Electronic structure of CdTe probed by Cd and Te M4,5 X-ray emission spectra (original) (raw)
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Local and electronic structure around Ga in CdTe: evidence of DX- and A-centers
Journal of Synchrotron Radiation, 2013
The lattice relaxation around Ga in CdTe is investigated by means of extended X-ray absorption spectroscopy (EXAFS) and density functional theory (DFT) calculations using the linear augmented plane waves plus local orbitals (LAPW + lo) method. In addition to the substitutional position, the calculations are performed for DX-and A-centers of Ga in CdTe. The results of the calculations are in good agreement with the experimental data, as obtained from EXAFS and X-ray absorption near-edge structure (XANES). They allow the experimental identification of several defect structures in CdTe. In particular, direct experimental evidence for the existence of DX-centers in CdTe is provided, and for the first time the local bond lengths of this defect are measured directly.
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.
Journal of Alloys and Compounds, 2009
Electronic properties of cadmium selenogallate CdGa 2 Se 4 , a very promising electro-optical material, were studied both from theoretical and experimental points of view employing the ab initio band-structure augmented plane wave + local orbitals (APW + LO) method with the WIEN2k code and X-ray photoelectron spectroscopy (XPS). The present APW + LO calculations reveal that the Se p-like states are the dominant contributors into the top of the valence band of CdGa 2 Se 4 , while the bottom of the band is dominated by contributions of the Cd d-like states. Additionally, the bottom of the conduction band of the compound under consideration is dominated by contributions of the Ga s-like states. The present calculations render that the valence-band maximum and conduction band minimum in CdGa 2 Se 4 are located at resulting in a direct energy gap. The imaginary part of the electronic dielectric function ε(ω) was also calculated for the compound under consideration based on its band-structure data. The ε xx (ω) and ε zz (ω) components of the imaginary part of the electronic dielectric function reveal a considerable anisotropy in CdGa 2 Se 4 . The XPS valence-band spectrum of CdGa 2 Se 4 has been derived and the binding energies of core-level electrons of the constituting atoms of the compound have been measured.
Journal of Modern Nanotechnology, 2024
Objective: Substantiation of the result of the interaction of thermal neutrons with CdTe crystals by quantum-chemical methods and comparison of the experimental results with the results of quantum-chemical calculations. Methods: Single crystal samples of cadmium telluride (CdTe) were obtained by a modified Bridgman method. The plates cut from the single crystal were subjected to mechanical grinding with abrasive paper of the type P1,200-P4,000, mechanical polishing with silver paste. To clean the surface of the plate from contamination, it was washed in ethyl alcohol. In the experimental part of the work, the influence of low thermal neutron fluxes on the structural parameters of CdTe was studied. The samples were irradiated with a thermal neutron flux in the range from 2.64×10 7 neutron/cm 2 to 1.85×10 9 neutron/cm 2. To study the structure of CdTe crystals, the method of X-ray structural analysis was used, using a DRON-3 X-ray diffractometer. In the second part of the work, computer modeling of the process of interaction of thermal neutrons on CdTe crystals was carried out. Results: X-ray diffraction analysis of the crystals showed that as a result of irradiation with low thermal neutron fluxes, the structural parameters of CdTe improve, as evidenced by the ordering of the crystallites. In addition, in this region of the thermal neutron flux the resistance of the crystals decreases. Conclusion: Calculations of the crystal lattice parameters of CdTe are in good agreement with experimental data. The electronic and optical properties of CdTe have been studied. A decrease in the band gap after irradiation with thermal neutrons has been shown.
