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Showing papers by "Alex Zunger published in 1995"


Journal ArticleDOI
TL;DR: In this article, the authors systematically calculated the alloy bowing coefficients, alloy mixing enthalpies, and interfacial valence and conduction band offsets for three mixed anion (CuInX2, X=S, Se, Te) and three mixedcation (CuMSe2, M=Al, Ga, In) chalcopyrite systems.
Abstract: Using first‐principles band‐structure theory we have systematically calculated the (i) alloy bowing coefficients, (ii) alloy mixing enthalpies, and (iii) interfacial valence‐ and conduction‐band offsets for three mixed‐anion (CuInX2, X=S, Se, Te) and three mixed‐cation (CuMSe2, M=Al, Ga, In) chalcopyrite systems. The random chalcopyrite alloys are represented by special quasirandom structures (SQS). The calculated bowing coefficients are in good agreement with the most recent experimental data for stoichiometric alloys. Results for the mixing enthalpies and the band offsets are provided as predictions to be tested experimentally. Comparing our calculated bowing and band offsets for the mixed‐anion chalcopyrite alloys with those of the corresponding Zn chalcogenide alloys (ZnX, X=S, Se, Te), we find that the larger p−d coupling in chalcopyrite alloys reduces their band offsets and optical bowing. Bowing parameters for ordered, Zn‐based II‐VI alloys in the CuAu, CuPt, and chalcopyrite structures are present...

429 citations


Journal ArticleDOI
TL;DR: This work inverts a set of self-consistently determined screened LDA potentials for a range of bulk crystal structures and unit cell volumes, thus determining spherically symmetric and structurally averaged atomic potentials (SLDA), and finds that the adjustment represents a reasonably small perturbation over the SLDA potential.
Abstract: Transferable screened atomic pseudopotentials were developed 30 years ago in the context of the empirical pseudopotential method (EPM) by adjusting the potential to reproduce observed bulk electronic energies. While extremely useful, such potentials were not constrained to reproduce wave functions and related quantities, nor was there a systematic way to assure transferability to different crystal structures and coordination numbers. Yet, there is a significant contemporary demand for accurate screened pseudopotentials in the context of electronic structure theory of nanostructures, where local-density-approximation (LDA) approaches are both too costly and insufficiently accurate, while effective-mass band approaches are inapplicable when the structures are too small. We can now improve upon the traditional EPM by a two-step process: {ital First}, we invert a set of self-consistently determined screened LDA potentials for a range of bulk crystal structures and unit cell volumes, thus determining spherically symmetric and structurally averaged atomic potentials (SLDA). These potentials reproduce the LDA band structure to better than 0.1 eV, over a range of crystal structures and cell volumes. {ital Second}, we adjust the SLDA to reproduce {ital observed} excitation energies. We find that the adjustment represents a reasonably small perturbation over the SLDA potential, so that the ensuing fitted potentialmore » still reproduces a {gt}99.9% overlap with the original LDA pseudowave functions despite the excitation energies being distinctly non-LDA. We apply the method to Si and CdSe in a range of crystal structures, finding excellent agreement with the {ital experimentally} {ital determined} band energies, optical spectra {epsilon}{sub 2}({ital E}), static dielectric constants, deformation potentials, and, at the same time, {ital LDA{minus}quality} wave functions.« less

160 citations


Journal ArticleDOI
TL;DR: In this paper, the role of subsurface strain in the ordering of Ga0.5In0P alloys was examined using a valence force field approach and ab initio pseudopotential calculations.
Abstract: Using a valence force field approach and ab initio pseudopotential calculations, we examine the role of subsurface strain in the ordering of Ga0.5In0.5P alloys. We show that depending on the orientation of the surface phosphorus dimers, these alloys can have (i) a CuPtA ordering for 1×2 or c(4×4) reconstruction; (ii) a CuPtB ordering for 2×1 or β2(2×4) reconstruction; and (iii) a triple period ordering for 2×3 or c(8×6) reconstruction. These results are in good agreement with recent experiments of Gomyo et al. [Phys. Rev. Lett. 72, 673 (1994); Jpn. J. Appl. Phys. 34, L469 (1995)].

