W. Y. Ching

A minimal basis semi-ab initio approach to the band structures of semiconductors

Abstract The band structures of 32 of the most important semiconductor crystals are calculated using an efficient, minimal basis, orthogonalized LCAO method. These include the diamond structure of C, Si, Ge, α-Sn; the zinc blende structure of β-SiC, BN, BP, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, β-ZnS, ZnSe, ZnTe, CdS, CdTe; the wurtzite structure of AlN, GaN, ZnO, α-ZnS, CdS, CdSe; the sodium chloride structure of CdO, GeTe, SnTe and trigonal Se and Te. The calculations, which involve diagonalizations of small size matrix equations yield results having the following characteristics: (1) satisfactory valence bands and lower conduction bands and bulk densities of states; (2) the gap sizes and the locations of valence band maximum and conduction band minimum in agreement with experiment; (3) reasonable values of fractional ionicity and electron and hole effective masses. These are achieved by fine-tuning the exchange parameters in the construction of the potentials. Application of this approach to the study of the electronic structures of disordered and other complex semiconductor systems is also discussed.

Electronic Structure and Electrical Conductivity of Undoped LiFePO4

The electronic structure of LiFePO 4 underpins transport properties important to its use as a lithium storage electrode. Here we have calculated the electronic structure of LiFePO 4 in the ordered olivine structure by a first-principles method to determine (i) the effective mass of carriers and (ii) the nature of the band structure. The electrical conductivity in high purity undoped LiFePO 4 has also been measured experimentally. Spin-polarized calculations show a large electron effective mass and a much smaller but highly anisotropic hole effective mass, suggesting that hole-doped compositions should have the greater electronic conductivity. More surprisingly, the calculations show that this polyanion compound is a half-metal with spin-sensitive band structure, like some other oxides being studied for spintronics applications. This previously unappreciated aspect of the LiFePO 4 electronic structure may play a role in determining transport properties including those relevant to electrochemical applications.

Electronic structure of aluminum nitride: Theory and experiment

Recent efforts to develop electronic.,’ optical, and electro-optical’ -” components and applications based on wide band-gap III-V materials have generated considerable practical interest in the electronic structure of AlN. While a number of theoretical calculations5m10 of the band structure exist, the experimental work”-‘” available for comparison has been limited in either energy range or energy resolution. Extant experimental studies are mostly on thin films due to the difficulty of growing high-purity bulk crystals of 41N. Aside from techuological applications, the electronic structure of AlN is also of fundamental interest. Calculation of the electronic structure of ceramics is a developing area of theoretical physics and the nitrides are of particular importance because the bonding is more covalent than in the oxides. Experimental information about the electronic structure of AlN provides insight as to how theoretical models based on oxides should be adapted to covalent ceramics. Quantitative comparison of theory and experiment affords the best tool for gaining this insight. To this end, we present our results in terms of analytical critical point models of the interband transition strength, J,, , for both a first principles calculation and a vacuum ultraviolet (VW) optical measurement. Our method emphasizes the relationship among critical points grouping them into sets representative of transitions between pairs of bands, while other recent work” emphasizes individual critical points. A single crystal (W201), with a thermal conductivity of 275 W m -’ K-r grown by a modified Bridgman technique,r6 was studied. The oxygen content of the single crystal was 3GO ppm, as previously reported.‘” Polishing with diamond powders suspended in high-purity dry kerosene yields an oxide-free surface for reRectance measurements. Oxidized surfaces give spurious results and were avoided by nonaqueous polishing. The polished face of the single crystal was near normal to the c axis. The VIJV reflectance spectra were obtained with a laser plasma sourced VUV spectrophotometer, described elsewhere.9 Above the band gap at 6.2 eV, t.he spectra show two main features. A sharp peak appears at about 9 eV and a smaller, somewhat brozder peak appears at about l&15 eV. A small feature also appears at about 35 eV. Small features below the band gap are due to vacancies. The single crystal response was found to be representative of commercial polycrystalline substrates also studied.” The spectrum was adjustedd, consistent with an index of jz[ 1.25 eV] = 2.1 A Kramers-Kriinig (KK) analysi?” recovered the phase information, S[h,\r], and permitted calculation of other optical properties,” including the complex dielectric function shown in Fig. 1. The energy band structure of AlN was calculated from first principles, using the orthogonalized linear combination of atomic orbitals (OL.CAOj method with the local density approximation (LDA

Order-disorder transformations in chemisorbed layers: Oxygen on W(110)

We have investigated the order-disorder transformation in oxygen adsorbed on W(110). An analysis of the ordering at T = 0 using the lattice gas formalism shows that there must be significant three-particle interactions to break the particle-hole symmetry. This is necessary since there is a p(2 × 2) phase at three-quarter coverage which is not present at one-quarter coverage. Monte Carlo techniques are used to obtain estimates of the strength of the two and three-particle interactions by matching calculated and measured LEED intensity curves. The qualitative characteristics of the phase diagram are discussed with emphasis on the multicritical points which must be present if the transition at half coverage is second order. Evidence in support of a second order transition is reviewed.

