Chemical trends for localized gap states in amorphous semiconductors

The absence or presence of unpaired spin and of variable range hopping due to the localized gap states (dangling bond states) in various amorphous semiconductors is explained based on the chemical nature of constituent atoms and randomness of the glass structure. The randomness is also correlated with optical absorption data.     

Two frequency light induced absorption and energy density of localized states in amorphous semiconductors

Infrared light induced absorption is proposed to determine the energy dependence of localized electrons' density in amorphous semiconductors. The possibility of the experimental test of energy independence of the recombination coefficients is shown.     

Transient photodecay measurements as a probe of the density of localized states in amorphous semiconductors

We show that structure in the density of localized states of an amorphous semiconductor beyond the apparently ubiquitous exponential band tails yield deviations from the usual power-law decay of the photocurrent, i(t) ∝ t^α?1, which can be described analytically. With our expression, dispersive-transport results can be deconvoluted to provide a spectroscopy of the localized-state distribution when well-defined defect centers and band tails are simultaneously present.     

An icosahedral quasicrystalline model for amorphous semiconductors

In the first article of this series an icosahedral quasicrystal with a tetracoordinated decoration of atoms was introduced as a model for amorphous semiconductors. The electronic structure is one of its most interesting features. Based upon a LCAO tight binding scheme with five orbitals sp³s* per atom the electronic density of states (DOS) is calculated by use of the recursion method. Various effects of the specific geometry on the DOS are investigated, including the topology of the tetracoordinated network, the corresponding dihedral angles, bond angles, bond lengths, and dangling bonds. These geometrical parameters are treated separately and therefore reveal their distinct influences on the DOS. The resulting effects are discussed regarding the DOS as a whole, while details are provided about variations of the outer band edges, shifts of maxima and first moments of uppermost valence band and lowest conduction band, and — most important — changes in the central gap. The properties of single wave functions are subject of the third article in this series.     

An icosahedral quasicrystalline model for amorphous semiconductors

In disordered structures localized electronic states are expected to be found having a strong impact on the electronic properties. They are also expected to exist in an icosahedral quasicrystalline semiconductor which is studied therefore with respect to its electronic wave functions. The quasicrystalline model was introduced in part one of this series of articles, its electronic DOS in part two. Based upon the same LCAO tight binding scheme with five orbitals sp³s* per atom individual electronic wave functions of the quasicrystalline semiconductor are calculated using the Lanczos method on a cubic supercell of 20 000 atoms. The wave functions are analyzed regarding projections on the atomic orbitals, influence of the dangling bonds, spatial distribution, and — most interesting — localization measures like the participation ratio and the localization length which allow the definition of mobility edges.     

Density of states and specific heat of elastic vibrations in layer structures

On the basis of a determination of normal modes in materials consisting of periodic arrangements of macroscopic layers (of period d), the low frequency density of states and the corresponding low temperature specific heat were calculated numerically. The average temperature dependence of the latter changes in the vicinity of a characteristic temperature T₀ (proportional to 1/d), from a low temperature (α₀T³)- to a higher temperature (αT³+βT²)- law. Depending on the material parameters, β may be positive (especially if Stonely waves are present) or negative. The coefficients α and α₀ can differ by a factor larger than 2. Characteristics of thick and thin layers and the implications of the results on the interpretation of experimental data are discussed.     

An icosahedral quasicrystalline model for amorphous semiconductors

One of the basic problems in modeling amorphous semiconductors is posed by the construction of a suitable geometry describing the positions of atoms and the network of bonds. Many attempts have been made to find reasonable models including hand-built ones and various computer-generated constructions. Not long ago a new approach was presented based upon an atomic decoration of the three-dimensional Penrose quasilattice. Therein, the structural elements form an icosahedral quasicrystalline tetracoordinated network. Its geometrical properties are analysed in detail in this paper. The electronic states of this quasicrystalline geometrical model for amorphous semiconductors are discussed in two following papers.     

Band effects on conduction electron density of states for the anderson model

Summary The effects of the shape of free conduction density of states on the physical quantities for the periodic Anderson model have been investigated.     

