Showing papers by "Alex Zunger published in 1997"

Size-Dependent Spectroscopy of InP Quantum Dots

Abstract: The spectroscopic behavior of colloidal InP quantum dots (QDs) has been investigated as a function of the mean QD diameter (which ranged from 26 to 60 A). Absorption spectra show up to three peaks or shoulders which reflect excited state transitions in the QDs. Global photoluminescence (PL) spectra (excitation well to the blue of the absorption onset and which consequently excites most of the QDs in the size distribution) show broad PL emission. The emission and absorption features shift to higher energy with decreasing QD size. Resonant PL spectra (size-selective excitation into the tail of the absorption onset) show increasing fluorescence line narrowing with increasing excitation wavelength; PL and photoluminescence excitation spectroscopy were used to derive the PL red shift as a function of QD size. The resonant red shifts for QDs of a single size were extracted from PL data that reflect the emission from an ensemble of QD diameters. An analysis of the single-dot resonant red shift (difference betwee...

Stabilization of Ternary Compounds via Ordered Arrays of Defect Pairs

Abstract: First-principles calculations show that the defect pair $({2V}_{\mathrm{Cu}}^{\ensuremath{-}}+{\mathrm{In}}_{\mathrm{Cu}}^{+})$ in CuInS${\mathrm{e}}_{2}$ has an unusually low formation energy, due both to the relative ease of forming Cu vacancies $({V}_{\mathrm{Cu}})$ and to the attractive interactions between ${V}_{\mathrm{Cu}}^{\ensuremath{-}}$ and ${\mathrm{In}}_{\mathrm{Cu}}^{2+}$. The defect pair is predicted to be electrically inactive. This explains the surprising electrical tolerance of CuInS${\mathrm{e}}_{2}$ to its huge $(\ensuremath{\sim}1%)$ concentration of native defects. An attractive interaction among the defect pairs is further predicted to lead to a crystallographic ordering of the pairs, explaining the observed, but hitherto surprising, structures CuI${\mathrm{n}}_{5}$S${\mathrm{e}}_{8}$, CuI${\mathrm{n}}_{3}$S${\mathrm{e}}_{5}$, C${\mathrm{u}}_{2}$I${\mathrm{n}}_{4}$S${\mathrm{e}}_{7}$, etc.

Electronic and structural anomalies in lead chalcogenides

Abstract: The rocksalt-structure PbS, PbSe, and PbTe semiconductors and their alloys exhibit a series of electronic-structure anomalies relative to the II-VI system, including the occurrence of direct gaps at the L point, anomalous order of band gaps and valence-band maximum energies versus anions, negative optical bowing, and negative band-gap pressure coefficients. We show that these anomalies result from the occurrence of the Pb s band below the top of the valence band, setting up coupling and level repulsion at the L point. Furthermore, we find that the topology of the frustrated octahedral structure leads to the occurrence in the random alloy of two distinct bonds for each anion-cation pair and to the predicted stabilization of {ital bulk} ordered Pb{sub 2}STe CuPt-like phase. {copyright} {ital 1997} {ital The American Physical Society}

Valence band splittings and band offsets of AlN, GaN and InN.

Abstract: First‐principles electronic structure calculations on wurtzite AlN, GaN, and InN reveal crystal‐field splitting parameters ΔCF of −217, 42, and 41 meV, respectively, and spin–orbit splitting parameters Δ0 of 19, 13, and 1 meV, respectively. In the zinc blende structure ΔCF≡0 and Δ0 are 19, 15, and 6 meV, respectively. The unstrained AlN/GaN, GaN/InN, and AlN/InN valence band offsets for the wurtzite (zinc blende) materials are 0.81 (0.84), 0.48 (0.26), and 1.25 (1.04) eV, respectively. The trends in these spectroscopic quantities are discussed and recent experimental findings are analyzed in light of these predictions.

