Showing papers by "Alex Zunger published in 1991"

First-principles statistical mechanics of structural stability of intermetallic compounds.

Abstract: While as elemental solids, Al, Ni, Cu, Rh, Pd, Pt, and Au crystallize in the face-centered-cubic (fcc) structure, at low temperatures, their 50%-50% compounds exhibit a range of structural symmetries: CuAu has the fcc-based L1o structure, CuPt has the rhombohedral L1& structure, and CuPd and A1Ni have the body-centered-cubic B2 structure, while CuRh does not exist (it phase separates into Cu and Rh). Phenomenological approaches attempt to rationalize this type of structural selectivity in terms of classical constructs such as atomic sizes, electronegativities, and electron/atom ratios. More recently, attempts have been made at explaining this type of selectivity in terms of the (quantum-mechanical) electronic structure, e.g. , by contrasting the self-consistently calculated total electron+ion energy of various ordered structures. Such calculations, however, normally select but a small, O(10) subset of "intuitive structures" out of the 2 possible configurations of two types of atoms on a fixed lattice with X sites, searching for the lowest energy. We use instead first-principles calculations of the total energies of O(10) structures to define a multispin Ising Hamiltonian, whose ground-state structures can be systematically searched by using methods of lattice theories. Extending our previous work on semiconductor alloys [S.-H. Wei, L. G. Ferreira, and A. Zunger, Phys. Rev. B 41, 8240 (1990)], this is illustrated here for the intermetallic compounds A1Ni, CuRh, CuPd, CuPt, and CuAu, for which the correct ground states are identified out of -65000 configurations, through the combined use of the densityfunctional formalism (to extract Ising-type interaction energies) with a simple configurational-search strategy (to find ground states). This establishes a direct and systematic link between the electronic structure and phase stability.

Surface-induced ordering in GaInP.

Abstract: ${\mathrm{Ga}}_{0.5}$${\mathrm{In}}_{0.5}$P alloys order spontaneously during growth into a (111) monolayer superlattice despite the fact that this is not the lowest-energy structure of the three-dimensional bulk compound. Using first-principles total-energy calculations, we show that a novel electronically driven surface reconstruction provides a driving force for such ordering.

Stability, Electronic Structure, and Phase Diagrams of Novel Inter- Semiconductor Compounds

Abstract: High-technology electronic devices are based on highly specialized core materials that make their operation pos sible : semiconductors. Unfortunately, the range of mate rial properties that make high-technology devices work is extremely narrow, since the sheer number of useful semiconductors is small. Hence, a major challenge has been to predict and develop new, potentially useful semiconductors. We describe here how the use of state- of-the-art techniques in both quantum and statistical mechanics can lead to predictions of new, stable, and ordered semiconductor alloys. A number of laboratories have already grown experimentally these new materials; efforts to characterize their useful material properties are ongoing in the United States, Japan, and Europe. This work describes the theoretical methodologies of our ap proach, and shows how supercomputers make possible the quantum-mechanical architectural analysis of new materials.

Disorder effects on the density of states of the II-VI semiconductor alloys Hg0.5Cd0.5Te, Cd0.5Zn0.5Te, and Hg0.5Zn0.5Te.

Abstract: The electronic structure of substitutionally random {ital A}{sub 1{minus}{ital x}B{ital x}C} alloys of zinc-blende semiconductors {ital AC} and {ital BC} departs from what a virtual-crystal approximation would grant both because of (i) a chemical perturbation, associated with an {ital electronicmismatch} between atoms {ital A} and {ital B}, and because of (ii) a structural perturbation (positional relaxation) induced by a {ital size} {ital mismatch} between {ital A} and {ital B}. Both effects on the electronic density of states are studied here for Hg{sub 0.5}Cd{sub 0.5}Te, Cd{sub 0.5}Zn{sub 0.5}Te, and Hg{sub 0.5}Zn{sub 0.5}Te in the context of first-principles self-consistent supercell models. We use our recently developed special quasirandom structures'' (A. Zunger, S.-H. Wei, L. G. Ferreira, and J. E. Bernard, Phys. Rev. Lett. 65, 353 (1990)) concept whereby lattice sites of a periodic structure are occupied by {ital A} and {ital B} atoms so as to closely reproduce the structural correlation functions of an infinite, perfectly random alloy. Total-energy minimization provides then the relaxed atomic positions while application of the local-density formalism, as implemented by the linearized augmented-plane-wave method, describes self-consistently the consequences of chemical and structural perturbations. We show how these perturbations lead both to (i) distinct {ital A}-like and {italmore » B}-like features in the density of states and the electronic charge densities, and even to (ii) different {ital C}-like features associated with fluctuations in the local environments around the common sublattice.« less

