Wide Gap Insulators Research Articles

Optimization of the Jastrow factor using the random-phase approximation and a similarity-transformed Hamiltonian: Application to band-structure calculation for some semiconductors and insulators

Based on the random-phase approximation and the transcorrelated (TC) method, we optimize the Jastrow factor together with one-electron orbitals in the Slater determinant in the correlated wave function with a new scheme for periodic systems. The TC method is one of the promising wave function theories for first-principles electronic structure calculation, where the many-body wave function is approximated as a product of a Slater determinant and a Jastrow factor, and the Hamiltonian is similarity-transformed by the Jastrow factor. Using this similarity-transformed Hamiltonian, we can optimize the one-electron orbitals without evaluating 3N-dimensional integrations for the N-electron system. In contrast, optimization of the Jastrow factor within the framework of the TC method is computationally much more expensive and has not been performed for solid-state calculations before. In this study, we also benefit from the similarity-transformation in optimizing the Jastrow factor. Our optimization scheme is tested in applications to some solids from narrow-gap semiconductors to wide-gap insulators, and it is verified that the band gap of a wide-gap insulator and the lattice constants of some solids are improved by this optimization with reasonable computational cost.

Electron Band Alignment at Interfaces of Semiconductors with Insulating Oxides: An Internal Photoemission Study

Evolution of the electron energy band alignment at interfaces between different semiconductors and wide-gap oxide insulators is examined using the internal photoemission spectroscopy, which is based on observations of optically-induced electron (or hole) transitions across the semiconductor/insulator barrier. Interfaces of various semiconductors ranging from the conventional silicon to the high-mobility Ge-based (Ge,Si1-xGex,Ge1-xSnx) andAIIIBVgroup (GaAs,InxGa1-xAs, InAs, GaP, InP, GaSb, InSb) materials were studied revealing several general trends in the evolution of band offsets. It is found that in the oxides of metals with cation radii larger than≈0.7 Å, the oxide valence band top remains nearly at the same energy (±0.2 eV) irrespective of the cation sort. Using this result, it becomes possible to predict the interface band alignment between oxides and semiconductors as well as between dissimilar insulating oxides on the basis of the oxide bandgap width which are also affected by crystallization. By contrast, oxides of light elements, for example, Be, Mg, Al, Si, and Sc exhibit significant shifts of the valence band top. General trends in band lineup variations caused by a change in the composition of semiconductor photoemission material are also revealed.

Single-layer graphene oxide films on a silicon surface

A method is proposed to produce large-area single-layer graphene oxide films on the surface of semiconductor silicon wafers by precipitation from aqueous suspensions. Graphene oxide is synthesized from natural crystalline graphite during chemical oxidation and represents a wide-gap insulator. Single-layer graphene with a homogeneous-fragment size up to 50 μm can be formed by the reduction of graphene oxide films, and this size is significantly larger than those achieved to date.

Bulk compound synthesis and oxygen deficiency effect on electronic and magnetic properties of the Zn-based oxyarsenide LaZnAsO1−δ

• Polycrystalline bulk compound LaZnAsO was synthesized for the first time. • Oxygen deficiency was introduced up to 35 at.% under a high pressure of 6 GPa. • The semiconductivity was highly robust against the oxygen deficiency up to 25 at.%. LaZnAsO has been studied mainly in the thin film form that was indicated as a wide-gap insulator with the gap size of ∼2.3 eV. We herein report the synthesis of polycrystalline bulk LaZnAsO compound for the first time under a high pressure of 6 GPa. Effect of oxygen deficiency on the electric and magnetic properties of the compound was also investigated by synchrotron X-ray diffraction and measurements of electrical resistivity, magnetic susceptibility, isothermal magnetization, and specific heat. The bulk LaZnAsO crystallized into the ZrCuSiAs-type structure with the space group P 4/ nmm , isostructural with the layered LaFeAsO . It is semiconducting with a thermal activation energy of ∼0.8 eV, and the semiconductivity was highly robust against oxygen deficiency up to 25 at.%. The virtue will benefit the development of a diluted magnetic semiconductor.

