Sub-band-gap photoconductivity in Co2+-doped ZnO

Variable-temperature and polarized scanning photoconductivity measurements on Zn1-xCoxO epitaxial films allow description of sub-band-gap Co2+-derived photoionization excited states in this archetypal diluted magnetic oxide. Low-temperature (27 K) measurements demonstrate spontaneous ionization from the photogenerated T-4(1)(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 T-4(1)(P) excited state at low temperatures is estimated to be E-a approximate to 43 meV.