Koster-Slater Model Research Articles

Substitutional impurity in single-layer graphene: The Koster–Slater and Anderson models

The Koster–Slater and Anderson models are used to consider substitutional impurities in free-standing single-layer graphene. The density of states of graphene is described using a model (the M model). For the nitrogen and boron impurities, the occupation numbers and the parameter η which defines the fraction of delocalized electrons of the impurity are determined. In this case, experimental data are used for both determination of the model parameters and comparison with the results of theoretical estimations. The general features of the Koster–Slater and Anderson models and the differences between the two models are discussed. Specifically, it is shown that the band contributions to the occupation numbers of a nitrogen atom in both models are comparable, whereas the local contributions are substantially different: the local contributions are decisive in the Koster–Slater model and negligible in the Anderson model. The asymptotic behavior of the wave functions of a defect is considered in the Koster–Slater model, and the electron states of impurity dimers are considered in the Anderson model.

Heterovalent interlayers and interface states: Anab initiostudy ofGaAs∕Si∕GaAs(110) and (100) heterostructures

We have investigated ab initio the existence of localized states and resonances in abrupt GaAs/Si/GaAs (110)- and (100)-oriented heterostructures incorporating 1 or 2 monolayers (MLs) of Si, as well as in the fully developed Si/GaAs (110) heterojunction. In (100)-oriented structures, we find both valence- and conduction-band related near-band edge states localized at the Si/GaAs interface. In the (110) systems, instead, interface states occur deeper in the valence band; the highest valence-related resonances being about 1 eV below the GaAs valence-band maximum. Using their characteristic bonding properties and atomic character, we are able to follow the evolution of the localized states and resonances from the fully developed Si/GaAs binary junction to the ternary GaAs/Si/GaAs (110) systems incorporating 2 or 1 ML of Si. This approach also allows us to show the link between the interface states of the (110) and (100) systems. Finally, the conditions for the existence of localized states at the Si/GaAs (110) interface are discussed based on a Koster-Slater model developed for the interface-state problem.

Pressure behavior of Te isoelectronic centers in S‐rich ZnS1–xTex alloy

AbstractThe photoluminescence from ZnS1–xTex alloy with 0 < x < 0.3 was investigated under hydrostatic pressure up to 7 GPa. Two peaks were observed in the alloys with x < 0.01, which are related to excitons bound to isolated Te isoelectronic impurities (Te1 centers) and Te pairs (Te2 centers), respectively. Only the Te2 related emissions were observed in the alloys with 0.01 < x < 0.03. The emissions in the alloys with 0.03 < x < 0.3 are attributed to the excitons bound to the Ten (n ≥ 3) cluster centers. The pressure coefficient of the Te1 related peak is 89(4) meV/GPa, about 40% larger than that of the band gap of ZnS. On the other hand, the pressure coefficient of the Te2 related emissions is only 52(4) meV/GPa, about 15% smaller than that of the ZnS band gap. A simple Koster–Slater model has been used to explain the different pressure behavior of the Te1 and Te2 centers. The pressure coefficient of the Te3 centers is 62(2) meV/GPa. Then the pressure coefficients of the Ten centers decrease rapidly with further increasing Te composition.

Pressure behavior of Te isoelectronic centers in ZnS:Te

ZnS:Te epilayers with Te concentration from 0.5% to 3.1% were studied by photoluminescence under hydrostatic pressure at 15 K. Two emission bands related to the isolated Te1 and Te2 pair isoelectronic centers were observed in the samples with Te concentrations of 0.5% and 0.65%. For the samples with Te concentrations of 1.4% and 3.1%, only the Te2-related peak was observed. The pressure coefficients of all the Te1-related bands were found to be unexpectedly much larger than that of the ZnS band gap. The pressure coefficients for all the Te2-related bands are, however, rather smaller than that of ZnS band gap as usually observed. Analysis based on a Koster–Slater model indicates that an increase of the valence bandwidth with pressure is the main reason for the faster pressure shift of the Te1 centers, and the huge difference in the pressure behavior of the Te1 and Te2 centers is due mainly to the difference in the pressure-induced enhancement of the impurity potential on the Te1 and Te2 centers.

Koster-Slater model for the interface-state problem

Koster-Slater model for the interface-state problem

Isoelectronic impurity states in GaAs:N

Using the one-band one-site Koster-Slater model, we explain the different behavior of isoelectronic impurities in GaAs:N and GaP:N in terms of their band-structure difference. We show that the two lowest nitrogen bound states, ${\mathrm{NN}}_{1}$ and ${\mathrm{NN}}_{2},$ are associated with the [220] and [110] nitrogen pairs, respectively, that the optical transition of the former is dipole allowed whilst the latter is forbidden in both systems, and that the order of the [220] and [110] pair levels are reversed in the two systems.

