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.

Prediction of low-Z collinear and noncollinear antiferromagnetic compounds having momentum-dependent spin splitting even without spin-orbit coupling

Abstract: Antiferromagnetic order offers a non-relativistic route to create spin splitting and spin polarization effects that do not rely on heavy element compounds (with strong spin-orbit coupling, SOC), and could exist even in centrosymmetric crystals. Cases enabling such non-relativistic, SOC-unrelated spin polarization effect are classified as SST-4 (SST-4A and SST-4B) of all seven possible spin splitting prototypes derived in this paper based on magnetic symmetry analysis. The authors uncovered 422 magnetic space groups (160 centrosymmetric and 262 non-centrosymmetric) and 201 candidate antiferromagnets that belong to the SST-4 category. DFT calculations for collinear and noncollinear cases are provided as basis for guiding future experiments.

Electronic structure of transition-atom impurities in semiconductors: Substitutional 3d impurities in silicon

Abstract: The electronic structure of neutral substitutional $3d$ transition-metal impurities in an infinite silicon host crystal has been calculated for the first time. The calculation is carried out self-consistently in the local-density-functional formalism to within a high precision. We use nonlocal, first-principles pseudopotentials and the recently developed quasiband crystal-field (QBCF) Green's-function method. The elements of the electronic structure of this system are discussed in detail. The calculation reveals the chemical trends in the defect energies (gap states as well as resonances) for the impurities Zn, Cu, Ni, Co, Fe, Mn, Cr, V, and Ti, as well as the regularities in the density of states, wave functions, charge distributions, and screening potentials. For charged impurities, the model explains the remarkable occurrence of many charge states in the narrow-band-gap region through a new self-regulating mechanism analogous to the homeostasis control in biological systems.

Pseudopotential-based multiband k

Abstract: The electronic structure of quantum wells, wires, and dots is conventionally described by the envelope-function eight-band k\ensuremath{\cdot}p method (the ``standard k\ensuremath{\cdot}p model'') whereby coupling with bands other than the highest valence and lowest conduction bands is neglected. There is now accumulated evidence that coupling with other bands and a correct description of far-from-\ensuremath{\Gamma} bulk states is crucial for quantitative modeling of nanostructure. While multiband generalization of the k\ensuremath{\cdot}p exists for bulk solids, such approaches for nanostructures are rare. Starting with a pseudopotential plane-wave representation, we develop an efficient method for electronic-structure calculations of nanostructures in which (i) multiband coupling is included throughout the Brillouin zone and (ii) the underlying bulk band structure is described correctly even for far-from-\ensuremath{\Gamma} states. A previously neglected interband overlap matrix now appears in the k\ensuremath{\cdot}p formalism, permitting correct intervalley couplings. The method can be applied either using self-consistent potentials taken from ab initio calculations on prototype small systems or from the empirical pseudopotential method. Application to both short- and long-period (GaAs${)}_{\mathit{p}}$/(AlAs${)}_{\mathit{p}}$ superlattices (SL) recovers (i) the bending down (``deconfinement'') of the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}(\ensuremath{\Gamma}) energy level of (001) SL at small periods p; (ii) the type-II--type-I crossover at p\ensuremath{\approxeq}8 SL, and (iii) the even-odd oscillation of the energies of the R\ifmmode\bar\else\textasciimacron\fi{}/X\ifmmode\bar\else\textasciimacron\fi{}(L) state of (001) SL and \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}(L) state of (111) SL. Introducing a few justified approximations, this method can be used to calculate the eigenstates of physical interest for large nanostructures. Application to spherical GaAs quantum dots embedded in an AlAs barrier (with \ensuremath{\sim}250 000 atoms) shows a type-II--type-I crossover for a dot diameter of 70 \AA{}, with an almost zero \ensuremath{\Gamma}-X repulsion at the crossing point. Such a calculation takes less than 30 min on an IBM/6000 workstation model 590. \textcopyright{} 1996 The American Physical Society.

New materials and structures for photovoltaics

Abstract: Despite the fact that over the years crystal chemists have discovered numerous semiconducting substances, and that modern epitaxial growth techniques are able to produce many novel atomic-scale architectures, current electronic and opto-electronic technologies are based but on a handful of ∼10 traditional semiconductor core materials. This paper surveys a number of yet-unexploited classes of semiconductors, pointing to the much-needed research in screening, growing, and characterizing promising members of these classes. In light of the unmanageably large number of a-priori possibilities, we emphasize the role that structural chemistry and modern computer-aided design must play in screening potentially important candidates. The basic classes of materials discussed here include nontraditional alloys, such as non-isovalent and heterostructural semiconductors, materials at reduced dimensionality, including superlattices, zeolite-caged nanostructures and organic semiconductors, spontaneously ordered alloys, interstitial semiconductors, filled tetrahedral structures, ordered vacancy compounds, and compounds based on d and f electron elements. A collaborative effort among material predictor, material grower, and material characterizer holds the promise for a successful identification of new and exciting systems.

Trends in ferromagnetism, hole localization, and acceptor level depth for Mn substitution in

Abstract: We examine the intrinsic mechanism of ferromagnetism in dilute magnetic semiconductors by analyzing the trends in the electronic structure as the host is changed from GaN to GaP, GaAs and GaSb, keeping the transition metal impurity fixed. In contrast with earlier interpretations which depended on the host semiconductor, we found that a single mechanism is sufficient to explain the ferromagnetic stabilization energy for the entire series.

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. ...