Showing papers by "Alex Zunger published in 2003"
TL;DR: Theoretical investigations of doping of several wide-gap materials suggest a number of rather general, practical "doping principles" that may help guide experimental strategies for overcoming doping bottlenecks as discussed by the authors.
Abstract: Theoretical investigations of doping of several wide-gap materials suggest a number of rather general, practical “doping principles” that may help guide experimental strategies for overcoming doping bottlenecks.
371 citations
TL;DR: First-principles modeling of grain boundaries (GB) in CuInSe2 semiconductors reveals that an energetic barrier exists for holes arriving from the grain interior (GI) to the GB, arising from reduced p-d repulsion due to Cu-vacancy surface reconstruction.
Abstract: First-principles modeling of grain boundaries (GB) in CuInSe2 semiconductors reveals that an energetic barrier exists for holes arriving from the grain interior (GI) to the GB Consequently, the absence of holes inside the GB prevents GB electrons from recombining At the same time, the GI is purer in polymaterials than in single crystals, since impurities segregated to the GBs This explains the puzzle of the superiority of polycrystalline CuInSe2 solar cells over their crystalline counterpart We identify a simple and universal mechanism for the barrier, arising from reduced p-d repulsion due to Cu-vacancy surface reconstruction This discovery opens up possibilities for the future design of superior polycrystalline devices
312 citations
TL;DR: In this paper, the atomistic pseudopotential method is used to accurately predict the electron-hole exchange-induced fine structure (FS) and polarization anisotropy in million-atom quantum dots of various shapes and compositions.
Abstract: The atomistic pseudopotential method is used to accurately predict the electron-hole exchange-induced fine structure (FS) and polarization anisotropy in million-atom ${\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dots of various shapes and compositions. The origin of the FS splittings is clarified using a simple model where the effects of atomistic symmetry and spin-orbit interaction are separately evident. Remarkably, polarization anisotropy and FS splittings are shown to occur, even in a cylindrically symmetric dot. Furthermore, ``dark excitons'' are predicted to be partially allowed. Trends in splittings among different shapes and compositions are revealed.
264 citations
TL;DR: The results agree quantitatively with recent measurements and offer new predictions on the dependence of Auger electron cooling on size, on correlation effects, and on the presence of a spectator exciton.
Abstract: Auger rates are calculated for CdSe colloidal quantum dots using atomistic empirical pseudopotential wave functions. We predict the dependence of Auger electron cooling on size, on correlation effects (included via configuration interaction), and on the presence of a spectator exciton. Auger multiexciton recombination rates are predicted for biexcitons as well as for triexcitons. The results agree quantitatively with recent measurements and offer new predictions.
256 citations
TL;DR: A cluster-doping approach in which a locally stable chemical environment is realized by using few dopant species is suggested, which explains two puzzling experimental observations, i.e., that monodoping N in ZnO via N2 fails to produce p-type behavior, whereas using an NO source produces metastable p- type behavior, which disappears over time.
Abstract: First-principles calculations on p-type doping of the paradigm wide-gap ZnO semiconductor reveal that successful doping depends much on engineering a stable local chemical bonding environment. We suggest a cluster-doping approach in which a locally stable chemical environment is realized by using few dopant species. We explain two puzzling experimental observations, i.e., that monodoping N in ZnO via N2 fails to produce p-type behavior, whereas using an NO source produces metastable p-type behavior, which disappears over time.
245 citations
TL;DR: In this paper, the authors investigated whether alternate lattice sites are favored and what the magnetic consequences of this might be for the ferromagnetic semiconductor GaAs under As-rich (Ga-poor) conditions prevalent at growth.
