ISOTROPIC AND ANISOTROPIC CONTRIBUTIONS TO THE OPTICAL REFLECTION OF SI(001)-2X1

Optical properties of the outermost layer of clean Si and Ge surfaces are different from the bulk, and can be changed by chemisorption. We s how that on Si and Ge(001), in contrast to O-2, H is not capable of re moving completely the optical activity of the outermost layer as usual ly is assumed. By ellipsometric measurements both near Brewster's angl e and at normal incidence, we are able to separate surface dielectric tensor components parallel and perpendicular to dimers on clean Si(001 ). The tensor components consist of at least two parts: a large isotro pic part and a much weaker anisotropic part. The isotropic part can be represented by an attenuation of the bulk dielectric function.