Spinodal Decomposition in the TiO2–VO2 System

Spinodal decomposition is a ubiquitous phenomenon leading to phase separation from a uniform solution. We show that a spinodal decomposition occurs in a unique combination of two rutile compounds of TiO2 and VO2, which are chemically and physically distinguished from each other: TiO2 is a wide-gap insulator with photo catalytic activities and VO2 is assumed to be a strongly correlated electron system which exhibits a dramatic metal-insulator transition at 342 K. The spinodal decomposition takes place below 830 K at a critical composition of 34 mol% Ti, generates a unidirectional composition modulation along the c axis with a wavelength of approximately 6 nm, and finally results in the formation of self-assembled lamella structures made up of Ti-rich and V-rich layers stacked alternately with 30-50 nm wavelengths. A metal-insulator transition is not observed in quenched solid solutions with intermediate compositions but emerges in the thin V-rich layers as the result of phase separation. Interestingly, the metal-insulator transition remains as sharp as in pure VO2 even in such thin layers and takes place at significantly reduced temperatures of 310-340 K, which is probably due to a large misfit strain induced by lattice matching at the coherent interface.

Local ab initio methods for calculating optical bandgaps in periodic systems. II. Periodic density fitted local configuration interaction singles method for solids

We present a density fitted local configuration interaction singles (CIS) method for calculating optical bandgaps in 3D-periodic systems. We employ an Ewald technique to carry out infinite lattice summations for the exciton-exciton interaction, and robust product-density specific local density fitting in direct space for the electron-hole interaction. Moreover, we propose an alternative to the usual cyclic model with Born-von Karman periodic boundary conditions, the so called Wigner-Seitz supercell truncated infinite model, which exhibits much improved convergence of the CIS excitation energy with respect to the size of the supercell. Test calculations on a series of prototypical systems demonstrate that the method at the present stage can be used to calculate the excitonic bandgaps of 3D periodic systems with up to a dozen atoms in the unit cell, ranging from wide-gap insulators to semiconductors.

Orbital symmetry reconstruction and strong mass renormalization in the two-dimensional electron gas at the surface of KTaO3

We study, using angle-resolved photoemission spectroscopy, the two-dimensional electron gas (2DEG) at the surface of KTaO${}_{3}$ (KTO), a wide-gap insulator with strong spin-orbit coupling (SOC). We find that this 2DEG is a genuinely different physical state with respect to the bulk: the orbital symmetries of its subbands are entirely reconstructed and their masses are renormalized. This occurs because the values of the SOC, the Fermi energy, and the subband splittings become comparable in the 2DEG. Additionally, we identify an $F$-center-like heavy band resulting from the polar nature of the KTO surface.

Effect of phonon spectrum heating on the formation of laser-induced electron plasma in wide-gap insulators

The effect of lattice heating by laser pulses on the dynamics of electron plasma generation in transparent solids has been theoretically studied. Several ways of taking into account the contribution of the phonon spectrum heating to the electron avalanche dynamics, depending on the type of the effective (with respect to the field energy transfer to electrons) phonons and laser pulse duration, have been proposed. A comparative analysis of the results of Monte Carlo computation of electron gas heating in the laser pulse field, which were obtained for cold and heated lattices, has been performed. It is shown that the consideration of the effect of lattice heating on the probabilities of electron-phonon and electron-phonon-photon scattering leads to an increase in the avalanche rate, which is more pronounced at longer wavelengths of the incident radiation and under longer laser pulses. Some qualitative features of the redistribution of the energy, absorbed during a pulse, between the electron plasma and lattice are revealed, which suggest initiation of irreversible microscopic changes in the insulator. In particular, the ratio R of the energy accumulated in the electron subsystem to the excess (with respect to the initial equilibrium state) energy in the phonon subsystem has been calculated for different initial lattice temperatures. It is shown that this ratio increases with a decrease in the laser wavelength in the computation scheme with lattice heating disregarded and decreases at all pulse durations when the lattice heating is taken into account.

Families of Superhard Crystalline Carbon Allotropes Constructed via Cold Compression of Graphite and Nanotubes

We report a general scheme to systematically construct two classes of structural families of superhard sp(3) carbon allotropes of cold-compressed graphite through the topological analysis of odd 5+7 or even 4+8 membered carbon rings stemmed from the stacking of zigzag and armchair chains. Our results show that the previously proposed M, bct-C(4), W and Z allotropes belong to our currently proposed families and that depending on the topological arrangement of the native carbon rings numerous other members are found that can help us understand the structural phase transformation of cold-compressed graphite and carbon nanotubes (CNTs). In particular, we predict the existence of two simple allotropes, R and P carbon, which match well the experimental x-ray diffraction patterns of cold-compressed graphite and CNTs, respectively, display a transparent wide-gap insulator ground state and possess a large Vickers hardness comparable to diamond.

