Pseudopotential Band Structure Research Articles

Ab initio study of the half-metallic full-Heusler compounds Co2ZAl [Z = Sc, Ti, V, Cr, Mn, Fe]; the role of electronic correlations

We study the structural, electronic, and magnetic properties of Co2ZAl compounds employing a pseudopotential electronic band structure method. The stability of the compounds is established through formation and cohesive energy calculations. The effect of the lattice parameter variation on the electronic and magnetic properties of the compounds is investigated and meticulous explanation is provided for the observed behavior. The variation of the individual spin magnetic moments and the stability of the total spin magnetic moment during the expansion and contraction of the lattice parameter is observed and an attempt is made to understand the obtained behavior. Finally, we implement DFT+U to examine its consequences on the electronic and magnetic properties of the Co2ZAl compounds. We find that the use of DFT+U is not justified for these compounds and in some cases like Co2MnAl it produces unrealistic properties. Exception is Co2FeAl where the desired half-metallicity is restored after the inclusion of on-site correlations. We explain why the on-site correlations might be important for Co2FeAl by comparing it with other Heusler alloys where the correlation was found to be meaningful to explain the observed magnetic moments.

Discontinuous Galerkin deterministic solvers for a Boltzmann–Poisson model of hot electron transport by averaged empirical pseudopotential band structures

The purpose of this work is to incorporate numerically, in a discontinuous Galerkin (DG) solver of a Boltzmann–Poisson model for hot electron transport, an electronic conduction band whose values are obtained by the spherical averaging of the full band structure given by a local empirical pseudopotential method (EPM) around a local minimum of the conduction band for silicon, as a midpoint between a radial band model and an anisotropic full band, in order to provide a more accurate physical description of the electron group velocity and conduction energy band structure in a semiconductor. This gives a better quantitative description of the transport and collision phenomena that fundamentally define the behavior of the Boltzmann–Poisson model for electron transport used in this work. The numerical values of the derivatives of this conduction energy band, needed for the description of the electron group velocity, are obtained by means of a cubic spline interpolation. The EPM-Boltzmann–Poisson transport with this spherically averaged EPM calculated energy surface is numerically simulated and compared to the output of traditional analytic band models such as the parabolic and Kane bands, numerically implemented too, for the case of 1D n+−n−n+ silicon diodes with 400 and 50 nm channels. Quantitative differences are observed in the kinetic moments related to the conduction energy band used, such as mean velocity, average energy, and electric current (momentum), as well as the IV-curves.

Optical absorption and intrinsic recombination in relaxed and strained InAs1–xSbx alloys for mid-wavelength infrared application

The intrinsic carrier recombination lifetime in relaxed and strained InAs1−xSbx alloys is investigated using the full-band Green's function theory. By computing the phonon-perturbed electron self-energy of the system, both direct and phonon-assisted indirect Auger and radiative processes are studied as functions of antimony molar fractions, lattice temperatures and applied in-plane biaxial strains. To improve the overall accuracy of the calculation, an empirical pseudopotential band structure for the alloy is also fitted to the measured band extrema and effective masses under different biaxial strains. A set of effective screened potentials valid for all the needed antimony fractions x and biaxial strains ϵ, therefore, is obtained and applied to the calculation. The results showed reduced total Auger recombination rates and enhanced radiative recombination rates in InAsSb alloys at room temperature when a compressive strain is applied. Furthermore, the study on the widely employed mid-wavelength infrared detector material, InAs0.91Sb0.09, strained by an InAs substrate, demonstrated that much longer minority carrier lifetime can be achieved compared to that in the lattice-matched situation when the lattice temperature is above 200 K.

