PNNL

Abstract

Variable-temperature and polarized scanning photoconductivity measurements on Zn1-xCoxO epitaxial films allow description of Co2+-derived sub-bandgap photoionization excited states in this archetypal diluted magnetic oxide. Low-temperature (27 K) measurements demonstrate spontaneous ionization from the photo-generated 4T1(P) d-d excited state, despite the highly localized nature of this excitation. Ionization involves relaxation from this d-d state to the lower Co2+/3+ donor-type photoionization level. Variable-temperature photoconductivity measurements reveal an additional thermally assisted ionization process that enhances the d-d photoconductivity at higher temperatures. The energy barrier to thermal ionization from the 4T1(P) excited state at low temperatures is estimated to be Ea ˜ 40 meV.