Abstract: While most calculations on the properties of the ferromagnetic semiconductor GaAs:Mn have focused on isolated Mn substituting the Ga site $({\mathrm{Mn}}_{\mathrm{Ga}}),$ we investigate here whether alternate lattice sites are favored and what the magnetic consequences of this might be. Under As-rich (Ga-poor) conditions prevalent at growth, we find that the formation energies are lower for ${\mathrm{Mn}}_{\mathrm{Ga}}$ over interstitial Mn $({\mathrm{Mn}}_{i}).$ As the Fermi energy is shifted towards the valence band maximum via external p doping, the formation energy of ${\mathrm{Mn}}_{i}$ is reduced relative to ${\mathrm{Mn}}_{\mathrm{Ga}}.$ Furthermore, under epitaxial growth conditions, the solubility of both substitutional and interstitial Mn are strongly enhanced over what is possible under bulk growth conditions. The high concentration of Mn attained under epitaxial growth of p-type material opens the possibility of Mn atoms forming small clusters. We consider various types of clusters, including the Coulomb-stabilized clusters involving two ${\mathrm{Mn}}_{\mathrm{Ga}}$ and one ${\mathrm{Mn}}_{i}.$ While isolated ${\mathrm{Mn}}_{i}$ are hole killers (donors), and therefore destroy ferromagnetism, complexes such as $({\mathrm{Mn}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{Mn}}_{i}\ensuremath{-}{\mathrm{Mn}}_{\mathrm{Ga}})$ are found to be more stable than complexes involving ${\mathrm{Mn}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{Mn}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{Mn}}_{\mathrm{Ga}}.$ The former complexes exhibit partial or total quenching of holes, yet ${\mathrm{Mn}}_{i}$ in these complexes provide a channel for a ferromagnetic arrangement of the spins on the two ${\mathrm{Mn}}_{\mathrm{Ga}}$ within the complex. This suggests that ferromagnetism in Mn-doped GaAs arises both from holes due to isolated ${\mathrm{Mn}}_{\mathrm{Ga}}$ as well as from strongly Coulomb stabilized ${\mathrm{Mn}}_{\mathrm{Ga}}\ensuremath{-}{\mathrm{Mn}}_{i}\ensuremath{-}{\mathrm{Mn}}_{\mathrm{Ga}}$ clusters.
109 citations
TL;DR: In this article, two s-like peaks appear, due to strong $s\ensuremath{-}d$ coupling, in the case of zinc blende semiconductors.
Abstract: Most zinc blende semiconductors have a single anion-like s state near the bottom of the valence band, found in density-of-states (DOS) calculations, and seen in photoemission. Here, we discuss the case where two s-like peaks appear, due to strong $s\ensuremath{-}d$ coupling. Indeed, away from the $\mathbf{k}=0$ Brillouin zone center, cation d states and anion s states can couple in zinc blende symmetry. Depending on the energy difference $\ensuremath{\Delta}{E}_{\mathrm{sd}}{=E}_{s}^{\mathrm{anion}}\ensuremath{-}{E}_{d}^{\mathrm{cation}},$ this interaction can lead to either a single or two s-like peaks in the DOS and photoemission. We find four types of behaviors. (i) In GaP, GaAs, InP, and InAs, $\ensuremath{\Delta}{E}_{\mathrm{sd}}$ is large, giving rise to a single cation d peak well below the single anion s peak. (ii) Similarly, in CdS, CdSe, ZnS, ZnSe, and ZnTe, we see also a single s peak, but now the cation d is above the anion s. In both (i) and (ii) the $s\ensuremath{-}d$ coupling is very weak. (iii) In GaN and InN, the local density approximation (LDA) predicts two s-like peaks bracketing below and above the cation d-like state. Correcting the too low binding energies of LDA by LDA+SIC (self-interaction correction) still leaves the two s-like peaks. The occurrence of two s-like peaks represents the fingerprint of strong $s\ensuremath{-}d$ coupling. (iv) In CdTe, LDA predicts a single s-like peak just as in case (ii) above. However, LDA+SIC correction shifts down the cation d state closer to the anion s band, enhancing the $s\ensuremath{-}d$ coupling, and leading to the appearance of two s-like peaks. Case (iv) is a remarkable situation where LDA errors cause not only quantitative energetic errors, but actually leads to a qualitative effect of a DOS peak that exists in LDA+SIC but is missing in LDA. We predict that the double-$s$ peak should be observed in photoemission for GaN, InN, and CdTe.
65 citations
TL;DR: In this paper, it was shown that MgO-ZnO is stable in the sixfold-coordinated structure for Zn concentrations below 67%, giving rise to spontaneously ordered alloys.
Abstract: Isostructural II-VI alloys whose components are either rocksalt stable (e.g., CaO-MgO) or zincblende stable (e.g., ZnS-ZnSe) are known to be thermodynamically unstable at low temperatures, showing a miscibility gap and nobulk ordering. In contrast, we show that heterostructural MgO-ZnO is stable, under certain conditions, in the sixfold-coordinated structure for Zn concentrations below 67%, giving rise to spontaneously ordered alloys. Using first-principles calculations, we explain the origin of this stability, the structures of their low-temperature ordered phases, short-range-order patterns, and their optical band-gap properties.
