ELECTRONIC-STRUCTURE OF SIXGE1-X SEMICONDUCTOR SOLID-SOLUTIONS

The electronic structure of Si//xGe//1// minus //x solid solutions is studied by means of a semi-ab initio method without involving the virtual crystal approximation. The calculations are performed on large cubic cells of the diamond lattice which contain a total of 64 Si and Ge atoms homogeneously distributed in different compositional ratios. The basis functions and the potentials used in the calculation give good band structures of elemental Si and Ge respectively. The calculated variation of band gap with x is in agreement with optical experiment: the two linear curves of gap vs concentration have different slopes at high and low x values with a crossing-over at about X equals 0. 28. The band gaps are direct for x epsilon (0, 0. 28) and indirect for x greater than equivalent to 0. 28. The density of states (DOS) of the solid solution can be very well approximated by the weighted average of the bulk Si and Ge DOS.