Changes in surface composition and Fermi level position during thermal desorption of ultraviolet radiation/ozone formed oxides on GaAs

Abstract

X-ray photoelectron spectroscopy (XPS) has been used to study the native oxide structure and surface Fermi level of GaAs exposed to ultraviolet light (UV)/ozone, as well as to monitor changes that occur during thermal desorption of the surface oxide. The XPS results confirm that the surface oxide consists of a mixture of gallium and arsenic oxide phases which desorb at two different temperature ranges: desorption of arsenic oxide phases and oxygen transfer from arsenic to gallium occur at 250–500 °C; and desorption of gallium oxide phases occurs at 550–600 °C. In addition, we found that the surface Fermi level of GaAs below the UV/ozone formed oxide was always at about 0.8 eV from the valence- band maximum (VBM) for n-type substrates and 0.5 eV for p-type substrates. Similar results were found for wet chemically treated and then air exposed GaAs surfaces. However, the surface Fermi levels of both n- and p-type substrates, during the desorption of arsenic oxide in the temperature range of 250–500 °C, moved further away from VBM by as much as 0.4 eV. When the samples were heated to higher temperatures, the surface Fermi level moved back to midgap even before complete desorption of the gallium oxide phases. Fermi level pinning on samples before and after oxide desorption has also been characterized by in situ deposition of 5 nm of silicon nitride on the sample surface in the XPS system and by monitoring the surface potential of the semiconductor with XPS as a function of charging potential on the dielectric. It was found that surfaces both before and after arsenic oxide desorption were pinned within 0.1 eV with a negative voltage of about 2 V on the thin dielectric.