Structural stability and electronic properties of M2TaN3,ε-TaN and MTa2N3(M = Ti, Zr, Hf) compounds

Using a Plane-Wave Pseudo-Potential (PWPP) method, total energy and band structure calculations for M2TaN3, e-TaN and MTa2N3 (M = a transition metal, TM) compounds have been performed in order to understand their structural stability and electronic properties. To do that, we first focus on e-TaN compounds. The exchange correlation is treated using the Generalized Gradient Approximation (GGA). The Virtual Crystal Approximation (VCA) is then used to examine the structural stability when substituting Ti, Zr or Hf to a Ta atom either in a cell corner [c(M)(Ta2N3)] or inside the cell [in(M2)(TaN3)]. Actually, substitution of Ta by a Ti, Zr or Hf atom at a corner site does slightly change the corresponding lattice constant. Also we calculate ground-state quantities such as elastic constants, shear moduli, Young’s modulus and bulk modulus as well as Poisson’s ratio. The corresponding results for band structures and densities of states are shown as well. As far as we know our work is a pioneer attempt to determine elastic, mechanic and electronic properties for M2TaN3 and MTa2N3 (M = TM) compounds.

ChemInform Abstract: KBiMS4 (M: Si, Ge): Synthesis, Structure, and Electronic Structure.

AbstractThe title compounds are prepared from mixtures of the elements using K2S3 as a reactive flux (873 K, 72 h) and characterized by single crystal XRD, UV/VIS diffuse reflectance spectroscopy, and plane‐wave pseudopotential band structure calculations.

Theoretical studies of positron annihilation in YN and ScN

Valence electron and positron charge densities in YN and ScN are obtained from wave functions derived in a model pseudopotential band structure calculation coupled with the independent particle model (IPM). It is observed that the positron density is maximum in the open interstices and is excluded not only, from the ion cores but also to a considerable degree from the valence bonds. Electron–positron momentum densities are calculated for the (0 0 1) and (1 1 0) planes. The results are used to analyze the positron effect in binary alloy semiconductors.

Carrier‐induced refractive index change in InN

AbstractRapid development of InN technology demands comprehensive assessment of the electronic and optoelectronic potential of this material. In this theoretical work the effect of free electrons on the optical properties of the wurtzite phase of InN is investigated. The blue shift of the optical absorption edge by the free‐carrier band filling is known as the Burstein‐Moss effect for which InN offers to be a very suitable candidate as has been recently demonstrated experimentally. Due to well known Kramers‐Kronig relations, a change in absorption is accompanied by a change in the index of refraction. Considering n‐type InN samples with free electron concentrations ranging from 5×1017 to 5×1020 cm−3, and employing a nonlocal empirical pseudopotential band structure, it is shown that this leads to a few percent change of the index of refraction. These carrier‐induced refractive index changes can be utilized in optical switches, futhermore it needs to be taken into account in the design of InN‐based optical devices such as lasers and optical modulators. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pseudo-potential band structure calculation of InSb ultra-thin films and its application to assess the n-metal-oxide-semiconductor transistor performance

The band structure of InSb thin films with ⟨1 0 0⟩ surface orientation is calculated using the empirical pseudopotential method (EPM) to evaluate the performance of nanoscale devices using a InSb substrate. Contrary to the predictions by simple effective mass approximation methods (EMA), our calculation reveals that the Γ valley is still the lowest lying conduction valley. Based on EPM calculations, we obtained the important electronic structure and transport parameters, such as effective mass and valley energy minimum, of InSb thin film as a function of the film thickness. Our calculations reveal that the ‘effective mass’ of Γ-valley electrons increases with the scaling down of the film thickness. We also provide an assessment of nanoscale InSb thin film devices using a non-equilibrium Green's function under the effective mass framework in the ballistic regime.

Experimental and theoretical studies of adsorption of [formula omitted] on α-Fe 2O 3 (0 0 0 1) surfaces

Experimental and theoretical studies of adsorption of methyl on hematite (0 0 0 1) were carried out using a novel source of the methyl radicals, TPD, AES, QMS, LEED and density functional theory. The crystal surface was not very well characterized experimentally, due to the possible co-existence of several phases and tendency to form magnetite overlayers. Thus observed weak methyl adsorption could be tentatively assigned to defect sites. DFT pseudopotential supercell band structure calculations on clean and adsorbed idealized surfaces predict that the partially oxidized “ferryl structure” has largest adsorption energy, and that regular sites could also adsorb methyl radicals. Embedded-cluster models were used to extract local bonding information and details of electronic structure.

