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.

Solving Schrödinger’s equation around a desired energy: Application to silicon quantum dots

Abstract: We present a simple, linear‐in‐size method that enables calculation of the eigensolutions of a Schrodinger equation in a desired energy window. We illustrate this method by studying the near‐gap electronic structure of Si quantum dots with size up to Si1315H460(≊37 A in diameter) using a plane wave pseudopotential representation.

Hidden spin polarization in inversion-symmetric bulk crystals

Abstract: Spin polarization due to spin–orbit coupling requires broken inversion symmetry. Now, calculations show that the effect arises from local site-asymmetry rather than global space-group asymmetry, and that a hitherto overlooked form of spin polarization should also exist in centrosymmetric structures.

Comparison of two methods for describing the strain profiles in quantum dots

Abstract: The electronic structure of interfaces between lattice-mismatched semiconductors is sensitive to the strain. We compare two approaches for calculating such inhomogeneous strain—continuum elasticity [(CE), treated as a finite difference problem] and atomistic elasticity. While for small strain the two methods must agree, for the large strains that exist between lattice-mismatched III-V semiconductors (e.g., 7% for InAs/GaAs outside the linearity regime of CE) there are discrepancies. We compare the strain profile obtained by both approaches (including the approximation of the correct C2 symmetry by the C4 symmetry in the CE method) when applied to C2-symmetric InAs pyramidal dots capped by GaAs.

n -type doping of CuIn Se 2 and CuGa Se 2

Abstract: The efficiency of $\mathrm{CuIn}{\mathrm{Se}}_{2}$ based solar cell devices could improve significantly if $\mathrm{CuGa}{\mathrm{Se}}_{2}$, a wider band gap chalcopyrite semiconductor, could be added to the $\mathrm{CuIn}{\mathrm{Se}}_{2}$ absorber layer. This is, however, limited by the difficulty of doping $n$-type $\mathrm{CuGa}{\mathrm{Se}}_{2}$ and, hence, in its alloys with ${\mathrm{CuInSe}}_{2}$. Indeed, wider-gap members of semiconductor series are often more difficult to dope than lower-gap members of the same series. We find that in chalcopyrites, there are three critical values of the Fermi energy ${E}_{F}$ that control $n$-type doping: (i) ${E}_{F}^{n,\mathrm{pin}}$ is the value of ${E}_{F}$ where the energy to form Cu vacancies is zero. At this point, the spontaneously formed vacancies ($=$acceptors) kill all electrons. (ii) ${E}_{F}^{n,\mathrm{comp}}$ is the value of ${E}_{F}$ where the energy to form a Cu vacancy equals the energy to form an $n$-type dopant, e.g., ${\mathrm{Cd}}_{\mathrm{Cu}}$. (iii) ${E}_{F}^{n,\text{site}}$ is the value of ${E}_{F}$ where the formation of Cd-on-In is equal to the formation of Cd-on-Cu. For good $n$-type doping, ${E}_{F}^{n,\mathrm{pin}}$, ${E}_{F}^{n,\mathrm{comp}}$, and ${E}_{F}^{n,\text{site}}$ need to be as high as possible in the gap. We find that these quantities are higher in the gap in $\mathrm{CuIn}{\mathrm{Se}}_{2}$ than in $\mathrm{CuGa}{\mathrm{Se}}_{2}$, so the latter is difficult to dope $n$-type. In this work, we calculate all three critical Fermi energies and study theoretically the best growth condition for $n$-type $\mathrm{CuIn}{\mathrm{Se}}_{2}$ and $\mathrm{CuGa}{\mathrm{Se}}_{2}$ with possible cation and anion doping. We find that the intrinsic defects such as ${\mathrm{V}}_{\mathrm{Cu}}$ and ${\mathrm{In}}_{\mathrm{Cu}}$ or ${\mathrm{Ga}}_{\mathrm{Cu}}$ play significant roles in doping in both chalcopyrites. For group-II cation (Cd, Zn, or Mg) doping, the best $n$-type growth condition is $\mathrm{In}∕\mathrm{Ga}$-rich, and maximal Se-poor, which is also the optimal condition for stabilizing the intrinsic ${\mathrm{In}}_{\mathrm{Cu}}∕{\mathrm{Ga}}_{\mathrm{Cu}}$ donors. Bulk $\mathrm{CuIn}{\mathrm{Se}}_{2}$ can be doped at equilibrium $n$-type, but bulk $\mathrm{CuGa}{\mathrm{Se}}_{2}$ cannot be due to the low formation energy of intrinsic Cu-vacancy. For halogen anion doping, the best $n$-type materials growth is still under $\mathrm{In}∕\mathrm{Ga}$-rich, and maximal Se-poor conditions. These conditions are not best for halogen substitutional defects, but are optimal for intrinsic ${\mathrm{In}}_{\mathrm{Cu}}∕{\mathrm{Ga}}_{\mathrm{Cu}}$ donors. Again, $\mathrm{CuGa}{\mathrm{Se}}_{2}$ cannot be doped $n$-type by halogen doping, while $\mathrm{CuIn}{\mathrm{Se}}_{2}$ can.

Switching a Normal Insulator into a Topological Insulator via Electric Field with Application to Phosphorene

Abstract: The study of topological insulators has generally involved search of materials that have this property as an innate quality, distinct from normal insulators. Here we focus on the possibility of converting a normal insulator into a topological one by application of an external electric field that shifts different bands by different energies and induces a specific band inversion, which leads to a topological state. Phosphorene is a two-dimensional (2D) material that can be isolated through mechanical exfoliation from layered black phosphorus, but unlike graphene and silicene, single-layer phosphorene has a large band gap (1.5-2.2 eV). Thus, it was unsuspected to exhibit band inversion and the ensuing topological insulator behavior. Using first-principles calculations with applied perpendicular electric field F⊥ on few-layer phosphorene we predict a continuous transition from the normal insulator to a topological insulator and eventually to a metal as a function of F⊥. The tuning of topological behavior with electric field would lead to spin-separated, gapless edge states, that is, quantum spin Hall effect. This finding opens the possibility of converting normal insulating materials into topological ones via electric field and making a multifunctional "field effect topological transistor" that could manipulate simultaneously both spin and charge carrier. We use our results to formulate some design principles for looking for other 2D materials that could have such an electrical-induced topological transition.

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. ...