Photoreflectance spectroscopy investigation of two-dimensional cesium metallic clusters on GaAs(100)

We have monitored, using photoreflectance spectroscopy, the change of photovoltage and of surface barrier induced by cesium adsorption at 90 K on GaAs(100). Over a total coverage range of only 0.1 monolayer (ML), near 0.6 ML, the photovoltage first increases abruptly by a factor of 5, and subsequently decreases by an even larger factor. The increase is due to the sudden change of surface density of states caused by the formation of cesium bidimensional clusters of size larger than the limit for metallicity. For further cesium exposure, the photovoltage decreases because, due to the increased cluster size, a significant fraction of these clusters overlap randomly-situated recombination centers, which efficiently discharge them through metallic transport in the cluster. Since at this temperature cesium does not diffuse on the surface, these clusters are disordered. Further analysis shows that the cesium metallic clusters are metastable and are irreversibly destroyed by annealing at room temperature. The disruption temperature is found to depend on cluster size, larger clusters being disrupted at a lower temperature. Based on simulations, we propose that the abrupt change of cesium sticking coefficient, which also occurs at a similar cesium coverage as the one of the photovoltage increase, reflects the change of the average coordinence of surface cesium.