InP quantum dots: Electronic structure, surface effects, and the redshifted emission

Abstract: We present pseudopotential plane-wave electronic-structure calculations on InP quantum dots in an effort to understand quantum confinement and surface effects and to identify the origin of the long-lived and redshifted luminescence. We find that (i) unlike the case in small GaAs dots, the lowest unoccupied state of InP dots is the ${\ensuremath{\Gamma}}_{1c}$-derived direct state rather than the ${X}_{1c}$-derived indirect state and (ii) unlike the prediction of $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ models, the highest occupied state in InP dots has a $1sd$-type envelope function rather than a (dipole-forbidden) $1pf$ envelope function. Thus explanations (i) and (ii) to the long-lived redshifted emission in terms of an orbitally forbidden character can be excluded. Furthermore, (iii) fully passivated InP dots have no surface states in the gap. However, (iv) removal of the anion-site passivation leads to a P dangling bond (DB) state just above the valence band, which will act as a trap for photogenerated holes. Similarly, (v) removal of the cation-site passivation leads to an In dangling-bond state below the conduction band. While the energy of the In DB state depends only weakly on quantum size, its radiative lifetime increases with quantum size. The calculated $\ensuremath{\sim}300\ensuremath{-}\mathrm{meV}$ redshift and the $\ensuremath{\sim}18$ times longer radiative lifetime relative to the dot-interior transition for the 26-\AA{} dot with an In DB are in good agreement with the observations of full-luminescence experiments for unetched InP dots. Yet, (vi) this type of redshift due to surface defect is inconsistent with that measured in selective excitation for HF-etched InP dots. (vii) The latter type of (``resonant'') redshift is compatible with the calculated screened singlet-triplet splitting in InP dots, suggesting that the slow emitting state seen in selective excitation could be a triplet state.

Direct pseudopotential calculation of exciton coulomb and exchange energies in semiconductor quantum dots

Abstract: The effects of electron-hole interaction on the exciton energy of semiconductor quantum dots are calculated using pseudopotential wave functions. A comparison with the widely used, but never tested, effective-mass approximation (EMA) shows that the electron-hole Coulomb energy is significantly ( $\ensuremath{\sim}40%$) overestimated by the EMA, and that the scaling with the dot size $R$ is sublinear in $1/R$. The exchange splitting is much smaller than the Coulomb energy, and in the case of CdSe quantum dots shows significant deviations from the ${1/R}^{3}$ scaling predicted by the EMA.

Cu-Au, Ag-Au, Cu-Ag and Ni-Au intermetallics: First-principles study of phase diagrams and structures

Abstract: The classic metallurgical systems -- noble metal alloys -- that have formed the benchmark for various alloy theories, are revisited. First-principles fully relaxed general potential LAPW total energies of a few ordered structures are used as input to a mixed-space cluster expansion calculation to study the phase stability, thermodynamic properties and bond lengths in Cu-Au, Ag-Au, Cu-Ag and Ni-Au alloys. (i) Our theoretical calculations correctly reproduce the tendencies of Ag-Au and Cu-Au to form compounds and Ni-Au and Cu-Ag to phase separate at T=0 K. (ii) Of all possible structures, Cu/sub 3/Au (L1/sub 2/) and CuAu (L1/sub 0/) are found to be the most stable low-temperature phases of Cu/sub 1-x/Au/sub x/ with transition temperatures of 530 K and 660 K, respectively, compared to the experimental values 663 K and 670 K. The significant improvement over previous first-principles studies is attributed to the more accurate treatment of atomic relaxations in the present work. (iii) LAPW formation enthalpies demonstrate that L1/sub 2/, the commonly assumed stable phase of CuAu/sub 3/, is not the ground state for Au-rich alloys, but rather that ordered superlattices are stabilized. (iv) We extract the non-configurational (e.g., vibrational) entropies of formation and obtain large values for the size mismatched systems: 0.48 k/sub B//atom in Ni/sub 0.5/Au/sub 0.5/ (T=1100 K), 0.37 k/sub B//atom in Cu/sub 0.14/Ag/sub 0.86/ (T=1052 K), and 0.16 k/sub B//atom in Cu/sub 0.5/Au/sub 0.5/ (T=800 K). (v) Using 8 atom/cell special quasirandom structures we study the bond lengths in disordered Cu-Au and Ni-Au alloys and obtain good qualitative agreement with recent EXAFS measurements.

Band gaps of GaPN and GaAsN alloys

Abstract: The importance of atomic relaxations, chemical disorder, and epitaxial constraints on the band gap of random, anion-mixed nitride alloys GaPN and GaAsN have been investigated, via pseudopotentials calculation. It has been demonstrated that simple approximations such as the virtual crystal approximation, or the use of high-symmetry ordered structure to mimic a random alloy, or the neglect of atomic displacements, are inadequate. It is found that a fully relaxed, large supercell calculation reproduces well the experimental band gaps of GaPN and GaAsN films.

