Showing papers by "Alex Zunger published in 1984"

Theory of the band-gap anomaly in AB C 2 chalcopyrite semiconductors

Abstract: Using self-consistent band-structure methods, we analyze the remarkable anomalies (g50%) in the energy-band gaps of the ternary $\mathrm{I}B\ensuremath{-}\mathrm{I}\mathrm{I}\mathrm{I}A\ensuremath{-}\mathrm{V}\mathrm{I}{A}_{2}$ chalcopyrite semiconductors (e.g., CuGa${\mathrm{S}}_{2}$) relative to their binary zinc-blende analogs $\mathrm{II}B\ensuremath{-}\mathrm{V}\mathrm{I}A$ (e.g., ZnS), in terms of a chemical factor $\ensuremath{\Delta}{E}_{g}^{\mathrm{chem}}$ and a structural factor $\ensuremath{\Delta}{E}_{g}^{S}$. We show that $\ensuremath{\Delta}{E}_{g}^{\mathrm{chem}}$ is controlled by a $p\ensuremath{-}d$ hybridization effect $\ensuremath{\Delta}{E}_{g}^{d}$ and by a cation electronegativity effect $D{E}_{g}^{\mathrm{CE}}$, whereas the structural contribution to the anomaly is controlled by the existence of bond alternation (${R}_{\mathrm{AC}}\ensuremath{ e}{R}_{\mathrm{BC}}$) in the ternary system, manifested by nonideal anion displacements $u\ensuremath{-}\frac{1}{4}\ensuremath{ e}0$. All contributions are calculated self-consistently from band-structure theory, and are in good agreement with experiment. We further show how the nonideal anion displacement and the cubic lattice constants of all ternary chalcopyrites can be obtained from elemental coordinates (atomic radii) without using ternary-compound experimental data. This establishes a relationship between the electronic anomalies and the atomic sizes in these systems.

Bond lengths around isovalent impurities and in semiconductor solid solutions

Abstract: Using a valence force field, we predict the symmetric lattice distortions around isovalent impurities in 64 semiconductor-impurity systems. For the five systems for which extended x-ray absorption fine-structure (EXAFS) data are available, the results are in excellent agreement with experiment. Our theory also explains quantitatively, without adjustable parameters, the observed bond-length variations in solid solutions ${A}_{1\ensuremath{-}x}{B}_{x}C$ of semiconductor alloys, as well as their excess enthalpies of mixing.

A universal trend in the binding energies of deep impurities in semiconductors

Abstract: Whereas the conventional practice of referring binding energies of deep donors and acceptors to the band edges of the host semiconductor does not produce transparent chemical trends when the same impurity is compared in different crystals, referring them to the vacuum level through the use of the photothreshold reveals a remarkable material invariance of the levels in III-V and II-VI semiconductors. It is shown that this is a consequence of the antibonding nature of the deep gap level with respect to the impurity atom-host orbital combinations.

Many-electron multiplet effects in the spectra of 3 d impurities in heteropolar semiconductors

Abstract: The excitation energies of impurities in semiconductors, as well as their donor and acceptor ionization energies, represent a combination of one-electron and many-electron multiplet effects, where the latter contribution becomes increasingly significant as localized states are formed. Analysis of the absorption and ionization data for 3d impurities is often obscured by the inability of contemporary multiplet theories (e.g., the Tanabe-Sugano approach) to separate these two contributions and by the inadequacy of mean-field, one-electron theories that neglect multiplet effects altogether. We present a novel theory of the multiplet structure of localized impurities in semiconductors that circumvents the major shortcomings of the classical Tanabe-Sugano approach and at the same time separates many-electron from mean-field effects. Excitation and ionization energies are given as a sum of mean-field (MF) and multiplet corrections (MC): AE =hEMF +AEMc. We determine EEMc from the analysis of the experimental data. This provides a way to compare experimentally deduced mean-field excitation and ionization energies AEMF —AE —AEMc with the results of electronicstructure calculations. The three central quantities of the theory —the eand t2-orbital deformation parameters and the effective crystal-field splitting —can be obtained from mean-field electronicstructure calculations, or, alternatively, can be deduced from experiment. In this paper, we analyze the absorption spectra of 3d impurities in ZnO, ZnS, ZnSe, and GaP, as well as those of the bulk Mott insulators NiO, CoO, and MnO, in light of the new approach to multiplet effects. These mean-field parameters are shown to display simple chemical regularities with the impurity atomic number and the covalency of the host crystal; they combine, however, to produce interesting nonmonotonic trends in the many-electron correction terms AEMC. These trends explain many of the hitherto puzzling discrepancies between one-electron (AEMF ) theory and experiment (hE). This approach unravels the chemical trends underlying the excitation and donor or acceptor spectra, pro-

Electronic structure of the ternary pnictide semiconductors ZnSiP 2 , ZnGeP 2 , ZnSnP 2 , ZnSiAs 2 , and MgSiP 2

