Electronic structure of CdTe probed by Cd and Te M4,5 X-ray emission spectra

We have investigated the bulk electronic structure of CdTe focusing on the Cd 5p and Te 5p valence states by X-ray emission spectroscopy (XES). Despite the very low fluorescence yields the Cd and Te M_4,5 (5p → 3d_3/2,5/2) spectra have been recorded successfully. A good correspondence has been found between the valence band XES and X-ray photoelectron spectra (XPS) by comparison on a common binding energy scale. We also performed a density functional theory calculation of the CdTe valence band, obtaining the Cd 4d, 5s, 5p and Te 5s, 5p local partial densities of states. The experimental Cd 5p and Te 5p derived from the X-ray emission spectra are in good agreement with the calculation. The intensity ratio of the Cd M_4,5 to the Te M_4,5 spectrum is obtained to be 0.25, in agreement with the ratio of the calculated Cd 5p to the Te 5p density of states in the CdTe upper valence band (0.22).     

Electronic structure and X-ray spectra of transition metal sulfides NiS,CuS, and ZnS

A numerical electronic band structure calculations for sulfides NiS, CuS, and ZnS are carried out. Using the results a detailed analysis of a valence states is performed; obtained partial densities of states are compared with X-ray SL _2,3 and $ SK_{\beta _{1,3} } $ SK_{\beta _{1,3} } -emission spectra. We showed that spectrum lineshape depends on hybridization strength between various Me(3d)-orbitals and 3p-states of sulfur. The hybridization strength and the symmetry of hybrid Me(3d)-orbitals are defined by crystal lattice structure. Finally a well splitted in energy bonding and antibonding states Me(3d)-S(3p) appear while weakly hybridized Me(3d)-states mainly contribute to spectra intensity in the energy between them. A good agreement between the theoretical and the experimental spectra of valence band for considered sulfides is obtained.     

Electronic structure and calculated x-ray photoemission spectra of substoichiometric cubic molybdenum nitride

Total and partial densities of states of substoichiometric cubic MoN _x have been calculated for several concentrationsx by means of the KKR-CPA method. There are significant differences to the results of Papaconstantopoulos and Pickett [1] obtained by the LCAO-CPA method. In particular, the concentration dependence of the DOS at the Fermi energy is quite different. Using the single-scatterer final-state approximation, Al–K-XPS spectra have been calculated. It is found that the first peak in the valence band spectra is due mainly to metal-p rather than nonmetal-p states. The vacancy states in the DOS of substoichiometric MoN _x give also rise to an additional peak in the XPS spectra.     

Ab-initio study of electronic and optical properties of InN in wurtzite and cubic phases

We have performed systematic first principle calculations for the electronic and optical properties of a narrow band gap semiconductor InN in cubic and wurtzite phases by ‘state-of-the-art’ DFT calculations within generalized gradient approximation (GGA) and Engel-Vosko's corrected generalized gradient approximation (EVGGA) using full potential linear augmented plane wave (FPLAPW) method as implemented in WIEN2k code. The total energy for the wurtzite phase of InN was found to be smaller by 0.0184 Ry/molecule by cubic phase which confirms the greater stability of the wurtzite structure than the cubic one. Band structure, effective masses, density of states, valence charge densities, and dielectric functions are computed and presented in detail. The critical points are extracted out of calculated dielectric function, compared with available measured data and are explained in terms of transitions occurred in the band structure along different symmetry and antisymmetry lines. The valence band maxima and conduction band minima are strongly dominated by N-2p states and located at the Γ-symmetrical line which predicts its direct band gap nature in both phases.     

