ENERGY-BAND STRUCTURE AND LINEAR OPTICAL-PROPERTIES OF SI AND GE STRAINED ALONG THE [111] AND [110] DIRECTIONS

The electronic and optical properties of Si and Ge coherently grown on (111) and (110) surfaces of Si1-xGex alloy substrates are calculated with the use of a realistic empirical tight-binding model. The depende nce of band-edge energies on strain and direction of growth is present ed and the results are compared to those of the linear deformation pot ential theory. Nonlinear effects in the deformation potential theory a re also considered for the conduction-band minima of Si along the Delt a direction. The calculations show that these strained materials remai n indirect gap semiconductors for all substrates and for both directio ns of growth. Their linear optical properties are analyzed in detail. Distortion along the [111] and [110] directions transforms the materia ls into uniaxial and biaxial crystals, respectively. The imaginary par t of the dielectric function, epsilon(2)(omega), along the principal a xes is also calculated. The resulted anisotropy, as well as the struct ures in the epsilon(2) spectra, are analyzed in terms of the band stru cture and transition probabilities.