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.

Defects in photovoltaic materials and the origin of failure to dope them

Abstract: I will review the basic physical principles underlying the formation energy of various intrinsic defects in common photovoltaic materials. I then use the above principles to explain why doping of semiconductors is in general, limited and which design principles can be used to circumvent such limits. This work can help design strategies of doping absorber materials as well as explain how TCOs work. Recent results on the surprising stability of polar [112] + (1~1~2~) surfaces of CIS will also be described in this context.

Reversal in the order of impurity binding energies with atomic energies

Abstract: Whereas atomistic models predict that binding energies of donor levels in semiconductors increase with the ionization potential of the free impurity atoms, we find that a special enhancement of the screening in the solid predicts, for chalcogen impurities in silicon, a reversal in this order

Theory of Surface Dimerization-induced Ordering in GaInP Alloys

Abstract: We identify via thermodynamic energy minimization the role of subsurface strain (caused by surface reconstruction and dimerization) in the ordering of Ga0.51n0.5P alloys. Depending on the growth conditions, the alloy surfaces can have either β2(2×4), c(4×4) or c(8×6) reconstructions, with characteristic 2×1, 1×2 and 2×3 RHEED patterns. We show that (i) the 1×2 reconstruction will lead to a CUPtA surface ordering, (ii) a 2×1 reconstruction will lead to a CuPtB ordering, (iii) a 2×3 reconstruction will lead to a 3-period ordering, and (iv) single (double) bilayer steps are stable at low (high) anion chemical potential. These results are in good agreement with recent experimental observations.

Atomistic Pseudopotential Theory of Droplet Epitaxial GaAs/AlGaAs Quantum Dots

Abstract: In this chapter, following the introduction to the basic electronic properties of semiconductor quantum dots (QDs), we first briefly introduce our atomistic methodology for multi-million atom nanostructures, which is based on the empirical pseudopotential method for the solution of the single-particle problem combined with the configuration interaction (CI) scheme for the many-body problem which were developed in the solid-state theory group at the National Renewable Energy Laboratory over the past two decades. This methodology, described in Sect. 14.2, can be used to provide quantitative predictions of the electronic and optical properties of colloidal nanostructures containing thousands of atoms as well as epitaxial nanostructures containing several millions of atoms. In Sect. 14.3, we show how the multi-exciton spectra of a droplet epitaxy QD encodes nontrivial structural information that can be uncovered by atomistic many-body pseudopotential calculations. In Sect. 14.4, we investigate the vertical electric field tuning of the fine-structure splitting (FSS) in several InGaAs and GaAs QDs using our atomistic methodology. We reveal the influence of the atomic-scale structure on the exciton FSS in QDs. Finally, a comprehensive and quantitative analysis of the different mechanisms leading to HH–LH mixing in QDs is presented in Sect. 14.5. The novel quantum transmissibility of HH–LH mixing mediated by intermediate states is discovered. The design rules for optimization of the HH–LH mixing in QDs are given in this section.

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. ...