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.

P-P and As-As isovalent impurity pairs in GaN: Interaction of deep t 2 levels

Abstract: The electronic and atomic structure of substitutional {ital n}th neighbor (1{le}n{le}6) P-P and As-As impurity pairs in zinc blende GaN is investigated using self-consistent plane-wave pseudopotential and empirical pseudopotential methods. A single impurity introduces a deep t{sub 2} gap level; we show that the interaction between the t{sub 2} defect orbitals of the impurity pairs leads to an interesting pattern of single-particle level splitting, being largest for the first (n=1) and fourth (n=4) neighbor pairs, both exhibiting a C{sub 2v} symmetry. The total energy of the {ital n}th order pair {Delta}E{sup (n)} relative to isolated (n{r_arrow}{infinity}) impurities indicates pairing tendency for n=1 and n=2 ({Delta}E{sup (1,2)}{lt}0) while n=4 pairs are unstable ({Delta}E{sup (4)}{gt}0). We explain this behavior of {Delta}E{sup (n)} vs {ital n} as a consequence of the interplay between two effects: an {open_quotes}elastic contribution{close_quotes} representing the interaction between the elastic strain fields of the two impurities and an {open_quotes}electronic contribution{close_quotes} describing the interaction of the defect orbitals of the two impurity atoms. The binding energies of the impurity-pair bound excitons are calculated for the n=1 As-As and P-P pairs and are found to be significantly larger than for the corresponding isolated impurities. The probabilities for electronic transitions between themore » defect levels and conduction band are calculated. The results predict existence of a rich series of spectroscopic features distinct from single impurities. {copyright} {ital 1999} {ital The American Physical Society}« less

Spontaneous Non-stoichiometry and Ordering in Degenerate but Gapped Transparent Conductors

Abstract: Summary We highlight a class of materials representing an exception to the Daltonian view that compounds maintain integer stoichiometry at low temperatures and use this behavior to select ordered vacancy compounds (OVCs) striking a wanted compromise between carrier concentration, transparency, and phase stability, crucial for transparent conductors (TCs). We show that carriers in the conduction band (CB) of degenerate gapped BaNbO3, Ca6Al7O16, and Ag3Al22O34 compounds can cause a self-regulating instability, whereby cation vacancies form exothermically because a fraction of the CB electrons decays into the hole states formed by such vacancies, and this electron-hole recombination offsets the positive energy associated with vacancy bond breaking. This Fermi level-induced spontaneous non-stoichiometry can lead to the formation of OVCs with different optoelectronic properties and stable in different ranges of chemical potentials. Thus, we demonstrate how a window of opportunity can be determined between opposing tendencies of transparency, conductivity, and stability to design TCs.

Coexistence and coupling of zero-dimensional, two-dimensional, and continuum resonances in nanostructures

Abstract: Quantum dots (QDs) embedded in a matrix exhibit a coexistence of ``zero-dimensional'' (0D) bound electron and hole states on the dot with ``three-dimensional'' (3D) continuum states of the surrounding matrix. In epitaxial QDs one encounters also ``two-dimensional'' (2D) states of a quantum well-like supporting structure (wetting layer). This coexistence of 0D, 2D, and 3D states leads to interesting electronic consequences explored here using multiband atomistic pseudopotential calculations. We distinguish strained dots (InAs in GaAs) and strain-free dots (InAs in GaSb) finding crucial differences: in the former case ``potential wings'' appear in the electron confining potential in the vicinity of the dot. This results in the appearance of localized electronic states that lie above the threshold of the 3D continuum. Such resonances are ``strain-induced localized states'' (SILSs) appearing in strained systems, whereas in strain-free systems the dot resonances in the continuum are the usual ``virtual bound states'' (VBSs). The SILSs were found to occur regardless of the thickness of the wetting layer and even in interdiffused dots, provided that the interdiffusion length is small compared to the QD size. Thus, the SILSs are well isolated from the environment by the protective potential wings, whereas the VBSs are strongly interacting. These features are seen in our calculated intraband as well as interband absorption spectra. Furthermore, we show that the local barrier created around the dot by these potential wings suppresses the 0D-2D (dot-wetting layer) hybridization of the electron states. Consequently, in contrast to findings of simple model calculations of envelope function, 0D-to-2D ``crossed transitions'' (bound hole-to-wetting layer electron) are practically absent because of their spatially indirect character. On the other hand, since no such barrier exists in the hole confining potential, a strong 0D-2D hybridization is present for the hole states. We show this to be the source for the strong 2D-to-0D crossed transitions determined experimentally.

Comparison of two cluster-expansion methods for the energetics of Pd-V alloys

Abstract: The formation energies of substitutional transition-metal alloys are examined by several means. First, two types of direct total-energy calculations are considered, namely, (i) the local-density approximation (LDA), and (ii) a tight-binding (TB) approximation thereof. Second, these directly calculated total energies are used to construct two Ising-like cluster expansions that, if sufficiently accurate, could be used to construct the full statistical mechanics of transition-metal alloys. These are (a) the Connolly-Williams (CW) method, and (b) direct configurational averaging (DCA). Finally, the ability of these two cluster expansions [(a) and (b)] to fit and predict a large number of the underlying directly calculated [(i) and (ii)] total energies is tested, by the average prediction error [chi]. These tests are performed for a large number of Pd-V alloys, and also, to a more limited extent, for the Pd-Rh, Pd-Ti, and Pt-V systems. We find for Pd-V that (i) direct TB calculations show significant overbinding (too-negative formation energies) relative to the LDA, with average error of [chi]=112 meV/atom (a typical formation energy of Pd[sub 0.50]V[sub 0.50] is [similar to][minus]250 meV/atom); (ii) the CW cluster expansion mimics quite well the results of the respective direct calculations, whether LDA ([chi]=19 meV/atom) or TB ([chi]=19 meV/atom); (iii) themore » DCA cluster expansions provides a less accurate depiction of the TB energies on which it is based ([chi]=65 meV/atom); (iv) the prediction errors for the equimolar random alloys are significantly larger using the DCA than using the CW method. In light of (i) above, it appears that the tight-binding model needs to be refined before it can be used systematically for (either DCA or CW) cluster expansions.« less

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. ...