Inhomogeneity in the Network Order of Device Quality a-Si:H
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INHOMOGENEITY IN THE NETWORK ORDER OF DEVICE QUALITY a-Si:H G. MORELL*, R.S. KATIYAR', S.Z. WEISZ', M. GOMEZ', AND I. BALBERG-* "Physics Department, University of Puerto Rico, Rio Piedras, PR 00931 "Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel ABSTRACT In this paper we show that the degree of order of the Si network in a-Si:H is increasing with two length scales from the surface into the bulk. The major manifestation of the disorder is the variation in the Si-Si bond-stretching rather than the variation in the width of the dihedral angle distribution. The results are interpreted in terms of the decrease of the hydrogen concentration from the free surface into the bulk. INTRODUCTION A considerable understanding of the lattice dynamics of covalent amorphous semiconductors in general, and of hydrogenated amorphous silicon in particular, has been gained1 3 between the mid 70's and the mid 80's. This understanding is based on the identification of the phonon spectra in these materials as associated with a coupling 4 parameter-weighed phonon density of states of the crystalline one phonon modes T. Following this development the Raman spectra of a-Si:H became a tool in evaluating the "order" of the amorphous silicon network. Both the tool and the "order" are simply defined by the similarity to the spectrum 3 of crystalline silicon, c-Si, while the amorphicity of the structure (i.e., the corresponding lack of a long range order or the presence of the corresponding amorphous radial distribution function ) is maintained. The most conspicuous feature of these Raman spectra is the phonon density band which originates from the transverse optical (TO) Si-Si stretching mode in the tetrahedral structure of c-Si. The closer the peak position of the corresponding band in the spectrum, WTo, to that of the c-Si line at 520 cm', and the narrower the peak width, ATo, the more "ordered" the material. The microscopic meanings of these features are that the Si-Si bond length is closer to that of c-Si, and that the width of the bond's dihedral angle distribution function,
A6, is narrower. In particular, models were presented 58- and predictions were given for an almost linear relationship between ATO and A6. Once this model became well established, attempts were made to relate the degree of order to other physical properties9" 2 , and to evaluate the effects of various film deposition parameters on the degree of disorder. The purpose of the present study is to follow the type and degree of network order from the free surface of a deposited a-Si:H film toward its bulk. Our approach in the present work is to study both, the effect of the laser excitation frequency and the effect of the film thickness on the Raman spectra. The need for such a simultaneous application of both approaches is that application of the first approach only, does not enable the association of the observations with other physical parameters (such as the optical band gap 6) that were studied in conjunction with the Raman spectra. On the oth
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