Photonic band gaps and defect states induced by excitations of Bose-Einstein condensates in optical lattices

We study the interaction of a Bose-Einstein condensate, which is confined in an optical lattice, with a largely detuned light field propagating through the condensate. If the condensate is in its ground state it acts as a periodic dielectric and gives rise to photonic band gaps at optical frequencies. The band structure of the combined system of condensed lattice-atoms and photons is studied by using the concept of polaritons. If elementary excitations of the condensate are present, they will produce defect states inside the photonic band gaps. The frequency of localized defect states is calculated using the Koster-Slater model.

Ion neutralization on composite catalysts

Ion neutralization near the surface of the composite catalysts Pt (or Ni)/ZnO is investigated by means of the Green-function method and the approximate many-level approach within the framework of the tight-binding model. The catalyst metal film is represented by a one-dimensional finite chain of d orbitals, the semiconductor support ZnO is characterized by a linear semi-infinite chain of alternating s and p orbitals, and the effects of a substitutional impurity atom at the surface of a substrate are studied using the Koster-Slater model. The calculated results show that the electronic structures of both the supported metal film and the support ZnO play important roles in the resonant charge-transfer process, upon which the impurity atom at the surface also exerts significant effects.

Effect of impurity on ion neutralization.

The influence of a substitutional impurity atom on the ion neutralization near the (100) surface of a simple cubic substrate is investigated using the Green's-function method. Ion neutralization is studied within the framework of the approximate many-level method. The pure metal substrate is described by the three-dimensional tight-binding scheme, and the impurity is described by the Koster-Slater model. The effects of the impurity site, orbital energy, and impact site on ion neutralization are discussed in detail. Calculations show that the ion occupancy depends strongly on the impact site, and the impurity can play a potentially dominant role in surface-ion neutralization.

Impurity Effects on Overlayer Structures

The Green function method and the tight-binding approximation are employed to study the indirect interaction between two adatoms. Overlayer structures of hydrogen, oxygen and haloid adsorbed on contaminated nickel substrates are explained. The impurity effects are described by the Koster-Slater model. It is found that the introduction of impurity atoms can even change the overlayer structures.

Vibronic Koster-Slater impurity: Exactly soluble model of deep levels in semiconductors.

The Koster-Slater model of a deep impurity, extended by a single phonon mode coupled to the electron only at the impurity site, can be solved exactly, i.e., not using an adiabatic or static approximation. Total (vibronic) eigenenergies and eigenfunctions are derived and compared with the adiabatic ansatz

THEORETICAL CALCULATIONS FOR HYPERFINE INTERACTIONS OF THE JAHN-TELLER DISTORTED DIVACANCY IN SILICON

For two main Jahn-Teller distortion modes of the divacancy in Si, hyperfine interactions of the unpaired electron of both negative and positive charged states (V2-,V2+) are calculated by using extended Koster-Slater model. The calculated results in agreement with the experiments are obtained., From these, we determe that the unpaired electron of V2+ is located at the symmetric state Ag of the energy level at 0.3eV and the V2- is located at the symmetric state Bu of the energy level at 0.63eV. We infer that the EG1 mode is the preferential mode of distortion for both V2- and V2+.

Change in density of states due to chemisorption on a substrate containing an impurity

The presence of an impurity atom within a metal substance affects the change in density of states (CDOS) upon chemisorption, and such effects can be studied theoretically by the Green's function method. A tight-binding model is employed for the (100) surface of a simple cubic solid substrate, and the impurity is described by the Koster-Slater model. The CDOS is computed as a function of the impurity locations for different impurity and adatom coupling parameters. It is found that the impurity effect is most significant when the impurity is closest to the adatom, although its effect can be felt when it is separated from the adatom by one or two substrate atoms. For the system of hydrogen chemisorbed on tungsten, it is predicted that introduction of an impurity rhenium atom on the surface produces an impurity-induced localised state below the substrate band.

Impurity effects on chemisorption

The influence of a substitutional impurity atom on the chemisorption properties of an adsorbate is investigated using the Green's function method. The pure solid substrate is approximated by the tight-binding scheme and the impurity is described by the Koster-Slater model. The adsorbate-impurity interaction is found to be an oscillatory function of the substrate Fermi level. Thus, depending on the kind of substrate, an impurity of a given type may strengthen or weaken the chemisorptive bond, which for molecular adsorption may lead to an increase or decrease of the rate of the subsequent dissociation process.

