ELECTRONIC-STRUCTURE AND OPTICAL-PROPERTIES OF SI/GE SUPERLATTICES

A new approach that combines various calculational methods is presented for studying the effect of the Si-Ge interface onto the electronic and optical properties of SinGem superlattices (SLs). In particular, our approach employs: i) the tight-binding (TB) method to calculate the band structure of the SinGem SLs which exhibit an abrupt or diffused Si-Ge interface, ii) the coherent potential approximation (CPA) method, in order to describe the interdiffusion across the Si-Ge interface and, finally, iii) the Kubo-Greenwood in order to obtain the optical-absorption coefficient of the SL. This approach is applied on a strained symmetrized Si5Ge5 SL and our results confirm recent theoretical and experimental ones that support the finding that interdiffusion across the interface of the SL degrades the strength of the optical transitions. According to our findings, this degradation is mainly attributed to the widening of the energy gap of the SL that follows the interdiffusion. The present method is more efficient than others based on calculating transition matrix elements explicitly, because it incorporates the effects of randomness on both the density of states and the optical-transition matrix elements through a single step in the framework of the CPA and the Kubo-Greenwood formula.