Optical Spectroscopy of Semiconductor Surface

Abstract

The achievement of optical spectroscopy in research on electronic states of surfaces are less significant because of experimental difficulty of discerning a weak optical signal coming from the surface against a strong background of much intense signal from the bulk. The recent progress in optical spectroscopy of surfaces is because of the elaboration of new techniques of discrimination between surface and bulk signals. Considerable amount of data has been collected concerning spectra of surface-state optical transitions, their relation to the surface reconstruction, and sensitivity to adsorbates. The main results have been obtained on elementary—that are, Si, Ge, and III-V semiconductors. The first observation of optical transitions among the electronic states of a semiconductor surface has been performed with the use of differential surface reflectivity (DSR) technique, which makes use of sensitivity of electron surface states to surface conditions. The DSR measurements on (111)2×1 surfaces of silicon and germanium both in multiple internal and external configurations have revealed a narrow peak, associated with dangling bond states, at 0.45 eV for Si and at 0.5 eV for Ge—that is, near the mid-gap of these materials. A very important result obtained with DSR method is the observation of polarization dependence of surface optical transitions. The measurements on single-domain Si(111)2×1 surface have shown a marked anisotropy of the mid-gap peak with a maximum of ΔR/R when the light is polarized along the direction. Analogous results have been obtained by photothermal displacement spectroscopy (PTDS). This technique is based on optical detection of the thermal expansion of a sample as it is heated by absorption of light. The technique does not require a difference spectrum between clean and oxidized surfaces, but it is obviously limited to the spectral range of low bulk absorption.