Composition dependence of interband transition intensities in GaPN, GaAsN, and GaPAs alloys

Abstract: Using large (512-atom) pseudopotential supercell calculations, we have investigated the composition dependence of the momentum matrix element ${M}_{v,c}$ for transitions between the valence-band maximum and the conduction-band minimum of three semiconductor alloys: ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ and ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x},$ exhibiting large chemical and size differences between their alloyed elements, and ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{As}}_{x},$ which is a weakly perturbed alloy In the composition ranges where these alloys have a direct band gap, we find that (i) in ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{As}}_{x},$ ${M}_{v,c}$ is large (like the virtual-crystal value) and nearly composition independent; (ii) in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x},$ ${M}_{v,c}$ is strongly composition dependent: large for small $x$ and small for large $x;$ and (iii) in ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{N}}_{x},$ ${M}_{v,c}$ is only slightly composition dependent and is significantly reduced relative to the virtual-crystal value The different behavior of ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{As}}_{x},$ ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{N}}_{x},$ and ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ is traced to the existence/absence of impurity levels at the dilute alloy limits: (a) there are no gap-level impurity states at the $x\ensuremath{\rightarrow}1$ or $x\ensuremath{\rightarrow}0$ limits of ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{As}}_{x},$ (b) an isolated As impurity in GaN ($\mathrm{GaN}:\mathrm{A}\mathrm{s}$) has a deep band gap impurity level but no deep impurity state is found for N in GaAs, and (c) $\mathrm{GaN}:\mathrm{P}$ exhibits a P-localized deep band-gap impurity state and $\mathrm{GaP}:\mathrm{N}$ has an N-localized resonant state The existence of deep levels leads to wave-function localization in real space, thus to a spectral spread in momentum space and to a reduction of ${M}_{v,c}$ These impurity levels are facilitated by atomic relaxations, as evident by the fact that unrelaxed $\mathrm{GaN}:\mathrm{A}\mathrm{s}$ and $\mathrm{GaN}:\mathrm{P}$, show no deep levels, have extended wave functions, and have large interband transition elements

Local-density-derived semiempirical nonlocal pseudopotentials for InP with applications to large quantum dots

Abstract: In the same way that atomic calculations have been used previously to extract bare ionic pseudopotentials, self-consistent bulk calculations can be used to construct screened atomic pseudopotentials We use such a method to construct screened nonlocal atomic pseudopotentials for InP A series of bulk, local-density-approximation (LDA) calculations are performed on a few InP crystal structures, covering a range of unit-cell volumes, to produce bulk potentials {${\mathrm{V}}_{\mathrm{LDA}}$ (G)} By solving a set of linear equations, we extract from these crystalline potentials the corresponding screened atomic 'spherical LDA' (SLDA) potentials ${\mathrm{v}}_{\mathrm{SLDA}}^{\mathrm{\ensuremath{\alpha}}g}$(|q|) for sites \ensuremath{\alpha}=In or P In combination with the nonlocal part of the usual LDA pseudopotentials, these SLDA potentials give band structures and wave functions that are virtually indistinguishable from the self-consistent LDA results for bulk InP In the next step, we apply linear changes to the local SLDA potentials (while keeping the nonlocal potentials at their LDA values), to fit the band structures to experiment Interestingly, this removal of LDA eigenvalue errors requires only small and subtle changes in the potential---mostly an upshift in the region near the cation core, with nearly no change at the bond center Furthermore, the linear changes to the SLDA potentials result mostly in an upshift of the conduction bands with little effect on the valence bands Because only small changes in the potential suffice to fit the bands to experimental results, the wave functions remain virtually unchanged relative to those in the original LDA calculation Hence, we obtain semiempirical pseudopotentials which can produce ab initio LDA-quality wave functions with experimentally measured band structures, effective masses, and deformation potentials The potentials obtained here were deposited on an FTP site and can be used by interested readers Since the resulting pseudopotentials are 'soft' (with small high-momentum components), they can be applied within a plane-wave basis in combination with a Gaussian correction to large systems for which LDA calculations are prohibitively expensive As an illustration, we apply our InP screened atomic pseudopotentials to calculate quantum size effects on the band gaps of InP dots with sizes up to 700 atoms Good agreement is found between the theoretical and the experimental band gaps Fitting the calculated band gaps ${\mathrm{E}}_{\mathrm{g}}$ (in unit of eV) versus the effective dot sizes D (in unit of \AA{}) gives ${\mathrm{E}}_{\mathrm{g}}$ =145+37295/${\mathrm{D}}^{116}$ This prediction differs significantly from the quadratic size dependence ${\mathrm{D}}^{\mathrm{\ensuremath{-}}20}$ expected from simple effective-mass theory

