Photo- and electroluminescence in short-period Si/Ge superlattice structures

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Abstract
Interband optical transitions have been studied in a variety of short-period Si/Ge superlattice structures by means of photocurrent spectroscopy, infrared absorption, photo- and electroluminescence. Furthermore, the bandgap photoluminescence from strain-adjusted SimGen (m=9, 6, 3; n=6, 4, 2) superlattices was studied under applied hydrostatic pressure. The strain adjustment was achieved by a thick, step-graded Si1-xGex buffer layer resulting in an improved quality of the superlattice with respect to dislocation density. The hydrostatic pressure dependence was modelled using an approach based on deformation potentials and effective-mass theory. In samples annealed at 500 degrees C and higher, a systematic shift of the bandgap was observed which is discussed in terms of a process involving interdiffusion of the Si and Ge atoms. Bandgap-related electroluminescence was observed in mesa diodes at room temperature, whereas the photoluminescence disappeared at about 40 K. The electroluminescence from samples based on different buffer-layer concepts is compared. Apart from the strain-symmetrized Si/Ge superlattices, another structure that has been proposed to act as an efficient, light-emitting device in the Si-based systems is an ultrathin Ge layer (1-2 monolayers) embedded in bulk Si. We report on the electroluminescence spectra at various temperatures from a sample based on this concept, namely a layer sequence consisting of two periods of Si17Ge2 grown pseudomorphically on an n+ Si substrate. A very intensive, well resolved electroluminescence was obtained at 55 K from the QW.