CRITICAL-POINTS OF SI1-YCY AND SI1-X-YGEXCY LAYERS STRAINED PSEUDOMORPHICALLY ON SI(001)

We investigate the influence of carbon on the optical transitions of S i1-yCy and Si1-x-yGexCy layers grown pseudomorphically on a Si(001) su bstrate also including full strain compensation. The layers were inves tigated by spectroscopic ellipsometry and electroreflectance spectrosc opy for carbon fractions y less than or equal to 1.2 at. % and germani um fractions up to x=16 at. %. The spectra were analyzed by measuring fitting electroreflectance spectra at 80 K and ellipsometry data at ro om temperature, resulting for both techniques in a weak and nearly lin ear dependence on the carbon fraction at all transitions. The results of both techniques are compared and discussed. The strong line broaden ing for increasing carbon fractions can be caused by a high scattering efficiency of the carbon. Our results indicate that the interpretatio n of optical spectra of carbon-containing alloys cannot be performed s traightforwardly by simple interpolating between the appropriate band structures of silicon, germanium, and carbon. An analysis based on str ain-induced contributions only also does not describe the experimental results correctly. For a complete description of the observed energy shifts detailed band structure calculations and further experimental d ata are necessary. (C) 1996 American Institute of Physics.