Abstract
Three-dimensional band structure of rock-salt (rs) CdxZn1−xO (x = 1.0, 0.83, and 0.60) have been determined by angle-resolved photoemission spectroscopy (ARPES) using synchrotron radiation. Valence-band features shift to higher binding energy with Zn content, while the conduction band position does not depend strongly on Zn content. An increase of the indirect band gap with Zn-doping is larger than that of the direct band gap, reflecting a weaker hybridization between Zn 3d and O 2p than that between Cd 4d and O 2p. Two-dimensional electronic states due to the quantization along surface normal direction are formed in the surface accumulation layer and show non-parabolic dispersions. Binding energy of the quantized two-dimensional state is well reproduced using an accumulation potential with the observed surface band bending and the characteristic width of about 30 Å.
Highlights
Transparent conducting oxides (TCOs) possess concurrent optical transparency and high electrical conductivity in a single material
Www.nature.com/scientificreports edge of the obtained spectra. Another important aspect of the CdO based semiconductors is the formation of electron accumulation layer at the surface, which is caused by the defect-induced pinning of the surface Fermi energy at the Fermi level stabilization energy, EFS10,11, or the charge neutrality level[12,13,14,15]
We have found that the changes of the indirect and direct band gap are mainly related to the shift of the valence band whereas the charge neutrality level in conduction band shows small shift
Summary
The quantized conduction sub-band with n = 1 has a smaller binding energy than that reported in previous work[16] where CdO films with the thickness of 380 nm were grown on r-plane sapphire using metal-organic vapor phase epitaxy and cleaned in UHV by an annealing at 600 °C, that is higher than our annealing temperature of 300 °C This could affect the concentration of electrically active surface adsorbates which, as has been shown in refs[28,29], are important factors determining the surface band bending. The bulk carrier density of UHV-annealed sample was estimated as 2.1 × 1019 cm−3 from the depth of surface potential that reproduce the energies of quantized states in the surface accumulation layer and from the Luttinger volume of conduction band whose bottom is located at 0.15 eV below the Fermi level. The decrease of carrier density upon the annealing have been reported previously and described as the decrease of unintentional carriers by the removal of defects or atoms at interstitial sites[26]
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