62 citations
TL;DR: In this article, the redshift of the exciton energy upon negative charging is rapidly reduced with increasing the In content and increasing the dot height, while the opposite behavior is observed upon positive charging.
Abstract: Atomistic pseudopotential many-body calculations of excitonic (X) recombination in charged, self-assembled ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ dots predict and explain remarkable trends. (i) The redshift of the exciton energy upon negative charging is rapidly reduced with increasing the In content and increasing the dot height. The opposite behavior is observed upon positive charging. (ii) The recombination peak energies of different charge states show intriguing symmetries and alignments, e.g., ${X}^{\ensuremath{-}}$ aligns with ${X}^{2\ensuremath{-}}$ and ${X}^{3\ensuremath{-}}$ aligns with ${X}^{4\ensuremath{-}}.$ (iii) The ${X}^{3\ensuremath{-}}$ spectrum shows that a triplet initial state is lower in energy for flat dots (yielding two spectral lines), whereas the singlet state is lower in energy for taller dots (yielding a single line). These trends are explained theoretically in terms of a crossover occurring at a critical In concentration and dot height at which the electron wave functions becomes more localized than the hole wave functions.
53 citations
TL;DR: It is discovered that Au-rich Cu1-xAux and Pt-rich Ni1- xPtx contain a composition range in which there is a quasicontinuum of stable, ordered "adaptive structures" made of (001) repeat units of simple structural motifs.
Abstract: One of the usual canons of solid state chemistry and metallurgy is that at low temperatures the equilibrium phase diagram of ordering alloys shows only a limited number of stable ordered phases (‘‘line compounds’’) [1– 3]. These line compounds appear at simple, Daltonian stoichiometric ratios and have fairly small unit cells. This fundamental paradigm led to the traditional description of ordered ‘‘ground state structures’’ in terms of Ising Hamiltonian [4 –8] with fairly short-range interatomic interactions. For example, if binary fcc A1� xBx compounds are described via an Ising Hamiltonian with nearest neighbor interatomic interactions of arbitrary magnitude [6 –8], a complete search of all possible ground states reveals a total of five possible zero-temperature structures, appearing at compositions x � 0, 0.25, 0.5, 0.75, and 1. However, in 1973, Anderson [9] noted experimentally the existence of a group of crystalline materials where, ‘‘within certain composition limits, every possible composition can attain a unique, fully ordered structure without defects.’’ Furthermore, such ‘‘infinitely adaptive structures’’ had a multiplicity of discrete higher energy, fully ordered structures which were separated energetically by only a small amount, for any one composition. An example of the then recognized [9] infinitely adaptive structures included phases related to lowtemperature Ta2O5, crystallographic ‘‘shear phases’’ of TiO2, ReO3-type, MoO3 and � -PbO2 structures, and the ‘‘microphases’’ of CeCd. In 1978, Kittel suggested [10] that long-range repulsive interactions can account for such infinite adaptivity [11]. However, even modern parametrizations of alloy Ising Hamiltonians from experimental data [12] or first-principles calculations [13,14] have been generally restricted to rather short-range interactions and thus could not reveal if a given alloy system has adaptive structures or not. We have previously developed a generalized Ising Hamiltonian (‘‘mixed basis cluster expansion’’) [15] in which the range of interactions and their types (pairs, triangle, tetrahedra, . . .) are decided via firstprinciples total-energy calculations on a set of configurations of various arrangements ApBq of A and B atoms on a lattice. The expansion is given in terms of chemical (pair and multiatom) interactions, as well as long-range, repulsive strain-driven elastic pair interactions, resulting from the forced coherence of the size-mismatched A and B planes along arbitrary crystallographic directions. The interaction energies are determined by the requirement that this cluster expansion reproduce the accurate configurational energies obtained by first-principles totalenergy calculations. This condition produces for many transition metal alloys fairly long-range chemical interactions (� 20–40 different pairs; 5–10 multibody terms), in addition to the formally infinite-range elastic interactions. We show here that a T � 0 ground state search of such a first-principles configurational Hamiltonian reveals for NiPt and CuAu a composition range, exclusively on the heavy-metal-rich side, exhibiting a dense, quasicontinuum of stable ground states, each having low-energy excited configurations. We identify the structural motifs in such ‘‘adaptive structures,’’ explain its stability. Although various long-period structures based on CuAu were previously identified [1] as entropy-stabilized finite temperature structures (e.g., CuAu-II), the T � 0 infinitely adaptive ground state structures are unsuspected [16,17].
