Observation of graded interfaces in strained Si/Si1-xGex layers (0.17<x<0.23) using photoreflectance

Photoreflectance has been used to study critical points along the (111) direction of Si/Si1-xGex (0.17 less than or equal to x less than or equal to 0.23) layers with SiGe layer widths between 30 Angstrom and 105 Angstrom. Transition energies of the SiGe E-1 and E-1 + Delta(1)/E-0' critical points, found from fits with third derivative functions, increase with decreasing well width. The fitted energies are compared with energies deduced from composition profiles of the SiGe layer showing graded SiGe interfaces. These energies agree well with experiment for SiGe layer thickness > 50 Angstrom, and so for these thicknesses the energy shifts can be explained by composition grading of the SiGe interfaces. For SiGe layers < 50 Angstrom, the energy shifts can be explained in terms of composition grading and quantum confinement. (C) 1998 Elsevier Science B.V.