Surface-reconstruction-enhanced solubility of N, P, As, and Sb in III-V semiconductors

Abstract: We show that surface reconstructions may play an essential role in determining the equilibrium solubilities of N, P, As, and Sb in various III-V compounds. In particular, anion–anion dimerization of the (001)-β2(2×4) surface can enhance the solubility of N near the surface in GaAs, GaP, and InP by five, three, and two orders of magnitudes, respectively, at 1000 K. With certain assumptions on the growth kinetics, this high concentration of N may be frozen in as the crystal grows.

Million-Atom Pseudopotential Calculation of γ - X Mixing in GaAs / AlAs Superlattices and Quantum Dots

Abstract: We have developed a ``linear combination of bulk bands'' method that permits atomistic, pseudopotential electronic structure calculations for $\ensuremath{\sim}{10}^{6}$ atom nanostructures. Application to $(\mathrm{GaAs}{)}_{n}/(\mathrm{AlAs}{)}_{n}$ (001) superlattices (SL's) reveals even-odd oscillations in the $\ensuremath{\gamma}\ensuremath{-}X$ coupling magnitude ${V}_{\ensuremath{\gamma}X}(n)$, which vanishes for $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{odd}$, even for abrupt and segregated SL's, respectively. Surprisingly, in contrast with recent expectations, 0D quantum dots are found here to have a smaller $\ensuremath{\gamma}\ensuremath{-}X$ coupling than equivalent 2D SL's. Our analysis shows that for large quantum dots this is largely due to the existence of level repulsion from many $X$ states.

Comparison of the k⋅p and the direct diagonalization approaches for describing the electronic structure of quantum dots

Abstract: It is shown that the standard (decoupled) 6×6 k⋅p effective-mass approach for semiconductor quantum dots overestimates significantly the hole and electron confinement energies, and, for dots made of materials with small spin-orbit coupling (e.g., phosphides, sulphides) produces a reverse order of s- and p-like valence states. By contrasting the electronic structures of dots as obtained by a direct diagonalization (multiband) pseudopotential approach and by its k⋅p approximation, we are able to trace the systematic errors of k⋅p in dots to the k⋅p errors in the underlying bulk solids. This suggests a “diagnostic tool” and a strategy for improving the k⋅p.

Ni-Au: A testing ground for theories of phase stability

Abstract: The theory of phase stability in the Ni-Au alloy system is a popular topic due to the large size mismatch between Ni and Au, which makes the effects of atomic relaxation critical, and also the fact that Ni-Au exhibits a phase separation tendency at low temperatures, but measurements at high-temperature show an ordering-type short-range order. We have clarified the wide disparity which exists in the previously calculated values of mixing energies and thermodynamic properties by computing ``state-of-the-art'' energetics (full-potential, fully-relaxed LDA total energies) combined with ``state-of-the-art'' statistics (k-space cluster expansion with Monte Carlo simulations) for the Ni-Au system. We find: (i) LDA provides accurate mixing energies of disordered Ni_{1-x}Au_x alloys (\Delta H_{mix} < +100 meV/atom) provided that both atomic relaxation (a ~100 meV/atom effect) and short-range order (~25 meV/atom) are taken into account properly. (ii) Previous studies using empirical potentials or approximated LDA methods often underestimate the formation energy of ordered compounds, and hence also underestimate the mixing energy of random alloys. (iii) Measured values of the total entropy of mixing combined with calculated values of the configurational entropy demonstrate that the non-configurational entropy in Ni-Au is large, and leads to a significant reduction in miscibillity gap temperature. (iv) The calculated short-range order agrees well with measurements, and both predict ordering in the disordered phase. (v) Consequently, using inverse Monte Carlo to extract interaction energies from the measured/calculated short-range order in Ni-Au would result in interactions which would produce ordering-type mixing energies, in contradiction with both experimental measurements and precise LDA energies.

