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. ...

/pdf/a-comprehensive-review-of-zno-materials-and-devices-21a3zjadem.pdf

A comprehensive review of zno materials and devices

While most calculations on the properties of the ferromagnetic semiconductor GaAs:Mn have focused on isolated Mn substituting the Ga site $({\mathrm{Mn}}_{\mathrm{Ga}}),$ we investigate here whether alternate lattice sites are favored and what the magnetic consequences of this might be. Under As-rich (Ga-poor) conditions prevalent at growth, we find that the formation energies are lower for ${\mathrm{Mn}}_{\mathrm{Ga}}$ over interstitial Mn $({\mathrm{Mn}}_{i}).$ As the Fermi energy is shifted towards the valence band maximum via external p doping, the formation energy of ${\mathrm{Mn}}_{i}$ is reduced relative to ${\mathrm{Mn}}_{\mathrm{Ga}}.$ Furthermore, under epitaxial growth conditions, the solubility of both substitutional and interstitial Mn are strongly enhanced over what is possible under bulk growth conditions. The high concentration of Mn attained under epitaxial growth of p-type material opens the possibility of Mn atoms forming small clusters. We consider various types of clusters, including the Coulomb-stabilized clusters involving two ${\mathrm{Mn}}_{\mathrm{Ga}}$ and one ${\mathrm{Mn}}_{i}.$ While isolated ${\mathrm{Mn}}_{i}$ are hole killers (donors), and therefore destroy ferromagnetism, complexes such as $({\mathrm{Mn}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{Mn}}_{i}\ensuremath{-}{\mathrm{Mn}}_{\mathrm{Ga}})$ are found to be more stable than complexes involving ${\mathrm{Mn}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{Mn}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{Mn}}_{\mathrm{Ga}}.$ The former complexes exhibit partial or total quenching of holes, yet ${\mathrm{Mn}}_{i}$ in these complexes provide a channel for a ferromagnetic arrangement of the spins on the two ${\mathrm{Mn}}_{\mathrm{Ga}}$ within the complex. This suggests that ferromagnetism in Mn-doped GaAs arises both from holes due to isolated ${\mathrm{Mn}}_{\mathrm{Ga}}$ as well as from strongly Coulomb stabilized ${\mathrm{Mn}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{Mn}}_{i}\ensuremath{-}{\mathrm{Mn}}_{\mathrm{Ga}}$ clusters.

Ferromagnetism in Mn-doped GaAs due to substitutional-interstitial complexes

Electron-hole exchange interactions can lead to spin-forbidden ''dark'' excitons in direct-gap quantum dots. Here, we explore an alternative mechanism for creating optically forbidden excitons. In a large spherical quantum dot made of a diamond-structure semiconductor, the symmetry of the valence band maximum (VBM) is t{sub 2}. The symmetry of the conduction band minimum (CBM) in direct-gap material is a{sub 1}, but for indirect-gap systems the symmetry could be (depending on size) a{sub 1}, e, or t{sub 2}. In the latter cases, the resulting manifold of excitonic states contains several symmetries derived from the symmetries of the VBM and CBM (e.g., t{sub 2}xt{sub 2}=A{sub 1}+E+T{sub 1}+T{sub 2} or t{sub 2}xe=T{sub 1}+T{sub 2}). Only the T{sub 2} exciton is optically active or ''bright,'' while the others A{sub 1}, E, and T{sub 1} are ''dark.'' The question is which is lower in energy, the dark or bright. Using pseudopotential calculations of the single-particle states of Si quantum dots and a direct evaluation of the screened electron-hole Coulomb interaction, we find that, when the CBM symmetry is t{sub 2}, the direct electron-hole Coulomb interaction lowers the energy of the dark excitons relative to the bright T{sub 2} exciton. Thus, the lowest energy exciton ismore » forbidden, even without an electron-hole exchange interaction. We find that our dark-bright excitonic splitting agrees well with experimental data of Calcott et al., Kovalev et al., and Brongersma et al. Our excitonic transition energies agree well with the recent experiment of Wolkin et al. In addition, and contradicting simplified models, we find that Coulomb correlations are more important for small dots than for intermediate sized ones. We describe the full excitonic spectrum of Si quantum dots by using a many-body expansion that includes both Coulomb and exchange electron hole terms. We present the predicted excitonic spectra. (c) 2000 The American Physical Society.« less

Dark excitons due to direct Coulomb interactions in silicon quantum dots

An atomistic direct diagonalization pseudopotential approach has been used to analyze the optical excitation spectra of CdSe quantum dots for up to 1.5 eV about the band gap. Good agreement is obtained with experiment for all the eight excitonic transitions, without resorting to fitting to the experimental data on dots. The observed excitonic transitions are identified in terms of specific pairs of valence and conduction single-particle states. For the lowest few transitions, the assignments agree with the conventional k.p effective-mass result, but this is not the case for the higher peaks. Indeed, we find in our atomistic approach that many more valence states exist within a given energy range than in the continuum k.p approach. Furthermore, we find that the mixing of even and odd angular momentum symmetry, disallowed in the contemporary simple k.p models, is actually permitted in the more general atomistic approach.

High-Energy Excitonic Transitions in CdSe Quantum Dots

Structural and electronic properties of epitaxial thin-layer SinGen superlattices.

The microscopic cause of conductivity in transparent conducting oxides like ZnO, In{2}O{3}, and SnO{2} is generally considered to be a point defect mechanism in the bulk, involving intrinsic lattice defects, extrinsic dopants, or unintentional impurities like hydrogen. We confirm here that the defect theory for O-vacancies can quantitatively account for the rather moderate conductivity and off-stoichiometry observed in bulk In{2}O{3} samples under high-temperature equilibrium conditions. However, nominally undoped thin-films of In{2}O{3} can exhibit surprisingly high conductivities exceeding by 4-5 orders of magnitude that of bulk samples under identical conditions (temperature and O{2} partial pressure). Employing surface calculations and thickness-dependent Hall measurements, we demonstrate that surface donors rather than bulk defects dominate the conductivity of In{2}O{3} thin films.

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Alex Zunger

Papers

Ferromagnetism in Mn-doped GaAs due to substitutional-interstitial complexes

Dark excitons due to direct Coulomb interactions in silicon quantum dots

High-Energy Excitonic Transitions in CdSe Quantum Dots

Structural and electronic properties of epitaxial thin-layer SinGen superlattices.

Surface Origin of High Conductivities in Undoped In2O3 Thin Films