95 citations


Journal ArticleDOI
TL;DR: The band-gap reduction and wave-function localization of alloys with short-range order is compared to the effects of long- range order, where the gap reduction is due to level repulsion between zone-folding conduction states.
Abstract: First-principles and empirical pseudopotentials are used to study the effects of short-range and long-range atomic order on the electronic properties of III-V semiconductor alloys. The alloy structure with a given degree of long- or short-range order is modeled by two types of supercells: (a) Small (16-32 atom) supercells are constructed in the fashion of the special quasirandom structures (SQS) used previously to simulate random alloys [A. Zunger et al., Phys. Rev. Lett. 65, 353 (1990)]. Their electronic structure is treated via first-principles pseudopotential methods. (b) Large (\ensuremath{\sim} 1000 atom) supercells are found by a simulated-annealing technique which optimizes the atomic configuration until a given degree of short-range order is reproduced. The electronic structure is then determined using the empirical pseudopotential method. Statistical tests prove that the small cell SQS mimic the much larger supercells and thus provide an efficient means of studying the electronic band structure of disordered alloys in a non-mean-field approach. For the direct band gaps of ideally random ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}\mathrm{As}$, ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{P}$, and ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{As}$ alloys, we find optical bowing parameters $b=0.48, 0.46, \mathrm{and} 0.52$ eV, respectively. In the presence of short-range order in the form of cation clustering, we find the following: (i) Clustering elongates the Ga-P bond and shortens the In-P bond in ${\mathrm{Ga}}_{0.5}$${\mathrm{In}}_{0.5}$P and (ii) the optical bowing of the direct band gap is greatly enhanced. This leads to an indirect-gap to direct-gap crossover in ${\mathrm{Al}}_{0.5}$${\mathrm{Ga}}_{0.5}$As with sufficient clustering. (iii) The band-gap reduction is accompanied by a localization of band-edge wave functions on certain types of clusters. The clusters act as "isoelectronic impurities" which localize states if their concentration (i.e., the degree of short-range order) is large enough. Electrons at the conduction-band minimum localize on the cations with lower $s$-orbital energies. The band-gap reduction and wave-function localization of alloys with short-range order is compared to the effects of long-range order, where the gap reduction is due to level repulsion between zone-folding conduction states. Numerical results are given for CuPt-type long-range order of AlGa${\mathrm{As}}_{2}$, GaIn${\mathrm{P}}_{2}$, and AlIn${\mathrm{As}}_{2}$. For complete ordering, the band-gap reduction relative to the random alloys are 0.36, 0.49, and 0.16 eV, respectively.

77 citations


Journal ArticleDOI
TL;DR: Electronic excitations and spin polarization are not fully responsible for the {ital T} dependence of {ital J}{sub {ital f}} used in the SRO calculations, by performing temperature-dependent self-consistent local-density-approximation calculations.
Abstract: The short-range order (SRO) and long-range order (LRO) of Ni-V and Pd-V alloys are studied theoretically by a combination of first-principles calculations of Ising-like interaction energies (${\mathit{J}}_{\mathit{f}}$) with a Monte Carlo solution of the Ising Hamiltonian. We find the following: (i) There are several compositions in these alloys for which the dominant wave vectors of LRO and those of SRO do not coincide, indicating that the low-temperature (T) LRO may not always be inferred from the high-T SRO. (ii) In ${\mathrm{Ni}}_{3}$V and ${\mathrm{Pd}}_{3}$V, the density of states at the Fermi level, n(${\mathrm{\ensuremath{\epsilon}}}_{\mathit{F}}$), is much larger in L${1}_{2}$ than in the stable D${0}_{22}$ structure. This has two consequences: (a) thermal electron-hole excitations across ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{F}}$ are energetically more favorable in the L${1}_{2}$ structure and lead to a T dependence of ${\mathit{J}}_{\mathit{f}}$, and (b) magnetization stabilization is larger in L${1}_{2}$, so spin polarization affects structural stability. As a result, (iii) calculations using T-dependent ${\mathit{J}}_{\mathit{f}}$'s are needed to obtain quantitative agreement with experimental measurements of LRO, SRO, and transition temperatures in ${\mathrm{Ni}}_{0.75}$${\mathrm{V}}_{0.25}$, ${\mathrm{Ni}}_{0.67}$${\mathrm{V}}_{0.33}$, and ${\mathrm{Pd}}_{0.75}$${\mathrm{V}}_{0.25}$.(iv) We provide predictions of the SRO patterns where there is currently no experimental evidence for ${\mathrm{Pd}}_{0.67}$${\mathrm{V}}_{0.33}$, ${\mathrm{Ni}}_{0.6}$${\mathrm{V}}_{0.4}$, ${\mathrm{Pd}}_{0.6}$${\mathrm{V}}_{0.4}$, and ${\mathrm{Pd}}_{0.5}$${\mathrm{V}}_{0.5}$. (v) For ${\mathrm{Ni}}_{3}$V and ${\mathrm{Pd}}_{3}$V, discrepancies are found between the total-energy differences of the L${1}_{2}$ and D${0}_{22}$ structures as determined by T=0 first-principles calculations and those inferred from diffuse neutron scattering measurements at high T. By performing temperature-dependent self-consistent local-density-approximation calculations, we find that electronic excitations are responsible for reducing the discrepancy by \ensuremath{\sim}25% and the combination of spin polarization and electronic excitations reduce the discrepancy by \ensuremath{\sim}30--50 %. Thus, electronic excitations and spin polarization are not fully responsible for the T dependence of ${\mathit{J}}_{\mathit{f}}$ used in the SRO calculations.

