Modulated photovoltage changes at the nonmetal–metal transition of the Na/GaAs(0 0 1) and K/GaAs(0 0 1) interfaces

A new model is proposed in order to interpret the surface dependence of the constant and modulated photovoltages in a photoreflectance spectroscopy experiment, with a particular emphasis on low temperature conditions, at which the time of establishment of equilibrium between surface and bulk is very long. In this model, the constant photovoltage depends on the efficiency of surface recombination, while the modulated photovoltage is related to the modulation of this efficiency induced by the modulation of the surface Fermi 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 1 ML, there first occurs an increase of the modulated photovoltage, followed by a decrease, whereas the constant photovoltage exhibits a featureless abrupt decrease, due in the same way to discharging of the surface by metallic transport to surface defects. The increase of the modulated photovoltage is caused by the metallic surface phase, and is qualitatively explained by the above simple model, in which the modulated photovoltage changes depend on the total number of metallic states which pin the Fermi level.