Quantitative theory of optical properties of SiGe heterostructure

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It has been shown that by growing a series of very thin alternating layers (superlattice) of silicon and germanium (or suitably chosen alloys) it is possible to break the bulk signature of the conduction band wave function characteristic of an indirect gap semiconductor. As a result the optical transition probability for the jump across the fundamental gap is significantly enhanced. A full account of the optical properties of ideal (infinite, defect-free) SiGe superlattices is now available. However, experimental results suggest that in order to have close agreement with optical data a more realistic account of the structures in question may be required. A step in this direction is made in the present study. First, we identify the key features of the optical spectra of finite length SiGe superlattices. Secondly, we examine the effects of an external electric field upon the finite length SiGe structures. We compare our results with those familiar from the idealized model. Both the transitions across the fundamental gap and those between adjacent minibands have been studied. We report significant changes which affect the final assessment of the strength and spectral distribution of the key optical transitions.