Field Effect of the Reflectivity in Germanium

Abstract

This paper reports on simultaneous measurements of the reflectivity at normal incidence between 0.7 and 4.6 eV and the field effect of the surface conductance over a wide range of different surface conditions in germanium samples. In the wavelength region of direct interband transitions, the reflectivity is a function of the surface potential. It is concluded that the electric fields in the surface potential barrier are strong enough to change the optical properties of the surface region by Franz—Keldysh effect. A theoretical estimate based on this assumption reproduces the experimental results qualitatively. Since the effect is observed in reflection rather than in transmission, the influence of an electric field on interband transitions can be studied well above the fundamental absorption edge. It is shown that they are affected by an electric field in a way similar to the transitions which produce the fundamental absorption edge. It is further concluded that the field‐induced change in the absorption coefficient ``undercuts'' the field‐free absorption curve in the top portion of the fundamental absorption edge as predicted by the theory of the Franz—Keldysh effect.