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

An impurity in a semiconductor can have either amphoteric behavior (no net production of electron or holes), or be an energetically deep center (carriers produced only at high temperature), or a shallow center (carriers produced even at low temperature). In most semiconductors (e.g., Si, GaAs, GaP, InP, and ZnSe) hydrogen impurities do not produce free carriers, being instead an amphoretic center; yet hydrogen does dope n-type some oxides such as ${\mathrm{SnO}}_{2}$ and ZnO. We studied theoretically whether or not H could dope chalcopyrite ${\mathrm{I}\ensuremath{-}\mathrm{I}\mathrm{I}\mathrm{I}\ensuremath{-}\mathrm{V}\mathrm{I}}_{2}$ compounds, ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}.$ Based on the first-principles calculations, we find that nonsubstitutionally incorporated hydrogen forms a deep donor in ${\mathrm{CuGaSe}}_{2},$ but a relatively shallow donor in ${\mathrm{CuInSe}}_{2}.$ The interaction of hydrogen with the abundant defect complex ${(2V}_{\mathrm{Cu}}+{\mathrm{In}}_{\mathrm{Cu}}{)}^{0}$ yields an even shallower donor, making ${\mathrm{CuInSe}}_{2}$ n type. In addition, our results show that hydrogen passivates the acceptorlike copper vacancies in both ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2},$ thus eliminating p type behavior. These findings, in conjunction with typical conditions under which ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$ are grown, indicate that ${\mathrm{CuInSe}}_{2}$ could be doped n type via hydrogen incorporation, whereas ${\mathrm{CuGaSe}}_{2}$ could not. The reason for the different behavior of ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$ towards hydrogen is that in the latter case the conduction-band minimum is at a considerably higher energy than in the former case. Despite this difference in electrical properties, it is predicted that hydrogen can be stored in both ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$ via implantation since the implanted hydrogens decorate copper atoms as well as preexisting copper vacancies.

n -type doping and passivation of CuInSe 2 and CuGaSe 2 by hydrogen

Using a cluster description of a coherent binary alloy, we predict and contrast bulk and epitaxial composition-temperature phase diagrams. For alloys with stable ordered compounds in bulk, epitaxial growth away from the lattice-matched composition distorts phase boundaries and changes ordering temperatures. For alloys insoluble in bulk below a miscibility-gap temperature ${T}_{\mathrm{MG}}$, the epitaxial alloy may remain stable to temperatures g 1200 K below ${T}_{\mathrm{MG}}$ even on lattice-matched substrates, with stoichiometric compounds possible at low temperature.

Epitaxial effects on coherent phase diagrams of alloys

Fox and Tabbernor [Acta metall. mater.39, 669 (1991)] have recently measured the four lowest structure factors F(G) of NiAl using highly accurate high energy electron diffraction. We present here a systematic comparison of their results with ab initio band theory, in the context of the local density formalism. We find very good agreement for the three of the four lowest measured structure factors, while our F(200) is ∼0.4 e/cell higher. We tentatively attribute this difference to uncertainties in the treatment of the temperature factors in non-monoatomic compounds. Indeed, comparing with experiment our calculation for the monoatomic Si crystal (where the temperature term factors out), we find that theory reproduces the measured structure factors to within a very small deviations of ∼0.02 e/atom. We have also examined the effect of high Fourier components that are not currently amenable to measurements on the ensuing NiAl deformation electron density distribution (DEDD) maps. We find that the truncation of the Fourier series after four structure factors misses the directional d-like charge lobes near the Ni sites. We show that static and dynamic DEDD give a similar picture of the bonding.

Theory of bonding charge density in β′ NiAl

Nonisovalent ({ital A}{sup III}{ital BV}){sub 1{minus}{ital x}C2{ital x}}{sup IV} semiconductor alloys exhibit a transition as a function of composition between a phase with the zinc-blende symmetry (where the two fcc sublattices of the diamond lattice are unequally occupied by {ital A}{sup III} and {ital B}{sup V} atoms) and a phase with the diamond symmetry (where the two sublattices have equal occupations). Previous thermodynamic models of this transition have considered only nearest-neighbor interactions between {ital neutral} atoms. This approach ignores the important electrostatic interactions associated with electron transfer between the electron-rich {ital C}{sup IV}-{ital B}{sup V} ( donor'') bonds and the electron-deficient {ital A}{sup III}-{ital C}{sup IV} ( acceptor'') bonds. We have reexamined the validity of a three-dimensional bulk thermodynamic model for (GaAs){sub 1{minus}{ital x}}Ge{sub 2{ital x}} and (GaP){sub 1{minus}{ital x}}Si{sub 2{ital x}} using an energy model that includes such electrostatic interactions and pairwise energies extracted from first-principles local-density total-energy calculations.

Structural phase transition in (GaAs)1-xGe2x and (GaP)1-xSi2x alloys: Test of the bulk thermodynamic description.

Direct pseudopotential band structure calculations of thin Si(001) films reveal a number of features that are unexpected on the basis of conventional quantum confinement models: (i) The energies of some valence‐band states exhibit oscillations when the number of monolayers in the film changes from even to odd, (ii) certain film wave functions have a cosine (rather than sine) envelope function, and (iii) the energy of the highest occupied film state remains pinned at a constant value for all even‐layered film. We demonstrate a simple alternative to the effective‐mass model which explains these results.

Alex Zunger

Papers

n -type doping and passivation of CuInSe 2 and CuGaSe 2 by hydrogen

Epitaxial effects on coherent phase diagrams of alloys

Theory of bonding charge density in β′ NiAl

Structural phase transition in (GaAs)1-xGe2x and (GaP)1-xSi2x alloys: Test of the bulk thermodynamic description.

Prediction of unusual electronic properties of Si quantum films