41 citations
TL;DR: In this paper, the authors present a pseudopotential calculation of the single particle and excitonic spectrum of CdSe nanocrystals, and show that the energy gaps can effectively slow the hole relaxation process.
Abstract: We present a pseudopotential calculation of the single particle and excitonic spectrum of CdSe nanocrystals. We find that in the excitonic manifold derived from the ground state electron and the first 60 hole states there are two energy gaps much larger than the typical LO phonon energy in bulk CdSe. Such gaps can effectively slow the hole relaxation process, as recently found experimentally. We show that they originate from two gaps in the hole spectrum and are therefore a single-particle effect, as opposed to an excitonic effect. The calculated width of the gaps increases with decreasing dot size, in agreement with the experimental trend of the energy loss rate that decreases with dot size. We find that the presence of the gaps is not limited to CdSe nanocrystals with the wurtzite crystal structure but is also found in spherical InAs zinc blende dots. Comparison with our results for quantum rods and cylinders of different aspect ratios, and with a single-band effective mass model, shows the origin of th...
TL;DR: The electronic structure of GaAsN alloys was previously described in terms of nitrogen "cluster states" (CS) that exist in the dilute alloy in the bandgap, and perturbation host states (PHS) inside the conduction band.
Abstract: The electronic structure of GaAsN alloys was previously described in terms of nitrogen “cluster states” (CS) that exist in the dilute alloy in the bandgap, and “perturbed host states” (PHS) inside the conduction band. As the nitrogen concentration increases, the PHS plunge down in energy overtaking the CS. We show theoretically that the CS respond to the application of pressure in two different ways: the highly localized deep CS emerge (or remain) in the gap, because their pressure coefficient is lower than that of the conduction band minimum. In contrast, the shallow CS (first to be overtaken) hybridize so strongly with the conduction band that their pressure coefficient becomes comparable to that of the conduction states. These states fail to emerge into the gap upon application of pressure because they move, with application of pressure, at a similar rate with conduction states.
TL;DR: In this article, it was shown that hydrogen impurity can be used to dope chalcopyrite compounds, such as CuInSe and CuGaSe, but not to dope oxides such as ZnO.
Abstract: An impurity in a semiconductor can have either amphoteric behavior (no net production of electron or holes), or be an energetically deep center (carriers produced only at high temperature), or a shallow center (carriers produced even at low temperature). In most semiconductors (e.g., Si, GaAs, GaP, InP, and ZnSe) hydrogen impurities do not produce free carriers, being instead an amphoretic center; yet hydrogen does dope n-type some oxides such as ${\mathrm{SnO}}_{2}$ and ZnO. We studied theoretically whether or not H could dope chalcopyrite ${\mathrm{I}\ensuremath{-}\mathrm{I}\mathrm{I}\mathrm{I}\ensuremath{-}\mathrm{V}\mathrm{I}}_{2}$ compounds, ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}.$ Based on the first-principles calculations, we find that nonsubstitutionally incorporated hydrogen forms a deep donor in ${\mathrm{CuGaSe}}_{2},$ but a relatively shallow donor in ${\mathrm{CuInSe}}_{2}.$ The interaction of hydrogen with the abundant defect complex ${(2V}_{\mathrm{Cu}}+{\mathrm{In}}_{\mathrm{Cu}}{)}^{0}$ yields an even shallower donor, making ${\mathrm{CuInSe}}_{2}$ n type. In addition, our results show that hydrogen passivates the acceptorlike copper vacancies in both ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2},$ thus eliminating p type behavior. These findings, in conjunction with typical conditions under which ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$ are grown, indicate that ${\mathrm{CuInSe}}_{2}$ could be doped n type via hydrogen incorporation, whereas ${\mathrm{CuGaSe}}_{2}$ could not. The reason for the different behavior of ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$ towards hydrogen is that in the latter case the conduction-band minimum is at a considerably higher energy than in the former case. Despite this difference in electrical properties, it is predicted that hydrogen can be stored in both ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$ via implantation since the implanted hydrogens decorate copper atoms as well as preexisting copper vacancies.
TL;DR: In this paper, the existence of stable, ordered $3d\ensuremath{-}5d$ intermetallics CuAu and NiPt, as opposed to the unstable 4dd$ isovalent analogs CuAg, results from relativity.
