REFLECTANCE ANISOTROPY SPECTROSCOPY - A NEW METHOD FOR SEMICONDUCTOR SURFACE-CHEMISTRY INVESTIGATION

We present three applications of reflectance anisotropy spectroscopy (RAS) to study the chemistry of (001) semiconductor surfaces, in the particular case of sulphide-passivated GaAs. This technique allows us both to investigate the dimers at the semiconductor surface and to measure the surface barrier. For Na2S-passivated GaAs, we have studied the effect of annealing in ultrahigh vacuum on the reflectance anisotropy spectrum. The appearance and the increase of a dimer signal were observed. For all annealing stages, these dimers are induced by the breaking of chemical bonds on the surface, which has been studied by X-ray photoemission spectroscopy (XPS). Secondly, for the (NH4)2S passivation, the as-treated surface contains arsenic dimers whereas XPS reveals a large number of As-S bonds. This is interpreted by assuming that the bonds observed by photoemission are in the residual film above the surface, whereas the arsenic dimers studied by RAS are on this surface. RAS then allows the semiconductor surface to be studied and therefore the relevant physics to be recorded in order to understand the semiconductor passivation. Thirdly, we have studied the chemical reactions during passivation by measuring the time variation in the barrier height while the sample is immersed in the sulphide solution.