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.

Dilute nonisovalent (II-VI)-(III-V) semiconductor alloys: Monodoping, codoping, and cluster doping in ZnSe-GaAs

Abstract: A dilute nonisovalent semiconductor alloy, made of a III-V semiconductor component (GaAs) mixed with a II-VI semiconductor (ZnSe), can be viewed as the doping of a host semiconductor with a lower (higher) valent cation and a higher (lower) valent anion. We have investigated different doping types, i.e., monodoping, triatomic codoping, and cluster doping, in the ZnSe-GaAs system using ab initio pseudopotential plane-wave calculations. We find the following: (i) The acceptor dopant clusters are stabilized in a chemical potential range different from that of the donor dopant clusters. This explains the experimental observation that a nonisovalent alloy has a distinct carrier polarity. (ii) Cluster doping, e.g., $(\mathrm{Zn}\ensuremath{-}{\mathrm{Se}}_{4}{)}^{3+}$ or $(\mathrm{Se}\ensuremath{-}{\mathrm{Zn}}_{4}{)}^{3\ensuremath{-}}$ in GaAs, is predicted to be stable at extreme chemical potential limits, and also to contribute free carriers. (iii) Triatomic codoping is predicted to be thermodynamically unstable. (iv) Cluster doping produces shallower acceptor/donor levels than monodoping and triatomic codoping. (v) There is a strong attractive interaction between positively charged donors and negatively charged acceptors. Therefore, a high concentration of the charge-neutral dopant pairs exists in the alloy. This finding explains why free carriers in a nonisovalent alloy have a high mobility. (vi) Our results also explain the asymmetric dependence of the band gap on the alloy composition. Specifically, adding a small amount of Ga+As into ZnSe leads to a sharp drop in the band gap of the host crystal, whereas adding Zn+Se into GaAs does not change the band gap very much.

Electronic asymmetry in self-assembled quantum dot molecules made of identical InAs/GaAs quantum dots

Abstract: We show that a diatomic dot molecule made of two identical, vertically stacked, strained $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ self-assembled dots exhibits an asymmetry in its single- and many-particle wave functions. The single particle wave function is asymmetric due to the inhomogeneous strain, while the asymmetry of the many-particle wave functions is caused by the correlation-induced localization: the lowest singlet $^{1}\ensuremath{\Sigma}_{g}$ and triplet $^{3}\ensuremath{\Sigma}$ states show that the two electrons are each localized on different dots within the molecule; for the next singlet states $^{1}\ensuremath{\Sigma}_{u}$ both electrons are localized on the same (bottom) dot for interdot separation $dg8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The singlet-triplet splitting is found to be $\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at interdot separation $d=9\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and as large as $100\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ for $d=4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, orders of magnitude larger than the few meV found in the large $(50--100\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ electrostatically confined dots.

Reinterpreting the Cu–Pd phase diagram based on new ground-state predictions

Abstract: Our notions of the phase stability of compounds rest to a large extent on the experimentally assessed phase diagrams. Long ago, it was assumed that in the Cu–Pd system for xPd≤25% there are at least two phases at high temperature (L12 and a L12-based superstructure), which evolve into a single L12-ordered phase at low temperature. By constructing a first-principles Hamiltonian, we predict a yet undiscovered Cu7Pd ground state at xPd = 12.5% (referred to as S1 below) and an L12-like Cu9Pd3 superstructure at 25% (referred to as S2). We find that in the low-temperature regime, a single L12 phase cannot be stable, even with the addition of anti-sites. Instead we find that an S2-phase with S1-like ordering tendency will form. Previous short-range order diffraction data are quantitatively consistent with these new predictions.

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. ...