75 citations


Journal ArticleDOI
TL;DR: The type-I behavior noted for some As-rich InAs/InSb 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.
Abstract: 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.

69 citations


Journal ArticleDOI
TL;DR: The ability of generalized Ising-like cluster expansions to describe the energetics and thermodynamics associated with large atomic displacements in alloys and combination of the expansion with Monte Carlo simulations is shown to provide an efficient means for calculating thermodynamic properties.
Abstract: We demonstrate the ability of generalized Ising-like cluster expansions to describe the energetics and thermodynamics associated with large atomic displacements in alloys. Although the expansion is constructed only from the energies of a few (small-unit-cell) ordered structures, it provides accurate predictions of the atomically relaxed energies of random, ordered, or partially ordered alloys, as compared with direct, large scale energy-minimizing simulations. Relaxed energies are obtained without having to compute relaxed geometries. Combination of the expansion with Monte Carlo simulations is shown to provide an efficient means for calculating thermodynamic properties.

68 citations


Journal ArticleDOI
TL;DR: These calculations provide a detailed picture of how atoms are arranged in substitutionally random but positionally relaxed alloys, and offer an explanation for the effects of site correlations, static atomic relaxations, and dynamic vibrations on the phase-diagram and displacement maps.
Abstract: Extended x-ray-absorption fine-structure experiments have previously demonstrated that for each composition x, the sample average of all nearest-neighbor A-C distances in an ${\mathit{A}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{B}}_{\mathit{x}}$C semiconductor alloy is closer to the values in the pure (x\ensuremath{\rightarrow}0) AC compound than to the composition-weighted (virtual) lattice average. Such experiments do not reveal, however, the distribution of atomic positions in an alloy, so the principle displacement directions and the degrees of correlation among such atomic displacements remain unknown. Here we calculate both structural and thermodynamic properties of ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$P alloys using an explicit occupation- and position-dependent energy functional. The latter is taken as a modified valence force field, carefully fit to structural energies determined by first-principles local-density calculations. Configurational and vibrational degrees of freedom are then treated via the continuous-space Monte Carlo approach. We find good agreement between the calculated and measured mixing enthalpy of the random alloy, nearest-neighbor bond lengths, and temperature-composition phase diagram. In addition, we predict yet unmeasured quantities such as (a) distributions, fluctuations, and moments of first- and second-neighbor bond lengths as well as bond angles, (b) radial distribution functions, (c) the dependence of short-range order on temperature, and (d) the effect of temperature on atomic displacements. Our calculations provide a detailed picture of how atoms are arranged in substitutionally random but positionally relaxed alloys, and offer an explanation for the effects of site correlations, static atomic relaxations, and dynamic vibrations on the phase-diagram and displacement maps. We find that even in a chemically random alloy (where sites are occupied by Ga or In according to a coin toss), there exists a highly correlated static position distribution whereby the P atoms are displaced deterministically in certain high-symmetry directions.

48 citations


Journal ArticleDOI
TL;DR: In this paper, the mixing enthalpies for PdPt and Rh-Pt solid solutions are negative, in agreement with experiment, and they also exhibit ordering tendencies.
Abstract: First-principles quantum-mechanical calculations indicate that the mixing enthalpies for Pd-Pt and Rh-Pt solid solutions are negative, in agreement with experiment. Calculations of the diffuse-scattering intensity due to short-range order also exhibits ordering tendencies. Further, the directly calculated enthalpies of formation of ordered intermetallic compounds are negative. These ordering tendencies are in direct conflict with a 1959 prediction of Raub that Pd-Pt and Rh-Pt will phase-separate below ~760 °C (hence their mixing energy will be positive), a position that has been adopted by all binary alloy phase diagram compilations. The present authors predict that Pd1-xPtx will order in the L12, L10, and L12 structures ([001] superstructures) at compositionsx = 1/4, 1/2, and 3/4, respectively, while the ordered structures of Rh1-xPtx are predicted to be superlattices stacked along the [012] directions. While the calculated ordering temperatures for these intermetallic compounds are too low to enable direct growth into the ordered phase, diffuse-scattering experiments at higher temperatures should reveal ordering rather than phase-separation characteristics (i.e., off-F peaks). The situation is very similar to the case of Ag-Au, where an ordering tendency is manifested both by a diffuse scattering intensity and by a negative enthalpy of mixing. An experimental reexamination of PdPt and Rh-Pt is needed.

