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BOND ANGLE DISORDER IN TETRAHEDRALLY BONDED AMORPHOUS SILICON

C.K. WONG and G. LUCOVSKY Department of Physics, N.C. State University, Raleigh N.C. 27965-8202

ABSTRACT There have been many studies of the vibrational and optical properties of aSi and a-Ge that have correlated systematic variations of these properties with films grown and/or annealing temperatures, and have attributed these variations to changes the local atomic structure and intrinsic network disorder that are correlated with the thermal history of a particular sample. The most frequently proposed atomic structure parameter associated with this intrinsic disorder has been the width of the bond angle distribution. We attempted to isolate the effects of bond angle disorder on the vibrational and electronic properties of a-Si using a Bethe Lattice structure that avoids some of the uncertainties introduced by the uncontrolled variation in other atom structure parameters. Using this approach, we show that increases in the width of the bond angle distribution can account for trends in the calculated vibrational and optical properties wth thermal hisrory that are in good qualitative agreement with trends reported in the experiments.

I INTRODUCTION Lanin and coworkers [1] and Tsu and cowrkers [2] have studied the Raman spectra of a-Si and a-Ge films and have attributed systematic variations in the spectra features with deposition conditions (e.g., substrate temperatures, annealing cycles and the degree of hydrogenation) to changes in local atomic structure that are associated with bond angle disorder (defined as the deviation of bond angles from the tetrahedral value of 109.470). Cody and coworkers [31, as well as Lannin and his group [1] have also attributed similar preparation dependent variations in the position and slope of the fundamental absorption edge of a-Si to changes in the intrinsic network disorder. Lannin and his group [1] have shown further that the changes in the optical absorption are correlated with the changes in the Raman spectra. In addition to these experimental studies, there have been attempts to develop models for the network disorder. The most extensive work relates to the calculation of the one phonon densities of states and the Raman spectrum that are based on large clusters (500 to 600 atoms) with varying degrees of bond angle disorder [2]. There is a problem with this approach in the sense that the construction of a space filling continuous random networks (CRN) also builds in correlations between bond angles, dihedral angles and the ring statistics that are not controlled in any systematic manner. This makes it difficult to identify in an unambiquous way the effects of due only to the various bond angle distributuons. Note that the various aspects of atomic scale structural order can be characterized in terms of the numbers of neighbors whose atomic positions are correlated. In this context, bond angles establish three body correlations, dihedral angles four body correlations, and ring formation generally involves correlatio