Electronic structure of single-crystal rocksalt CdO studied by soft x-ray spectroscopies andab initiocalculations (original) (raw)
Related papers
Hard x-ray photoelectron spectroscopy as a probe of the intrinsic electronic properties of CdO
Physical Review B, 2014
Hard x-ray photoelectron spectroscopy (HAXPES) is used to investigate the intrinsic electronic properties of single crystal epitaxial CdO(100) thin films grown by metal organic vapor phase epitaxy (MOVPE). The reduced surface sensitivity of the HAXPES technique relaxes stringent surface preparation requirements, thereby allowing the measurement of as-grown samples with intrinsically higher carrier concentration (n = 2.4 × 10 20 cm −3). High-resolution HAXPES spectra of the valence band and core levels measured at photon energy of 6054 eV are presented. The effects of conduction band filling and band gap renormalization are discussed to explain the observed binding energy shifts. The measured bandwidth of the partially occupied conduction band feature indicates that a plasmon contribution may be present at higher carrier concentrations. The Cd 3d 5/2 and O 1s core-level line shapes are found to exhibit an increased asymmetry with increased carrier concentration, interpreted as evidence for final state screening effects from the carriers in the conduction band. Alternatively the core-level line shape is interpreted as arising from strong conduction electron plasmon satellites. The nature of these two competing models to describe core-level line shapes in metallic oxides is explored.
Ab-initio calculation and B 1 to B 2 structural phase transformation in CdO at high pressure
Indian Journal of Pure & Applied Physics, 2007
The electronic and structural properties of CdO at high pressure have been studied by using the self-consistent tight binding linear muffin tin orbital method. The relative stabilities of CdO at high pressure in the rock salt and CsCl structures have been examined. The rock-salt structure is found to be the ground state structure of CdO. The phase transition from NaCl (B,)- to CsCl (B 2 )-type structure for CdO is found to occur at 81.7 GPa, which is close to experimental value 90.6 GPa. Calculated lattice parameter and bulk modulus in both the phases are in good agreement with experimental results. The calculated band structure shows a small overlap between the conduction and valence bands with a direct band-gap of ∼1.37 eV. A broadening of the upper Cd-4d and 0-2p bands with some intermixing between them is observed in total DOS histogram of CdO.
Physical Review B, 2014
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.
Physical Review B, 2009
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.
Quasiparticle band structure of rocksalt-CdO determined using maximally localized Wannier functions
Journal of Physics: Condensed Matter, 2012
CdO in the rocksalt structure is an indirect band gap semiconductor. Thus, in order to determine its band gap one needs to calculate the complete band structure. However, in practice, the exact evaluation of the quasiparticle band structure for the large number of k-points which constitute the different symmetry lines in the Brillouin zone can be an extremely demanding task compared to the standard density functional theory (DFT) calculation. In this paper we report the full quasiparticle band structure of CdO using a plane-wave pseudopotential approach. In order to reduce the computational effort and time, we make use of maximally localized Wannier functions (MLWFs). The MLWFs offer a highly accurate method for interpolation of the DFT or GW band structure from a coarse k-point mesh in the irreducible Brillouin zone, resulting in a much reduced computational effort. The present paper discusses the technical details of the scheme along with the results obtained for the quasiparticle band gap and the electron effective mass.
Physical Review B, 2010
X-ray absorption near edge structure ͑XANES͒ at the cadmium L 3 and oxygen K edges for CdO thin films grown by pulsed laser deposition method, is interpreted within the real-space multiple scattering formalism, FEFF code. The features in the experimental spectra are well reproduced by calculations for a cluster of about six and ten coordination shells around the absorber for L 3 edge of Cd and K edge of O, respectively. The calculated projected electronic density of states is found to be in good agreement with unoccupied electronic states in experimental data and allows to conclude that the orbital character of the lowest energy of the conductive band is Cd 5s-O 2p ء. The charge transfer has been quantified and not purely ionic bonding has been found. Combined XANES and resonant inelastic x-ray scattering measurements allow us to determine the direct and indirect band gap of investigated CdO films to be ϳ2.4 eV and ϳ0.9 eV, respectively.
CdXO3 (X = C, Si, Ge, Sn, Pb) electronic band structures
Chemical Physics Letters, 2009
Electronic properties for a set of CdXO 3 (X = C, Si, Ge, Sn, Pb) crystals were investigated using the density functional theory (DFT) formalism considering both the local density and generalized gradient approximations, LDA and GGA, respectively. Hexagonal CdCO 3 and triclinic CdSiO 3 have indirect main energy band gaps while orthorhombic CdGeO 3 and CdSnO 3 exhibit direct interband transitions. Orthorhombic CdPbO 3 has a very small indirect band gap. The Kohn-Sham minimum electronic band gap oscillates as a function of the X ns level, changing from 2.94 eV (hexagonal CdCO 3 ; LDAÞ to 0.012 eV (orthorhombic CdPbO 3 ; LDA).
CdK-edge extended X-ray absorption fine-structure spectroscopic studies were carried out on Cd1–xCaxO(0x0.9) solid solutions and the first and second nearest neighbour (NN) distances and their mean square relative displacement 2 were estimated. The first NN distance,dCd–O(x), was found to be smaller than its expected value,a(x)/2, obtained from the X-ray diffraction measurements. It increases monotonically and non-linearly with a negative curvature, comparable with that of thea(x) value variation. The variation 2 of the 1NN withxis consistent with a disordered solid solution model. The 2NN distancesdCd–Cd(x) and dCd–Ca(x) are found to follow the average values obtained by X-ray diffraction withdCd–Ca(x)>dCd–Cd(x). From detailed analysis it is argued that the solid solution exhibits a bimodal distribution of the 1NN distances,dCd–O(x)and dCa–O(x), and that the system belongs to a persistent type.
Electronic structure of the rocksalt-structure semiconductors ZnO and CdO
Physical review. B, Condensed matter, 1991
ZnO, which normally occurs in the hexagonal wurtzite structure, can be transformed to the cubic rocksalt (NaC1) structure by the application of high pressure; this cubic phase has been reported to be metastable at atmospheric pressure. The band structure of this phase is calculated by the ab initio correlated Hartree-Fock method. Not surprisingly, the band structure of rocksalt ZnO is very similar to that of CdO, which has the same crystal structure; we present a band-structure calculation for CdO, which we believe is more accurate than any in the literature. A hallmark of these band structures is that the valence-band maximum is not at the center of the Brillouin zone, in contrast to the situation in tetrahedrally coordinated II-VI semiconductors. We confirm by direct calculation that this peculiarity of the band structure is a consequence of the hybridization of oxygen 2p-derived orbitals with Zn 3d or Cd 4d states, combined with octahedral point symmetry.
Physical Review B, 2008
The electronic structure of well-ordered single-crystal thin films of CdO͑100͒ has been studied using angleresolved photoemission spectroscopy. Quantized electron subbands are observed above the valence-band maximum. The existence of these states provides evidence of an intrinsic electron accumulation space-charge layer near the CdO surface, an interpretation supported by coupled Poisson-Schrödinger calculations. The origin of the accumulation layer result is discussed in terms of the bulk band structure of CdO calculated using quasiparticle-corrected density-functional theory, which reveals that the conduction-band minimum at the Brillouin-zone center lies below the charge neutrality level.