Abstract: (where each anion C is coordinated by two A-type cations and two 8-type cations) At the same time, the advantage of the EPM approach lies in its flexibility to emulate the experimentally known optical gaps into the band-structure calculation through small adjustments of the parameters, usually producing a perfect agreement with the available data The present authors have previously described a series of all-electron, nonempirical self-consistent studies of the electronic structure of I-III-VI2 chalcopyrites (eg, CuInSe2) where the presence of chemically active Cu 3d orbitals at the vicinity of the top of the valence band makes the EPM particularly problematic In the present paper we extend these calculations to five of the II-IV-Vz chalcopyrite-structure ternary pnictides While existing EMP calculations for many of these compounds are adequate and generally produce fundamental band gaps in

Separation of one- and many-electron effects in the excitation spectra of 3d impurities in semiconductors

Abstract: We present a new multiplet theory that separates mean-field from multiplet effects in the excitation and donor-acceptor ionization spectra of localized impurities. Analysis of the experimental data for all 3d impurities in ZnO, ZnS, ZnSe, and GaP for which sufficient data exist and for the bulk Mott insulators CoO, MnO, and NiO reveals, for the first time, regular chemical trends in many-electron effects with the impurity and the host-crystal covalency and delineates the regime where one-electron theory is applicable from the region where it is not.

Localization and magnetism of an interstitial iron impurity in silicon

Abstract: The apparent dichotomy between the covalently delocalized nature of Si: Fe (as suggested by its reduced hyperfine field, its extended spin density, and the occurrence of two closely spaced stable charge states within 0.4 eV) and the atomically localized picture (suggested, among other reasons, by the stability of a high-spin ground-state configuration) is resolved through a self-consistent Green's-function calculation with the self-interaction-corrected local-spin-density formalism.

Breathing-mode relaxation around tetrahedral interstitial 3d impurities in silicon

Abstract: Transition-metal (TM) atoms from a unique class of impurities in silicon' Having highly localized 1 orbitals unmatched by the sp bonded host crystal, they form deep impurity states that couple literally to thousands of host bands3 and, in sharp contrast to isolated TM ions, they can sustain a multitude of stable spin and charge states in a relatively narrow energy range2 4 (the Si band gap). This is accomplished without forming ionic bonds, but rather through an intra-atomic population inversion s d" 0"+ and a polarization of the valence band. 3 The ability of these impurities to accomodate themselves electronically to the host crystal (electrical self-regulating response4) despite profound chemical dissimilarities to Si, raises the question of their ability to accomodate themselves structurally (lattice relaxation). Electron paramagnetic resonance (EPR) studies'2 suggest that under normal preparation conditions most TM impurities occupy in Si the tetrahedral interstitial sites (coordination number of four), preserving the cubic symmetry of the host crystal (i.e. , no symmetry breaking static Jahn-Teller distortions). Symmetry-conserving distortions (i.e., breathing mode) were not studied experimentally. In this Rapid Communication we report the first theoretical prediction of lattice relaxation around interstitial TM impurities in a semiconductor, Previous calculations on TMinduced relaxation were specialized to ionic solids' and were based on wave-functionless models with phenomenological pair potentials. We have calculated self-consistently the electronic structure of undistorted Si:TM for TM = Cr, Mn, Fe, Co, and Ni using in the local density formalism and the quasi-band-crystal-field (QBCF) Green's function method. 6 We obtained the ab initio impurity-induced forces using the pseudopotential Hellmann-Feynman theorem7 8 and calculated the breathing distortions of the first 9 shells (82 atoms) by using an empiricial Si force field. 7 We find that for all impurities studied the first shell of nearest neighbors (1NN) containing four Si atoms moves away from the impurity, whereas the second nearest neighbor shell (2NN) containing six Si atoms moves towards the impurity. This deformation pattern tends to equalize the TM —Si bonds to 2.4—2.6 A, producing a rare molecular species of an approximately tenfold coordinated TM. Interestingly, the Coulombic part of the host pseudopotential, —8/r predicts a reverse (i.e., inward) motion of the 1NN but the repulsive part /sF, '= —J) [r'duv, (r)/drh pI t(r)dr, (2)

Interstitial transition atom impurities in silicon: electronic structure and lattice relaxation

Abstract: Both the electronic structure and the `breathing-mode' relaxation for tetrahedral interstitial 3D transition atom impurities in silicon are studied in the local-density approximation. The calculations show that although the interstitial 3d impurities constitute a very large perturbation locally, they interact rather weakly with the surrounding crystal in the sense that they perturb the spatial distribution of electrons on the surrounding atoms only weakly. A special pattern of relaxation is predicted, with an outward relaxation of the first-nearest neighbours and an inward relaxation of the second-nearest neighbours. It is explained in terms of the impurity-induced charge rearrangement.(19 refs)