Anomalies in the photoconductivity of nominally O-doped material, when subjected to strong light with energies ranging from 1.1 to 1.5eV at temperatures below 150K, were studied. They were identified as being due to a metastable population of holes which were situated at levels ranging from 0.4 to 0.7eV above the valence band. The main feature which was observed was a spectral distribution that was centered at just about the band-gap energy, plus some structural evidence for the existence of levels at about 0.57, 0.63 and 0.70eV above the valence band; with a photo-sensitivity that was perhaps enhanced by replicas of 0.03eV phonons. The population of holes was sufficient to provoke a change from n-type to p-type, under extrinsic excitation in the above energy range. It required an activation energy of 0.0338eV at temperatures below 80K, and a deactivation energy of 0.0429eV at higher temperatures. The difference between these values (0.0767eV) seemed to be related to a negative potential barrier to electron recapture in these centers. The photo-Hall data revealed an electron level at 0.5eV below the conduction band which, in turn, could take part in photo-quenching. A model was therefore proposed which was based upon a level-to-level transition that involved a shallow acceptor level which was 0.03eV above the valence band. However, the present data could not reveal whether the photo-quenching and/or hole effect were linked to the supposed presence of O that had nominally been introduced into the material.

W.V.Machado, M.A.Amato, A.F.S.Landim, B.K.Ridley: Materials Science Forum, 1995,