Charge-transfer absorption band in Zn1−xMxO (M: Co, Mn) investigated by means of photoconductivity, Ga doping, and optical measurements under pressure

Abstract

The nature of the charge-transfer absorption band in undoped and Ga-doped Zn1−xMxO (M: Co, Mn) thin films is investigated by means of photoconductivity and optical absorption measurements under pressure. Internal transitions in the crystal field split Co 3d shell do not contribute to the photoconductivity spectrum and have very low pressure coefficient. Broad absorption bands at photon energies just below the band gap in both ZnMnO and ZnCoO clearly contribute to the photoconductivity spectra, indicating that they create free carriers and are consequently charge-transfer transitions. Under pressure, charge transfer bands have a pressure coefficient close to or larger than the band gap, in contrast to the expected low or negative pressure coefficient in a valence-band-to-localized level transition. Finally, the expected Burstein–Moss shift in the fundamental edge of heavily Ga-doped samples of ZnMO is associated to a larger shift and intensity decrease in the pre-edge band, confirming that charge-transfer transitions in ZnMO should be ascribed to transitions from the Co or Mn 3d shell to the conduction band.

Calculated optical properties of Co in ZnO: internal and ionization transitions

“…In the absorption measurements, only one ionization channel is seen at energies just below the band gap [2,4,6,9,10]. Based on our results, we assign this transition to t 2↑ → CBM, i.e.…”

Synchrotron photoemission study of (Zn,Co)O films with uniform Co distribution

“…Diluted magnetic semiconductors (DMS) have been recently intensively studied for spintronic applications (Bonanni and Dietl, 2010;Ueda et al, 2001) and ZnCoO focuses a great part of attention here (Khare et al, 2006;Assadi et al, 2009;De Carvalho et al, 2010;Zhang et al, 2009;Gilliland et al, 2010). This is possible because high cobalt concentration in ZnO is expected due to the nearly identical Co 2 þ and Zn 2 þ ionic radii.…”

Visible-light photoconductivity of Zn1−xCoxO and its dependence onCoJohnson1, Cohn2, Kaspar3 et al. 2011Phys. Rev. B385391View full textAdd to dashboardCite

“…24,25 In semiconductors, these are generally band-to-band transitions, but may also include mid-gap impurity photoionization transitions. 28 In some studies, photo-thermal ionization has been suggested, 18,19,22 and in others the 4 T 1 (P) absorption band does not produce any photoconductivity signal, 23 as might have been expected for a localized d-d excitation. The precise locations of the Co 2+ ionization thresholds relative to the ZnO band edges thus remain poorly defined, although these relationships are of central importance not just to photoconductivity but also to the magnetic properties of this same material.…”

“…10,11,[16][17][18][19][20][21][22][23] This observation is unusual because photoconductivity selectively probes the excited states of a material that generate mobile charge carriers, i.e., a photocurrent. 24,25 In semiconductors, these are generally band-to-band transitions, but may also include mid-gap impurity photoionization transitions.…”