Bond-length distribution in tetrahedral versus octahedral semiconductor alloys: The case of Ga 1 − x In x N

Abstract: Large $(\ensuremath{\approx}1000\mathrm{atoms})$ supercell valence force-field simulations are used to investigate the nearest-neighbor bond-length distribution in relaxed tetrahedral (zinc blende and wurtzite) and octahedral (rocksalt) ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{N}$ alloys. We find that, due to the rigidity of the octahedron, the distribution of each anion-cation bond length in rocksalt alloys has two contributions: unrelaxed bonds and relaxed bonds. These two peaks have a large width and overlap slightly, leading to a broad nearest-neighbor distance distribution. On the other hand, the anion-cation nearest-neighbor distribution in zinc-blende alloys can be decomposed into a sum over four closely spaced and sharp peaks associated with different clusters, leading to a narrow, single-peaked nearest-neighbor distribution. Finally the wurtzite alloys exhibit bond-length distributions that are very similar to the corresponding ones in the zinc-blende alloys, leading to a nearly identical strain energy in random zinc-blende and wurtzite alloys.

Pseudopotential theory of nanometer silicon quantum dots

Abstract: We present a systematic approach to the study of the electronic structure of thousand atom (nanometer scale) quantum structures. This approach uses the empirical pseudopotential method to approximate the Hamiltonian and a plane wave basis to expand the wavefunctions. Two complementary, newly developed methods are used to calculate the electronic structure of the system. The first method solves for the discrete near-edge states (the valence band maximum and the conduction band minimum). Its computational time scales linearly with the size of the system. The second method calculates statistically the electronic density of states and optical absorption spectra. For a given resolution and statistical accuracy, its computational time is independent of the size of the system for systems smaller than ≈10,000 atoms. The combination of these two methods is used to study the electronic and optical properties of up to thousand Si atom quantum dots passivated by hydrogen. The properties studied include: (1) band gap vs size; (2) band gap vs shape; (3) analysis of band edge states in terms of bulk Bloch functions; (4) total electronic density of state and optical absorption spectra; (5) dielectric constant vs size; (6) photoluminescence radiative lifetime vs luminescence photon energy. The results are compared with tight binding and other model calculations. Comparison with experimental data is made whenever possible. Good agreements with experiment are obtained for photoluminescence lifetime and for the ratio between conduction band shift and valence band shift.

Magnitude and Size Scaling of Intervalley Coupling in Semiconductor Alloys and Superlattices

Abstract: Coupling between different \ensuremath{\Gamma}, $X,$ and $L$ band-structure valleys is responsible for (a) level anticrossing in superlattices as a function of period, pressure, and electric field and for (b) ``optical bowing'' of band gaps in random alloys. We investigate the symmetry, magnitude, and size scaling of intervalley coupling in semiconductor superlattices and alloys by direct supercell calculations, performed with screened pseudopotentials and a plane-wave basis, considering up to ${10}^{6}$ atoms/supercell. Projecting the calculated electronic wave functions ${\ensuremath{\psi}}_{i}$ of alloys or superlattices onto the bulk states of the constituent zinc-blende materials shows that ${\ensuremath{\psi}}_{i}$ contain a ``majority representation'' from one or more zinc-blende states \ensuremath{\gamma}. The intervalley coupling $E(i,j)$ between the alloy states ${\ensuremath{\psi}}_{i}$ and ${\ensuremath{\psi}}_{j}$ then includes a term $2F(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})V(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ due to the ``majority representations'' \ensuremath{\gamma} and ${\ensuremath{\gamma}}^{\ensuremath{'}}$ of ${\ensuremath{\psi}}_{i}$ and ${\ensuremath{\psi}}_{j},$ respectively, plus residual terms due to the minority representations. We find the following: (i) In alloys, the orbital overlap function $F(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ is large, since the wave functions are extended. The intervalley coupling element $V(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ exhibits simple selection rules: being zero for $({\ensuremath{\Gamma}}_{1c}{,X}_{1c}),$ $({\ensuremath{\Gamma}}_{1c}{,L}_{3c}),$ ${(X}_{1c}^{x}{,X}_{1c}^{y}),$ etc. (``weak coupling''), and nonzero for $({\ensuremath{\Gamma}}_{1c}{,X}_{3c}),$ $({\ensuremath{\Gamma}}_{1c}{,L}_{1c}),$ ${(L}_{3c}{,X}_{1c}),$ etc. (``strong coupling''). This explains why the $\overline{\ensuremath{\Gamma}}$-like conduction band of mixed-cation alloys contains zinc-blende ${\ensuremath{\Gamma}}_{1c}$ and ${L}_{1c}$ character, but not ${X}_{1c}.$ In the case of strong coupling, $E(i,j)$ scales as $1/\sqrt{\ensuremath{\Omega}},$ where \ensuremath{\Omega} is the volume, while in the weak-coupling case the entire coupling originates from the ``minority representation,'' and is 20--100 times smaller. The minority representation, however, contributes to the bowing of the band gap vs composition. (ii) In superlattices, although the above selection rule for $V(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ still exists, the magnitude of the intervalley coupling is governed by the overlap function $F(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}}).$ For simple superlattices, $F(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ is small, since the wave functions are localized in particular segments (``weak coupling''). Consequently, the ``majority representation'' contributes 5--100 times less than in the analogous case of alloys. Furthermore, $E(i,j)$ scales as ${1/n}^{3},$ where $n$ is the superlattice period.

