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.

Relationships between the band gaps of the zinc-blende and wurtzite modifications of semiconductors

Abstract: While the direct band gaps of wurtzite (W) and zinc-blende (ZB) structures are rather similar, the W and ZB gaps can differ enormously (e.g., \ensuremath{\sim}1 eV in SiC) in indirect gap materials. This large difference is surprising given that the structural difference between wurtzite and zinc blende starts only in the third neighbor and that total energy differences are only \ensuremath{\sim}0.01 eV/atom. We show that zinc-blende compounds can be divided into five types (I--V) in terms of the order of their ${\mathrm{\ensuremath{\Gamma}}}_{1\mathit{c}}$, ${\mathit{X}}_{1\mathit{c}}$, and ${\mathit{L}}_{1\mathit{c}}$ levels and that this decides the character (direct, indirect, pseudodirect) of the wurtzite band gap. The observation of small ${\mathit{E}}_{\mathit{g}}^{\mathrm{W}}$-${\mathit{E}}_{\mathit{g}}^{\mathrm{ZB}}$ differences in direct band-gap systems (``type II,'' e.g., ZnS), and large differences in indirect gap systems (``type IV,'' e.g., SiC) are explained. We further show that while both type-III systems (e.g., AIN) and type-V systems (e.g., GaP) have an indirect gap in the zinc-blende form, their wurtzite form will have direct and pseudodirect band gaps, respectively. Furthermore, a direct-to-pseudodirect transition is predicted to occur in type-I (e.g., GaSb) systems.

Band gaps and spin-orbit splitting of ordered and disordered AlxGa1-xAs and GaAsxSb1-x alloys.

Abstract: Spontaneous long-range ordering of the otherwise disordered isovalent semiconductor alloys A/sub x/B/sub 1-x/C has been recently observed in numerous III-V alloy systems exhibiting the CuAu-I, CuPt, and chalcopyrite structures. We present a theory for the ordering-induced changes in the Brillouin-zone-center electronic properties, with application to the Al/sub x/Ga/sub 1-//sub x/As and GaAs/sub x/Sb/sub 1-//sub x/ alloys. The dominant effect for these systems is shown to be level repulsion between different-symmetry states of the binary constituents which fold into equal-symmetry states in the ordered ternary structures. Strong variations in the band gaps, spin-orbit splittings, and charge densities among the three basic ordered structures reflect the different magnitudes of the symmetry-enforced coupling between the folded states. An extension of the model to the disordered alloys yields good agreement with the observed optical bowing parameters for the fundamental gaps; however, the positive (downward concave) bowing of the spin-orbit splitting observed in some common-cation semiconductor alloy remains an unexplained puzzle.

Local-density-derived semiempirical pseudopotentials

Abstract: Transferable screened atomic pseudopotentials were developed 30 years ago in the context of the empirical pseudopotential method (EPM) by adjusting the potential to reproduce observed bulk electronic energies. While extremely useful, such potentials were not constrained to reproduce wave functions and related quantities, nor was there a systematic way to assure transferability to different crystal structures and coordination numbers. Yet, there is a significant contemporary demand for accurate screened pseudopotentials in the context of electronic structure theory of nanostructures, where local-density-approximation (LDA) approaches are both too costly and insufficiently accurate, while effective-mass band approaches are inapplicable when the structures are too small. We can now improve upon the traditional EPM by a two-step process: {ital First}, we invert a set of self-consistently determined screened LDA potentials for a range of bulk crystal structures and unit cell volumes, thus determining spherically symmetric and structurally averaged atomic potentials (SLDA). These potentials reproduce the LDA band structure to better than 0.1 eV, over a range of crystal structures and cell volumes. {ital Second}, we adjust the SLDA to reproduce {ital observed} excitation energies. We find that the adjustment represents a reasonably small perturbation over the SLDA potential, so that the ensuing fitted potentialmore » still reproduces a {gt}99.9% overlap with the original LDA pseudowave functions despite the excitation energies being distinctly non-LDA. We apply the method to Si and CdSe in a range of crystal structures, finding excellent agreement with the {ital experimentally} {ital determined} band energies, optical spectra {epsilon}{sub 2}({ital E}), static dielectric constants, deformation potentials, and, at the same time, {ital LDA{minus}quality} wave functions.« less

Linear combination of bulk bands method for large-scale electronic structure calculations on strained nanostructures

Abstract: A {open_quotes}strained linear combination of bulk bands{close_quotes} method is introduced for calculating the single-particle electronic states of strained, million-atom nanostructure systems, within an empirical pseudopotential Hamiltonian. This method expands the wave functions of a nanostructure (superlattice, wire, and dot) as linear combinations of bulk Bloch states of the constituent materials, over band indices {ital n} and wave vectors {ital k}. This allows one to use physical intuition in selecting the {ital n} and {ital k} that are most relevant for a given problem. This constitutes a useful approximation over the {open_quotes}direct diagonalization{close_quotes} approach where the basis is complete (individual plane waves) but unintuitive. It also constitutes a dramatic improvement upon the {bold k}{center_dot}{bold p} approach, where the continuum model Hamiltonian is used, losing the atomistic details of the system. For a pyramidal InAs quantum dot embedded in GaAs, we find electronic eigenenergies that are within 20 meV of the exact direct diagonalization calculation, while the speed of the current method is 100{endash}1000 times faster. The sublinear scaling of the current method with the size of the system enables one to calculate the atomistic electronic states of a million-atom system on a personal computer in about 10 h. Sufficient detail ismore » provided in the formalism, so that the method can be promptly implemented. {copyright} {ital 1999} {ital The American Physical Society}« 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. ...