47 citations


Journal ArticleDOI
TL;DR: It is found that random fluctuations in layer thicknesses lead to a {ital direct} gap in the planar Brillouin zone, strong wave function localization along the growth direction, short radiative lifetimes, and a significant band-gap reduction, in agreement with experiments on such intentionally grown disordered superlattices.
Abstract: We use realistic pseudopotentials and a plane-wave basis to study the electronic structure of nonperiodic, three-dimensional, 2000-atom (AlAs${)}_{\mathit{n}}$/(GaAs${)}_{\mathit{m}}$ (001) superlattices, where the individual layer thicknesses $n,m\ensuremath{\in}{1,2,3}$ are randomly selected. We find that while the band gap of the equivalent ( $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}m\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2$) ordered superlattice is indirect, random fluctuations in layer thicknesses lead to a direct gap in the planar Brillouin zone, strong wave function localization along the growth direction, short radiative lifetimes, and a significant band-gap reduction, in agreement with experiments on such intentionally grown disordered superlattices.

42 citations


Journal ArticleDOI
TL;DR: In the case of spherical GaAs quantum dots embedded in an Al{sub {ital x}}Ga{sub 1{minus} x}}As alloy, it is shown that, as a function of the dot radius and the alloy composition, different alignments of the band-edge states lead to different regimes of the lowest-energy optical transition.
Abstract: Large GaAs domains embedded in an Al{sub {ital x}}Ga{sub 1{minus}{ital x}}As matrix act as potential wells for both electrons and holes, resulting in a direct band-gap system. When the GaAs domains become small, however, quantum-confinement effects may push the {Gamma}-like conduction-band state localized on GaAs above the {ital X}-like conduction-band state of the Al{sub {ital x}}Ga{sub 1{minus}{ital x}}As alloy, leading to an indirect band-gap system. Using a pseudopotential band-structure method, as well as the conventional one-band effective-mass approximation, we investigate the nature of the direct{r_arrow}indirect ({Gamma}{r_arrow}{ital X}) transition in GaAs/Al{sub {ital x}}Ga{sub 1{minus}{ital x}}As quantum films, wires, and dots. In the case of an {ital isolated} GaAs quantum structure embedded in AlAs, we find that the critical size for the onset of the {Gamma}{r_arrow}{ital X} transition increases from {similar_to}31 A in a two-dimensional film through {similar_to}56 A in a one-dimensional cylindrical wire to {similar_to}80 A in a zero-dimensional spherical dot. The interaction between GaAs quantum structures tends to {ital reduce} the critical size for the {Gamma}{r_arrow}{ital X} transition. We further study the effect of the alloy composition on the {Gamma}{r_arrow}{ital X} transition, finding that the critical size {ital decreases} when the Ga concentration of the alloy {ital increases}. In themore » case of spherical GaAs quantum dots embedded in an Al{sub {ital x}}Ga{sub 1{minus}{ital x}}As alloy, we show that, as a function of the dot radius and the alloy composition, different alignments of the band-edge states lead to different regimes of the lowest-energy optical transition.« less

Journal ArticleDOI
TL;DR: It is shown that spin-polarized electronic structure calculations are crucial for predicting the correct $T\phantom{\rule{0ex}{0ex}}0$ crystal structures for ${Pd}}_{3}X$ and ${Pt}X-X compounds.
Abstract: Spin polarization is known to lead to important magnetic and optical effects in open-shell atoms and elemental solids, but has rarely been implicated in controlling structural selectivity in compounds and alloys. Here we show that spin-polarized electronic structure calculations are crucial for predicting the correct $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ crystal structures for ${\mathrm{Pd}}_{3}X$ and ${\mathrm{Pt}}_{3}X$ compounds. Spin polarization leads to (i) stabilization of the ${L1}_{2}$ structure over the ${D0}_{22}$ structure in ${\mathrm{Pt}}_{3}\mathrm{Cr}$, ${\mathrm{Pd}}_{3}\mathrm{Cr}$, and ${\mathrm{Pd}}_{3}\mathrm{Mn}$, (ii) stabilization of the ${D0}_{22}$ structure over the ${L1}_{2}$ structure in ${\mathrm{Pd}}_{3}\mathrm{Co}$, and (iii) ordering (rather than phase separation) in ${\mathrm{Pt}}_{3}\mathrm{Co}$ and ${\mathrm{Pd}}_{3}\mathrm{Cr}$. The results are analyzed in terms of first-principles local spin density calculations.