Strain-induced shift in the elastically soft direction of epitaxially grown fcc metals

Abstract: The theory of epitaxial strain energy is extended beyond the harmonic approximation to account for large film/substrate lattice mismatch. We find that for fcc noble metals (i) directions and soften under tensile biaxial strain (unlike zincblende semiconductors) while (ii) and soften under compressive biaxial strain. Consequently, (iii) upon sufficient compression becomes the softest direction (lowest elastic energy), but (iv) is the hardest direction for large tensile strain. (v) The dramatic softening of in fcc noble metals upon biaxial tensile strain is caused by small fcc/bcc energy differences for these materials. These results can be used in selecting the substrate orientation for effective epitaxial growth of pure elements and A/sub p/B/sub q/ superlattices, as well as to explain the shapes of coherent precipitates in phase separating alloys.

Predicion of charge separation in GaAs/AlAs cylindrical Russian Doll nanostructures

Abstract: We have contrasted the quantum confinement of (i) multiple quantum wells of flat GaAs and AlAs layers, i.e. $(\GaAs)_{m}/(\AlAs)_n/(\GaAs)_p/(\AlAs)_q$, with (ii) ``cylindrical Russian Dolls'' -- an equivalent sequence of wells and barriers arranged as concentric wires. Using a pseudopotential plane-wave calculation, we identified theoretically a set of numbers ($m,n,p$ and $q$) such that charge separation can exist in ``cylindrical Russian Dolls'': the CBM is localized in the inner GaAs layer, while the VBM is localized in the outer GaAs layer.

Revisiting the defect physics in CuInSe/sub 2/ and CuGaSe/sub 2/

Abstract: Using first-principles self-consistent electronic structure theory, we have calculated defect formation energies and defect energy levels in CuInSe/sub 2/. Contrary to previously accepted assumptions in the analysis of defects in CuInSe/sub 2/ we find that (i) it is much easier to form Cu vacancy in CuInSe/sub 2/ than to form cation vacancies in II-VI's. (ii) Defect formation energies vary considerably both with the Fermi energy and the chemical potential of the atomic species and (iii) defect pairs such as (2V/sub Cu/+In/sub Cu/) have a remarkably low formation enthalpy. This explains the massive nonstoichiometry of CuInSe/sub 2/ and the appearance of ordered defect compounds CuIn/sub 5/Se/sub 8/, CuIn/sub 3/Se/sub 5/, Cu/sub 2/In/sub 4/Se/sub 7/ and Cu/sub 3/In/sub 5/Se/sub 9/. The fact that CuInSe/sub 2/ has good electrical properties despite this off-stoichiometry reflects the mutual passivation of In/sub Cu/ by V/sub Cu/. Similar results are found for CuGaSe/sub 2/, except that (iv) it is more difficult to form (2V/sub Cu//sup -/+Ga/sub Cu//sup 2+/) in CuGaSe/sub 2/ than to from (2V/sub Cu//sup -/+In/sub Cu//sup 2+/) in CuInSe/sub 2/, and (v) the Ga/sub Cu/ donor levels are much deeper than the In/sub Cu/ donor levels. Thus, it is more difficult to dope CuGaSe/sub 2/ n-type.