Abstract: We show that the existence of stable, ordered $3d\ensuremath{-}5d$ intermetallics CuAu and NiPt, as opposed to the unstable $3d\ensuremath{-}4d$ isovalent analogs CuAg and NiPd, results from relativity. First, in shrinking the equilibrium volume of the $5d$ element, relativity reduces the atomic size mismatch with respect to the $3d$ element, thus lowering the elastic packing strain. Second, in lowering the energy of the bonding $6s,p$ bands and raising the energy of the $5d$ band, relativity enhances (diminishes) the occupation of the bonding (antibonding) bands. The raising of the energy of the $5d$ band also brings it closer to the energy of the $3d$ band, improving the $3d\ensuremath{-}5d$ bonding.
TL;DR: In this article, first-principles calculations on the chalcopyrite semiconductor CuInSe 2 reveal that facets terminated by the (112)-cation and (112-Se polar surfaces are lower in energy than the unfaceted (110) plane, despite the resulting increased surface area.
TL;DR: In this paper, the acceptor dopant clusters are stabilized in a chemical potential range different from that of the donor dopant cluster, which explains the experimental observation that a nonisovalent alloy has a distinct carrier polarity.
Abstract: A dilute nonisovalent semiconductor alloy, made of a III-V semiconductor component (GaAs) mixed with a II-VI semiconductor (ZnSe), can be viewed as the doping of a host semiconductor with a lower (higher) valent cation and a higher (lower) valent anion. We have investigated different doping types, i.e., monodoping, triatomic codoping, and cluster doping, in the ZnSe-GaAs system using ab initio pseudopotential plane-wave calculations. We find the following: (i) The acceptor dopant clusters are stabilized in a chemical potential range different from that of the donor dopant clusters. This explains the experimental observation that a nonisovalent alloy has a distinct carrier polarity. (ii) Cluster doping, e.g., $(\mathrm{Zn}\ensuremath{-}{\mathrm{Se}}_{4}{)}^{3+}$ or $(\mathrm{Se}\ensuremath{-}{\mathrm{Zn}}_{4}{)}^{3\ensuremath{-}}$ in GaAs, is predicted to be stable at extreme chemical potential limits, and also to contribute free carriers. (iii) Triatomic codoping is predicted to be thermodynamically unstable. (iv) Cluster doping produces shallower acceptor/donor levels than monodoping and triatomic codoping. (v) There is a strong attractive interaction between positively charged donors and negatively charged acceptors. Therefore, a high concentration of the charge-neutral dopant pairs exists in the alloy. This finding explains why free carriers in a nonisovalent alloy have a high mobility. (vi) Our results also explain the asymmetric dependence of the band gap on the alloy composition. Specifically, adding a small amount of Ga+As into ZnSe leads to a sharp drop in the band gap of the host crystal, whereas adding Zn+Se into GaAs does not change the band gap very much.
TL;DR: In this paper, the effect of 1:1 long range order on optical properties of alloys is investigated by means of first-principles calculations combined with large-scale atomistic empirical-pseudopotential simulations.
Abstract: The effect of 1:1 long range order on optical properties of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys is investigated by means of first-principles calculations combined with large-scale atomistic empirical-pseudopotential simulations. We propose an intra-band mechanism of ordering-induced band gap reduction for different optical polarization. The scaling of band gap reductions with order parameter is analyzed. Our simulations of inhomogeneous ordering suggest that coexistence of ordered and random domains may explain the large magnitude of the observed redshifts upon ordering.
TL;DR: Most studies of the anomalous electronic properties of the GaAs1-N-x alloy have focused on near-edge states, but x-ray spectroscopic experiments [V. N. Strocov et al. as mentioned in this paper, Phys. Status Solidi B 233, RI...
Abstract: Most studies of the anomalous electronic properties of the GaAs1-N-x alloy have focused on near-edge states, but x-ray spectroscopic experiments [V. N. Strocov et al., Phys. Status Solidi B 233, RI ...
TL;DR: In this paper, total energy calculations for hydrogen impurities in CuInSe2 and CuGaSe2 (CIS) showed that H+ takes up the Cu-Se bond center position, whereas H0 and H− take up tetrahedral interstitial site next to In (in CIS) or Ga (in CGS).