Mechanical properties, electronic structure and bonding of α- and β-tricalcium phosphates with surface characterization

The mechanical properties and electronic structure of alpha- and beta-tricalcium phosphate (TCP) crystals are studied by using two ab initio density functional methods, the Vienna Ab initio Simulation Package (VASP) and the orthogonalized linear combination of atomic orbitals method. Based on the VASP optimized crystal structures, the elastic constants of alpha- and beta-TCP are obtained using an effective stress-strain computational scheme. From the calculated elastic constants, the bulk modulus, shear modulus, Youngs modulus and Poissons ratios are obtained. The results show that the mechanical properties of the two crystals are comparable and that alpha-TCP is somewhat softer than beta-TCP. Comparison with experimental extrapolations of the elastic constants shows significant differences, which attest to the difficulty of obtaining single crystal samples. The calculated electronic structure results show that both crystals are large gap insulators with a direct band gap of 4.89 eV for alpha-TCP and 5.25 eV for beta-TCP. Effective charge calculations show that, on average, beta-TCP has slightly less charge transfer per Ca than alpha-TCP. The (010) ((001)) surface model for alpha-TCP (beta-TCP) is studied using a supercell slab geometry and fully relaxed to obtain the optimized structures. The estimated surface formation energies are 0.777 and 0.842 J m(-2) for alpha-TCP and beta-TCP, respectively. The electronic structures of the two surface models are compared with the bulk models. Charge density analysis shows that the surfaces of both TCP crystals are positively charged overall owing to the presence of Ca ions near the surfaces.

Critical point analysis of the interband transition strength of electrons

Optical and electron-energy-loss spectroscopies are well established methods of probing the electronic structure of materials. Comparison of experimental spectroscopic results with theory is complicated by the fact that the experiments extract information about the interband transition strength of electrons, whereas theoretical calculations provide information about individual valence and conduction bands. Based on the observation that prominent features in the optical response arise from critical points in the joint density of states, critical point modelling was developed to gain an understanding of these spectral features in terms of specific critical points in the band structure. These models were usually applied to derivative spectra and restricted to the consideration of isolated critical points. The authors present a new approach to critical point modelling of the undifferentiated spectra and interpret the model in terms of balanced sets of critical points which describe the interband transition strength arising from individual pairings of valence and conduction bands. This approach is then applied to achieve a direct, quantitative comparison of theoretical and experimental data on aluminium nitride.

Electronic structures of three phases of zirconium oxide

Abstract The band structures and density of states of zirconium dioxide in the cubic, the tetragonal and the monoclinic phases have been calculated using a first-principles self-consistent orthogonalized LCAO method. Band gaps of 3.84, 4.11 and 4.51 eV are obtained respectively for the three phases. The valence bands and the lower conduction bands are all quite similar but become more flat in going from the cubic to the monoclinic phase. The valence bands are composed of two parts, an O 2s band centered at about −16 eV, and a higher mixed O 2p, Zr 4d band of about 4.97–5.90 eV width. The conduction band is mainly derived from the Zr 4d orbitals. Mulliken charge analysis gives effective charges of about 2.0 electrons for Zr and 7.0 electrons for O.

X-ray absorption near-edge structure in alpha-quartz and stishovite: Ab initio calculation with core–hole interaction

Ab initio calculation of the XANSE/ELNES spectra for α quartz and stishovite were carried out using a large-supercell approach that includes the electron–core–hole interaction. Excellent agreements with experimental spectra were obtained for Si–K, Si–L2,3, and O–K edges. The usual interpretation using orbital-resolved local density of states in the conduction band is unsatisfactory.

An effective dipole theory for band lineups in semiconductor heterojunctions

An effective dipole theory is presented to estimate the band lineups at the interface of a lattice‐matched or nearly matched semiconductor heterojunction. The theory is based on the formation of an effective dipole at the interface which causes additional shift ΔEv in the difference of the band edges. A set of equations are derived from which δEv can be solved iteratively. The calculation requires the values of the top of the valence band and several bulk band‐structure parameters of the constituent semiconductors as input. The dipole effect is evaluated by considering the charge transfer induced by the penetration of the effective mass electrons representing the bulk band states into the quantum barrier of the neighboring semiconductor. The theory is applied to predict the band offset values of more than 100 heterojunctions involving group IV, III‐V, and II‐VI semiconductors. Of the 30 heterojunctions for which the experimental data have been reported, the predicted values differ from the data by only ab...

Electronic and optical properties of γ-Al2O3 from ab initio theory

We report on a density functional theory calculation of the electronic structure and optical properties of gamma-Al2O3. We have made a comparison between the optical and electronic properties of th ...