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

Recent measurements surprisingly show that the lowest valence-to-conduction confined transitions in narrow $(\mathrm{InAs}{)}_{8}/(\mathrm{GaSb}{)}_{n}$ and $(\mathrm{InAs}{)}_{6}/(\mathrm{GaSb}{)}_{n}$ superlattices increase in energy as the barrier thickness n increases. We show that in addition to the mesoscopic geometric quantities (well and barrier sizes), an atomic-scale description of interdiffused interfaces is needed to correctly reproduce the observed spectroscopic trend. The interdiffused interface is modeled via diffusion equations. We compare our atomistic empirical pseudopotential calculation in which only the bulk binary data are fit to experiment, with contemporary methods in which agreement with experiment is forced using ideally abrupt interfaces.

Predicting interband transition energies for InAs/GaSb superlattices using the empirical pseudopotential method

Crystallographic point group symmetry (CPGS) such as polar and nonpolar crystal classes have long been known to classify compounds that have spin-orbit-induced spin splitting. While taking a journey through the Brillouin zone (BZ) from one $k$-point to another for a fixed CPGS, it is expected that the wave vector point group symmetry (WPGS) can change, and consequently, a qualitative change in the texture of the spin polarization can occur [the expectation value of spin operator ${\stackrel{P\vec}{S}}_{n{k}_{0}}$ in Bloch state $u(n,k)$ and the wave vector ${k}_{0}$]. However, the nature of the spin texture (ST) change is generally unsuspected. In this paper, we determine a full classification of the linear-in-$k$ ST patterns based on the polarity and chirality reflected in the WPGS at ${k}_{0}$. The spin-polarization vector ${\stackrel{P\vec}{S}}_{n{k}_{0}}$ controlling the ST is bound to be parallel to the rotation axis and perpendicular to the mirror planes, and hence, symmetry operation types in WPGSs impose symmetry restriction to the ST. For instance, the ST is always parallel to the wave vector $k$ in nonpolar chiral WPGSs since they contain only rotational symmetries. Some consequences of the ST classification based on the symmetry operations in the WPGS include the observation of ST patterns that are unexpected according to the symmetry of the crystal. For example, it is usually established that spin-momentum locking effect (spin vector always perpendicular to the wave vector) requires the crystal inversion symmetry breaking by an asymmetric electric potential. However, we find that polar WPGS can have this effect even in compounds without electric dipoles or external electric fields. We use the determined relation between WPGS and ST as a design principle to select compounds with multiple STs near band edges at different $k$ valleys. Based on high-throughput calculations for 1481 compounds, we find 37 previously fabricated materials with different STs near band edges. The ST classification as well as the predicted compounds with multiple STs can be a platform for potential application for spin-valleytronics and the control of the ST by accessing different valleys.

Different shapes of spin textures as a journey through the Brillouin zone

A previously published interaction potential between ${\mathrm{N}}_{2}$ molecules has been utilized to compute the lattice mode frequencies of solid $\ensuremath{\alpha}$- and $\ensuremath{\gamma}$-${\mathrm{N}}_{2}$ at the equilibrium crystal structure corresponding to various pressures. Dispersion curves and the density of states are given. These are then used to calculate the lattice heat capacity, Gr\"uneisen mode parameters throughout the Brillouin zone, linear thermal-expansion coefficient, Debye temperatures, and temperature-dependent root-mean-square amplitudes of vibrations. Whenever comparison with experimental data is possible, good agreement is obtained.

Lattice dynamics of solid α- and γ-N 2 crystals at various pressures

Using first-principles defect theory, we investigate the role of intrinsic point defects in the limitation of the opencircuit voltage (V OC ) in Cu(In,Ga)Se 2 solar cells. We find that the intrinsic donors In Cu (In-on-Cu antisite defect) and V Se (Selenium vacancy) and their defect complexes with V Cu (Cu vacancies) represent two independent mechanisms that are expected to cause saturation of V OC around 1 eV, when the absorber band gap is increased towards Ga-rich compositions. Strategies to avoid these sources of V OC limitation are discussed.

https://www.nrel.gov/docs/fy08osti/43275.pdf

Limitation of the open-circuit voltage due to metastable intrinsic defects in Cu(In,Ga)Se 2 and strategies to avoid these defects

We investigated the adatom states for different Al coverages of InP(110) by synchrotron‐radiation photoemission, including ultralow coverages below 0.2 monolayer. The adatom states below 0.1 monolayer and above ∼3 monolayer appear similar to the corresponding Al–adatom states on GaAs(110). In particular, the results for both systems appear consistent with the formation of Al clusters at 0.1–2 monolayer coverage, and the Fermi‐level pinning occurs when the cluster formation starts. However, the similarity between the two systems is limited at intermediate (0.1–2 monolayer) coverages. At those coverages we observe a new bonded state for Al on InP, which is not observed on GaAs. Our results emphasize, in general, the need to extend the experiments to ultralow coverages when studying the Schottky barrier formation process.

Alex Zunger

Papers

Predicting interband transition energies for InAs/GaSb superlattices using the empirical pseudopotential method

Different shapes of spin textures as a journey through the Brillouin zone

Lattice dynamics of solid α- and γ-N 2 crystals at various pressures

Limitation of the open-circuit voltage due to metastable intrinsic defects in Cu(In,Ga)Se 2 and strategies to avoid these defects

Schottky barrier formation and the initial metal-atom bonding state: InP(110)-Al vs GaAs(110)-Al