Journal ArticleDOI
TL;DR: The mean-field theory of SRO is seen to qualitatively fail in describing fcc electrostatics, and electrostatic point-ion interactions lead to significant SRO correlations.
Abstract: We discuss the influence of point-ion electrostatics on the long- (LRO) and short-range order (SRO) in binary fcc-, bcc-, and simple-cubic-based (sc) alloys. The electrostatic problem is studied by a combination of (a) a model for the distribution of point charges on lattice sites, motivated by recent first-principles calculations, (b) a mapping of the infinite-ranged Coulomb interaction onto a rapidly convergent series of effective interactions, and (c) Monte Carlo simulated annealing of the ensuing Ising-like expansion. This provides a means to identify the lowest energy structures (``ground states'') at zero temperature and the dominant wave vectors of the SRO at high temperatures, which are stabilized by ionic interactions. (i) We confirm previous results that the three ground states of the fcc Madelung lattice are the D${0}_{22}$ (${\mathit{A}}_{3}$B and ${\mathit{AB}}_{3}$) and ``40'' (AB) structures, which can all be described as 〈210〉 superlattices. We further find that the ground states of the bcc and sc Madelung lattices are CsCl and NaCl, respectively. (ii) Despite the fact that the structure ``40'' has the lowest electrostatic energy of any fcc-type compound, this structure is very rare in nature. We find that this rarity could imply that a highly ionic fcc AB compound will transform to the bcc structure CsCl that is electrostatically more stable for the same charge distribution. The exception is when the energy required to promote the elemental solids A+B from fcc to bcc is larger than the gain in electrostatic energy. (iii) Monte Carlo and mean-field calculations both demonstrate that the dominant wave vectors of LRO and SRO coincide for the bcc and sc Madelung lattices.However, for compositions x\ensuremath{\lesssim}0.33 and x\ensuremath{\gtrsim}0.67 on the fcc lattice, mean-field calculations incorrectly predict SRO peaks at the 〈11/20〉 points, whereas Monte Carlo calculations show SRO peaks at the 〈100〉 points. Thus, in describing fcc electrostatics, the mean-field theory of SRO is seen to qualitatively fail. (iv) Electrostatic point-ion interactions lead to significant SRO correlations. Near the transition temperature, these correlations account for a \ensuremath{\gtrsim}60% change in the energy of the random alloy.

Journal ArticleDOI
TL;DR: In this article, a complete band-structure approach was proposed to predict that the electron effective mass in the ordering direction of Ga{sub 05}In{sub 5}P alloys increases with the degree of long-range order.
Abstract: The conventional eight-band {bold k}{center_dot}{bold p} model predicts a {ital decrease} of the electron effective mass and {ital no} {ital dependence} of the (001) strain band-gap deformation potential with the degree {eta} of long-range order in Ga{sub 05}In{sub 05}P alloys We show that a complete band-structure approach predicts instead that (i) the electron effective mass in the ordering direction {ital increases} from 0092{ital m}{sub 0} for {eta}=0 (random alloy) to 0133{ital m}{sub 0} for {eta}=1 (ordered alloy), and (ii) the strain deformation potential {ital decreases} in magnitude from 826 eV for {eta}=0 to 634 eV for {eta}=1 These two effects are caused by the mixing of the conduction-band minimum with the {ital L}-derived conduction band, neglected in the standard eight-band model

Journal ArticleDOI
TL;DR: Self-consistent solutions to such a constrained LDA problem reveal that the final hole state is sufficiently localized to trigger a self-interaction correction of 3--4 eV, needed to explain the discrepancy with experiment.
Abstract: Local-density approximation (LDA) band-structure calculations place the 3d band of zinc-blende ZnO, ZnS, ZnSe, and ZnTe at 54, 64, 68, and 75 eV below the valence-band maximum (VBM), while photoemission measurements place them at 78, 90, 94, and 98 eV below the VBM, respectively We show that this \ensuremath{\sim}3-eV LDA error can be accounted for using a ``broken symmetry'' band-structure approach In this approach, a d core hole is placed in an impuritylike splitoff d subband resulting from the creation of the hole on a particular Zn sublattice Self-consistent solutions to such a constrained LDA problem reveal that the final hole state is sufficiently localized to trigger a self-interaction correction of 3--4 eV, needed to explain the discrepancy with experiment This 3--4 eV shift is reduced, by screening effects, from the 97-eV value in a free Zn atom Finally, we calculated the binding energy ${\mathit{E}}_{\mathrm{Mn}}$ for Mn 3d states in ZnTe:Mn and the effective Coulomb interaction parameter ${\mathit{U}}_{\mathrm{eff}}$ Significant improvements over the results of local-spin-density calculations were found The calculated ${\mathit{E}}_{\mathrm{Mn}}$=${\mathit{E}}_{\mathrm{VBM}}$-393 eV and ${\mathit{U}}_{\mathrm{eff}}$=685 eV are in good agreement with experiments