Spin-dependent valence-band density of states of 3d ferromagnets

Spin-resolved X-ray photoelectron spectroscopy (SRXPS) and high resolution X-ray photoelectron spectroscopy (HRXPS) studies of the valence bands of ferromagnetic Fe, Co, Co₆₆Fe₄Ni₁B₁₄Si₁₅ and Ni are reported. The SRXPS and HRXPS spectra are compared with theoretical densities of states (DOS) that are corrected for photoelectric cross section variations within the valence band. Agreement between theory and experiment is very good for ferromagnetic Fe and Co₆₆Fe₄Ni₁B₁₄Si₁₅. For Co metal, experiment agrees poorly with theory incorporating a 1.5 eV exchange splitting. Agreement is improved if a reduced Co exchange splitting of 1.2 eV is adopted theoretically. The reduced exchange splitting is attributed to valence electron correlation in Co metal. Ferromagnetic Ni shows poor agreement between experiment and theory. The SRXPS Ni spectra demonstrate that most of the disagreement concerns the ↑-spin channel.     

掺杂GaN/AlN超晶格第一性原理计算研究

第一性原理计算方法在解释实验现象和预测新材料结构及其性质上有着重要作用. 因此, 通过基于密度泛函理论的第一性原理的方法, 本文系统地研究了Mg和Si掺杂闪锌矿和纤锌矿两种晶体结构的GaN/AlN超晶格体系中的能量稳定性以及电学性质. 结果表明: 在势阱层(GaN 层)中, 掺杂原子在体系中的掺杂形成能不随掺杂位置的变化而发生变化, 在势垒层(AlN层)中也是类似的情况, 这表明对于掺杂原子来说, 替代势垒层(或势阱层)中的任意阳离子都是等同的; 然而, 相比势阱层和势垒层的掺杂形成能却有很大的不同, 并且势阱层的掺杂形成能远低于势垒层的掺杂形成能, 即掺杂元素(Mg_Ga, Mg_Al, Si_Ga和Si_Al)在势阱区域的形成能更低, 这表明杂质原子更易掺杂于结构的势阱层中. 此外, 闪锌矿更低的形成能表明: 闪锌矿结构的超晶格体系比纤锌矿结构的超晶格体系更易于实现掺杂; 其中, 闪锌矿结构中, 负的形成能表明: 当Mg原子掺入闪锌矿结构的势阱层中会自发引起缺陷. 由此, 制备以闪锌矿结构超晶格体系为基底的p型半导体超晶格比制备n型半导体超晶格需要的能量更低并且更为容易制备. 对于纤锌矿体系来说, 制备p型和n型半导体的难易程度基本相同. 电子态密度对掺杂体系的稳定性和电学性质进一步分析发现, 掺杂均使得体系的带隙减小, 掺杂前后仍然为第一类半导体. 综上所述, 本文内容为当前实验中关于纤锌矿结构难以实现p型掺杂问题提供了一种新的技术思路, 即可通过调控相结构实现其p型掺杂.     

Valence band densities of states of CdIn2S4 and In2S3 from x-ray photoelectron spectroscopy

X-ray photoelectron spectra of the valence band of CdIn₂S₄ and In₂S₃ single crystals have been measured. The spectrum of CdIn₂S₄ has a strong resemblance to its synthesized spectrum from the In₂S₃ and CdS spectra, which is in good agreement with the theoretical density of states (DOS). The contribution of constituent atoms to the valence band DOS in CdIn₂S₄ is corresponding to those in In₂S₃ and CdS.     

A KKR-CPA study of the electronic states in Ti0.75Al0.25N

The partial densities of states (DOS) and the X-ray emission spectra of the random alloy Ti_0.75Al_0.25N were calculated using the KKR-CPA method. While the Ti and N atoms behave much the same as in TiN, the Al bonding is found to be different from both A1N and metallic Al.     

Specific features of the electronic band structure and x-ray spectra of boron nitride in sphalerite and wurtzite modifications

The electronic band structure of boron nitride compounds with crystal lattices of the sphalerite (c-BN) and wurtzite (w-BN) types is calculated by the local coherent potential method in the cluster muffin-tin approximation within the framework of the multiple scattering theory. The local partial densities of 2p states for boron and nitrogen in c-BN and w-BN modifications are compared with the experimental boron and nitrogen K x-ray emission spectra and band-structure calculations. A comparison of the total densities of states in c-BN and w-BN with the x-ray photoelectron spectra and the band calculations has revealed both similarities and differences in the electronic structures of these modifications. The fine structure in the vicinity of the valence band top of boron nitride in different crystal modifications is theoretically calculated for the first time. The specific features of the electronic structure and the x-ray spectra of boron nitride in different modifications are discussed.     

