signature=7704a1dededd24b9f87474f33c085310,Phys. Rev. B 64, 085310 (2001)

该研究通过光反射光谱学和电子能量损失光谱学，详细探讨了85K至300K温度范围内钠和钾在GaAs(001)表面的吸附行为。在低于200K的温度下，单层覆盖时会发生非金属到金属的相变。金属相在0.4-0.5ML时突然出现，并存在共存的过渡区域。随着后续吸附，非金属相会迅速消失。低温下，吸附物呈无序状态，因为钠和钾的表面扩散受限。金属相是亚稳态的，在退火到室温时会不可逆地消失，此时表面扩散变得热力学允许。金属相的存在直接与吸附物的无序状态相关。

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.Received 20 November 2000