PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE IN SHORT-PERIOD SI GE SUPERLATTICE STRUCTURES/

Interband optical transitions have been studied in a variety of short- period Si/Ge superlattice structures by means of photocurrent spectros copy, 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 p ressure. The strain adjustment was achieved by a thick, step-graded Si 1-xGex buffer layer resulting in an improved quality of the superlatti ce with respect to dislocation density. The hydrostatic pressure depen dence was modelled using an approach based on deformation potentials a nd effective-mass theory. In samples annealed at 500-degrees-C and hig her, 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 r oom temperature, whereas the photoluminescence disappeared at about 40 K. The electroluminescence from samples based on different buffer-laye r concepts is compared. Apart from the strain-symmetrized Si/Ge superl attices, another structure that has been proposed to act as an efficie nt, light-emitting device in the Si-based systems is an ultrathin Ge l ayer (1-2 monolayers) embedded in bulk Si. We report on the electrolum inescence 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 electroluminesence was obtained at 55 K from the QW.