Long-range order in binary late-transition-metal alloys

Abstract: A ground-state search of a generalized, many-body Ising Hamiltonian whose interaction energies are determined from first-principles local-density calculations reveals that PtX intermetallics for X=Ni, Cu, Rh, and Pd will form stable ordered structures at low temperatures. In contrast, d-band tight-binding models universally predict phase separation in all late-transition-metal alloys. It is shown that the previously neglected s-electron cohesion is responsible for this phase stability.

Proposal for III‐V ordered alloys with infrared band gaps

Abstract: It is shown theoretically that the recently observed spontaneous ordering of III‐V alloys that yields alternate monolayer (111) superlattices provides the opportunity for achieving infrared band gaps in systems such as (InAs)1(InSb)1 and (GaSb)1(InSb)1. A substantial reduction in the direct band gap is predicted to result from the L‐point folding that repel the Γ band‐edge states.

Strain energy and stability of Si-Ge compounds, alloys, and superlattices

Abstract: First-principles total-energy pseudopotential calculations are carried out for Si, Ge, zinc-blende-structure SiGe, (${\mathrm{Si}}_{2}$${)}_{\mathit{p}}$/(${\mathrm{Ge}}_{2}$${)}_{\mathit{p}}$ superlattices in various layer orientations G and with various choices of substrate lattice parameter ${\mathit{a}}_{\mathit{s}}$, and for the ${\mathrm{Si}}_{0.5}$${\mathrm{Ge}}_{0.5}$ random alloy. A subset of the results is used to construct an energy model, incorporating both strain (via an anharmonic valence force field) and chemical interactions (via a rapidly convergent cluster expansion) that closely reproduces the first-principles results, including those not used as input to the model. The model is applied to the study of larger superlattices than are amenable to first-principles treatment, revealing trends in (i) constituent strain energies, (ii) interfacial ``strain-relief'' relaxation energies, and (iii) interfacial chemical energies. The analysis reveals the major regularities in the dependence of superlattice stability on {p,G,${\mathit{a}}_{\mathit{s}}$}, and permits investigation of the nature of interactions at interfaces, including the substrate-film interface.

Electronic structure of random Ag0.5Pd0.5 and Ag0.5Au0.5 alloys.

Abstract: The electronic density of states and mixing enthalpies of random substitutional ${\mathit{A}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{B}}_{\mathit{x}}$ alloys have often been described within the single-site coherent-potential approximation (SCPA). There one assumes that each atom interacts with a fictitious, highly symmetric average medium and that at a given composition x, all A atoms (and separately, all B atoms) are equivalent (i.e., have the same charges and atomic sizes). In reality, however, a random alloy manifests a distribution of different (generally, low-symmetry) local environments, whereby an atom surrounded locally mostly by like atoms can have different charge-transfer or structural relaxations than an atom surrounded mostly by unlike atoms. Such ``environmental effects'' (averaged out in the SCPA) were previously studied in terms of simple model Hamiltonians. We offer here an efficient method capable of describing such effects within first-principles self-consistent electronic-structure theory. This is accomplished through the use of the ``special-quasirandom-structures'' (SQS) concept [Zunger et al., Phys. Rev. Lett. 65, 353 (1990)], whereby the lattice sites of a periodic ``supercell'' are occupied by A's and B's in such a way that the structural correlation functions closely mimic those of a perfectly random infinite alloy. The self-consistent charge density, total and local density of states, and mixing enthalpies are then obtained by applying band theory (here, the linearized augmented-plane-wave method) to the SQS. Application to ${\mathrm{Ag}}_{0.5}$${\mathrm{Pd}}_{0.5}$ and ${\mathrm{Ag}}_{0.5}$${\mathrm{Au}}_{0.5}$ alloys clearly reveals environmental effects; that is, the charge distribution and local density of states of a given atomic site depend sensitively not only on the composition and occupation of the site but also on the distribution of atoms around it. This SQS approach provides a rather general framework for studying the electronic density of states of alloys.

