Optical determination of the space-charge region in semiconductors by ellipsometry

Recent studies indicate that the spatial extent of the space-charge region in semiconductors can be determined from oblique incidence electroreflect-ance measurements with elliptically polarized light. The ellipsometric method yields experimental inforomation about the optical modulation that cannot be obtained from electroreflectance measurements at normal incidence or with linearly polarized light. When an electric field is applied normal to the reflecting surface of a semiconductor, the optical properties at photon energies in the vicinity of an absorption threshold are perturbed slightly due to the Franz-Keldysh effect. Three independent experimental quantities may be obtained from Series of ellipsometric measurements at a given photon energy. This information is then used to calculate the perturbations in the real and imaginary parts of the optical dielectric constant and, most importantly, the spatial extent of these perturbations. By using an electric field profile based on a numerical solution of Poisson´s equation, the effective depth of the space-charge region is obtained. Previously, etectroreflectance experiments have been analyzed by assuming that the electric field is uniform over the penetration depth of the light, or by independently determining the depth of the space-charge region trom field-effect conductivity measurements. At photon energies in the vicinity of the fundamental absorption edges in typical semiconductors, space-charge, depths of up to 0.1µ may be determined with this method. Experimental results on thin semiconducting films deposited on dielectric substrates will be discussed.