Lester Guttman

Ring structure of the crystalline and amorphous forms of silicon dioxide

Abstract The concept of a ring in a covalent substance is defined, and a prescription for counting rings is given and exemplified by various forms of SiO2.

Elastic moduli of random network models of amorphous Ge and Si

Abstract The bulk and shear moduli of periodic random network models of amorphous Ge and Si are computed. The Youngs modulus is reduced to about 90% of the average in the crystalline form.

Vibrational spectra of four‐coordinated random networks with periodic boundary conditions

Examples of perfectly four‐coordinated networks satisfying periodic boundary conditions are constructed by a pseudo‐random process, starting from a crystalline region. The unphysical features (high density, large deviations from the tetrahedral bond‐angle) are removed by systematic modification of the bonding scheme. The vibrational spectra are calculated, using a valence‐force potential, and the neutron scattering is computed by a phonon‐expansion approximation.

Computer modeling of the structure and vibrations of amorphous silica

Abstract We have constructed models of amorphous SiO2 by using a periodic, realistic, continuous random network model. A modified Keating potential which includes only short range interactions was used. Comparison of the structure factor with recent elastic neutron scattering experiments shows very good agreement. The vibrational density of states also shows fairly good agreement with experiments. It is noteworthy that the double maximum observed at high frequencies of the spectrum is also found in the simulation, using only short range forces. Some of the normal modes are found to be highly localized.

On the melting point of amorphous silicon

Abstract The entropy and melting point of amorphous silicon cannot be measured in a thermodynamic sense.

Atomic structure and self‐consistent electronic structure of periodic models of amorphous hydrogenated silicon

Computer models of hydrogenated amorphous silicon have been constructed by adding hydrogen atoms to broken bonds in periodic random network models of pure amorphous silicon. The average neutron scattering computed for 12 examples containing 10–13 at.% H is in good agreement with recent diffraction data, supporting the assumption that most of the hydrogen in this material is normally covalently bonded to silicon. The one‐electron wave‐functions and associated energies have been computed by the self‐consistent pseudopotential method for a number of these models. Soft core Si and H pseudopotentials are used. The occupied states associated with hydrogen are identified from the electron density, and are found to lie in four well‐defined bands within the valence band region of the silicon network.

Vibrational properties of pure and hydrogenated amorphous silicon

Abstract The vibrational densities of states of pure and hydrogenated amorphous silicon have been calculated from several computer models. First, the standard phonon method is applied to a supercell using the Keating potential in conjunction with a short range repulsive potential between the hydrogen atoms and their neighboring silicon atoms. The results are compared to experimental and the effect of the repulsive potential is also discussed. Second, the cluster-Bethe-lattice method is used with the Born potential. The results from both approaches are compared to each other.

Simulation of Continuous Random Network Models with Periodic Boundary Conditions

Examples of quasi‐random networks with perfect 4‐fold coordination bounded by identical copies of themselves can be simulated on a digital computer, starting from an arrangement of the same atoms on a lattice, and stochastically joining each atom to four of a set of its neighbors. After relaxation under a simple valence‐force law, these nets bear an encouraging resemblance to the structure of amorphous germanium. A simple calculation of the vibrational spectrum is also presented.

Vibrational and electronic properties of hydrogenated amorphous silicon with dangling bonds

Abstract The vibrational density of states of several models of hydrogenated amorphous silicon with one or two dangling bonds have been calculated using a modified Keating potential. The corresponding charge densities near the dangling bonds have also been calculated using the self-consistent pseudopotential method. The effect of mutual interaction between dangling bonds on the vibrational density of states and charge densities is also investigated.

Chapter 11 Relation between the Atomic and the Electronic Structures

Publisher Summary This chapter discusses the present knowledge of the atomic structure of amorphous hydrogenated silicon (a-Si:H) and reviews the quantum-mechanical calculations of its electronic structure that have been based on that knowledge. In contrast to the case of crystals, the structure of a substance like a-Si:H must necessarily be very incompletely known. For a perfect crystal, the relative positions of any number of atoms are determined—namely, the size and shape of the unit cell and the locations of the atoms within the cell. The absence of long-range order in a-Si:H prevents such a concise description. This information is available only in probabilistic terms (i.e., as distribution functions), because the disorder of an amorphous phase consists in part of spatial fluctuations in interatomic distances. Moreover, only pair distribution functions can presently be measured with any accuracy.