Susceptibility and specific heat of intermediate valence compounds studied in mean-field theory of a periodic anderson model

The periodic Anderson model for a lattice of magnetic ions is investigated in Hartree-Fock approximation. Attention is paid to different solutions of the self-consistency equations corresponding to ferromagnetic or antiferromagnetic ordering of the local magnetic moments. The effect of hybridization leading to reduced magnetic moments strongly depends on the position of the localizedf levels relative to the conduction band. For paramagnetic solutions with a non-integer value for thef level occupation number comparison is made with properties of intermediate valence rare earth compounds. The mean-field results for the susceptibility and specific heat agree with essential features found for these substances.Work performed within the research program of the Sonderforschungsbereich 125 Aachen-Jülich-Köln     

Self-consistent alloy treatment of the periodic anderson model: Susceptibility and specific heat of intermediate valence compounds

To describe the electronic properties of mixed valence compounds we study the periodic Anderson model within the frame of the alloy analog approximation. In this approach the model Hamiltonian is replaced by the sum of two single-particle alloy Hamiltonians the parameters of which have to be determined self-consistently. The alloy problem is solved within the coherent potential approximation. In contrast to other treatments of the periodic Anderson model this approximation scheme is exact in both trivially solvable limits of vanishing hybridization and Coulomb repulsion, respectively. For model parameters corresponding to a mixed valence situation only nonmagnetic solutions of the self-consistency equations exist. After discussing the limit of small hybridization analytically we numerically calculate the magnetic susceptibility and the electronic specific heat as a function of temperature for realistic values of the hybridization and Coulomb repulsion. The results are in very good qualitative agreement with experimental data.Work performed within the research program of the Sonderforschungsbereich 125 Aachen/Jülich/Köln     

Density of states of impurity bands in semiconductors

The formalism of Matsubara-Toyozawa for impurity bands in semiconductors is extended by including the correlation effect on electron hopping matrix elements. The density of states of impurity bands for various impurity concentrations is computed numerically. The comparison with Matsubara-Toyozawa's result indicates that the impurity bandwidth is drastically reduced by electron correlation.     

Electronic behaviors of the gap states in amorphous semiconductors

An extension of the Adler-Yoffa's calculation on the localized gap states in amorphous semiconductors has been made with a more realistic model which takes into account an influence of the diffused gap states and impurity states. A considerable large difference in the electron occupation probability of the gap states with sign of the correlation energy U has been examined on the tetrahedrally bonded and chalcogenide glasses. Variations of the Fermi level as a function of doped impurity concentration and gap state density have been studied.     

A cellular model for the specific heat of amorphous solids at low temperatures

The wave equation for a non-homogeneous continuum with a statistical mass-density distribution is studied as a model for the vibrational specific heat of amorphous solids. The frequency spectrum is calculated for the simple case of small homogeneous cells separated from each other by hard and soft boundary walls distributed at random. The usual bulk mode density is enhanced by a positive constant term yielding the specific heat AT + BT³. From experimental values of the constants A, an upper limit of about 70 Å is calculated for the average cell diameter in vitreous SiO₂ and GeO₂.     

Antiferromagnetic interactions between localized states in amorphous germanium

The temperature dependence of the magnetic susceptibility (300 → 1.6°K) and ESR (300 → 6°K) of amorphous germanium have been determined. There is a temperature dependent paramagnetic term to the magnetic susceptibility due to a density of localized unpaired spins (dangling bonds) of 10¹⁹ spins/cm³. There is an antiferromagnetic interaction between at least some of these localized unpaired spins with an exchange energy estimated by various models to be on the order of a degree Kelvin.     

Optically induced localized paramagnetic states in amorphous As

Photoluminescence and optically induced ESR and absorption due to localized states in the forbidden gap have been observed in amorphous As below 77 K. Analysis of the ESR spectru, indicates that these centers are highly localized and in orbitals which are predominantly p-like.     

Electronic density of states of amorphous III–V semiconductors

A short-range disorder model is used to predict the predominant features of the density of states of amorphous III–V semiconductors. Confirmation of this model would help establish the existence of III-III and V-V bonds in these materials.     

Density of phonon states in amorphous germanium and silicon

The density of phonon states in amorphous germanium and silicon is calculated by statistically averaging the crystalline phonon density of states according to the radial distribution function. A simple rigid ion model is used to calculate the density of phonon states at various lattice spacings. The appropriate model parameters are obtained from the pressure dependent elastic constants and the Raman frequency. The calculated results compare favorably to experimental data obtained by infrared and Raman scattering and the results of other theoretical calculations.