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.

Cluster-doping approach for wide-gap semiconductors: the case of p-type ZnO.

Abstract: First-principles calculations on p-type doping of the paradigm wide-gap ZnO semiconductor reveal that successful doping depends much on engineering a stable local chemical bonding environment. We suggest a cluster-doping approach in which a locally stable chemical environment is realized by using few dopant species. We explain two puzzling experimental observations, i.e., that monodoping N in ZnO via N2 fails to produce p-type behavior, whereas using an NO source produces metastable p-type behavior, which disappears over time.

Cu-Au, Ag-Au, Cu-Ag, and Ni-Au intermetallics: First-principles study of temperature-composition phase diagrams and structures

Abstract: The classic metallurgical systems---noble-metal alloys---that have formed the benchmark for various alloy theories are revisited. First-principles fully relaxed general-potential linearized augmented plane-wave (LAPW) total energies of a few ordered structures are used as input to a mixed-space cluster expansion calculation to study the phase stability, thermodynamic properties, and bond lengths in Cu-Au, Ag-Au, Cu-Ag, and Ni-Au alloys. (i) Our theoretical calculations correctly reproduce the tendencies of Ag-Au and Cu-Au to form compounds and Ni-Au and Cu-Ag to phase separate at $T=0$ K. (ii) Of all possible structures, ${\mathrm{Cu}}_{3}\mathrm{Au}$ ${(L1}_{2})$ and CuAu ${(L1}_{0})$ are found to be the most stable low-temperature phases of ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Au}}_{x}$ with transition temperatures of 530 K and 660 K, respectively, compared to the experimental values 663 K and \ensuremath{\approx}670 K. The significant improvement over previous first-principles studies is attributed to the more accurate treatment of atomic relaxations in the present work. (iii) LAPW formation enthalpies demonstrate that ${L1}_{2}$, the commonly assumed stable phase of ${\mathrm{CuAu}}_{3}$, is not the ground state for Au-rich alloys, but rather that ordered (100) superlattices are stabilized. (iv) We extract the nonconfigurational (e.g., vibrational) entropies of formation and obtain large values for the size-mismatched systems: 0.48 ${k}_{B}$/atom in ${\mathrm{Ni}}_{0.5}{\mathrm{Au}}_{0.5}$ $(T=1100$ K), 0.37 ${k}_{B}$/atom in ${\mathrm{Cu}}_{0.141}{\mathrm{Ag}}_{0.859}$ $(T=1052$ K), and 0.16 ${k}_{B}$/atom in ${\mathrm{Cu}}_{0.5}{\mathrm{Au}}_{0.5}$ $(T=800$ K). (v) Using 8 atom/cell special quasirandom structures we study the bond lengths in disordered Cu-Au and Ni-Au alloys and obtain good qualitative agreement with recent extended x-ray-absorption fine-structure measurements.

Spatial correlations in GaInAsN alloys and their effects on band-gap enhancement and electron localization.

Abstract: In contrast to pseudobinary alloys, the relative number of bonds in quaternary alloys cannot be determined uniquely from the composition. Indeed, we do not know if the ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}{\mathrm{As}}_{0.5}{\mathrm{N}}_{0.5}$ alloy should be thought of as $\mathrm{InAs}+\mathrm{GaN}$ or as $\mathrm{InN}+\mathrm{GaAs}$. We study the distribution of bonds using Monte Carlo simulation and find that the number of In-N and Ga-As bonds increases relative to random alloys. This quaternary-unique short range order affects the band structure: we calculate a blueshift of the band gap and predict the emergence of a broadband tail of localized states around the conduction band minimum.

InP quantum dots: Electronic structure, surface effects, and the redshifted emission

Abstract: We present pseudopotential plane-wave electronic-structure calculations on InP quantum dots in an effort to understand quantum confinement and surface effects and to identify the origin of the long-lived and redshifted luminescence. We find that (i) unlike the case in small GaAs dots, the lowest unoccupied state of InP dots is the ${\ensuremath{\Gamma}}_{1c}$-derived direct state rather than the ${X}_{1c}$-derived indirect state and (ii) unlike the prediction of $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ models, the highest occupied state in InP dots has a $1sd$-type envelope function rather than a (dipole-forbidden) $1pf$ envelope function. Thus explanations (i) and (ii) to the long-lived redshifted emission in terms of an orbitally forbidden character can be excluded. Furthermore, (iii) fully passivated InP dots have no surface states in the gap. However, (iv) removal of the anion-site passivation leads to a P dangling bond (DB) state just above the valence band, which will act as a trap for photogenerated holes. Similarly, (v) removal of the cation-site passivation leads to an In dangling-bond state below the conduction band. While the energy of the In DB state depends only weakly on quantum size, its radiative lifetime increases with quantum size. The calculated $\ensuremath{\sim}300\ensuremath{-}\mathrm{meV}$ redshift and the $\ensuremath{\sim}18$ times longer radiative lifetime relative to the dot-interior transition for the 26-\AA{} dot with an In DB are in good agreement with the observations of full-luminescence experiments for unetched InP dots. Yet, (vi) this type of redshift due to surface defect is inconsistent with that measured in selective excitation for HF-etched InP dots. (vii) The latter type of (``resonant'') redshift is compatible with the calculated screened singlet-triplet splitting in InP dots, suggesting that the slow emitting state seen in selective excitation could be a triplet state.

First-Principles Prediction of Vacancy Order-Disorder and Intercalation Battery Voltages in Li x CoO 2

Abstract: We present a first-principles technique for predicting the ordered vacancy ground states, intercalation voltage profiles, and voltage-temperature phase diagrams of Li intercalation battery electrodes. Application to the Li{sub x}CoO {sub 2} system yields correctly the observed ordered vacancy phases. We further predict the existence of additional ordered phases, their thermodynamic stability ranges, and their intercalation voltages in Li{sub x}CoO {sub 2}/Li battery cells. Our calculations provide insight into the remarkable electronic stability of this system with respect to Li removal: A rehybridization of the Co-O orbitals acts to restore charge to the Co site ({open_quotes}self-regulating response{close_quotes}), thereby minimizing the effect of the perturbation. {copyright} {ital 1998} {ital The American Physical Society}

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. ...