Electronic structure, lattice energies and Born exponents for alkali halides from first principles

First principles calculations based on DFT have been performed on crystals of halides (X = F, Cl, Br and I) of alkali metals (M = Li, Na, K, Rb and Cs). The calculated lattice energies (U0) are in good agreement with the experimental lattice enthalpies. A new exact formalism is proposed to determine the Born exponent (n) for ionic solids. The values of the Born exponent calculated through this ab-initio technique is in good agreement with previous empirically derived results. Band Structure calculations reveal that these compounds are wide-gap insulators that explains their optical transparency. Projected density of states (PDOS) calculations reveal that alkali halides with small cations and large anions, have small band gaps due to charge transfer from X → M. This explains the onset of covalency in ionic solids, which is popularly known as the Fajans Rule.

Inverse design approach to hole doping in ternary oxides: Enhancingp-type conductivity in cobalt oxide spinels

Holes can be readily doped into small-gap semiconductors such as Si or GaAs, but corresponding $p$-type doping in wide-gap insulators, while maintaining transparency, has proven difficult. Here, by utilizing design principles distilled from theory with systematic measurements in the prototype ${A}_{2}B$O${}_{4}$ spinel Co${}_{2}$ZnO${}_{4}$, we formulate and test practical design rules for effective hole doping. Using these, we demonstrate a 20-fold increase in the hole density in Co${}_{2}$ZnO${}_{4}$ due to extrinsic (Mg) doping and, ultimately, a factor of 10${}^{4}$ increase for the inverse spinel Co${}_{2}$NiO${}_{4}$, the $x$ $=$ 1 end point of Ni-doped Co${}_{2}$Zn${}_{1\ensuremath{-}x}$Ni${}_{x}$O${}_{4}$.

Electron States at Interfaces of Semiconductors and Metals with Insulating Films

Evolution of electron energy barriers at interfaces of AIV (Si, Ge), AIVBIV (GexSi1-x, SiC), and AIIIBV (GaAs, InxGa1-xAs, InP, GaSb) semiconductors with wide-gap oxide insulators is overviewed. Over a wide spectrum of studied materials no significant interface dipole contribution to the barrier height is found. Instead, it appears that the energy position of the top of the O2p-derived oxide valence band remains nearly constant, which can be used to evaluate the interface band alignment between oxides and semiconductors as well as between dissimilar oxides materials. By contrast, barriers for electrons at metal/oxide interfaces appear to be strongly affected by the interface processing indicating that near-interface oxide charges in combination with a resulting polarized metal surface may provide significant dipole contribution to the interface barrier height.

Electron and hole trapping in polycrystalline metal oxide materials

Electron and hole trapping by grain boundaries and dislocations in polycrystalline materials is important for wide ranging technological applications such as solar cells, microelectronics, photo-catalysts and rechargeable batteries. In this article, we first give an overview of the computational and methodological challenges involved in modelling such effects. This is followed by a discussion of two recent studies we have made on electron/hole trapping in wide gap insulators. The results suggest that such effects can be important for many applications which we discuss. These computationally demanding calculations have made extensive use of both the HPCx and HECToR services.

Band Alignment at Interfaces of Oxide Insulators with Semiconductors

Experimentally observed trends in energy barriers at interfaces of different semiconductors (Si, Ge, GaAs, InP) with wide-gap oxide insulators are overviewed, revealing general features over a wide spectrum of studied materials. In particular, we found that the energy position of the O2p-derived oxide valence band remains nearly constant in a large number of oxides and, therefore, can be used as a reference level to evaluate the interface band alignment between oxides and semiconductors as well as between dissimilar oxides materials.

Characteristics of SrTiO3 Field-Effect Transistors with DyScO3 Gate Insulators

We have developed a top-gate type of field-effect transistor with a single-crystal SrTiO3 channel and a DyScO3 gate insulator stack consisting of an epitaxial interface layer and an amorphous breakdown barrier layer. We show that the zero-bias conductivity of the transistor channel is strongly affected by the presence of charged traps in the amorphous gate insulator. Low off-state current could only be achieved in devices that were fabricated at an oxygen ambient pressure of 10 mTorr. At lower pressures, metallic channel interfaces were obtained, even after post-annealing in air. When both epitaxial and amorphous DyScO3 films were grown at 10 mTorr of oxygen, the on/off ratio of the field-effect transistors (FETs) reached 106. We argue that when designing oxide FETs, it is necessary to consider not only breakdown characteristics, but also the charged trap density in wide-gap oxide insulators.

