Alex Zunger

Bio: Alex Zunger is an academic researcher from University of Colorado Boulder. The author has contributed to research in topics: Band gap & Quantum dot. The author has an hindex of 128, co-authored 826 publications receiving 78798 citations. Previous affiliations of Alex Zunger include Tel Aviv University & University of Wisconsin-Madison.

Type I to type II transition at the interface between random and ordered domains of AlxGa1−xN alloys

Abstract: We analyze the optical and transport consequences of the existence of ordered and random domains in partially ordered samples of AlxGa1−xN alloys. Using atomistic empirical pseudopotential simulations, we find that the band alignment between random and ordered domains changes from type I to type II at x≃0.4. This leads to an increase by two to three orders of magnitude in the radiative lifetime of the electron–hole recombination. This can explain the experimentally observed mobility-lifetime product behaviors with changing Al concentration. The type I to type II transition results from a competition between the ordering-induced band folding effect and hole confinement on Ga-rich monolayers within the ordered structure.

Fitting of accurate interatomic pair potentials for bulk metallic alloys using unrelaxed LDA energies

Abstract: We present a general and simple method for obtaining accurate, local density approximation (LDA-) quality interatomic potentials for a large class of bulk metallic alloys. The method is based on our analysis of atomic relaxation, which reveals that the energy released in the relaxation process can be approximated by calculating the epitaxially constrained energies of the constituents {ital A} and {ital B}. Therefore, the pair potential is fitted to the LDA-calculated epitaxial energies of the constituents (to capture the relaxation energies), and to the unrelaxed energies of ordered A{sub n}B{sub m} compounds (to capture the fixed-lattice {open_quotes}chemical{close_quotes} energy). The usefulness of our approach is demonstrated by carrying out this procedure for the Cu{sub 1{minus}x}Au{sub x} alloy system. The resulting pair potential reproduces the relaxed LDA formation energies of ordered compounds rather accurately, even though we used only unrelaxed energies as input. We also predict phonon spectra of the elements and ordered compounds in very good agreement with the LDA results. From the calculations for {approx}10000 atom supercells representing the random alloy, we obtain the bond lengths and relaxation energies of the random phase that are not accessible to direct LDA calculations. We predict that, while in Cu-rich alloys the Cu-Cumore » bond is shorter than the Cu-Au bond, at higher Au compositions this order is switched. Furthermore, we find that Au-rich Cu{sub 1{minus}x}Au{sub x} alloys have ground states that correspond to (001) superlattices of {ital n} monolayers of fcc Au stacked on {ital m} monolayers of the L1{sub 0} CuAu-I structure. The potential developed in this work is available at the site http://www.sst.nrel.gov/data/download.html for interested users. {copyright} {ital 1999} {ital The American Physical Society}« less

Effects of configurational, positional and vibrational degrees of freedom on an alloy phase diagram: a Monte Carlo study of Ga1-xInxP

Abstract: A large number of ab initio calculated total energies of different GaP/InP superlattices are used to fit a Born-Oppenheimer energy surface. Monte Carlo simulations are then performed on this surface, including treatment of configurational, positional and vibrational degrees of freedom. This permits isolation of the effects of these degrees of freedom on the thermodynamic behaviour. We find the following. (i) Positional relaxation of the atoms to equilibrium, (off-site) locations lowers enormously both the mixing enthalpy (by approximately 50%) and the miscibility gap (MG) temperature (from TMG=1746 K to TMG=833 K). (ii) Allowance for configurational correlations (absent in a mean-field treatment) reduces both the entropy and the enthalpy, leading to a net increase of approximately 70 K in TMG. (iii) Vibrations reduce TMG by approximately 30 K leading to a final TMG=870 K. The calculated phase diagram is in accord with experiment.

Long- versus short-range order in Ni3V and Pd3V alloys.