Comparison of two methods for describing the strain profiles in quantum dots

Abstract: The electronic structure of interfaces between lattice-mismatched semiconductor is sensitive to the strain. We compare two approaches for calculating such inhomogeneous strain -- continuum elasticity (CE, treated as a finite difference problem) and atomistic elasticity (AE). While for small strain the two methods must agree, for the large strains that exist between lattice-mismatched III-V semiconductors (e.g. 7% for InAs/GaAs outside the linearity regime of CE) there are discrepancies. We compare the strain profile obtained by both approaches (including the approximation of the correct C_2 symmetry by the C_4 symmetry in the CE method), when applied to C_2-symmetric InAs pyramidal dots capped by GaAs.

Prediction of charge separation in GaAs/AlAs cylindrical nanostructures

Abstract: It is well known that in a sequence of flat, type-I (GaAs){sub m}/(AlAs){sub n}/(GaAs){sub p}/(AlAs){sub q}{center_dot}{center_dot}{center_dot} multiple quantum wells (MQWs), the wave functions of both the valence-band maximum and the conduction-band minimum are localized on the widest well. Thus, electron-hole charge separation is not possible. On the other hand, for short-period superlattices (type II), the electron and hole are localized on different materials (electron on AlAs and hole on GaAs) and different band-structure valleys (hole at {Gamma} and electron at X). Using a plane-wave pseudopotential direct-diagonalization approach, we predict that electron-hole charge separation on different layers of the {ital same} material (GaAs) and {ital same} valley ({Gamma}) is possible in {ital curved} (but not in flat) {ital geometries}. This is predicted for a set of concentric, nested cylinders of GaAs and AlAs. Since the flat multiple-quantum-well structure and the nested cylindrical structure with the same layer thicknesses have the same band offset diagram, the difference in behavior is not due to the potential. Rather, it reflects different interband coupling and kinetic energy confinement induced by the {ital curvature}, present in the nested-cylinder geometry but absent in the MQW. This identifies a geometric degree of freedom (curvature) that can be usedmore » to tailor electronic properties of nanostructures. {copyright} {ital 1997} {ital The American Physical Society}« less

Point-ion versus density functional calculations of electric field gradients in ordered GaInP2

Abstract: We investigate whether the electric field gradient (EFG) at an atomic site in the unit cell of a periodic solid can be modeled via the electrostatic field gradient set up by atomic point charges outside that site. To test this approach we contrast the EFG predicted by such point-ion models for long-range ordered GaInP2 alloys with the results obtained from self-consistent all-electron calculations in the local density approximation (LDA). We first tested our LDA approach for ZnAl2O4, for which experimental data exist, finding the quadrupole coupling constant Qcc(27Al)=3.94 MHz, compared with the measured value of |Q|=3.68 MHz. Applying next the LDA approach to perfectly ordered GaInP2 (for which experimental data do not exist), we find the LDA quadrupole coupling constant Qcc=−4.83, −2.84, and 13.08 MHz for 69Ga, 71Ga, and 115In, respectively. We further find that more than 95% of these EFGs originate from the anisotropic electron charge distribution within a small sphere of radius ∼0.2 A about the respec...

Why is CuInSe2 tolerant to defects and what is the origin of “Ordered Defect Structures”

Abstract: This paper explains both the (1) remarkable electronic passivity of CuInSe2 to its many structural defects, and (2) the occurance of previously noted but unexplained series of structures CuIn5Se8, CuIn3Se5, Cu2In4Se7, etc. in terms of the unusual stability of the charge-compensated defect pair (2VCu−+InCu2+).

First-Principles Theory of Cation and Intercalation Ordering in LixCoO2

Abstract: Several types of cation- and vacancy-ordering exist in the LixCoO2 battery material. The ordering patterns are of interest due to the fact that they can control the voltage in rechargeable Li batteries. We present a first-principles total energy theory which can predict both cation-and vacancy-ordering patterns at both zero and finite temperatures. Also, by calculating the energetics of the Li intercalation reaction, this theory can provide first-principles predictions of battery voltages of LixCoO2/Li cells. Our calculations allow us to search the entire configurational space to predict the lowest-energy ground state structures, search for large voltage cathodes, explore metastable low-energy states, and extend our calculations to finite temperatures, thereby searching for order-disorder transitions and states of partial disorder.