Journal ArticleDOI
TL;DR: In this article, the effects of a few types of atomic disorder on the electronic and optical properties of AlAs/GaAs (001) and (111) superlattices were studied.
Abstract: We study the effects of a few types of atomic disorder on the electronic and optical properties of AlAs/GaAs (001) and (111) superlattices: (i) atomic intermixing across the interfaces; (ii) replacing a single monolayer in a superlattice by one containing the opposite atomic type (isoelectronic δ doping); and (iii) random layer‐thickness fluctuations in superlattices (SL). Type (i) is an example of lateral disorder, while types (ii) and (iii) are examples of vertical disorder. Using three‐dimensional empirical pseudopotential theory and a plane‐wave basis, we calculate the band gaps, electronic wave functions, and optical matrix elements for systems containing up to 2000 atoms in the computational unit cell. Spin‐orbit interactions are omitted. Computationally much less costly effective‐mass calculations are used to evaluate the density of states and eigenstates away from the band edges in vertically disordered SLs. Our main findings are: (i) Chemical intermixing across the interface can significantly shi...

Journal ArticleDOI
TL;DR: In this paper, a large number of ab initio calculated total energies of different GaP/InP superlattices are used to fit a Born-Oppenheimer energy surface.
Abstract: A large number of ab initio calculated total energies of different GaP/InP superlattices are used to fit a Born-Oppenheimer energy surface. Monte Carlo simulations are then performed on this surface, including treatment of configurational, positional and vibrational degrees of freedom. This permits isolation of the effects of these degrees of freedom on the thermodynamic behaviour. We find the following. (i) Positional relaxation of the atoms to equilibrium, (off-site) locations lowers enormously both the mixing enthalpy (by approximately 50%) and the miscibility gap (MG) temperature (from TMG=1746 K to TMG=833 K). (ii) Allowance for configurational correlations (absent in a mean-field treatment) reduces both the entropy and the enthalpy, leading to a net increase of approximately 70 K in TMG. (iii) Vibrations reduce TMG by approximately 30 K leading to a final TMG=870 K. The calculated phase diagram is in accord with experiment.

Journal ArticleDOI
TL;DR: In this article, a generalized Ising model was used to map the energy of a binary substitutional A1-xBx alloy to the LDA energy of all 2N configurations obtained by occupying N sites by A and B atoms.
Abstract: Accurate information of energetics is essential to map out the temperature versus composition phase diagram of a binary substitutional A1-xBx alloy. Since it is computationally prohibitive to calculate the total energies of all 2N configurations obtained by occupying N sites by A and B atoms, we map instead the ab initio calculated total energies of only O(10) simple structures (with

Journal ArticleDOI
TL;DR: This work uses a recently published set of high-accuracy structure factors, based on {gamma}-ray measurements, to derive the charge density distribution in crystalline germanium with a millielectron-level resolution and finds a considerable improvement in the fit residuals.
Abstract: We use a recently published set of high-accuracy structure factors, based on {gamma}-ray measurements [Dewey {ital et} {ital al}., Phys. Rev. B 50, 2800 (1994)] to derive the charge density distribution in crystalline germanium with a millielectron-level resolution. We use a multipole expansion model of the charge densities represented as a superposition of orbital-dependent, nonspherical atomic charge densities. We include in the model anharmonic and nonrigid atomic thermal motions. This model is then fit to the measured structure factors. We find (i) a considerable improvement in the fit residuals (especially for the low-order structure factors) relative to our previous analysis, based on earlier measurements; (ii) a factor-of-2 improvement in the agreement between experiment and our earlier {ital ab} {ital initio} density-functional calculated structure factors; (iii) the evidence for the existence of nonrigid atomic thermal motion is marginal; (iv) a clear {similar_to}15% expansion of the valence orbital density relative to the free Ge atom; and (v) a twofold reduced upper limit on the anharmonic force constant. These bring the structure of germanium into better agreement with those of silicon and diamond.

