Bethe Lattice Method Research Articles

Electronic theory of ordering in (GaAs)1-xGe

The electronic energy of (GaAs${)}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{2\mathrm{x}}$ alloys is investigated as a function of the degree of short-range order in these ternary systems. The generalized cluster Bethe-lattice method, with a minimal ${\mathrm{sp}}^{3}$ set of orbitals, is used in the calculation. The results are parametrized in terms of nearest-neighbor pair interaction energies which are independent of the degree of short-range or long-range order and consistent with known defect energies. This confirms the validity of previously used Ising-like Hamiltonians for such systems. The theory allows a critical examination of both the thermodynamic and growth models proposed for the zinc-blende--diamond transition observed in these alloys. The transition temperature calculated from the pair interaction energies is much higher than actual sample preparation temperatures, indicating that kinetics must determine the observed structure. However, previous kinetic models have essentially neglected Ge correlations which are shown to be important.

Vibrational excitations in a-Si3N4:H(D) alloys

The authors report here the first results of a theoretical study of the vibrational excitations in the amorphous Si3N4 and a-Si3N4:H(D) alloys using a cluster Bethe lattice method. The a-Si3N4 network has a nearest-neighbour tetrahedral coordination of N atoms around the Si atom and a threefold planar coordination of Si atoms around the N atom. The computed phonon density of states for a-Si3N4 and a-Si3N4:H alloys is in very good agreement with the available infrared and Raman data. The study shows the occurrence of the mono- and dihydrides both at the Si and N atoms. Detailed infrared and Raman measurements need to be performed on the a-Si3N4:H alloys for the different concentrations of H atoms, especially in the low-frequency region, to elucidate the microscopic structure of the alloys.

Cohesive properties in the Al-Gd system

The standard molar enthalpies of formation of AlGd 2, Al 2Gd 3, AlGd, Al 2Gd and Al 3Gd compounds were determined by dissolution calorimetry, using a calorimeter based on liquid aluminium. Experimental results are compared with model predictions. More particularly the alloy cluster Bethe lattice method has been used to analyse the chemical trend in the bonding properties of these compounds.

Phonons in a-SiC:F alloys

A theoretical study of the vibrational excitations induced by F atoms in amorphous SiC has been made in a cluster Bethe lattice method formalism. The calculated results are in agreement with the available infrared data. However, two predicted peaks near 200 and 1000 cm −1 have not been detected so far.

Electronic structure of hydrogenated/fluorinated a-Si 1−xGe x alloys

A very good description of the electron energy states in a-Si 1−xGe x:F(H) alloys has been obtained throughout the entire energy region by the inclusion of a high energy excited s ∗ state in the basis of the atomic orbital set in the cluster Bethe lattice method (CBLM) for the first time . For a-Si 1−xGe x alloys, the computed variations of the magnitude of the fundamental energy gap for the random and chemically ordered sequences are seen to be in excellent agreement with the available photoemission data. Great improvement in terms of the number and location of F- and H-induced peaks in a-Si alloy has been obtained where the calculated results are in very good agreement with the photoemission data. The F(H)-induced peaks remain unaffected by the presence of the different concentrations of the constituent host atoms in a-Si 1−xGe x alloys both for the random and chemically ordered sequences.

Vibrational excitations in a-Si 3N 4:H(D) alloys

We report here the first results of a theoretical study of vibrational excitations in the a-Si 3N 4 : H(D) alloys using a cluster Bethe lattice method. The computed phonon density of states for the a-Si 3N 4 and a-Si 3N 4:H alloys is in very good agreement with the available infrared and Raman data. The study shows the occurence of the mono- and di-hydrides both at the Si and N atoms.

Electronic structure of a two-dimensional Penrose lattice: Single- and two-component systems.

The electronic structure of a two-dimensional Penrose lattice is calculated within the tight-binding Hamiltonian. By means of the cluster Bethe-lattice method, we study the dependence of the local density of states (LDS) on the cluster size for two different geometries. We find that very large clusters are necessary to obtain the main features of the LDS. The method is applied to a single-component system and to an ordered binary alloy.

