A Computer Modelling Study of the Structure of a-C:H
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A COMPUTER MODELLING STUDY OF THE STRUCTURE OF a-C:H D.W.HUXLEY, R.J.NEWPORT, A.N.NORTH and J.K.WALTERS Physics Laboratory, The University, Canterbury, CT2 7NR, Kent, UK.
ABSTRACT Neutron diffraction data has been used as input to a computer-modelling algorithm based on the "Reverse Monte Carlo" technique. Using this method the positions of - 5000 "atoms" in a box, with full periodicity, are altered until the associated model structure factor agrees with the analogous experimental curve to within errors. It is then possible to estimate the partial pair distribution functions (i.e. those associated with C-C, C-H and H-H correlations), bond angle distributions, coordination number distributions, etc. Whilst X-ray data is well-conditioned for the study of the carbon-carbon network, neutrons are sensitive to the interference terms involving hydrogen. We present an exploratory study of the effectiveness of the RMC method in this context, and suggest viable options for the future use of the method in model building. INTRODUCTION The detailed microscopic study of liquids and amorphous solids using neutron (and X-ray) scattering techniques continues to provide new insights into condensed matter physics and materials science. In the last decade there was a rapid growth of interest in the field of thin amorphous films which have often proven themselves to be of considerable importance in a technological, as well as in a fundamental sense. An example of this is the now extensive work on amorphous hydrogenated silicon, a-Si:H [1]. Of great contemporary interest however are the family of plasma-deposited materials based on carbon [2]. THE MATERIAL
The material that poses the most intriguing questions is ostensibly the simplest, in terms of compositon - amorphous hydrogenated carbon, a-C:H. The deposition of various forms of carbon from suitable plasmas is not a new process: their production in carbon-arc equipment is well known. It is only in more recent years however that a systematic study of the films for their own sake has begun. Because of their unusual properties they can, under suitable deposition conditions, be prepared denser, harder and more chemically resistant than any other hydrocarbon or carbonaceous polymer. It is important to note, however, that the
properties of a-C:H (and its alloys) depend critically on the conditions under which it was prepared - from the "diamond-like" form at one end of the spectrum to soft polymeric or porous graphitic films at the other; it is the dense, hard form that is of central concern here. a-C:H has been found to be transparent in the IR [3] and
furthermore it has been shown that a-C:H may be doped by alloying with phosphorus or boron [4]. The potential applications of this family of novel materials are legion, ranging from wear/corrosion resistance to "rugged" semiconductors to biocompatible coatings; indeed, there are already significant Mat. Res. Soc. Symp. Proc. Vol. 270. 01992 Materials Research Society
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commercial uses. Despite this there remains a dearth of accura
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