Journal ArticleDOI
TL;DR: It is shown that upon superlattice ordering these transitions are altered dramatically---some states shift up in energy, some shift down, and new ``ordering-induced`` transitions, absent in the disordered phase, now become allowed.
Abstract: We investigated theoretically the ordering-induced change of the {ital E}{sub 1}, {ital E}{sub 2}, and {ital E}{sub 0}{sup {prime}} transitions in Ga{sub 0.5}In{sub 0.5}P using symmetry arguments and first-principles band-structure calculations. We show that upon (111) superlattice ordering these transitions are altered dramatically---some states shift up in energy, some shift down, and new ``ordering-induced`` transitions, absent in the disordered phase, now become allowed. Experimental observation of these changes could serve as new fingerprints of ordering. We have also studied the pressure dependence of the energies of the {ital X}{sub 1{ital c}} and {ital L}{sub 1{ital c}} derived states in the ordered superlattice. The recent experimental observation of the ``{ital X}{sub 1{ital c}}-like`` state at higher pressure is identified as a mixture of the folded {ital L}{sub 1{ital c}} and {ital X}{sub 1{ital c}} states. A microscopic explanation is given.

Journal ArticleDOI
TL;DR: In this paper, the formation energies of ApBq superlattices with arbitrary periods p and q and layer orientation Ĝ can be predicted via a "cluster expansion" technique, given the formation energy of short period structures from first-principles calculations.

Journal ArticleDOI
TL;DR: In this paper, the authors derived the motif energies from a set of pseudopotential total energy calculations for flat GaAs(001) surfaces and for point defects in bulk GaAs.
Abstract: The energies of various steps on the As-terminated GaAs(001)-2 × 4 surface are evaluated using a novel, approximate method of “linear combination of structural motifs”. It is based on the observation that previous total energy minimizations of semiconductor surfaces produced invariably equilibrium structures made of the same recurring local structural motifs, e.g. tetrahedral fourfold Ga, pyramidal threefold As, etc. Furthermore, such surface structures were found to obey consistently the octet rules as applied to the local motifs. We thus express the total energy of a given semiconductor surface as a sum of (i) the energies eM of the local structural motifs appearing in the surface under consideration and (ii) an electrostatic term representing the Madelung energy of point charges resulting from application of the octet rule. The motif energies are derived from a set of pseudopotential total energy calculations for flat GaAs(001) surfaces and for point defects in bulk GaAs. This set of parameters suffices to reproduce the energies of other (001) surfaces, calculated using the same pseudopotential total energy approach. Application to GaAs(001)-2 × 4 surfaces with steps reveals the following. (i) “Primitive steps”, defined solely according to their geometries (i.e. step heights, widths and orientations) are often unstable. (ii) Additional, non-geometric factors beyond step geometries such as addition of surface adatoms, creation of vacancies and atomic rebonding at step edges are important to lower step energies. So is step-step interaction. (iii) The formation of steps is generally endothermic. (iv) The formation of steps with edges parallel to the direction of surface As dimers (A steps) is energetically favored over the formation of steps whose edges are perpendicular to the As dimers (B steps).

Journal ArticleDOI
TL;DR: It is shown that the previously neglected coupling between the spin-orbit split-off band and the crystal-field split-offs band is crucial for determining the RDS intensity of the Ga{sub {ital x}In{sub 1{minus}{ital x}}P alloy.
Abstract: Spontaneous CuPt-like ordering in III-V alloys causes an anisotropy in the intensities of the transitions between the split valence-band maximum states and the conduction-band minimum. This optical anisotropy has been detected in ordered III-V alloys using modulated reflectivity, and more recently, using reflectance-difference spectroscopy (RDS). We derive here a general formula relating the ordering-induced bulk RDS intensity with the degree of long-range order. We show that the previously neglected coupling between the spin-orbit split-off band and the crystal-field split-off band is crucial for determining the RDS intensity of the Ga{sub {ital x}}In{sub 1{minus}{ital x}}P alloy.

Journal ArticleDOI
10 Jul 1995-EPL
TL;DR: In this paper, the authors consider (AlAs)n/(GaAsn)n superlattices with random thickness fluctuations Δn around the nominal period n and show that any amount Δn/n of thickness fluctuations leads to band edge wave function localization.
Abstract: We consider (AlAs)n/(GaAs)n superlattices with random thickness fluctuations Δn around the nominal period n. Using three-dimensional pseudopotential plane-wave band theory, we show that i) any amount Δn/n of thickness fluctuations leads to band edge wave function localization, ii) for small Δn/n the SL band gap is pinned at the gap level produced by a single layer with "wrong" thickness n + Δn, iii) the bound states due to monolayer thickness fluctuations lead to significant band gap reductions, e.g., in n = 2, 4, 6, and 10 monolayer SLs the reductions are 166, 67, 29, and 14 meV for 111 SLs, and 133, 64, 36, and 27 meV for 001 SLs, iv) 001 AlAs/GaAs SLs with monolayer thickness fluctuations have a direct band gap, while the ideal 001 SLs are indirect for n < 4.

