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A9.2.1

Identification of Possible Bonding Sites for Post Deposition Oxygen Absorption in Microcrystalline Silicon L. M. Gedvilas and A. H. Mahan NREL, 1617 Cole Blvd., Golden, CO 80401 ABSTRACT Using infrared (IR) spectroscopy and x-ray diffraction, the nature of the grain boundaries in two µc-Si films deposited by hot wire CVD, displaying similar crystalline volume fractions but very different behavior upon exposure to atmospheric contaminants, is analyzed. For the film exhibiting significant post deposition oxidation, the IR spectrum in the 2100 cm-1 Si-H stretch mode region contains two sharp and very narrow peaks, suggesting that the crystallites have been incorporated into the µc-Si films with their hydrogenated surfaces relatively intact. By comparing these peak frequencies to those in the literature for Si-H bonding on c-Si surfaces, we identify certain crystallite orientations which, when comprising the µc-Si grain boundaries, are particularly susceptible to oxidation. We further suggest that the distribution of H in this grain boundary/crystallite surface region is crucial for depositing µc-Si films with good electronic properties and minimal post deposition oxidation. INTRODUCTION Microcrystalline silicon (µc-Si) has recently been very successfully used as an active layer in single junction solar cells and as the low bandgap material in tandem solar cell structures, as evidenced by solar cell efficiencies exceeding 9% and 13% respectively for these cell structures (1-3). The material’s low band gap and lack of Staebler-Wronski Effect in high crystalline volume fraction (vf) (2) films make it attractive for solar cell use. However, to achieve these results, it has been necessary to limit the amount of oxygen incorporated into the film. Although there are ways to minimize oxygen during film growth, in this paper we will only address post deposition oxygen uptake, a separate but important issue significantly affecting device performance. There appears to be general agreement that oxygen uptake depends on the µc-Si film structure or grain boundaries. In the µc-Si literature, as seen using IR spectroscopy, the shape of the 2100 cm-1 stretch mode in as grown films is normally rather broad and featureless (4). However sometimes this IR absorption can consist of multiple peaks that are sharp and narrow, resembling Si-H bonding on c-Si surfaces (5-8). This latter case suggests the crystallites may be incorporated into film with their hydrogenated surfaces, at least those comprising the grain boundaries, intact. By comparing these peak positions to those in the literature for H bonding on different c-Si surfaces, we can identify surface sites that comprise the grain boundaries. Also, by tracking these IR features with exposure time, we can identify those sites that undergo oxidation and confirm this vulnerability with the growth of the 900-1100 cm-1 Si-O-Si modes (9). EXPERIMENT The µc-Si films examined in this study were deposited by hot wire CVD (HWCVD) in a load locked deposition reactor described elsewhere (10)