Valence-band electronic structure of CdO, ZnO, and MgO from x-ray photoemission spectroscopy and quasi-particle-corrected density-functional theory calculations

Abstract

The valence-band density of states of single-crystalline rock-salt CdO͑001͒, wurtzite c-plane ZnO, and rocksalt MgO͑001͒ are investigated by high-resolution x-ray photoemission spectroscopy. A classic two-peak structure is observed in the VB-DOS due to the anion 2p-dominated valence bands. Good agreement is found between the experimental results and quasi-particle-corrected density-functional theory calculations. Occupied shallow semicore d levels are observed in CdO and ZnO. While these exhibit similar spectral features to the calculations, they occur at slightly higher binding energies, determined as 8.8 eV and 7.3 eV below the valence band maximum in CdO and ZnO, respectively. The implications of these on the electronic structure are discussed.

Hard x-ray photoelectron spectroscopy as a probe of the intrinsic electronic properties of CdO

“…These two possibilities are discussed later in the context of the core-level line shapes. Both the CB state and valence feature at ∼5.5 eV are strongly enhanced relative to the top of the VB, compared to previous lower photon energy measurements [14,15]. This is due to the pronounced Cd 5s character of both states which exhibit a slower decrease in photoionization cross section with increasing photon energy than either the O 2p or Cd 4d states which compose the rest of the valence band [15,23].…”

“…The situation is very similar in the Cd 3d region, where only a peak shift due to the band bending, and a broadening due to reduced experimental resolution are observed on the annealed sample and no significant contamination component is visible. However, the as-grown sample displays a large contamination component at ∼405.1 eV, on the Cd 3d 5/2 assigned to CdCO 3 or Cd(OH) 2 [15]. This component dominates the spectra, in this case, making the core-level shape appear almost mirrored.…”

“…In principle, the Cd 4d peak should also exhibit a similar phenomenon. However, the Cd 4d levels in CdO are extremely shallow (∼10 eV) and hybridize strongly with the O 2p levels complicating detailed peak fitting [15,24]. Therefore, we have directed our focus on the Cd 3d and O 1s core levels.…”

Polarity effects in the x-ray photoemission of ZnO and other wurtzite semiconductors

“…The VB-XPS spectra consist of two main peaks-peak I at ϳ5.0 eV which has been attributed to predominately O 2p derived states and peak II at ϳ7 eV which has a hybridized O 2p, Zn 4s, and ͑possibly͒ Zn 3d character. 17,18 While the intensity of peak II is similar on all three faces, a striking polarity-related effect involving the intensity of peak I is evident. Peak I is significantly more intense than peak II on the Zn-polar face, while on the O-polar face it is slightly weaker.…”

“…4 Here, we focus on the shape of the VB-XPS spectra which is closely related to the total valence band density of states. 17 Figure 3͑a͒ shows VB-XPS spectra collected from the Zn-polar, O-polar, and m-plane faces of HT ZnO using Al K ␣ radiation. The VB-XPS spectra consist of two main peaks-peak I at ϳ5.0 eV which has been attributed to predominately O 2p derived states and peak II at ϳ7 eV which has a hybridized O 2p, Zn 4s, and ͑possibly͒ Zn 3d character.…”