappears when strain. relaxation sets on upon annealing at around 600-degrees-C.