Electronic structure of hydrogenated-fluorinated a-Si1-xGex alloys.

A very good description of the electron energy states in a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$:F(H) alloys has been obtained throughout the entire energy region in the cluster Bethe-lattice method for the first time. The calculation considers a high-energy excited ${s}^{\mathrm{*}}$ state in the basis of the atomic-orbital set. For a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$ alloys, the computed variations of the magnitude of the fundamental energy gap for the random and chemically ordered sequences are seen to be in excellent agreement with the available photoemission data. Great improvement in terms of the number and location of F-induced peaks in fluorinated a-Si alloy has been obtained where the calculated results are in very good agreement with the photoemission data. The F(H)-induced peaks remain unaffected by the presence of the different concentrations of the constituent host atoms in a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$ alloys both for the random and chemically ordered sequences.

Analytic CPA approach to nonstoichiometric PdHx. I

The nonstoichiometric PdHx is approximated by two analytic model descriptions. The first, which incorporates only diagonal disorder, makes use of the doubly periodic Bethe lattice. The second, which is capable of incorporating both diagonal and off-diagonal disorder, employs the cluster Bethe lattice method. Both of these model descriptions are incorporated within the self-consistent scheme of the coherent potential approximation. The electron densities of states of PdH/sub /x obtained with these models are presented and compared with the results of other methods.

Calculation of the Electronic Structure! of Disordered Hydrogen Adsorption on the Si(111) Surface

AbstractThe local density of electronic states is calculated on the (111) surface of silicon with disordered adsorption of atomary hydrogen with an arbitrary degree of coverage. Computations are made by the recursion method and the method of generalized Bethe lattices. The dependence of the electronic density of states on the degree of coverage of the surface by hydrogen is investigated. The results obtained by the recursion method and the generalized Bethe lattice method are compared with those obtained by other methods and by the experiment and a good overall agreement is found. The contribution of hydrogen and silicon atomic states into the electronic structure of the Si(111) surface is analyzed.

Electronic structure of hydrogenated fluorinated amorphous GaAs alloys.

The electronic structure of amorphous GaAs:H:F alloys is discussed by using a cluster--Bethe-lattice method. The various hydrogenated and fluorinated conformations, e.g., GaF, ${\mathrm{GaF}}_{2}$, ${\mathrm{GaF}}_{3}$, AsF, ${\mathrm{AsF}}_{2}$, ${\mathrm{AsF}}_{3}$, GaH, ${\mathrm{GaH}}_{2}$, ${\mathrm{GaH}}_{3}$, AsH, ${\mathrm{AsH}}_{2}$, ${\mathrm{AsH}}_{3}$, AsFH, ${\mathrm{AsF}}_{2}$H, ${\mathrm{AsFH}}_{2}$, GaFH, ${\mathrm{GaF}}_{2}$H, and ${\mathrm{GaFH}}_{2}$ in a-GaAs have been investigated. Along with the features seen earlier in the elemental semiconducting alloys like a-Si:F:H and a-Ge:F:H alloys, novel peaks are seen to arise from dangling bonds as well as from the incorporated H atoms. Photoemission and other types of experiments are greatly needed to detect the predicted structure in the a-GaAs alloys.

Self-consistent model of hydrogen chemisorption on ferromagnetic transition metals.

The chemisorption of hydrogen on ferromagnetic transition metals is studied self-consistently with use of a spin-dependent Anderson-Hubbard Hamiltonian. The electronic structure of the crystal is treated with the Bethe-lattice method. The present treatment differs from previous ones in that the dominant effects of the change in occupancy of the substrate orbitals are taken into account. The consequent modification of the substrate Hamiltonian leads to the result that the chemisorption energy changes very little with the change of the magnetization of the substrate. This finding agrees with recent experimental evidence that a magnetic phase transition has no influence on the catalytic activity.