Abstract: Measurements of the short-range-order (SRO) diffuse scattered intensity show peaks at the 〈11/20〉 and 〈100〉 points in ${\mathrm{Ni}}_{3}$V and ${\mathrm{Pd}}_{3}$V, respectively, although the stable ground state in both systems (D${0}_{22}$) is a 〈11/20〉-type structure. Mean-field theory predicts 〈11/20〉 SRO in both materials, in contradiction with experiment for ${\mathrm{Pd}}_{3}$V. The 〈100〉-type SRO in ${\mathrm{Pd}}_{3}$V has been explained previously as a non-mean-field effect. Via a combination of first-principles total-energy calculations and Monte Carlo simulated annealing, we show that non-mean-field effects are insufficient to explain the observed SRO of ${\mathrm{Pd}}_{3}$V. However, the inclusion of electronic excitations leads to a temperature dependence in the interaction energies which correctly explains both the SRO and phase stability in ${\mathrm{Pd}}_{3}$V and ${\mathrm{Ni}}_{3}$V.

Bond disproportionation, charge self-regulation and ligand holes in s-p and in d electron ABX3 perovskites by density functional theory

Abstract: Some ABX3 perovskites exhibit different local environments (DLE) for the same B atoms in the lattice, an effect referred to as disproportionation, distinguishing such compounds from perovskites that have single local environments (SLE). The basic phenomenology of disproportionation involves the absence of B-atom charge ordering, the creation of different B-X bond length for different local environments, the appearance of metal (in SLE) to insulator (in DLE) transition, and the formation of ligand holes. We point out that this phenomenology is common to a broad range of chemical bonding patterns in ABX3 compounds, either with s-p electron B-metal cations (BaBiO3, CsTlF3), or noble metal cation (CsAuCl3), as well as d-electron cations (SmNiO3, CaFeO3). We show that underlying much of this phenomenology is the self-regulating response, whereby in strongly bonded metal-ligand systems with high lying ligand orbitals, the system protects itself from creating highly charged cations by transferring ligand electrons to the metal, thus preserving a nearly constant metal charge in DLE, while creating B-ligand bond alternation and ligand-like conduction band (ligand hole). We are asking what are the minimal theory ingredients needed to explain the main features of this SLE-to-DLE phenomenology, such as its energetic driving force, bond length changes, possible modifications in charge density and density of state changes. Using as a guide the lowering of the total energy in DLE relative to SLE, we show that density functional calculations describe this phenomenology across the whole chemical bonding range without resort to special strong correlation effects, beyond what DFT naturally contains. In particular, lower total energy configurations (DLE) naturally develop bond alternation, gaping of the metallic SLE state, and absence of charge ordering with ligand hole formation.

Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium.

Abstract: We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.

Self-interaction correction to density-functional approximations for many-electron systems

Abstract: The exact density functional for the ground-state energy is strictly self-interaction-free (i.e., orbitals demonstrably do not self-interact), but many approximations to it, including the local-spin-density (LSD) approximation for exchange and correlation, are not. We present two related methods for the self-interaction correction (SIC) of any density functional for the energy; correction of the self-consistent one-electron potenial follows naturally from the variational principle. Both methods are sanctioned by the Hohenberg-Kohn theorem. Although the first method introduces an orbital-dependent single-particle potential, the second involves a local potential as in the Kohn-Sham scheme. We apply the first method to LSD and show that it properly conserves the number content of the exchange-correlation hole, while substantially improving the description of its shape. We apply this method to a number of physical problems, where the uncorrected LSD approach produces systematic errors. We find systematic improvements, qualitative as well as quantitative, from this simple correction. Benefits of SIC in atomic calculations include (i) improved values for the total energy and for the separate exchange and correlation pieces of it, (ii) accurate binding energies of negative ions, which are wrongly unstable in LSD, (iii) more accurate electron densities, (iv) orbital eigenvalues that closely approximate physical removal energies, including relaxation, and (v) correct longrange behavior of the potential and density. It appears that SIC can also remedy the LSD underestimate of the band gaps in insulators (as shown by numerical calculations for the rare-gas solids and CuCl), and the LSD overestimate of the cohesive energies of transition metals. The LSD spin splitting in atomic Ni and $s\ensuremath{-}d$ interconfigurational energies of transition elements are almost unchanged by SIC. We also discuss the admissibility of fractional occupation numbers, and present a parametrization of the electron-gas correlation energy at any density, based on the recent results of Ceperley and Alder.

A comprehensive review of zno materials and devices

Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...