Highly disordered amorphous selenium studied by x-ray photoemission spectroscopy

X-ray photoemission spectroscopy (XPS) has been applied to a highly disordered amorphous Se deposited onto a cooled (150-K) substrate. XPS spectral changes have been observed in the 4p bonding and 4s states upon thermal annealing of the film. The thermally induced growth of the upper 4p bonding band is in good agreement with the previously reported UPS (ultraviolet photoemission spectroscopy) measurements [Takahashi et al., Phys. Rev. B 21, 3399 (1980)]. The 4s band of the highly disordered film shows a round and narrow feature and transforms into a rectangular shape upon the thermal annealing. The observed change of the 4s band is explained by a thermally induced increase of the cluster size in the cluster-junction model proposed in the previous UPS study.