Spontaneous surface-induced long-range order in Ga0.5In0.5P alloys.

Abstract: Previous total-energy calculations for bulk ${\mathrm{Ga}}_{0.5}$${\mathrm{In}}_{0.5}$P alloys have demonstrated that the lowest-energy configuration at T=0 corresponds to phase separation into GaP+InP, followed by the ordered ${\mathrm{GaInP}}_{2}$ chalcopyrite phase as the next lowest state; the (111)-ordered CuPt-like superstructure is predicted to lie at a much higher energy. Yet, vapor-phase crystal growth has shown CuPt-like long-range ordering in relatively thick ${\mathrm{Ga}}_{0.5}$${\mathrm{In}}_{0.5}$P films grown on a lattice-matched (001) GaAs substrate. We present here first-principles local-density total-energy calculations for ${\mathrm{Ga}}_{0.5}$${\mathrm{In}}_{0.5}$P/GaAs(001) in various two-dimensional structures, each having a free surface. For one-monolayer coverage, we find electronically driven surface reconstructions, consisting not only of the previously known cation dimerization, but also of buckling and tilting of the surface dimers. These considerably stabilize the CuPt-like surface topology over all other forms of surface order, including phase separation. Furthermore, a Ga/In layer covered by three monolayers still exhibits a significant energy preference (relative to ${\mathit{kT}}_{\mathit{g}}$, where ${\mathit{T}}_{\mathit{g}}$\ensuremath{\simeq}900 K is the growth temperature) for the CuPt structure. If complete atomic mobility were to exist irrespective of how deeply buried the atoms are, we would then expect that the surface-stable CuPt ordering would exist in the near-surface regions, whereas deeper layers would revert to the bulk-stable structures. Since, however, surface atomic mobilities are far larger than bulk mobilities, it is possible that surface-stabilized structures will be frozen in and consequently ordering will propagate into macroscopic film dimensions. In light of our results, we describe several possible ways that surface effects could lead to long-range CuPt-like ordering.

Electronic structure and density of states of the random Al0.5Ga0.5As, GaAs0.5P0.5, and Ga0.5In0.5As semiconductor alloys

Abstract: The electronic density of states (DOS), charge densities, equilibrium bond lengths, and optical bowing of the direct band gaps are calculated for three perfectly random semiconductor alloys within the first-principles pseudopotential method using the concept of ``special quasirandom structures'' (SQS's). The SQS's are periodic structures with moderately large unit cells whose sites are occupied by atoms in a way designed to reproduce the structural features of the infinite, perfectly random substitutional alloys. In avoiding averaging over atoms (as in the virtual-crystal approximation) or over atomic environments (as in the site-coherent-potential approximation), this approach is capable of revealing the multisite nature of chemical disorder, as well as atomic-relaxation effects. We show how the existence of different local environments about chemically identical sites leads to splittings and fine structures in the density of states, and how atomic relaxations are induced by such nonsymmetric environments and lead to significant modifications in these DOS features. The calculated alloy bond lengths and optical-bowing coefficients are found to be in good agreement with experiment. Relaxation-induced splittings in the DOS are offered as predictions for future photoemission studies.

Large lattice-relaxation-induced electronic level shifts in random Cu1-xPdx alloys.

Abstract: Mean-field theories of unrelaxed ${\mathrm{Cu}}_{0.75}$${\mathrm{Pd}}_{0.25}$ alloys exhibit a deep (${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{F}}$-5.5 eV) Pd bonding state at the bottom of the Cu d band that does not show up in photoemission experiments. We have applied the ``special quasirandom structures'' construct to ${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pd}}_{\mathit{x}}$ alloys in the context of local-density total-energy minimization, finding a distribution of Cu-Cu, Cu-Pd, and Pd-Pd bonds whose lengths deviate significantly from the single, unrelaxed value assumed in mean-field models. Such lattice distortions are found to induce a \ensuremath{\sim}1-eV shift in the Pd bonding state to lower binding energies, thus removing much of the discrepancy with experiment.

