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.

Intrinsic defects in ZnO calculated by screened exchange and hybrid density functionals

Abstract: The formation energies of intrinsic defects in ZnO are calculated by a family of screened exchange and hybrid density functionals, which include different fractions of Fock exchange and range separation in the hybrids. All functionals improve on local-density methods and agree remarkably well for formation energies of neutral vacancies but show significant variations for the energy of charge transition levels in the gap. This result highlights that a correct prediction of the band gap by a functional does not guarantee a high accuracy for the defect levels. Hybrid functionals obtain the correct localization of trapped hole states at the Zn vacancy.

Systematization of the stable crystal structure of all AB -type binary compounds: A pseudopotential orbital-radii approach

Abstract: We discuss the role of the classical crossing points of the nonlocal density-functional atomic pseudopotentials in systematizing the crystal structures of all binary $\mathrm{AB}$ compounds (with $A\ensuremath{ e}B$). We show how these pseudopotential radii ${{r}_{l}}$ can be used to "predict" the stable crystal structure of all known (565) binary compounds. We discuss the correlation between ${{r}_{l}}$ and semiclassical scales for bonding in solids.

Assessing capability of semiconductors to split water using ionization potentials and electron affinities only

Abstract: We show in this article that the position of semiconductor band edges relative to the water reduction and oxidation levels can be reliably predicted from the ionization potentials (IP) and electron affinities (AE) only. Using a set of 17 materials, including transition metal compounds, we show that accurate surface dependent IPs and EAs of semiconductors can be computed by combining density functional theory and many-body GW calculations. From the extensive comparison of calculated IPs and EAs with available experimental data, both from photoemission and electrochemical measurements, we show that it is possible to sort candidate materials solely from IPs and EAs thereby eliminating explicit treatment of semiconductor/water interfaces. We find that at pH values corresponding to the point of zero charge there is on average a 0.5 eV shift of IPs and EAs closer to the vacuum due to the dipoles formed at material/water interfaces.

Instilling defect tolerance in new compounds.

Abstract: The properties of semiconducting solids are determined by the imperfections they contain. Established physical phenomena can be converted into practical design principles for optimizing defects and doping in a broad range of technology-enabling materials.

Direct pseudopotential calculation of exciton coulomb and exchange energies in semiconductor quantum dots

Abstract: The effects of electron-hole interaction on the exciton energy of semiconductor quantum dots are calculated using pseudopotential wave functions. A comparison with the widely used, but never tested, effective-mass approximation (EMA) shows that the electron-hole Coulomb energy is significantly ( $\ensuremath{\sim}40%$) overestimated by the EMA, and that the scaling with the dot size $R$ is sublinear in $1/R$. The exchange splitting is much smaller than the Coulomb energy, and in the case of CdSe quantum dots shows significant deviations from the ${1/R}^{3}$ scaling predicted by the EMA.

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. ...