Journal ArticleDOI
TL;DR: Using three-dimensional pseudopotential plane-wave band theory, the authors showed that any amount Delta-n/n of thickness fluctuations leads to band-edge wavefunction localization.
Abstract: We consider (AlAs)_n/(GaAs)_n superlattices with random thickness fluctuations Delta-n around the nominal period n. Using three-dimensional pseudopotential plane-wave band theory, we show that (i) any amount Delta-n/n of thickness fluctuations leads to band-edge wavefunction localization, (ii) for small Delta-n/n the SL band gap is pinned at the gap level produced by a single layer with ``wrong'' thickness n + Delta-n, (iii) the bound states due to monolayer thickness fluctuations lead to significant band-gap reductions, (iv) AlAs/GaAs SL's with monolayer thickness fluctuations have a direct band gap, while the ideal SL's are indirect for n<4.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the evolution of effective masses, deformation potentials and pressure coefficients with the degree of long-range order in spontaneously ordered Ga 0.5 P alloys.
Abstract: Using ab initio all-electron methods, we investigate the evolution of effective masses, deformation potentials and pressure coefficients with the degree of long-range order in spontaneously ordered Ga 0.5 In 0.5 P alloys. We find that (i) the electron effective mass in the ordering direction increases significantly with the degree of order, while the effective mass in the perpendicular direction decreases ; this produces a strong anisotropy of the electron effective mass which is not present in the disordered alloy; (ii) the band-gap deformation potential for (001) epitaxial strain decreases with increasing degree of order, reflecting the extent of the ordering-induced Γ - L coupling; (iii) the band-gap pressure coefficient decreases from 8.4 meV/kbar in the disordered alloy to 6.6 meV/kbar in the ordered CuPt structure; interpolation to partial degrees of order leads to good agreement with recent high-pressure experiments.

Journal ArticleDOI
01 Sep 1995-EPL
TL;DR: In this article, Monte Carlo simulations of a chemically random Ga1-xInxP alloy are carried out and an interatomic potential has been parametrized to fit first-principle total energy calculations of various ordered GaP/InP structures.
Abstract: Monte Carlo simulations of a chemically random Ga1-xInxP alloy are carried out. An interatomic potential has been parametrized to fit first-principle total-energy calculations of various ordered GaP/InP structures. Using this potential, good agreement between predicted and measured properties of the Ga1-xInxP alloy are obtained. This potential is then used to predict positions of atoms in Ga1-xInxP yielding for the phosphorous atomic-positions probability peaks at several distinct lattice locations which emerge from correlated atomic displacements. The evolution of these displaced positions with temperature is followed by molecular-dynamics calculations showing that the average thermal r.m.s. displacements Ū obey ŪIn > ŪP > ŪGa.

Journal ArticleDOI
TL;DR: In this paper, the role of subsurface strain (caused by surface reconstruction and dimerization) in the ordering of Ga0.51n0.5P alloys was identified via thermodynamic energy minimization.
Abstract: We identify via thermodynamic energy minimization the role of subsurface strain (caused by surface reconstruction and dimerization) in the ordering of Ga0.51n0.5P alloys. Depending on the growth conditions, the alloy surfaces can have either β2(2×4), c(4×4) or c(8×6) reconstructions, with characteristic 2×1, 1×2 and 2×3 RHEED patterns. We show that (i) the 1×2 reconstruction will lead to a CUPtA surface ordering, (ii) a 2×1 reconstruction will lead to a CuPtB ordering, (iii) a 2×3 reconstruction will lead to a 3-period ordering, and (iv) single (double) bilayer steps are stable at low (high) anion chemical potential. These results are in good agreement with recent experimental observations.

Journal ArticleDOI
TL;DR: In this article, the authors studied the ordering induced changes in the optical properties of the semiconductor alloys and proposed a theory of reflectance-difference spectroscopy in ordered alloy.
Abstract: Many III–V semiconductor alloys exhibit spontaneous (111) alternate monolayer ordering when grown from the vapor phase. We have studied theoretically the ordering induced changes in the optical properties of the semiconductor alloys. We describe (i) the change of the band gap ΔEg and the valence band splitting ΔE12 as functions of the long range order parameter η, (ii) the consequence of coexistence of (001) epitaxial strain and (111) chemical ordering on the optical properties, (iii) optical anisotropy and spin polarization effects in the ordered alloy, (iv) theory of reflectance-difference spectroscopy in ordered alloy, and (v) the ordering-induced changes on high energy E1 and E2 transitions. Specific, experimentally testable predictions are listed in the summary section.