Ultrafast relaxation of electrons and selfconsistent electrical charging in insulators

Abstract Low energy electron scattering in insulators is described by a Monte Carlo program based on acoustic and optical phonon scattering as well as on impact ionization of valence band electrons, especially aimed to the scattering of ballistic and drifting electrons in wide gap dielectrics. There is a rapid relaxation of excited electrons within the conduction band of wide gap insulators like SiO 2 and Al 2 O 3 over femtoseconds. The field-dependent transport and trapping parameters allow us to model the selfconsistent charge transport and charging-up of insulating samples during electron irradiation. The resulting spatial and time-dependent distributions of all currents j ( x ,t), charges ρ ( x ,t), the electric field F ( x ,t) and the potential V ( x ,t) are obtained and compared with experimental measurements.

Monte Carlo simulation of low energy electron injection and scattering in insulating layers

Low energy electrons scattered in the conduction band of a dielectric solid should behave like Bloch electrons and will interact with perturbations of the atomic lattice, i.e. with phonons. Optical as well as acoustic phonons are included. Moreover, the inelastic scattering is described by the dielectric energy loss function Im ( − 1 / ϵ ) especially reflecting the excitation of valence band electrons, i.e. secondary electrons (SE). With these collective scattering models we have performed the simulation of electron injection and excited electron relaxation and attenuation in the insulator SiO 2. After secondary electron excitation to a mean initial energy of several eV their energy relaxation occurs within a short time interval of 200 fs to full thermalization. There is a very rapid cooling by impact ionization connected with cascading of electrons at the beginning during the first 20 fs, followed by much slower attenuation due to phonon losses in wide-gap dielectrics and insulators. These attenuation times are connected with SE escape depths, even with and against the direction of an electric field within the nonconductive isolating sample.

Temperature-dependent optical conductivity of undoped cuprates with weak exchange

The optical conductivity $\ensuremath{\sigma}(\ensuremath{\omega})$ is calculated at finite temperature $T$ for ${\text{CuO}}_{2}$ chain clusters within a $pd$-Hubbard model. Data at $T=300\text{ }\text{K}$ for ${\text{Li}}_{2}{\text{CuO}}_{2}$ are reanalyzed within this approach. The relative weights of Zhang-Rice singlet and triplet charge excitations near 2.5 and 4 eV, respectively, depend strongly on $T$, and a rather dramatic dependence of $\ensuremath{\sigma}(\ensuremath{\omega})$ on the ratio of the first to second neighbor exchange integrals is predicted. On the basis of these results, information about exchange interactions for frustrated edge-shared cuprates can be obtained from $T$-dependent optical spectra. Our results are also relevant for magnetically weakly coupled wide-gap insulators in general.

Injection and selfconsistent charge transport in bulk insulators

The new flight-drift model (FDM) of selfconsistent electron transport and electrical charge storage in wide-gap insulators reflects a more realistic simulation of these processes in dielectric and insulating materials than the former mainly ballistic model. Thus, electron-hole creation, their ballistic flight, followed by field-drift transport, and finally trapping in localized states and/or recombination are taken into account. The experimentally accessable quantities of field assisted secondary electron emission σ as well as the resulting surface potential V 0 due to internal current j ( x , t ) , charge ρ ( x , t ) , field F ( x , t ) , and potential V ( x , t ) distributions are obtained. The calculations are performed for bulk Al 2O 3 ceramics with open and metal-coated and grounded surfaces.

Ab initio calculation of charge- and spin-controlled Sr 1−x−yLa x+yTi 1−xCr xO 3

Perovskite SrTiO 3 is a wide-gap band insulator. Because of the stability of its crystal structure, various nonmagnetic and magnetic atoms can be doped into the A-site and the B-site, respectively. A-site substitution, namely La 3+ for Sr 2+, introduces free electrons in the conduction band, while B-site substitution, namely Cr 3+ for Ti 4+, introduces not only localized spins but also free holes in the valence band. Therefore, we can independently introduce x localized spins and y free electrons (or holes) in SrTiO 3 by co-doping La and Cr, by the composition Sr 1− x − y La x + y Ti 1− x Cr x O 3. In this paper, we present an ab initio calculation of the electronic structure of Sr 1− x − y La x + y Ti 1− x Cr x O 3 by the KKR–CPA–LDA method, changing the doping concentrations x and y systematically within 0< x<0.2 and −0.2< y<0.2. We found that the Cr t 2g-states are well localized, and due to the large exchange splitting, only the majority spin states are occupied. However, the energy of the occupied Cr t 2g-states are considerably shallow compared to the photoemission experiment, suggesting the importance of the electron correlation for these localized states. The total magnetic moment is almost proportional to the Cr concentration as expected; however, it considerably changes with La concentration.