Frederick Wooten

Diffusons, locons and propagons: Character of atomie yibrations in amorphous Si

Abstract Numerical studies of amorphous Si show that the lowest 4% of vibrational modes are piane wave like (‘propagons’) and the highest 3% of modes are localized (‘locons’). The rest are neither piane wave like nor localized. We cali them ‘diffusons’. Since diffusons are by far the most numerous, we try to characterize them by calculating such properties as the wave-vector and polarization (which do not seem to be useful), ‘phase auotient’ (a measure of the change of vibrational phase between first-neighbour atoms), spadal polarization memory and diffusivity. Localized states are characterized by finding decay lengths, inverse participation ratios and coordination numbers of the participating atoms.

Generation of random network models with periodic boundary conditions

Abstract We have investigated a novel procedure for the generation of tetrahedral random network models compatible with periodic boundary conditions. Data for a sequence of such models is presented and critically examined.

Electronic and optical properties of SbSBr, SbSI and SbSeI

Abstract Band structures of SbSBr and SbSeI have been obtained by using the empirical pseudopotential method (EPM) to fit our measured optical reflectivity data and earlier gap measurements. An SbSI band structure has been determined by fitting to earlier reflectivity and Raman spectroscopic data, and the results agree better with the data than do the results of an earlier preliminary EPM calculation. Secondary conduction band minima may in part be responsible for the observed microwave oscillation (Gunn effect) in SbSI. Similar minima in SbSBr and SbSeI are reported, suggesting these crystals might also show microwave properties. The total densities of states are presented.

The d-p hybridized valence charge distribution in TiC

Abstract The d-p hybridized charge distribution of the transition metal compound TiC has been calculated using the wavefunctions from an earlier empirical pseudopotential method (EPM) band structure calculation. The charge distribution explicitly shows a partial transfer of charge from the titanium atom to the carbon atom, and the bonding associated with this charge has both an ionic and a covalent character. There is no indication of strong metal-metal (TiTi) bonding.

Reflectivity of uniaxial absorbing crystals

Equations for the reflectances of ordinary and extraordinary waves from the basal plane of strongly absorbing uniaxial crystals are extended to reflectances from planes parallel to the optic axis for configurations likely to be useful and convenient for experimentalists.

Structure, odd lines and topological entropy of disorder of amorphous silicon

A continuous random network model of amorphous silicon, subject to periodic boundary conditions, is partitioned into cells bounded by irreducible rings. An algorithm has been developed to find the cells and the rings that bound them. A thread can be imagined to pass through odd rings (rings containing an odd number of atoms) without passing through even rings. Such a thread is an algorithmic realization of an odd line, which is the only topological defect in glass or amorphous condensed matter. The topological entropy of disorder associated with these odd lines is found to be approximately 80% of the value for an ideal tetrahedrally bonded random network of atoms for which the rings that bound the cells are statistically independent.

New applications of the equation-of-motion method: Optical properties

The equation-of-motion method offers advantages in the calculation of properties of large structural models, of the kind recently developed for a-Si (104 or more atoms). Originally developed for calculating densities of states, it has been shown to be adaptable to a wide variety of other purposes, including the study of localization, conductivity and the Hall effect. Recently we have shown how it may be used to calculate linear optical properties as a function of frequency. Here we present new calculations for a-Si and a-Si:H, using various structural models. The method may be further extended to evaluate nonlinear optical coefficients χ(2)(ω) and χ(3)(ω); we are applying it to the study of χ(3)(ω) for crystalline and amorphous silicon. A conventional approach to this problem, such as calculating eigenstates, would be totally impractical.

A structural model for hydrogenated amorphous silicon

Abstract A ball-and-stick model has been built to simulate the structure of hydrogenated amorphous silicon. The hydrogen content of the model, which incorporates 314 Si atoms, is approximately 20 atomic percent, and the density is in agreement with experiment. It is found that it is necessary to form local clusters of hydrogen atoms if no dangling bonds are to be left, but these are not of any simple recurrent type. The model contains only a few hydrogen atoms which are bonded in pairs to silicon atoms. Preliminary results are reported for the radial distribution functions, ring statistics and broadening of vibrational spectra of the model structure, and these are compared with recent experimental results.

The density of states in amorphous Si

Abstract We have used the equation of motion method in k -space to compute the spectral function and density of states for two different models of amorphous Si containing 216 atoms. The method has considerable potential for self consistent calculations and the calculation of transport properties.

A computer-generated model of the crystalline/amorphous interface in silicon

Abstract We have generated a crystalline/amorphous interface in silicon. Starting with a 512-atom supercell of silicon in the diamond cubic structure, a region containing 192 atoms bounded by (100) faces was held at zero temperature. The remaining region of 320 atoms was randomized at the “melting” point and then annealed. The result is a sharp interface model in which the structure factor drops from its value in the crystal to a value characteristic of the bulk amorphous region over a distance of only 3 A. If the cell is held at half the melting point for a time very much longer than the time required to produce a “good” interface model, the amorphous region will crystallize.