Electronic structure and optical properties of RbPb2Br5

We report on density functional theory (DFT) calculations of the total and partial densities of states of rubidium dilead pentabromide, RbPb₂Br₅, employing the augmented plane wave+local orbitals (APW+lo) method as incorporated in the WIEN2k package. The calculations indicate that the Pb 6s and Br 4p states are the dominant contributors to the valence band: their main contributions are found to occur at the bottom and at the top of the band, respectively. Our calculations reveal that the bottom of the conduction band is formed predominantly from contributions of the unoccupied Pb 6p states. Data of total DOS derived in the present DFT calculations are found to be in agreement with the experimental X-ray photoelectron valence-band spectrum of this compound. The predominant contributions of the Br 4p states at the top of the valence band of rubidium dilead pentabromide are confirmed by comparison on a common energy scale of the X-ray emission band representing the energy distribution of the valence Br p states and the X-ray photoelectron valence-band spectrum of the RbPb₂Br₅ single crystal. Main optical characteristics of RbPb₂Br₅, such as dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity are explored for RbPb₂Br₅ by the DFT calculations.     

Cd掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法,对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构,态密度以及分态密度的计算和比较,发现Cd_xZn_1-xO价带顶端(VBM)始终由O-2p占据;而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加,决定带隙宽度的CBM的位置下降,同时VBM的位置上升,从而导致了带隙的变窄,出现了红移现象. 此外,Cd掺杂会使晶胞发生膨胀,这种张应变也是导致Cd 关键词： 密度泛函理论 电子结构 Cd掺杂ZnO     

Cd掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法，对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构，态密度以及分态密度的计算和比较，发现Cd_xZn_1-xO价带顶端(VBM)始终由O-2p占据；而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加，决定带隙宽度的CBM的位置下降，同时VBM的位置上升，从而导致了带隙的变窄，出现了红移现象. 此外，Cd掺杂会使晶胞发生膨胀，这种张应变也是导致Cd     

Electronic structure and optical properties of ordered compounds potassium tantalate and potassium niobate and their disordered alloys

The electronic energy band structure, site and angular momentum decomposed density of states (DOS) of cubic perovskite oxides KNbO₃ and KTaO₃ have been obtained from a first principles density functional based full potential linearized augmented plane wave (FLAPW) method within a generalized gradient approximation (GGA). The total DOS in valence region is compared with the experimental photo-emission spectra (PES). The calculated DOS is in good agreement with the experimental energy spectra and the features in the spectra are interpreted by comparison with the projected density of states (PDOS). The valence band PES is mainly composed of Nb-4d/Ta-5d and O 2p states in KNbO₃ and KTaO₃, respectively. Using the PDOS and the band structure we have analyzed the inter-band contribution to the optical properties of these materials. The real and imaginary parts of the dielectric function have been calculated and compared with experimental data. They are found to be in a reasonable agreement. The role of band structure on the optical properties have been discussed.     

The structure of theL 2,3 VV-auger line of silicon and silicon compounds

TheL _2,3 VV Auger transitions of Si, SiO₂, and SiC have been measured and compared with the self-fold electron density of states. The data indicate that Auger matrix effects must be included to explain the structure of the Auger lines. A comparison with soft X-ray measurements of Wiech shows, that the measured Auger line shape is nearly identical with the self-foldK _β emission band. The selection rules for X-ray emission lead then to the conclusion that mostlyp-like valence electrons are involved in the Auger transition. This result indicates the relative importance ofs andp states in Auger transitions which is in accordance with theoretical calculations of Feibelman et al.     

