Vibrational properties of amorphous silicon from tight-binding O(N) calculation

We present an O(N) algorithm to study the vibrational properties of amorphous silicon within the framework of tight-binding approach. The dynamical matrix elements have been evaluated numerically in the harmonic approximation exploiting the short-range nature of the density matrix to calculate the vibrational density of states which is then compared with the same obtained from a standard O($N^4$) algorithm. For the purpose of illustration, an 1000-atom model is studied to calculate the localization properties of the vibrational eigenstates using the participation numbers calculation.Comment: 5 pages including 5 ps figures; added a figure and a few references; accepted in Phys. Rev.

Observation and Theoretical Prediction of Structure in Amorphous Carbon and Related Materials

Covalently bonded materials of the light elements B, C, N, O and Al form an interesting series of structures which are readily prepared by physical vapour deposition methods. The most important structural issues are the degree of positional order (that is, whether the material is amorphous or nano phase crystalline) and for alloys, the degree of site occupancy order (that is, the extent of ‘chemical’ order). We propose that the degree of positional order is dependent on the bond angle stiffness of the covalent bonding and the degree of chemical order is dependent on the energy difference of bonds between like and unlike atoms (see figure 1).The observation of structure in these materials is conveniently carried out by electron optical techniques in an electron microscope. The techniques of value are (a) Energy Filtered Electron Diffraction, and (b) Electron Energy Loss Spectroscopy (EELS).</jats:p

Experimental and Theoretical Characterisation of Structure in Thin Disordered Films

The electron microscope provides an ideal environment for the structural analysis of small volumes of amorphous and polycrystalline materials by collecting scattering information as a function of energy loss and momentum transfer. The scattering intensity at zero energy loss can be readily processed to a reduced density function G(r), providing information on nearest neighbour distances and bond angles[l]. Figure 1(a) shows the G(r) for glassy carbon, a turbostratic form of graphite. The three nearest neighbours in glassy carbon (labelled 1-3 in figure 1) are at 1.42 Å, 2.44 Å and 3.75 Å respectively. These distances correspond to the first three nearest neighbours in a graphite sheet and are expected in glassy carbon which is know to have good in-plane graphitic order. In figure 1(b) the G(r) of cathodic arc deposited tetrahedral amorphous carbon is shown. This material contains a high fraction of diamond-like bonding[2] and has a 1st nearest neighbour peak at 1.52 Å.</jats:p