ISSN - 10980121, Physical Review B - Condensed Matter and Materials Physics, 2001, vol. 64, p. 853101-8531011
Metallicity and disorder at the alkali-metal/GaAs(001) interface
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 formation of a metallic phase through, respectively, the photovoltage value and the presence of surface plasmons in the loss spectrum. Reflectance-anisotropy spectroscopy allows us to characterize the disorder of the adsorbate as found 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 at 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 nonmetallic 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 potassium is prohibited. The metallic phase is metastable and irreversibly disappears under annealing to RT, at a temperature at which surface diffusion becomes 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 determined the diagram of existence of the disordered metallic phase as a function of temperature and coverage, as well as of the transition region.