Photoelectron Spectroscopic Study of Coadsorbed States of Cs and O on GaAs(100)

The electronic states of O/Cs/GaAs(100) have been investigated by using ultraviolet photoelectron spectroscopy in order to understand the initial coadsorption kinetics on the way to the negative electron affinity formation. It was found that the energy positions of the Ga-3d and As-3d peaks shift to the lower binding energy side by the first Cs deposition, while they shift to the higher binding energy side during the n-th Cs deposition ( n≧3). It was also found that besides the appearance of oxidized As, the width of the Ga-3d peak becomes larger by the first oxygen exposure. In the case of the n-th Cs deposition and oxygen exposure, all photoelectron peaks and the work function made parallel shifts in energy, which can be well interpreted in terms of the change in band bending. These results indicate that the coadsorption kinetics can be classified into two stages: At first Cs deposition and oxygen exposure, both of Ga and As react with Cs and O, resulting in the rearrangement in the surface layers, whil...