Pseudopotential band structures of rocksalt MgO, ZnO, and Mg1−xZnxO

The electronic properties of the rocksalt group-II oxides MgO and ZnO are investigated by means of the empirical pseudopotential method. Using a simple empty core model potential and experimentally known low-temperature transition energies of rocksalt MgO and wurtzite ZnO, we obtained cationic model potential parameters for Mg and Zn atoms, respectively. Making use of the transferability of ionic model potential parameters, we obtained one single set of anionic model potential parameters for the O atom. The electronic properties of the Mg1−xZnxO alloy system are investigated by means of the virtual crystal approximation for x<0.5 in the rocksalt phase.

Positron annihilation in boron nitride

Electron and positron charge densities are calculated as a function of position in the unit cell for boron nitride. Wave functions are derived from pseudopotential band structure calculations and the independent particle approximation (IPM), respectively, for electrons and positrons. It is observed that the positron density is maximum in the open interstices and is excluded not only from ion cores but also to a considerable degree from valence bonds. Electron-positron momentum densities are calculated for (001,110) planes. The results are used in order to analyse the positron effects in BN.

Monte Carlo simulation of the effect of impact ionization in thin-film electroluminescent devices

By fitting the empirical pseudopotential band structure data using piecewise pol ynomials, an analytical band model of ZnS is presented for thin-film electrolumi nescent devices. The density of states and scattering rates are calculated using the above model. As compared with the results from the full band model, we have shown that our model, which takes less time, has the same precision as that obt ained from the full band model. Using Monte Carlo method, we simulated the field -dependent electron occupation functions of 1st and 2nd bands, electron energy d istribution functions under four-electron fields and the dependence of electron energy on time with or without impact ionization. This shows that the inter-vall ey scattering, inter-band scattering and impact ionization are important for tra nsporting electrons among valleys. Another important result is that the effect o f impact ionization on current multiplication and electron energy distribution i s also discussed.

Nonlinear algorithm for the solution of the Kohn–Sham equations in solids

We apply a nonlinear multigrid algorithm, named the full approximation storage (FAS)scheme, to the Kohn–Sham equations for pseudopotential band structure calculations.Traditionally, the nonlinear self-consistent problem is linearized into successive fixedpotential eigenvalue problems with potentials updated between them. In the new method,the self-consistent problem is solved directly with the FAS scheme. First, the error ofself-consistence in density is calculated; then, an FAS coarse grid problem is defined andsolved; finally, a correction is interpolated to the fine grid to modify the density. Theeigenvalue problem is integrated inside the FAS scheme, and evolves along with theself-consistent problem within the FAS frame. Calculations are demonstrated for Si and Al.

Theoretical assessment of electronic transport in InN

Among the group-III nitrides, InN displays markedly unusual electronic transport characteristics due to its smaller effective mass, high peak velocity and high background electron concentration. First, a non-local empirical pseudopotential band structure of InN is obtained in the light of recent experimental and first-principles results. This is utilized within an ensemble Monte Carlo framework to illuminate the interesting transport properties. It is observed that InN has a peak velocity which is about 75% higher than that of GaN while at higher fields its saturation velocity is lower than that of GaN. Because of the strongly degenerate regime brought about by the high background electron concentration, the electron–electron interaction is also investigated, but its effect on the steady-state and transient velocity–field characteristics is shown to be negligible. Finally, hot phonon generation due to excessive polar optical phonon production in the electron scattering and relaxation processes is accounted for. The main findings are the appreciable reduction in the saturation drift velocity and the slower recovery from the velocity overshoot regime. The time evolution of the hot phonon distribution is analysed in detail and it is observed to be extremely anisotropic, predominantly along the electric force direction.