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

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

First-principles study of intervalley mixing: Ultrathin GaAs/GaP superlattices.

Abstract: Application des calculs de pseudopotentiel de premiers principes aux proprietes structurales, aux discontinuites de bande et aux etats electroniques (GaP) n (GaAs) n n≤3. Developpement d'une theorie systematique reliant les niveaux des superreseaux a ceux de leurs constituants en traitant le superreseau dans l'approximation de cristal virtuel. Description semiquantitative des etats de superreseau resultants

Real-space description of semiconducting band gaps in substitutional systems

Abstract: The goal of ``band-gap engineering'' in substitutional lattices is to identify atomic configurations that would give rise to a desired value of the band gap. Yet, current theoretical approaches to the problems, based largely on compilations of band structures for various latice configurations, have not yielded simple rules relating structural motifs to band gaps. We show that the band gap of substitutional AlAs/GaAs lattices can be usefully expanded in terms of a hierarchy of contributions from real-space ``atomic figures'' (pairs, triplets, quadruplets) detemined from first-principles band-structure calculations. Pair figures (up to fourth neighbors) and three-body figures are dominant. In analogy with similar cluster expansions of the total energy, this permits a systematic search among all lattice configurations for those having ``special'' band gaps. This approach enables the design of substitutional systems with certain band-gap properties by assembling atomic figures. As an illustration, we predict that the [01\ifmmode\bar\else\textasciimacron\fi{}2]-oriented (AlAs${)}_{1}$/(GaAs${)}_{4}$/(AlAs${)}_{1}$/(GaAs${)}_{2}$ superlattice has the largest band gap among all ${\mathrm{Al}}_{0.25}$${\mathrm{Ga}}_{0.75}$As lattices with a maximum of ten cations per unit cell.

Surface reconstructions and surface energies of monolayer‐coverage cation‐terminated Ga0.5In0.5P(001) surfaces

Abstract: Using the first‐principles pseudopotential method we have studied the fully covered cation‐terminated (001) surfaces of Ga0.5In0.5P alloys. We find that among several possible Ga and In surface patterns (within a 2×2 unit cell), the one corresponding to CuPt‐like bulk ordering is stabilized by ∼100 meV per surface atom. This structure has been observed experimentally in thick films, yet is known to be bulk unstable. The stability of a CuPt‐like surface layer is related to electronically driven surface reconstructions—dimerization, buckling, and tilting—which are discussed in detail.

Predicting structural energies of atomic lattices.

Abstract: The complexity of current {ital ab} {ital initio} quantum-mechanical calculations of the total energy of given distributions of atoms on a periodic lattice often limits explorations to just a few configurations. We show how such a small number of calculations can be used instead to compute the interaction energies of a generalized Ising model, which then readily provides predicted energies of many more interesting configurations. This is illustrated for AlAs/GaAs systems.

Thermodynamic instability of ordered (001) AlGaAs2 in bulk form.

Abstract: Recent tight-binding calculations of bulk electronic total energies by Koiller, Davidovich, and Falicov (KDF) [Phys. Rev. B 41, 3670 (1990)] indicated the tendency for ${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As alloys to form ordered structures. The stablest structure they predicted was the monolayer (AlAs${)}_{1}$(GaAs${)}_{1}$[001] superlattice, which was recently observed in homogeneous vapor-phase growth. In light of these results we have examined the possibility that bulk energetics can explain this ordering. We have subjected KDF's tight-binding total-energy calculations and, separately, our own first-principles pseudopotential total-energy calculations to a statistical-mechanics analysis of order-disorder transitions. We find that bulk thermodynamics is inconsistent with the observed ordering; hence, explanations must be sought elsewhere (e.g., surface thermodynamics or kinetic effects).