The effect of local atomic structure on the optical properties of GeSi self-assembled islands buried in silicon matrix

The local atomic structure of GeSi self-assembled islands buried in a silicon matrix strongly influences the optical properties of such systems. In the present paper this structure was determined by x-ray absorption fine-structure (XAFS) spectroscopy and high resolution transmission electron microscopy (HRTEM) and used to build a schematic description of the band structure model. Quantitative analysis of the extended XAFS (EXAFS) spectrum was performed for three coordination shells around the Ge absorbing atom with multiple scattering taken into account. It was proved that the coordination number of elements in an alloy resulting from EXAFS analysis for all three coordination spheres (i.e.?'mixing degree' parameters) cannot be taken as the concentration of alloy but can be used together with a proper model of the alloy unit cell to calculate a realistic concentration. The fraction of Ge calculated in this way is consistent with HRTEM results. The found model of the unit cell was used to generate a x-ray absorption near edge structure spectrum by ab initio calculations. This approach yielded a spectrum in good agreement with the experimental one. The information gained from XAFS and HRTEM was then used for calculation of the band structure diagram. Results of the calculation are discussed and compared with the experimental photoluminescence spectrum.