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.

Method of linear combination of structural motifs for surface and step energy calculations: Application to GaAs(001)

Abstract: First-principles calculations of the atomic structure and formation energies of semiconductor surfaces and surface steps are often complicated by the existence of complex structural patterns. We suggest here a simpler, algebraic (not differential) approach that is based on two observations distilled from previous first-principles calculations. First, a relatively large collection of equilibrium structures of surfaces and bulk point defects can be built from a limited number of recurring local ``structural motifs,'' including for GaAs tetrahedrally bonded Ga and As and miscoordinated atoms such as threefold-coordinated pyramidal As. Second, the structure is such that band-gap levels are emptied, resulting in charged miscoordinated atoms. These charges compensate each other. We thus express the total energy of a given surface as a sum of the energies of the motifs, and an electrostatic term representing the Madelung energy of point charges. The motif energies are derived by fitting them to a set of pseudopotential total-energy calculations for flat GaAs(001) surfaces and for point defects in bulk GaAs. This set of parameters is shown to suffice to reproduce the energies of other (001) surfaces, calculated using the same pseudopotential approach. Application of the ``linear combination of structural motif'' (LCSM) method to flat GaAs(001) surfaces reveals the following: (i) The observed h(2\ifmmode\times\else\texttimes\fi{}3) surface may be a disordered c(8\ifmmode\times\else\texttimes\fi{}6) surface. (ii) The observed (2\ifmmode\times\else\texttimes\fi{}6) surface is a metastable surface, only 0.03 eV/(1\ifmmode\times\else\texttimes\fi{}1) higher than the \ensuremath{\alpha}(2\ifmmode\times\else\texttimes\fi{}4) surface having the same surface coverage. (iii) We confirm the recent suggestion by Hashizume et al. that the observed \ensuremath{\gamma}(2\ifmmode\times\else\texttimes\fi{}4) phase of the (2\ifmmode\times\else\texttimes\fi{}4) surface is a mixture of the \ensuremath{\beta}2(2\ifmmode\times\else\texttimes\fi{}4) and c(4\ifmmode\times\else\texttimes\fi{}4) surfaces. In particular, we examined an 8\ifmmode\times\else\texttimes\fi{}7 surface structure which has a lower energy than the earlier proposed \ensuremath{\gamma}(2\ifmmode\times\else\texttimes\fi{}4) structure. Application of the LCSM method to prototype steps on the GaAs(001)-(2\ifmmode\times\else\texttimes\fi{}4) surface is illustrated, comparing the LCSM results directly to pseudopotential results. \textcopyright{} 1996 The American Physical Society.

Long-range order in binary late-transition-metal alloys

Abstract: A ground-state search of a generalized, many-body Ising Hamiltonian whose interaction energies are determined from first-principles local-density calculations reveals that PtX intermetallics for X=Ni, Cu, Rh, and Pd will form stable ordered structures at low temperatures. In contrast, d-band tight-binding models universally predict phase separation in all late-transition-metal alloys. It is shown that the previously neglected s-electron cohesion is responsible for this phase stability.

s-d coupling in zinc-blende semiconductors

Abstract: Most zinc blende semiconductors have a single anion-like s state near the bottom of the valence band, found in density-of-states (DOS) calculations, and seen in photoemission. Here, we discuss the case where two s-like peaks appear, due to strong $s\ensuremath{-}d$ coupling. Indeed, away from the $\mathbf{k}=0$ Brillouin zone center, cation d states and anion s states can couple in zinc blende symmetry. Depending on the energy difference $\ensuremath{\Delta}{E}_{\mathrm{sd}}{=E}_{s}^{\mathrm{anion}}\ensuremath{-}{E}_{d}^{\mathrm{cation}},$ this interaction can lead to either a single or two s-like peaks in the DOS and photoemission. We find four types of behaviors. (i) In GaP, GaAs, InP, and InAs, $\ensuremath{\Delta}{E}_{\mathrm{sd}}$ is large, giving rise to a single cation d peak well below the single anion s peak. (ii) Similarly, in CdS, CdSe, ZnS, ZnSe, and ZnTe, we see also a single s peak, but now the cation d is above the anion s. In both (i) and (ii) the $s\ensuremath{-}d$ coupling is very weak. (iii) In GaN and InN, the local density approximation (LDA) predicts two s-like peaks bracketing below and above the cation d-like state. Correcting the too low binding energies of LDA by LDA+SIC (self-interaction correction) still leaves the two s-like peaks. The occurrence of two s-like peaks represents the fingerprint of strong $s\ensuremath{-}d$ coupling. (iv) In CdTe, LDA predicts a single s-like peak just as in case (ii) above. However, LDA+SIC correction shifts down the cation d state closer to the anion s band, enhancing the $s\ensuremath{-}d$ coupling, and leading to the appearance of two s-like peaks. Case (iv) is a remarkable situation where LDA errors cause not only quantitative energetic errors, but actually leads to a qualitative effect of a DOS peak that exists in LDA+SIC but is missing in LDA. We predict that the double-$s$ peak should be observed in photoemission for GaN, InN, and CdTe.

Split Dirac cones in HgTe/CdTe quantum wells due to symmetry-enforced level anticrossing at interfaces

Abstract: HgTe is a band-inverted compound which forms a two-dimensional topological insulator if sandwiched between CdTe barriers for a HgTe layer thickness above the critical value. We describe the fine structure of Dirac states in the HgTe/CdTe quantum wells of critical and close-to-critical thicknesses and show that the necessary creation of interfaces brings in another important physical effect: the opening of a significant anticrossing gap between the tips of the Dirac cones. The level repulsion driven by the natural interface inversion asymmetry of zinc-blende heterostructures considerably modifies the electron states and dispersion but preserves the topological transition at the critical thickness. By combining symmetry analysis, atomistic calculations, and extended $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ theory with interface terms, we obtain a quantitative description of the energy spectrum and extract the interface mixing coefficient. We discuss how the fingerprints of the predicted zero-magnetic-field splitting of the Dirac cones could be detected experimentally by studying magnetotransport phenomena, cyclotron resonance, Raman scattering, and THz radiation absorption.

Zinc-blende half-metallic ferromagnets are rarely stabilized by coherent epitaxy

Abstract: The need for spin-injectors having the same zinc-blende-type crystal structure as conventional semiconductor substrates has created significant interests in theoretical predictions of possible metastable ``half-metallic'' zinc-blende ferromagnets, which are normally more stable in other structure-types, e.g., NiAs. Such predictions were based in the past on differences ${\ensuremath{\Delta}}_{\text{bulk}}$ in the total energies of the respective bulk crystal forms (zinc blende and NiAs). We show here that the appropriate criterion is comparing difference ${\ensuremath{\Delta}}_{\mathrm{epi}}({a}_{s})$ in epitaxial total energies. This reveals that even if ${\ensuremath{\Delta}}_{\text{bulk}}$ is small, still for MnAs, CrSb, CrAs, CrTe, ${\ensuremath{\Delta}}_{\mathrm{epi}}({a}_{s})g0$ for all substrate lattice constant ${a}_{s}$, so the zinc-blende phase is not stabilized. For CrS we find ${\ensuremath{\Delta}}_{\mathrm{epi}}({a}_{s})l0$, but the system is antiferromagnetic, thus not half-metallic. Finally, zinc-blende CrSe is predicted to be epitaxially stable for ${a}_{s}g6.2\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and is half metallic.

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. ...