Structure of Hydrogen in Microcrystalline and Amorphous Silicon: an NMR Comparison
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STRUCTURE OF HYDROGEN IN MICROCRYSTALLINE AND AMORPHOUS SILICON: AN NMR COMPARISON S. E. Ready, J. B. Boyce, and C. C. Tsai, XEROX Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304. ABSTRACT Hydrogen NMR data on three microcrystalline silicon samples grown by plasma decomposition using different H2/SiH4 gas ratios are examined. A decrease of the total atomic percent hydrogen content from 12 at. % to 6 at. % is seen as the gas ratio is reduced from 98/1 to 20/1. 20 to 40% of this total hydrogen content is determined to be non-bonded molecular hydrogen. In addition, evidence for SiH2 units is seen in the spectrum. INTRODUCTION Microcrystalline hydrogenated silicon (uc-Si:H) is an attractive material due to its technological compatibility with current amorphous and crystalline Si processing techniques and due to the fact that it provides a logical extension to the knowledge base already in existence for silicon materials. uc-Si:H provides a means to increase the carrier mobility towards the crystalline Si limit while retaining the features of a low temperature process. If we assume good electrical behavior in the crystalline portion of the film, the electrical characteristics will therefore be determined by the nature of the surrounding amorphous matrix and/or the crystalline/amorphous interface. As with a-Si:H material, hydrogen plays a significant role in the passivation of defect centers in the amorphous matrix. Here we report NMR results on the hydrogen local environment in samples produced by glow discharge with three different H2/SiH4 gas dilutions. Details on the form in which hydrogen is incorporated
into microcrystalline silicon (uc-Si:H) is discussed and contrasted with results from similar studies of a-Si:H. We find that, like a-Si:H, uc-Si:H exhibits both a broad and a narrow hydrogen NMR line at room temperature. But unlike a-Si:H, the narrow line in uc-Si:H is considerably narrower by about an order of magnitude (less than 0.4 kHz full width at half maximum versus 3 kHz). Previous studies[1,2] of uc-Si:H have suggested that this narrow line is due to a motionally narrowed species, and H2 and SiH2 were suspected but no definitive evidence presented. One ofthese studies[ 1] rules out H2 and concludes that the narrow line is due to mobile SiH2. We show that H2 and SiH2 can be distinguished from each other and from H with low temperature NMR and we find that a significant fraction of the motionally narrowed line is due to molecular H2. EXPERIMENT Three uc-Si:H samples were deposited on aluminum substrates by plasma decomposition of H2 and SiH4 at varying gas ratios (see Table 1), using a power density of 0.26 Watts/cm2 and substrate temperature of 250C. To provide the amount of material needed for the NMR work, a deposition time of 16-17 hours was necessary. The aluminum foil was then removed in diluted HCl. Pulsed NMR data on H nuclei was acquired over a temperature range of 1.45 300K at a H Larmor frequency of 113 MHz. The spin-lattice relaxation time, T1, was
Mat. Res. Soc. Symp. Pro
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