Metallicity and disorder at the alkali-metal/GaAs(001) interface - art. no. 085310

We have investigated the adsorption of sodium and potassium on GaAs(001) at temperatures ranging between 85 K and 300 K. Photoreflectance spectroscopy and electron-energy-loss spectroscopy are used to characterize the formati on of a metallic phase through, respectively, the photovoltage value and th e presence of surface plasmons in the loss spectrum. Reflectance-anisotropy spectroscopy allows LIS to characterize the disorder of the adsorbate as f ound from the width of the line at 2.3 eV, which characterizes the spectrum of the gallium-rich surface. If the alkali-metal adsorption is performed a t a temperature lower than 200 K, there occurs a nonmetal-metal transition at a submonolayer coverage. The metallic phase appears abruptly near 0.4-0. 5 ML, and there exists a transition regime where both metallic and nonmetal lic phases coexist. Subsequent adsorption leads to the abrupt disappearance of the nonmetallic phase. At low temperature, the adsorbate is found to be disordered since surface diffusion of sodium and to some extent of potassi um is prohibited. The metallic phase is metastable and irreversibly disappe ars under annealing to RT, at a temperature at which surface diffusion beco mes thermally allowed. As a result, the presence of metallicity is directly related to the disorder of the adsorbate. For Na and for K, we have determ ined the diagram of existence of the disordered metallic phase as a functio n of temperature and coverage, as well as of the transition region.