Chemisorption on a contaminated BCC metal

The presence of an impurity atom within a metal substrate affects the change in density of states (CDOS) upon chemisorption, and these effects are investigated via the Green's function method. A tight-binding model is employed for the (100) surface of a bcc substrate, and the impurity is described by the Koster-Slater model. The CDOS is computed as a function of different impurity and adatom-substrate coupling parameters for on-site and centered-site adsorption.

Resonant states and their influence on optical properties of Hg1-xCdxTe type crystals

A three-band Koster-Slater model is applied to resonant states induced by a highly localised potential in a Hg1-xCdxTe type band structure. The density of states is obtained as a function of four parameters; resonant or discrete states can appear in various energy regions. The statistics of these states is discussed as well as their influence on optical transitions (interband and intraband) for various positions of the Fermi level. Characteristic dips, peaks and thresholds appear in the absorption spectrum.

Phase-shift analysis of the impurity problem in cubic metals. II. Change of electron density near impurity

The phase-shift approach to the tight-binding method has been applied to the calculation of the change of the electron density due to a single substitutional impurity in a metal. The electron density is given by the band of states of a metal and its change is calculated at the lattice site occupied by the impurity. To get a comparison with the Koster-Slater model the calculation is limited to a special case of a simple cubic lattice with the nearest-neighbor interaction between $s$ atomic states, but this calculation can be generalized to many-metal cases.

Nitrogen states in Ga(As,P) and the Koster-Slater model

Nitrogen states in Ga(As,P) and the Koster-Slater model

Nitrogen isoelectronic trap inGaAs1−xPx: II. Model calculation of the electronic statesNΓandNXat low temperature

A semiphenomenological theory of the isolated N isoelectronic trap in $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ is presented, based on an extended (multisite), one-band Koster-Slater model of the electron-impurity interaction, including the effects of both the central-cell atomic pseudopotential difference and the spatially extended lattice distortion surrounding the substitutional nitrogen impurity. The alloy host is treated in a virtual-crystal approximation. The parameters of the model are determined by fitting low-temperature photoluminescence data from ion-implanted materials of two compositions selected near $x\ensuremath{\approx}0.35$. The model yields both a spatially localized ${N}_{X} (or A)$ state which evolves continuously with decreasing $x$ from the $A$ line of GaP, and a spatially-diffuse state ${N}_{\ensuremath{\Gamma}}$ which is present in near-direct and direct-band-gap alloys ($0.3\ensuremath{\lesssim}x\ensuremath{\lesssim}0.5$). The theory quantitatively describes the energies of these luminescence lines as a function of alloy composition $x$. In addition, good agreement with the data is found for the following calculated quantities: (i) the composition dependences of the ${N}_{\ensuremath{\Gamma}}$ and ${N}_{X}$ luminescence intensities, (ii) the pressure dependences of the ${N}_{\ensuremath{\Gamma}}$ and ${N}_{X}$ intensities, (iii) the composition dependence of the ${N}_{X}$ lifetime, and (iv) the binding energy of N${\mathrm{N}}_{1}$ pairs in GaP. The model leads to a tentative interpretation of ${N}_{\ensuremath{\Gamma}}$ luminescence as originating, at least in part, from excitonic molecules.

Radiative recombination efficiencies in Ga(As,P) : N and (In,Ga)P : N

The recombination kinetic equations for a III-V mixed crystal alloy containing isolated nitrogen impurities [e.g., Ga(As,P) : N and (In,Ga)P : N] and acceptor impurities are solved in steady state, and the internal quantum efficiency is calculated for compositions greater than that at the direct-indirect crossover. The model which is considered treats three states of the nitrogen: neutral nitrogen, electrons bound to the nitrogen by a short-range potential (i.e., the Koster-Slater model), and hydrogenic excitons bound to the nitrogen. In addition, intervalley scattering is included. Because of the large densities of states of the indirect X minimum and the valence band, a simple appoximation is derived which is roughly valid for nitrogen and acceptor concentrations less than the corresponding densities of states associated with the X minimum and the valence band. From the short thermalization times for bound electrons and bound excitons, the roles of the X minimum in efficiently pumping electrons into the nitrogen level and of the valence band in supplying holes to the bound excitonic states are elucidated. In addition, the bound-exciton radiative lifetime is found to be dominant in determining the composition dependence of the efficiency. The internal quantum efficiency for a large excitonic hole capture cross section is proportional to the product of the nitrogen and free-hole concentrations and is in good agreement with light-emitting-diode experiments. These results emphasize the importance of nitrogen for light emission in these systems.