Abstract: First-principles total-energy calculations for hydrogen impurities in CuInSe2 (CIS) and CuGaSe2 (CGS) show that H+ takes up the Cu–Se bond center position, whereas H0 and H− take up tetrahedral interstitial site next to In (in CIS) or Ga (in CGS). Hydrogen creates a negative-U center (i.e., H0 is never stable), with a (+/−) transition level at Ec−0.39 eV in CIS, and Ec−0.57 eV in CGS. However, once combined with the 2VCu−+IIICu2+ complex, hydrogen forms shallower centers with transition levels at Ec−0.15 eV in CIS, and Ec−0.39 eV in CGS. We conclude that hydrogen could convert CIS to n type, but not CGS.
TL;DR: In this article, an atomic-scale description of interdiffused interfaces is provided to correctly reproduce the observed spectroscopic trend of the lowest valence-to-conduction confined transitions in narrow superlattices.
Abstract: Recent measurements surprisingly show that the lowest valence-to-conduction confined transitions in narrow $(\mathrm{InAs}{)}_{8}/(\mathrm{GaSb}{)}_{n}$ and $(\mathrm{InAs}{)}_{6}/(\mathrm{GaSb}{)}_{n}$ superlattices increase in energy as the barrier thickness n increases. We show that in addition to the mesoscopic geometric quantities (well and barrier sizes), an atomic-scale description of interdiffused interfaces is needed to correctly reproduce the observed spectroscopic trend. The interdiffused interface is modeled via diffusion equations. We compare our atomistic empirical pseudopotential calculation in which only the bulk binary data are fit to experiment, with contemporary methods in which agreement with experiment is forced using ideally abrupt interfaces.
TL;DR: In this paper, the effects of different interfacial bonding compositions on the electronic and optical properties of the InAs/GaSb superlattices were investigated using a simple kinetic model of molecular beam epitaxy growth.
Abstract: Interfacial interdiffusion in quantum wells and superlattices could alter the interfacial strain, band alignment, and even the atomic symmetry at the interface, thus potentially changing the electronic and optical properties. We consider the InAs/GaSb system describing the interdiffused interfaces via a simple kinetic model of molecular beam epitaxy growth. The predicted atomic positions after interdiffusion are then used in a pseudopotential theory to describe the electronic and optical consequences of interdiffusion. We determine (i) the effects of different interfacial bonding compositions on the electronic and optical properties; (ii) the segregation profiles at the normal and inverted interfaces; and (iii) the effect of structural disorder on band gaps. The application of our method to the InAs/GaSb superlattices allows us to explain numerous observed results and trends.
10 Nov 2003
TL;DR: In this article, the effects of interfacial atomic segregation on the electronic and optical properties of InAs/GaSb superlattices were studied and a pseudopotential method was proposed to predict the band structure dependence on the detailed atomic configuration.
Abstract: The authors study the effects of interfacial atomic segregation on the electronic and optical properties of InAs/GaSb superlattices. They describe their atomistic empirical pseudopotential method and test its performance against the available experimental data. They show its ability to predict the band structure dependence on the detailed atomic configuration, and thus to properly account for the effects of interfacial atomic segregation and structural disorder. They also show how their method avoids the approximations underlying the pseudopotential method of Dente and Tilton, which gives different results. The application of the proposed method to the InAs/GaSb superlattices allows the explanation of some observed experimental results, such as: the bandgap difference between (InAs)8/(GaSb)8 superlattices with almost pure InSb-like or GaAs-like interfaces; the large blue shift of the bandgap when the growth temperature of the superlattice increases; and the blue shift of the bandgap of (InAs)8/(GaSb)n superlattices with increasing GaSb period n. They present a detailed comparison of their predicted blue shift with that obtained by other theories.
01 Mar 2003
TL;DR: In this paper, the existence of stable, ordered 3D-5d intermetallics, as opposed to the unstable 3d-4d isovalent analogs CuAg and NiPd, results from relativity.
Abstract: We show that the existence of stable, ordered 3d-5d intermetallics CuAu and NiPt, as opposed to the unstable 3d-4d isovalent analogs CuAg and NiPd, results from relativity. First, in shrinking the equilibrium volume of the 5d element, relativity reduces the atomic size mismatch with respect to the 3d element, thus lowering the elastic packing strain. Second, in lowering the energy of the bonding 6s,p bands and raising the energy of the 5d band, relativity enhances (diminishes) the occupation of the bonding (antibonding) bands. The raising of the energy of the 5d band also brings it closer to the energy of the 3d hand, improving the 3d-5d bonding.