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.

The Failure of LDA and GGA to describe Relative Stability, Electronic Structure and Magnetism of MnN and (Ga,Mn)N Alloys

Abstract: Pure MnN and (Ga,Mn)N alloys are investigated using the ab initio generalized gradient approximation $+U$ $(\\text{GGA}+U)$ or the hybrid-exchange density-functional (B3LYP) methods. These methods are found to predict dramatically different electronic structure, magnetic behavior, and relative stabilities compared to previous density-functional theory (DFT) calculations. A unique structural anomaly of MnN, in which local-density calculations fail to predict the experimentally observed distorted rocksalt as the ground-state structure, is resolved under the $\\text{GGA}+U$ and B3LYP formalisms. The magnetic configurations of MnN are studied and the results suggest the magnetic state of zinc-blende MnN might be complex. Epitaxial calculations are used to show that the epitaxial zinc-blende MnN can be stabilized on an InGaN substrate. The structural stability of (Ga,Mn)N alloys was examined and a crossover from the zinc-blende-stable alloy to the rocksalt-stable alloy at an Mn concentration of $\\ensuremath{\\sim}65%$ was found. The tendency for zinc-blende (Ga,Mn)N alloys to phase separate is described by an asymmetric spinodal phase diagram calculated from a mixed-basis cluster expansion. This predicts that precipitates will consist of Mn concentrations of $\\ensuremath{\\sim}5$ and $\\ensuremath{\\sim}50%$ at typical experimental growth temperatures. Thus, pure antiferromagnetic MnN, previously thought to suppress the Curie temperature, will not be formed. The Curie temperature for the $50%$ phase is calculated to be ${T}_{C}=354\\text{ }\\text{K}$, indicating the possibility of high-temperature ferromagnetism in zinc-blende (Ga,Mn)N alloys due to precipitates.

Band structure of the one-dimensional metallic (SN) x crystal

Abstract: A self‐consistent LCAO tight binding calculation of the band structure of the one‐dimensional (SN)x crystal is performed. Convergence of the band structure as a function of the interaction range and the number of translational irreducible representations that are allowed to interact (via the Hartree–Fock elements) as well as the SCF iteration cycle convergence are examined. The crystal is shown to possess a partially occupied valence band in accord with its experimentally established metallic behavior. Bond alternancy is investigated by examining the stability of the total energy with respect to conformational changes. The alternant structure is shown to be more stable than the equal‐bond structure. Analysis of the charge distribution in the system reveals a low ionicity of the S+δN−δ bond (δ=0.18e) and points to the possibitlity of formation of cross bonds between nonbonded S–S pairs. The possibility of the occurrence of a Peierls instability was investigated by searching for a superlattice of model conf...

Crystal structures and metastability of carbon-boron compounds C3B and C5B

Abstract: The recent discovery of the diamondlike C${}_{3}$B and C${}_{5}$B compounds has raised hopes of revealing interesting properties and also elicits questions about the stability of such compounds. Using our implementation of the evolutionary global space-group optimization method, we have found ordered structural models for C${}_{3}$B (layered hexagonal) and C${}_{5}$B (diamondlike) with lower energies than previously obtained and revealing unusual layer-stacking sequences. The compounds are less stable than a mixture of freestanding lowest-energy phases of B, C, and C${}_{4}$B, thus C${}_{3}$B and C${}_{5}$B are not ground-state structures. Nevertheless, disordered diamondlike C${}_{3}$B and C${}_{5}$B can be formed exothermically at high temperature in the reaction [graphitelike C${}_{3}$B] $+$ 2C \ensuremath{\rightarrow} [diamondlike C${}_{5}$B] and [graphitelike C${}_{3}$B] \ensuremath{\rightarrow} [diamondlike C${}_{3}$B]. Thus, the disorder on the C and B sites of diamondlike C${}_{3}$B and C${}_{5}$B is responsible for the observed phases.

Anticrossing and coupling of light-hole and heavy-hole states in (001) GaAs/Al x Ga 1-x As heterostructures

Abstract: Heterostructures sharing a common atom such as AlAs/GaAs/AlAs have a ${D}_{2d}$ point-group symmetry which allows the bulk-forbidden coupling between odd-parity light-hole states (eg, lh1) and even-parity heavy-hole states (eg, hh2) Continuum models, such as the commonly implemented (``standard model'') $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ theory miss the correct ${D}_{2d}$ symmetry and thus produce zero coupling at the zone center We have used the atomistic empirical pseudopotential theory to study the lh1-hh2 coupling in (001) superlattices and quantum wells of $\mathrm{GaAs}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ By varying the Al concentration x of the barrier we scan a range of valence-band barrier heights $\ensuremath{\Delta}{E}_{v}(x)$ We find the following: (i) The lh1 and hh2 states anticross at rather large quantum wells width or superlattice periods $60l{n}_{c}l70$ monolayers (ii) The coupling matrix elements ${V}_{\mathrm{lh}1,hh2}^{{\mathbf{k}}_{\ensuremath{\Vert}}=0}$ are small (002--007 meV) and reach a maximum value at a valence-band barrier height $\ensuremath{\Delta}{E}_{v}\ensuremath{\approx}100 \mathrm{meV},$ which corresponds to an Al composition ${x}_{\mathrm{Al}}=02$ in the barrier (iii) The coupling matrix elements obtained from our atomistic theory are at least an order of magnitude smaller than those calculated by the phenomenological model of Ivchenko et al [Phys Rev B 54, 5852 (1996)] (iv) The dependence of ${V}_{\mathrm{lh}1,hh2}$ on the barrier height $\ensuremath{\Delta}{E}_{v}(x)$ is more complicated than that suggested by the recent model of Cortez et al, [J Vac Sci Technol B 18, 2232 (2000)], in which ${V}_{\mathrm{lh}1,hh2}$ is proportional to the product of $\ensuremath{\Delta}{E}_{v}(x)$ times the amplitudes of the lh1 and hh2 envelopes at the interfaces Thus, atomistic information is needed to establish the actual scaling

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. ...