OPTICAL-PROPERTIES AND BAND-STRUCTURE OF GE1-YCY AND GE-RICH SI1-X-YGEXCY ALLOYS

We measured the dielectric function of Ge1-yCy and Ge-rich Si1-x-yGexC y alloys from 1.6 to 5.2 eV using spectroscopic ellipsometry. These al loys were grown by molecular beam epitaxy at 600 degrees C on (001) Si substrates. Analytic lineshapes fitted to numerically calculated deri vatives of their dielectric functions determined the critical-point pa rameters of the E-1, E-1 + Delta(1), E-0', and E-2 transitions. The cr itical-point energies of the Ge1-yCy alloys were found to be indisting uishable from those of bulk Ge. This indicates that the presence of C in these alloys has no detectable influence on the band structure. The amplitude of the ellipsometric spectra is much lower than for bulk Ge , which can be attributed to surface roughness and explained within th e framework of the Kirchhoff theory of diffraction or using effective medium theory. The degree of surface roughness indicated by optical me asurements was verified by atomic force microscopy. (C) 1998 Elsevier Science S.A.