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.

Thermodynamic instability of ultrathin semiconductor superlattices: The (001) (GaAs)1(AlAs)1 structure.

Abstract: First-principles total-energy pseudopotential and all-electron calculations predict (001) (GaAs)i(A1As)1 and (CdTe)~(HgTe)~ superlattices to be intrinsically unstable towards disproportionation into compounds. This instability is traced to unfavorable charge redistribution in the system. Many disordered' or artificially ordered semiconductor systems are manifestly metastable in temperature and composition ranges in which they are usually characterized and utilized. Such are disordered GaAs Sb~ alloys (grown in the range of thermodynamic immiscibility of GaAs and GaSb), and ordered (A'"B )t (C2 )„alloys (judged by their equilibrium phase diagrams to spinodally decompose). Metastable systems come to exist through kinetic rather than thermodynamic control, e.g. , by nonequilibrium growth techniques. ' They owe their thermal stability' to large reorientation activation barriers, small thermodynamic driving forces, ' and exceedingly low diAusion coefficients at laboratory temperatures. ' Despite extensive study, it is as yet unclear whether artificial semiconductor superlattices (AC) (BC)„are thermodynamically (intrinsically) stable or metastable. Current understanding can be characterized as follows. Disordered (D) isovalent alloys A„Bt-„Care known to have positive enthalpies of mixing hH (I), so that at a sufticiently low temperature T, the negative entropy term — T,BS is overwhelmed by the positive dH, leading eventually to disproportionation. Most contemporary theoretical models analyze this instability of A„Bj C alloys via models that do not distinguish them from ordered compounds A~B„C~+„ofthe same composition. Such are, e.g. , elastic models which attribute dH & 0 to the destabilizing role of microscopic strain associated with a mismatch h, a between lattice constants of AC and BC. Since thin superlattices (AC)~(BC)„are most naturally regarded as ordered compounds — e.g. , an m =n =1 superlattice in the (001) orientation is crystallographically identical to an ABC2 compound with the simple tegragonal p4m2 space groups 6 (having a CuAu-I-like A-B sublattice) — these models would judge both alloys and superlattices (having nearly the same ha) intrinsically unstable at low temperatures. However, Srivastava, Martins, and Zunger demonstrated that hH ) 0 does not require ordered (0) phases to be unstable too because (i) a chemical energy term, neglected by other models, may render AH negative, and (ii) coherently ordered arrangements of bonds can reduce strain imposed by bond-length mismatch better than do disordered arrangements. Perhaps the best-studied superlattice — (GaAs) (AIAs)„— exhibits, however, a delicate energy balance: It has a nearly vanishing h, a =R&1A, — R~,A, =0.0009 A at growth temperatures — 800 K and consequently a nearly vanishing hH (hence, ordering offers but a small reduction in strain), yet Al differs (slightly) from Ga in electronegativity (hence, charge transfer may stabilize the system). This delicacy is highlighted by the disparate views on stability of the (GaAs)1(AIAs) t superlattice. Kuan et al. , having observed ordered GaAlAs2 even in spontaneous growth, characterized it as the thermodynamic equilibrium state of Ga Al~ As, as did Petroff for the layer-by-layer-grown superlattice. On the other hand, Phillips' suggested that this phase was intrinsically unstable but stabilized via pinning by oxygen impurities, and Ourmazd and Bean" suggested that it was stable only because of extrinsic substrate strain eAects. Theoretical estimates for the formation enthalpy of the ordered (0) superlattice similarly range (referring all energies to a primitive cell of four atoms) between stability (AHo = — 1.5 meV, obtained from empirical tight binding i2 or AHO — 20 meV, from a cluster calculation after optimization of bond lengths' ) and instability (AHo=+9. 2 meV in a recent calculation using relativistic pseudopotentials ' ).

The enabling electronic motif for topological insulation in ABO3 perovskites and its structural stability

Abstract: Stable oxide topological insulators (TIs) that could bring together the traditional oxide functionalities with the dissipationless surface states of TIs have been sought for years but none was found. Yet, heavier chalcogenides (selenides, tellurides) were readily found to be TIs. We clarify here the basic contradiction between TI-ness and stability which is maximal for oxides, and trace the basic design principles necessary to identify the window of opportunity of stable TIs. We first identify the electronic motif that can achieve topological band inversion ("topological gene") in ABO3 as being a lone-pair electron-rich B atom (e.g. Te, I, Bi) at the octahedral site. We then illustrate that poorly screened oxide systems with large inversion energies can undergo energy-lowering atomic distortions that remove the band inversion. We identify the coexistence windows of TI functionality and structure stability for different pressures and find that the common cubic ABO3 structures have inversion energies lying outside this coexistence window at zero pressure but could be moved into the coexistence window at moderate pressures. Our study demonstrates the interplay between topological band inversion and structural stability and traces the basic principles needed to design stable oxide topological insulators at ambient pressures.

Structurally unstable A III BiO 3 perovskites are predicted to be topological insulators but their stable structural forms are trivial band insulators

Abstract: Structurally unstable compounds can be predicted to be topological insulators (TIs) but their stable structural forms are trivial band insulators. The authors illustrate the danger in such false positive predictions for the A${}^{I\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}I}$BiO${}_{3}$ bismuth oxides that have been recently proposed as TIs in a hypothetical ``perovskite'' crystal structure-type. They show that the inversion of anti-bonding with bonding electron levels may destabilize the material and argue that to predict new candidate TIs by quantum atomistic modeling, one should simultaneously test structural stability and topological character of the candidate materials.

The Rashba Scale: Emergence of Band Anti-crossing as a Design Principle for Materials with Large Rashba Coefficient

Abstract: Summary The spin-orbit-induced spin splitting of energy bands in low-symmetry compounds (the Rashba effect) has a long-standing relevance to spintronic applications and the fundamental understanding of symmetry breaking in solids, yet the knowledge of what controls its magnitude in different materials is difficult to anticipate. Indeed, rare discoveries of compounds with large Rashba coefficients are invariably greeted as pleasant surprises. We advance the understanding of the “Rashba Scale” using the “inverse design” approach by formulating theoretically the relevant design principle and then identifying compounds satisfying it. We show that the presence of energy band anti-crossing provides a causal design principle of compounds with large Rashba coefficients, leading to the identification via first-principles calculations of 34 rationally designed strong Rashba compounds. Since topological insulators must have band anti-crossing, this establishes an interesting cross-functionality of “topological Rashba insulators” that may provide a platform for the simultaneous control of spin splitting and spin polarization.

Electric field control and optical signature of entanglement in quantum dot molecules

Abstract: The degree of entanglement of an electron with a hole in a vertically coupled self-assembled dot molecule is shown to be tunable by an external electric field Using atomistic pseudopotential calculations followed by a configuration interaction many-body treatment of correlations, we calculate the electronic states, degree of entanglement, and optical absorption We offer a way to spectroscopically detect the magnitude of electric field needed to maximize the entanglement

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. ...