Electronic structure of Zn1−xCoxO using photoemission and x-ray absorption spectroscopy

Abstract

Electronic structures of homogeneous bulk samples of Zn0.9Co0.1O which do not exhibit diluted ferromagnetic semiconducting (DMS) behavior have been investigated using photoemission spectroscopy and x-ray absorption spectroscopy. We have found that the Co ions in Zn1−xCoxO are in the divalent Co2+(d7) states under the tetrahedral symmetry. Our finding indicates that the DMS properties will not be produced when Co ions are properly substituted for Zn sites, implying that the DMS properties observed in Zn1−xCoxO thin films are likely to be extrinsic.

Role of defects in ferromagnetism inZn1−xCoxO: A hybrid density-functional study

“…The majority-spin conduction-band structure of Co Zn is nearly identical to that of bulk ZnO, but there is a considerable difference between the majority-spin valenceband structure for ZnO and Co Zn . Results from resonant photoemission experiments 39,52 show a well-defined peak at the top of the valence band as well as a broader density of states due to Co extending down to 8 eV below the Fermi level; the photoemission peak near the Fermi level may therefore arise predominantly from emission from 3d minority-spin states. The large energy splitting of occupied e and unoccupied t minority-spin d electrons found here is caused by differences in exchange energies of these levels and not crystal field splitting; a similar study of Zn 1−x Mn x O, 22 in which d levels are entirely occupied or entirely empty, shows a crystal field ͑e and t͒ splitting of less than 1 eV.…”

“…However, optical spectra in materials with highly localized excited states yield excitation energies for the N-electron state, but do not provide good estimates of ionization potential and electron affinity differences because the large electron-hole attraction energy in the optically excited N-electron state reduces the excitation energy well below the difference in ionization potential and electron affinity. Photoemission experiments 39,52 and band structure calculations, which involve N + 1 and N − 1 electron energy levels, provide the best estimates of the relevant energy level positions. However, observation of photocurrents in Zn 1−x Co x O ͑Ref.…”

Investigation of pure and Co2+-doped ZnO quantum dot electronic structures using the density functional theory: choosing the right functional

“…Recently reported photoemission studies on bulk Co 2+ -doped ZnO [49,57,58] describe two key results: (1) pure 3d Co 2+ states are located at the edge of the ZnO VB, well separated from the conduction band; (2) a broad satellite peak is observed at about 4 eV into the VB. While the pure DFT methods shift the occupied Co 2+ 3d levels and put them too close to the conduction band to be consistent with experiment, the hybrid PBE1 correctly places the occupied Co 2+ 3d levels above the ZnO VB edge, cleanly separated from the conduction band.…”

Soft-x-ray spectroscopic investigation of ferromagnetic Co-doped ZnO

“…Hence, it is important to verify that the transition-metal dopants occupy the Zn site and that ferromagnetism can result. In light of earlier reports such as that of Wi et al 8 substitutional cobalt may be a necessary but not sufficient condition for ferromagnetism. This realization may reconcile some of the apparently conflicting experimental reports.…”

“…Following their work many conflicting reports have attributed the origin of ferromagnetic behavior as being due to substitution by Co atoms in the ZnO 2-6 or due to clustering of Co atoms in secondary phases that are ferromagnetic; 7 while others report no ferromagnetic behavior even though Co occupies the substitutional sites. 8 Three different mechanisms have been proposed to explain the origin of ferromagnetism in transition-metal-doped ZnO: ͑I͒ A model of ferromagnetism where there is an exchange interaction mediated by carriers in the valence or conduction band and the localized moment of the ion, 1 ͑II͒ a double exchange mechanism in which hopping of 3d electrons from one ion to the next results in ferromagnetic behavior. 9 , and ͑III͒ an impurity band model 10 where localized ionic moments create magnetic polarons in a defect-related donor impurity band.…”