Scaling and strain dependence of nanoscale strained-Si p-MOSFET performance

Self-consistent fullband Monte Carlo simulations based on nonlocal empirical pseudopotential band structures including spin-orbit splitting are employed to estimate the on-current in nanoscale strained-Si p-MOSFETs. Effective gate lengths from L/sub eff/ = 75 nm down to L/sub eff/ = 25 nm and strain levels corresponding to germanium contents of up to x = 0.4 in the relaxed Si/sub 1-x/Ge/sub x/ substrate are considered. It is found that the on-current continuously increases for growing substrate germanium contents. The strain-induced performance enhancement moderately decreases with scaling, but the improvement at L/sub eff/ = 25 nm still attains 20% for x = 0.4. In contrast to strained-Si n-MOSFETs, increasing the substrate germanium content beyond x = 0.2 is essential for p-MOSFET performance improvement by strain in the sub 0.1 /spl mu/m regime. However, even for x = 0.4 the on-current in a strained-Si p-MOSFET is still smaller than in a corresponding unstrained-Si n-MOSFET.

Pressure dependence of positron annihilation in boron phosphide

We have obtained the valence electron and positron charge densities in boron phosphide from wavefunctions derived in a model pseudopotential band structure calculation. The charge densities are calculated at pressures ranging from 0 to 90 GPa. We find that the positron density is maximum in the open interstices and is excluded not only from the ion cores, but also to a considerable degree from the valence bonds. Electron momentum densities are calculated in the (001–110) plane. The observation that the total positron annihilation rate increases with pressure has been explained in terms of increased positron penetration into the ion cores.

POSITRON ANNIHILATION IN SiC

Valence electron and positron charge densities in SiC are obtained from wave functions derived in a model pseudopotential bandstructure calculation. It is observed that the positron density is maximum in the open interstices and is excluded not only from the ion cores but also, to a considerable degree, from the valence bonds. Electron–positron momentum densities are calculated for the (001–110) plane. The results are used to analyze the positron effect in large gap semiconductors.

Electronic structure of the layer compounds GaSe and InSe in a tight-binding approach

The three-dimensional band structure of the III-VI layer compounds GaSe and InSe has been investigated in the tight-binding approach. The pseudo-Hamiltonian matrix elements in the ${\mathrm{sp}}^{3}{s}^{*}$ basis are fit in order to reproduce the nonlocal pseudopotential band structure, in the framework of constrained optimization techniques using the conjugate gradient method. The results are in good agreement with the optical and photoemission experimental data. The scaling laws appropriate to the covalent bonding are violated by a fraction of eV only, which suggests that the interlayer interactions are not solely of the van der Waals type.

Segregation effects on the optical properties of (InAs)/(GaSb) superlattices

We combine a kinetic model of MBE growth with the empirical pseudopotential band structure method to study the effects of interfacial disorder and segregation on the optical properties of InAs/GaSb superlattices. We fit the layer-by-layer growth model to the observed STM segregated profiles, extracting surface-to-subsurface atomic exchange energies. These are then used to obtain a detailed simulated model of segregated InAs/GaSb superlattices with atomic resolution. The application of the pseudopotential calculations to such structures reveals remarkable electronic consequences of segregation, including a blue shift of band gap with increasing sample growth temperature.

Optical Properties of High Pressure Phases in ZnTe 1− x Se x

We report on the optical properties of high pressure semiconducting phases in ZnTe 1 m x Se x . In the Te rich side, the cinnabar phase is observed in the upstroke between typically 9.5 and 12.5 GPa with a pressure interval of existence that decreases with increasing the Se content. In most studied samples, the indirect absorption edge could be determined, with values of the bandgap increasing with the Se content and ranging from 1.2 to 1.7 eV. In the downstroke, the cinnabar phase is observed in the whole composition range but its bandgap can not be unambiguously determined in the Se-rich side, as it coexists with rocksalt or zincblende phases. The indirect semiconducting rocksalt phase is observed in the Se-rich side, with an indirect bandgap of the order of 0.7 eV. Within the experimental errors, the bandgaps of both the cinnabar and NaCl phases are pressure insensitive, in agreement with first-principles pseudopotential band structure calculations, that predict very low pressure coefficients for both indirect transitions.