Modulated photovoltage changes at the nonmetal-metal transition of the Na/GaAs(001) and K/GaAs(001) interfaces

A new model is proposed in order to interpret the surface dependence of the constant and modulated photovoltages in a photo reflectance spectroscopy e xperiment, with a particular emphasis on low temperature conditions, at whi ch the time of establishment of equilibrium between surface and bulk is ver y long. In this model, the constant photovoltage depends on the efficiency of surface recombination, while the modulated photovoltage is related to th e modulation of this efficiency induced by the modulation of the surface Fe rmi level, This model is illustrated by the photovoltage changes induced by formation of a metallic phase under adsorption of Na and K on GaAs(0 0 1). At a coverage situated between 0.5 and I ML, there first occurs an increas e of the modulated photovoltage, followed by a decrease, whereas the consta nt photovoltage exhibits a featureless abrupt decrease, due in the same way to discharging of the surface by metallic transport to surface defects. Th e increase of the modulated photovoltage is caused by the metallic surface phase, and is qualitatively explained by the above simple model, in which t he modulated photovoltage changes depend on the total number of metallic st ates which pin the Fermi level. (C) 2001 Elsevier Science B.V. All rights r eserved.