The Research of Electronic Properties of Hydrogenated a-Si

We first used Bethe lattice method and coherent potential approximation method to calculate the densities of bulk and surface states for a–Si, a–Si:H. The ideal vacancies were also studied. The results agree with experiments and other works. Dihedral disorder contributes to the submerging of a peak and sp peak of the valence band. In hydrogenated case, extra structure appears in the spectrum of densities of surface states. Doped hydrogen affects the densities of bulk states in the gap in principle, but hydrogen is not important for densities of surface states in the gap. Finally, we obtained self-energies which are in good agreement with the empirical results from experiments and other methods[1].

Many-body effects in the paramagnetic and antiferromagnetic states of the (111) silicon face.

The electron density of surface states has been calculated for both the paramagnetic and the antiferromagnetic states of Si(111). Many-body effects associated with correlation and electron-phonon coupling are included. Our results have been obtained by using a self-consistent tight-binding approach, a Bethe-lattice method, and Green-function techniques. The paramagnetic phase shows a very narrow Kondo-like peak at the Fermi level ${E}_{F}$, two satellites at ${E}_{F}$\ifmmode\pm\else\textpm\fi{}0.2 eV, and two broader peaks at ${E}_{F}$\ifmmode\pm\else\textpm\fi{}0.8 eV. The antiferromagnetic phase shows two peaks at ${E}_{F}$\ifmmode\pm\else\textpm\fi{}0.8 eV and two others at ${E}_{F}$\ifmmode\pm\else\textpm\fi{}0.4 eV. In this case the energy gap of 0.6 eV due to correlation effects is reduced by electron-phonon coupling up to 0.25 eV.

A theoretical analysis of bonding in transition metal-polyvalent metal compounds

An electronic theory for the total energy of transition metal-polyvalent metal binary alloys is developed using an extended Cluster Bethe Lattice Method. This approach allows the energy of formation of these alloys to be calculated and the chemical trends in the bonding properties to be analyzed.

INFLUENCE OF THE LOCAL CHEMICAL ORDER ON THE ELECTRONIC PROPERTIES IN TRANSITION METAL-POLYVALENT METAL ALLOYS

The Cluster Bethe Lattice Method is used for a quantitative study of hybridization effects in transition metal-polyvalent metal alloys. It is shown that the electronic density of states around equiatomic composition is characterized by the occurence of a pseudo-gap near the top of the d band. Implication of this pseudogap for anomalous physical properties are discussed.

Electronic structure of pure and fluorinated silane chains

The electronic density of states for amorphous silicon containing various conformations like SiH n (n = 1, 3), SiF mH n (m, n = 1, 2, m+n ⩽ 3), and the chain like (SiH) n, (SiH 2) n and (SiFH) 2 ones have been computed using a cluster Bethe lattice method (CBLM). The results are in excellent agreement with the available photoemission data for a-Si:H alloys. Experiments on a-Si:F:H alloys are very much needed.

An analytic study of the friedel criterion in disordered magnetic states in transition metals in the presence ofS-D hybridisation, using the cluster bethe lattice approach

In this paper we present a new theory for including both short range order and long range order simultaneously in the well known cluster Bethe lattice method for binary alloys. We have used this theory for obtaining the Friedel criterion for the appearance of magnetic moments in disordered states using the single band Hubbard model. This is followed by a study of this criterion in a two-bands-d hybridised Hubbard model, which is considered as a simulation of real transition metals. A new technique for solving this problem in the Bethe lattice network is presented, which yields an analytic solution for the critical correlation strength in the presence of hybridisation and short range order. It is found that in all cases hybridisation tends to diminish the tendency for magnetisation, which is in accord with physical expectations.

Electronic structure of Si and Ge (111) surfaces and of the Si-Ge(111) interface.

The electronic density of states for Si and Ge (111) surfaces and of the Si-Ge(111) interface has been calculated using a cluster Bethe-lattice method. The calculated surface and interface states are in reasonable agreement with the available photoemission data. No interface state is seen in the mutual energy gap of Si and Ge, a fact predicted earlier by the author for the Ge-GaAs and Ge-ZnSe interfaces.

Electronic structure of defects in amorphous silicon

The effects of a variety of defect centres such as T 0 3, T 0 2, T 0 1 and their interacting units on the electronic structure of the amorphous silicon has been studied in a cluster Bethe lattice method. The gap states arising from these defects may explain a number of experimental observations.