XPS INVESTIGATION ON VACUUM THERMAL-DESORPTION OF UV OZONE TREATED GAAS(100) SURFACES

In order to prepare suitable surfaces for molecular beam epitaxy (MBE), sacrificial thin oxide layers on HCl etched GaAs(100) surfaces were grown by both air and UV/ozone exposure. Passive films were subsequently removed by vacuum thermal desorption to achieve surfaces that were smooth and clean on an atomic scale. The evolution of the surface chemical composition, as a function of vacuum desorption temperature, has been studied by means of X-ray photoelectron spectroscopy (XPS). XPS results have evidenced for air and UV/ozone exposed GaAs(100) surfaces a relationship between desorption temperature and surface chemical composition; indeed, the oxide removal is temperature dependent and sequentially selective as follows: As2O3, AsO and Ga2O3. Furthermore, XPS results have shown that air-grown films have a chemical composition and thermal desorption behaviour different from UV/ozone treated materials. Indeed, these latter have an As2O3/Ga2O3 and an unoxidized As/Ga ratio close to unity for as grown and thermal treated at 580-degrees-C surfaces, respectively. By contrast, air-exposed GaAs(100) materials are Ga2O3-enriched and after vacuum thermal desorption treatments have never a stoichiometric composition (As(GaAs)/Ga(GaAs) = 1). Furthermore UV/ozone treated GaAs(100) surfaces subjected to a vacuum thermal treatment at 580-degrees-C, have a troublesome organic contamination level below XPS detectability, whereas from air-exposed surfaces, carbon is not completely thermally removable.