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Comparative Fluctuation Microscopy Study of Medium-Range Order in Hydrogenated Amorphous Silicon Deposited by Various Methods P. M. Voyles,1,2 M. M. J. Treacy,2 H-C. Jin,3 J. R. Abelson,3 J. M. Gibson,4 J. Yang,5 S. Guha,5 and R. S. Crandall6 1

Dept. of Physics, Univ. of Illinois at Urbana-Champaign, 1110 W. Green St, Urbana, IL 61801 NEC Research Institute, 4 Independence Way, Princeton, NJ 08540 3 Dept. of Materials Science and Engineering and Coordinated Science Laboratory, Univ. of Illinois at Urbana-Champaign, 1110 W. Springfield, Urbana, IL 61801 4 Materials Science Division, Argonne National Laboratory, 9700 Cass Ave, Argonne, IL 60439 5 United Solar Systems Corp., 1110 West Maple Road, Troy, MI 48084 6 National Renewable Energy Laboratory, Golden, CO 80401 2

ABSTRACT We have characterized by fluctuation electron microscopy the medium-range order of hydrogenated amorphous silicon thin films deposited by a variety of methods. Films were deposited by reactive magnetron sputtering, hot-wire chemical vapor deposition, and plasma enhanced chemical vapor deposition with and without H2 dilution of the SiH4 precursor gas. All of the films show the signature of the paracrystalline structure typical of amorphous Si. There are small variations in the degree of medium-range order with deposition method and H content. The PECVD film grown with high H2 dilution contains Si crystals ~5 nm in diameter at a density of ~109 cm-2. The amorphous matrix surrounding these crystals shows no difference in mediumrange order from the standard PECVD film. This supports explanations of the resistance of the H-dilution material to light-induced degradation that depend only on the presence of crystalline grains without modifications of the amorphous matrix. INTRODUCTION Fluctuation electron microscopy is a technique for characterizing medium-range order (MRO) in disordered materials. It has been used previously to demonstrate a reduction in MRO in a-Ge thin films on thermal annealing [1] and in hydrogenated amorphous silicon (a-Si:H) thin films on light soaking [2]. It has also lead to the development of the paracrystalline model for the structure of as-deposited amorphous semiconductor thin films [3]. Fluctuation microscopy works by examining the statistical properties of dark-field electron micrographs acquired at deliberately low image resolution. Electron micrographs of amorphous materials typically show a speckle pattern of bright and dark dots (see Figure 2(a)); the size of each dot is governed by the microscope point-spread function. At low-resolution (~1 nm) one dot represents the scattering from a mesoscopic volume containing ~1000 atoms. A dark-field electron micrograph is an image formed only with radiation that has been diffracted by a particular scattering vector k. Putting these facts together, one can qualitatively view a lowresolution dark-field electron micrograph as a position-resolved map of the diffracted intensity from mesoscopic volumes in the sample. In fluctuation microscopy we compute the normalized variance of t