Vibrational signatures of O Te and O Te– V Cd in CdTe: A first-principles study
Computational Materials Science, 2010
Chen et al. [Phys. Rev. Lett. 96 (2006) 035508] experimentally observed vibrational signatures related to defects in oxygen-doped CdTe using ultrahigh resolution Fourier transform infrared (FTIR) spectroscopy. They observed an absorption peak at 350 cm À1 . In addition, for samples grown under certain conditions, they observed two higher frequency peaks (1097 and 1108 cm À1 ) at low temperature that merged into one at room-temperature. They attributed the low-frequency peak (350 cm À1 ) to the vibration of O Te and the two higher frequency peaks to the vibrational modes of a O Te -V Cd complex. Subsequently, they reported similar modes around 1100 cm À1 in O-doped CdSe [Phys. Rev. Lett. 101 (2008) 195502] which were attributed to an O Se -V Cd complex. We employed first-principles DFT calculations to calculate the vibrational modes of O Te and O Te -V Cd complex in CdTe. Our calculations show that the 350 cm À1 mode is consistent with O Te . However, the frequencies of the modes around 1100 cm À1 are more than twice the expected frequencies for O Te -V Cd complexes in CdTe (or O Se -V Cd in CdSe), indicating that the O Te -V Cd complex cannot be the cause of the observed 1100 cm À1 modes. A search for a new defect model is in order.
New Journal of Physics, 2015
Using first-principles density functional calculations, we investigate the relative stability and electronic structure of the grain boundaries (GBs) in zinc-blende CdTe. Among the low-Σ-value symmetric tilt Σ3 (111), Σ3 (112), Σ5 (120), and Σ5 (130) GBs, we show that the Σ3 (111) GB is always the most stable due to the absence of dangling bonds and wrong bonds. The Σ5 (120) GBs, however, are shown to be more stable than the Σ3 (112) GBs, even though the former has a higher Σ value, and the latter is often used as a model system to study GB effects in zinc-blende semiconductors. Moreover, we find that although containing wrong bonds, the Σ5 (120) GBs are electrically benign due to the short wrong bond lengths, and thus are not as harmful as the Σ3 (112) GBs also having wrong bonds but with longer bond lengths.
Electronic structure, structural and optical properties of thermally evaporated CdTe thin films
Physica B: Condensed Matter, 2007
Thin films of CdTe were deposited on glass substrates by thermal evaporation. From the XRD measurements it is found that the films are of zinc-blende-type structure. The lattice parameter was determined as a = 6.529 Å, which is larger than 6.48 Å of the powder sample, because the recrystallized lattice of the grown films is subjected to a compressive stress aroused as a result of the lattice mismatch and/or differences in thermal expansion coefficient between the CdTe and the underlying substrate. Transmittance, absorption, extinction, and refractive coefficients are measured. Electronic structure, band parameters and optical spectra of CdTe were calculated from ab initio studies within the LDA and LDA+U approximations. It is shown that LDA underestimates the band gap, energy levels of the Cd-4d states, s-d coupling and band dispersion. However, it calculates the spin-orbit coupling correctly. LDA+U did not increase much the band gap value, but it corrected the s-d coupling by shifting the Cd-4d levels towards the experimentally determined location and by splitting the LDA-derived single s peak into two peaks, which originates from admixture of s and d states. It is shown that the s-d coupling plays an important role in absorption and reflectivity constants. The calculated optical spectra fairly agree with experimental data. Independent of wave-vector scissors operator is found to be a good first approximation to shift rigidly the band gap of CdTe underestimated by LDA.
Study of the photoluminescence spectrum in high purity CdTe
Journal of Luminescence, 1983
In this work, a study of the photoluminescence produced by a high purity sample of n-type CdTe of IN D-NA[ < 1014 impurities per cm 3 was done at several temperatures, varying from 10 to 35 K. Several sharp lines were observed in the spectral region between 1.5 to 1.6 eV, plus the well-known 1.4 eV band with several well-defined structures on it. The observed temperature behaviour of the line positions, linewidths and relative intensities allowed us to establish the presence of a new transition, located at 10 K 21.3 meV below in energy from the free exciton (FE) line, as well as its first phonon replica. Its nature seems to be transitions originating from the cotiduction band to an acceptor level, 32 meV above the valence band. These two lines appear at the same position where previous works had reported the first and second phonon replicas of the FE. A scheme of impurity levels is proposed to explain the observed transitions in terms of previously established levels and this new acceptor level.