Valence-band orbital character of CdO: A synchrotron-radiation photoelectron spectroscopy and density functional theory study

Abstract

N -type CdO is a transparent conducting oxide (TCO) which has promise in a number of areas including solar cell applications. In order to realize this potential a detailed knowledge of the electronic structure of the material is essential. In particular, standard density functional theory (DFT) methods struggle to accurately predict fundamental material properties such as the band gap. This is largely due to the underestimation of the Cd 4d binding energy, which results in a strong hybridization with the valence-band (VB) states. In order to test theoretical approaches, comparisons to experiment need to be made. Here, synchrotron-radiation photoelectron spectroscopy (SR-PES) measurements are presented, and comparison with three theoretical approaches are made. In particular the position of the Cd 4d state is measured with hard x-ray PES, and the orbital character of the VB is probed by photon energy dependent measurements. It is found that LDA + U using a theoretical U value of 2.34 eV is very successful in predicting the position of the Cd 4d state. The VB photon energy dependence reveals the O 2p photoionization cross section is underestimated at higher photon energies, and that an orbital contribution from Cd 5p is underestimated by all the DFT approaches.

Lifetime effects and satellites in the photoelectron spectrum of tungsten metal

“…Mudd et al [108] encountered a similar challenge, in the case of CdO, where the unoccupied Cd 5p state contributes to the valence p character. The Cd 5p orbital, much like the W 6p orbital, is unoccupied in the ground state of the atom, and so Mudd et al approached the problem by multiplying the In 5p/In 5s cross section ratio to the cross section of the Cd 5s orbital to estimate the Cd 5p cross section.…”

Galore: Broadening and weighting for simulation of photoelectron spectroscopy

“…19 A best-practice approach is to integrate the relevant equations over a range of angles depending on the equipment geometry. 20 Currently this data is not included in Galore, but users are able to include corrected cross-sections from a JSON-formatted data file if available.…”

Valence-band density of states and surface electron accumulation in epitaxialSnO2films

“…Among the III-V semiconductors, InAs [6] and InN [7] are known to have surface electron accumulation layers. Most binary TCOs that have attracted intensive research into their optical and electronic properties, namely, In 2 O 3 [8,9], CdO [10][11][12][13][14], ZnO [11,15,16], and SnO 2 [17][18][19], have all been shown to exhibit surface electron accumulation. This follows from the fact that the charge neutrality level (CNL), defined as the boundary energy at which defect states change from donor type to acceptor type with reference to the Fermi level, lies above the conduction-band minimum (CBM) in these materials.…”