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.

Surface energetics and ordering in GaInP

Abstract: Although bulk III-V semiconductor alloys generally exhibit phase separation, vapor-phase epitaxial growth of Ga0.5In0.5P on GaAs (001) shows spontaneous ordering into a [111] oriented monolayer superlattice. This ordering is believed to be induced at the surface during growth. We have performed total-energy first-principles pseudopotential calculations for the surface layers of cation- and anion-terminated (001) surfaces of Ga0.5In0.5P on GaAs substrates and valence-forcefield model calculations for deeper layers. These calculations show that the relative stability of the various Ga/In arrangements is a strong function of the layer depth below the surface. At the cation-terminated surface a Ga/In atomic arrangement corresponding to the observed bulk ordering is preferred over other arrangements by 90 meV per surface atom. This preference is intimately tied to electronically driven surface reconstructions. For subsurface Ga/In layers the energy differences are generally small (<30 meV per atom), but at the fourth subsurface layer they play a critical role in controlling the three-dimensional stacking of the ordered (001) layers. Finally, thermodynamic calculations based on a configurational Hamiltonian whose interaction energies are fit to the total energy calculations show that the observed ordering can be explained as a thermodynamically stable surface phase at growth temperatures, which, depending on the atomic mobilities, may remain as a metastable bulk phase as the growth proceeds. On the basis of these results we propose a possible mechanism for the development of the ordered phase.

Dependence of band gaps in d-electron perovskite oxides on magnetism

Abstract: Understanding the controlling principles of band gaps trends in d electron perovskites is needed both for gauging metal-insulator transitions, as well as their application in catalysis and doping. The magnitude of this band gap is rather different for different magnetic spin configurations. We find via electronic structure theory that the factors that connect gapping magnitudes to magnetism depend on the nature of the band edge orbital character (BEOC) and surprisingly scale with the number of antiferromagnetic contacts z$_i$ between neighboring transition metal ions. The dependence is weak when the BEOC are (d,d)-like ("Mott insulators"), whereas this dependence is rather strong in (p,d)-like ("charge transfer" insulators). These unexpected rules are traced to the reduced orbital interactions through the increase in the number of antiferromagnetic contacts between transition metal ions. The impact of magnetic order is not limited to the band gap magnitude and includes also the magnitude of lattice distortions connected to the electronic structure. These results highlight the importance of establishing in electronic structure theory of gap-related phenomena (doping, transport, metal-insulator transitions, conductive interfaces) the appropriate magnetic order.

Enhancement of interactions between magnetic ions in semiconductors due to declustering

Abstract: It is often assumed that the exchange interaction between two magnetic ions in a semiconductor host depends only on the distance and orientation of the magnetic ions. Using first-principles electronic structure calculations of Mn impurities in GaAs, we show that the exchange interaction between two magnetic ions depends also on the concentration and spatial arrangement of the other, “spectator” magnetic ions. Thus, such systems cannot be described by a Heisenberg Hamiltonian with fixed exchange interactions. Specifically, we find that at fixed Mn concentration, association “clustering” of Mn impurities leads to a decrease of the Curie temperature, while dissociation “declustering” leads to higher Curie temperatures. We conclude that clustering is the major impediment to achieve high Curie temperatures in Mn-doped GaAs.

Why is CuInSe2 tolerant to defects and what is the origin of “Ordered Defect Structures”

Abstract: This paper explains both the (1) remarkable electronic passivity of CuInSe2 to its many structural defects, and (2) the occurance of previously noted but unexplained series of structures CuIn5Se8, CuIn3Se5, Cu2In4Se7, etc. in terms of the unusual stability of the charge-compensated defect pair (2VCu−+InCu2+).

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. ...