Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set

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.

Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium.

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.

Self-interaction correction to density-functional approximations for many-electron systems

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

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. ...

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A comprehensive review of zno materials and devices

The wider-gap members of a semiconductor series such as diamond→Si→Ge or AlN→GaN→InN often cannot be doped n-type at equilibrium. We study theoretically if this is the case in the chalcopyrite family CuGaSe2→CuInSe2, finding that: (i) Bulk CuInSe2 (CIS, Eg=1.04eV) can be doped at equilibrium n-type either by Cd or Cl, but bulk CuGaSe2 (CGS, Eg=1.68eV) cannot; (ii) result (i) is primarily because the Cu-vacancy pins the Fermi level in CGS farther below the conduction band minimum than it does in CIS, as explained by the “doping limit rule;” (iii) Cd doping is better than Cl doping, in that CdCu yields in CIS a higher net donor concentration than ClSe; and (iv) in general, the system shows massive compensation of acceptors (CdIII,VCu) and donors (ClSe,CdCu,InCu).

Why can CuInSe2 be readily equilibrium-doped n-type but the wider-gap CuGaSe2 cannot?

We use our previously reported method for solving self-consistently the local-density one-particle equations in a numerical-basis-set linear combination of atomic orbitals expansion to study the ground-state charge density, x-ray structure factors, directional Compton profile, total energy, cohesive energy, equilibrium lattice constant, and behavior of one-electron properties under pressure of diamond. Good agreement is obtained with available experiment data. The results are compared with those obtained by the restricted Hartree-Fock model: the role of electron exchange and correlation on the binding mechanism, the charge density, and the momentum density is discussed. I. INTRODU(.'TION The local density functional (LDF) formalism of Hohenberg, Kohn, and Sham, " and its recent extension as a local spin-density functional formalism, ' form the basis of a new approach to the study of electronic structure in that the effects of exchange and correlation are incorporated directly into a charge- density- dependent potential term that is determined self -consistently from the solution of an effective one-particle equation. Applications of the LDF formalism to atoms+' and molecules' have yielded encouraging results. Similar applications for solids are complicated by (i) the need to con sider both the short-range and the long- range multicenter crystal potential having nonspherical components, (ii) the difficulties in obtaining full self-consistency in a periodic system, and (iii) the need to provide a basis set with sufficient variational flexibility. Hence, theoretical. studies of ground- state electronic properties of solids in the LDF formalism have been mainly l.imited to muffin-tin models for the potential, " non-selfconsistent schemes, ' treatments of simplified jellium models'0 or spherical cellular schemes We have recently proposed"" a general selfconsistent method for solving the LDF formalism one-particle equation for realistic solids using a numerical-basis-set LCAO (linear combination of atomic orbitals) expansi. on and retaining all nonspherical parts of the crystal potential. We have demonstrated a rapid convergence of the selfconsistent (SC) cycle when the treatment of the full crystal charge density is suitably apportioned between real- space and Fourier- transformed reciprocal-space parts and have indicated the large degree of variational flexibility offered by a nonlinearly optimized (exact) numerical atomiclike basis set. We have shown that all multicenter interactions as well as the nonconstant parts of the crystal potential are efficiently treated by a three- dimensional Diophantine integration scheme.

Ground-state electronic properties of diamond in the local-density formalism

Using first-principles band-structure calculations we have studied the valence-band alignment of InAs/InSb, deducing also the offset at the ${\mathrm{InAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$/${\mathrm{InAs}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Sb}}_{\mathit{y}}$ heterostructure. We find the following: (i) Pure InAs/InSb has a ``type-II broken gap'' alignment both with and without strain. (ii) For Sb-rich ${\mathrm{InAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$/InSb heterostructures, the unstrained band alignment is type II; both epitaxial strain and CuPt ordering enhance the type-II character in this Sb-rich limit. (iii) For As-rich InAs/${\mathrm{InAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ heterostructures the top of the valence band is always on the alloy layer while the conduction-band minimum can be localized either on the alloy layer (type-I) or on the InAs layer (type-II), depending on the balance between concentration, strain, and degree of ordering/phase separation. In this case, epitaxial strain enhances the type-II character, while ordering enhances the type-I character. Our results are compared with recent experimental observations. We find that the type-I behavior noted for some As-rich InAs/${\mathrm{InAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ interfaces and the type-II behavior noted in other such samples could be explained in terms of the dominance of ordering and strain effects, respectively.

InAsSb/InAs: A type-I or a type-II band alignment.

Using atomistic pseudopotential wave functions, we calculate the electron and hole addition energies and the quasi-particle gap of InAs quantum dots. We find that the addition energies and the quasi-particle gap depend strongly on the dielectric constant eout of the surrounding material, and that when eout is much smaller than the dielectric constant of the dot the electron−electron and hole−hole interactions are dominated by surface polarization effects. We predict the addition energies and the quasi-particle gap as a function of size and eout, and compare our results with recent single-dot tunneling spectroscopy experiments.

Addition Spectra of Quantum Dots: the Role of Dielectric Mismatch

The electronic structure of substitutionally random {ital A}{sub 1{minus}{ital x}B{ital x}C} alloys of zinc-blende semiconductors {ital AC} and {ital BC} departs from what a virtual-crystal approximation would grant both because of (i) a chemical perturbation, associated with an {ital electronicmismatch} between atoms {ital A} and {ital B}, and because of (ii) a structural perturbation (positional relaxation) induced by a {ital size} {ital mismatch} between {ital A} and {ital B}. Both effects on the electronic density of states are studied here for Hg{sub 0.5}Cd{sub 0.5}Te, Cd{sub 0.5}Zn{sub 0.5}Te, and Hg{sub 0.5}Zn{sub 0.5}Te in the context of first-principles self-consistent supercell models. We use our recently developed special quasirandom structures'' (A. Zunger, S.-H. Wei, L. G. Ferreira, and J. E. Bernard, Phys. Rev. Lett. 65, 353 (1990)) concept whereby lattice sites of a periodic structure are occupied by {ital A} and {ital B} atoms so as to closely reproduce the structural correlation functions of an infinite, perfectly random alloy. Total-energy minimization provides then the relaxed atomic positions while application of the local-density formalism, as implemented by the linearized augmented-plane-wave method, describes self-consistently the consequences of chemical and structural perturbations. We show how these perturbations lead both to (i) distinct {ital A}-like and {italmore » B}-like features in the density of states and the electronic charge densities, and even to (ii) different {ital C}-like features associated with fluctuations in the local environments around the common sublattice.« less

Alex Zunger

Papers

Why can CuInSe2 be readily equilibrium-doped n-type but the wider-gap CuGaSe2 cannot?

Ground-state electronic properties of diamond in the local-density formalism

InAsSb/InAs: A type-I or a type-II band alignment.

Addition Spectra of Quantum Dots: the Role of Dielectric Mismatch

Disorder effects on the density of states of the II-VI semiconductor alloys Hg0.5Cd0.5Te, Cd0.5Zn0.5Te, and Hg0.5Zn0.5Te.