First principle study of nitrogen vacancy in aluminium nitride

In the framework of density functional theory, using the plane-wave pseudopotential method, the nitrogen vacancy ($V_{\rm N})$ in both wurtzite and zinc-blende AlN is studied by the supercell approach. The atom configuration, density of states, and formation energies of various charge states are calculated. Two defect states are introduced by the defect, which are a doubly occupied single state above the valance band maximum (VBM) and a singly occupied triple state below the conduction band minimum (CBM) for wurtzite AlN and above the CBM for zinc-blende AlN. So $V_{\rm N}$ acts as a deep donor in wurtzite AlN and a shallow donor in zinc-blende AlN. A thermodynamic transition level $E({3 + } \mathord{\left/ {\vphantom {{3 + } + }} \right. \kern-\nulldelimiterspace} + )$ with very low formation energy appears at 0.7 and 0.6eV above the VBM in wurtzite and zinc-blende structure respectively, which may have a wide shift to the low energy side if atoms surrounding the defect are not fully relaxed. Several other transition levels appear in the upper part of the bandgap. The number of these levels decreases with the structure relaxation. However, these levels are unimportant to AlN properties because of their high formation energy.     

Chemical bonding and pseudogap formation in D022- and L12-structure (V, Ti)Al3

To obtain a rigorous definition of the chemical bonds in binary transition-metal aluminides, topological analyses were performed for VAl₃ and TiAl₃ in the D0₂₂ and L1₂ structures. The analyses were based on the valence charge densities calculated with the ab initio density functional theory. To better understand the formation mechanism of the pseudogap in these compounds, the band structure, the density of states (DOS) and the band decomposed charge density (BDCD) were calculated. The topological analyses reveal that the interactions between the (V, Ti) and Al atoms are all pure shared-shell interactions, the bonds are covalent and clearly have π-bond character. The study of the band structure, DOS and BDCD shows that the formation of the pseudogap is due to the crystal field energy splitting of the (V, Ti)-3d orbitals combined with the inter-unit-cell orbital interaction.     

The genesis of energy bands formed by sublattice states in alkaline-earth metal oxides and sulfides

The self-consistent electron energy band spectra of crystals and charged sublattices of alkaline-earth metal oxides and sulfides are calculated in the framework of the density functional theory within the pseudopotential approximation in the basis set of localized orbitals. The charge states of sublattices (such as neutral sublattices, empty metal sublattices, and doubly charged anion sublattices) are analyzed with due regard for the electrical neutrality of the crystal. It is demonstrated that the valence bands of the studied crystals are very similar to the valence bands of the doubly charged anion sublattices. The distributions of the valence electron densities of the crystals are virtually identical to those of the anion sublattices. The lower conduction bands of the crystals and doubly charged anion sublattices also almost coincide with each other for MgO and MgS but differ substantially for CaO and CaS. This is associated with the difference between the contributions from the anions and cations to the conduction band of the crystals. It is found that these contributions depend on the relative energy positions of p and d unoccupied states.

     

Electronic structure of bismuth ferrite and hematite single crystals: X-ray photoelectron study and calculation

The X-ray photoelectron spectra of the inner levels and valence bands of the surfaces of BiFeO₃ and Fe₂O₃ single crystals have been measured using X-ray photoelectron spectroscopy on an ESCALAB 250 X-ray photoelectron microprobe with monochromatization of X-ray radiation of the AlK _α line. The ab initio calculations have been performed by the full-potential pseudopotential method in terms of the density functional theory (Quantum-Espresso program package). The band structure and the total and partial densities of states have been calculated for the BiFeO₃ and Fe₂O₃ systems. A comparison of the theoretical and experimental results has demonstrated good agreement between theory and experiment.     

Electronic structure of PbI2 from photoelectron spectra and the exciton problem

The valence band density of states for PbI₂ is determined from X-ray and u.v. induced photoelectron spectra. It is shown that the band derived from Pb 6s states is at 8 eV binding energy and not at the top of the valence bands as suggested by band structure and charge density calculations. A rigid shift in the predominantly iodine 5p derived bands to lower binding energy brings the band structure calculations into essential agreement with experiment. Pb 5d core level binding energies determined here are used to derive core level exciton energies of 0.7 eV from published reflectivity spectra.