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 4$d$ 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 4$d$ 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 4$d$ state. The VB photon energy dependence reveals the O 2$p$ photoionization cross section is underestimated at higher photon energies, and that an orbital contribution from Cd 5$p$ is underestimated by all the DFT approaches.