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.

Predictions of new AB O 3 perovskite compounds by combining machine learning and density functional theory

Abstract: We apply machine learning (ML) methods to a database of 390 experimentally reported $AB{\mathrm{O}}_{3}$ compounds to construct two statistical models that predict possible new perovskite materials and possible new cubic perovskites. The first ML model classified the 390 compounds into 254 perovskites and 136 that are not perovskites with a 90% average cross-validation (CV) accuracy; the second ML model further classified the perovskites into 22 known cubic perovskites and 232 known noncubic perovskites with a 94% average CV accuracy. We find that the most effective chemical descriptors affecting our classification include largely geometric constructs such as the $A$ and $B$ Shannon ionic radii, the tolerance and octahedral factors, the $A$-O and $B$-O bond length, and the $A$ and $B$ Villars' Mendeleev numbers. We then construct an additional list of $625AB{\mathrm{O}}_{3}$ compounds assembled from charge conserving combinations of $A$ and $B$ atoms absent from our list of known compounds. Then, using the two ML models constructed on the known compounds, we predict that 235 of the 625 exist in a perovskite structure with a confidence greater than 50% and among them that 20 exist in the cubic structure (albeit, the latter with only $\ensuremath{\sim}50%$ confidence). We find that the new perovskites are most likely to occur when the $A$ and $B$ atoms are a lanthanide or actinide, when the $A$ atom is an alkali, alkali earth, or late transition metal atom, or when the $B$ atom is a $p$-block atom. We also compare the ML findings with the density functional theory calculations and convex hull analyses in the Open Quantum Materials Database (OQMD), which predicts the $T=0$ K ground-state stability of all the $AB{\mathrm{O}}_{3}$ compounds. We find that OQMD predicts 186 of 254 of the perovskites in the experimental database to be thermodynamically stable within 100 meV/atom of the convex hull and predicts 87 of the 235 ML-predicted perovskite compounds to be thermodynamically stable within 100 meV/atom of the convex hull, including 6 of these to be in cubic structures. We suggest these 87 as the most promising candidates for future experimental synthesis of novel perovskites.

Stability of ordered bulk and epitaxial semiconductor alloys.

Abstract: Using first-principles self-consistent total-energy calculations for unconstrained and epitaxially confined models of Si-C and Si-Ge alloys we study the general classes of stability of ordered phases of semiconductor alloys. The unusual ordering observed in SiGe grown on a Si substrate is explained.

Deep electronic gap levels induced by isovalent P and As impurities in GaN

Abstract: The electronic and atomic structure of isovalent substitutional P and As impurities in GaN is studied theoretically using a self-consistent plane-wave pseudopotential method. In contrast with the conventional isovalent III-V systems, $\mathrm{GaN}\mathrm{}:\mathrm{P}$ and $\mathrm{GaN}\mathrm{}:\mathrm{A}\mathrm{s}$ are shown to exhibit deep gap levels. The calculated donor energies are $\ensuremath{\epsilon}(+/0)={\ensuremath{\epsilon}}_{v}+0.22$ and ${\ensuremath{\epsilon}}_{v}+0.41$ eV, respectively, and the double donor energies are $\ensuremath{\epsilon}(++/+)={\ensuremath{\epsilon}}_{v}+0.09$ and ${\ensuremath{\epsilon}}_{v}+0.24$ eV, respectively. The $p$-like gap wave function is found to be strongly localized on the impurity site. Outward atomic relaxations of $\ensuremath{\sim}13%$ and $\ensuremath{\sim}15%$ are calculated for the nearest-neighbor Ga atoms surrounding neutral ${\mathrm{GaN}\mathrm{}:\mathrm{P}}^{0}$ and ${\mathrm{GaN}\mathrm{}:\mathrm{A}\mathrm{s}}^{0},$ respectively. The relaxation increases by $\ensuremath{\sim}1%$ for the positively charged impurities. The impurity-bound exciton binding energy is calculated at ${E}_{b}=0.22$ and ${E}_{b}=0.41$ eV for $\mathrm{GaN}:P$ and $\mathrm{GaN}:As.$ The former is in good agreement with the experimental data ${(E}_{b}=0.232$ eV) whereas the latter is offered as a prediction. No clear Jahn-Teller symmetry lowering ${(T}_{d}\ensuremath{\rightarrow}{C}_{3v})$ distortion, suggested by the one-electron configuration, is found for $\mathrm{GaN}:{\mathrm{P}}^{+}$ and $\mathrm{GaN}:{\mathrm{As}}^{+}.$

Using genetic algorithms to map first-principles results to model Hamiltonians: Application to the generalized Ising model for alloys

Abstract: The cluster expansion method provides a standard framework to map first-principles generated energies for a few selected configurations of a binary alloy onto a finite set of pair and many-body interactions between the alloyed elements. These interactions describe the energetics of all possible configurations of the same alloy, which can hence be readily used to identify ground state structures and, through statistical mechanics solutions, find finite-temperature properties. In practice, the biggest challenge is to identify the types of interactions which are most important for a given alloy out of the many possibilities. We describe a genetic algorithm which automates this task. To avoid a possible trapping in a locally optimal interaction set, we periodically “lock out” persistent near-optimal cluster expansions. In this way, we identify not only the best possible combination of interaction types but also any near-optimal cluster expansions. Our strategy is not restricted to the cluster expansion method alone, and can be applied to select the qualitative parameter types of any other class of complex model Hamiltonians.

Composition dependence of interband transition intensities in GaPN, GaAsN, and GaPAs alloys

Abstract: Using large (512-atom) pseudopotential supercell calculations, we have investigated the composition dependence of the momentum matrix element ${M}_{v,c}$ for transitions between the valence-band maximum and the conduction-band minimum of three semiconductor alloys: ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ and ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x},$ exhibiting large chemical and size differences between their alloyed elements, and ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{As}}_{x},$ which is a weakly perturbed alloy In the composition ranges where these alloys have a direct band gap, we find that (i) in ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{As}}_{x},$ ${M}_{v,c}$ is large (like the virtual-crystal value) and nearly composition independent; (ii) in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x},$ ${M}_{v,c}$ is strongly composition dependent: large for small $x$ and small for large $x;$ and (iii) in ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{N}}_{x},$ ${M}_{v,c}$ is only slightly composition dependent and is significantly reduced relative to the virtual-crystal value The different behavior of ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{As}}_{x},$ ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{N}}_{x},$ and ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ is traced to the existence/absence of impurity levels at the dilute alloy limits: (a) there are no gap-level impurity states at the $x\ensuremath{\rightarrow}1$ or $x\ensuremath{\rightarrow}0$ limits of ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{As}}_{x},$ (b) an isolated As impurity in GaN ($\mathrm{GaN}:\mathrm{A}\mathrm{s}$) has a deep band gap impurity level but no deep impurity state is found for N in GaAs, and (c) $\mathrm{GaN}:\mathrm{P}$ exhibits a P-localized deep band-gap impurity state and $\mathrm{GaP}:\mathrm{N}$ has an N-localized resonant state The existence of deep levels leads to wave-function localization in real space, thus to a spectral spread in momentum space and to a reduction of ${M}_{v,c}$ These impurity levels are facilitated by atomic relaxations, as evident by the fact that unrelaxed $\mathrm{GaN}:\mathrm{A}\mathrm{s}$ and $\mathrm{GaN}:\mathrm{P}$, show no deep levels, have extended wave functions, and have large interband transition elements

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. ...