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0910-A06-05

Microstructure Characterization of Amorphous Silicon-Nitride Films by Effusion Measurements W. Beyer1, and H.F.W. Dekkers2 1 Institut für Photovoltaik, Forschungszentrum Jülich GmbH, Jülich, D-52425, Germany 2 IMEC vzw, Kapeldreef 75, Leuven, B-3001, Belgium

ABSTRACT The microstructure of a-Si:N:H films, which are applied as antireflection coating and for defect passivation in multicrystalline silicon (mc-Si) solar cells, was studied by gas effusion experiments. The results show for as-deposited material of low substrate temperatures (TS = 200 – 300°C) a predominant diffusion of molecular hydrogen for temperatures up to 800°C in agreement with the presence of interconnected openings (voids). At higher substrate temperatures, the material has a more compact structure and atomic hydrogen is the dominant diffusing species in the accessible temperature range. Annealing effects were also studied. The results are consistent with the concept that atomic hydrogen released from the a-Si:N:H coating serves for defect passivation in mc-Si solar cells. INTRODUCTION Deposition of hydrogenated amorphous silicon-nitride (a-Si:N:H) as an anti-reflection coating is a standard process step in the production of crystalline silicon solar cells. In case of multicrystalline Si (mc-Si) solar cells, these coatings are also of interest for defect passivation in the mc-Si absorber material. Commonly, such defect passivation is achieved after applying rapid annealing (“firing”) procedures. The passivation process is thought to involve hydrogen diffusion from the a-Si:N:H layer [1] or from the mc-Si/ a-Si:N:H interface [2] into the bulk multicrystalline silicon. Thus, in particular in the case when a-Si:N:H is the primary source of passivating hydrogen, the microstructure of the a-Si:N:H layer is expected to be of substantial importance for the resulting H passivation effects. In this article, we report on studies of the microstructure of a-Si:N:H films by effusion measurements. Effusion of hydrogen (present in these films in concentrations of 10 to 20 at.%) is known to be highly sensitive to the material microstructure, since, depending on the microstructure, the diffusing hydrogen species can be either atomic or molecular hydrogen [3-5]. Effusion studies of stacked samples of hydrogenated and deuterated material [3-6] as well as of He and Ne implanted material [7] allow further insight in material microstructure and in hydrogen diffusion processes. EXPERIMENTAL DETAILS The a-Si:N:H films were prepared at IMEC by low frequency plasma-enhanced chemical vapor deposition (LF-PECVD) at a plasma excitation frequency of 440 kHz [6] using SiH4 and NH3 precursor gases. Isotopically enriched (>98%) gases SiD4 and ND3 were employed for the deuterated silicon nitride (a-Si:N:D) layers. Using ERDA (elastic recoil detection analysis), the

nitrogen to silicon ratio for the investigated films was found to range between 1.0 and 1.3, i.e. the films are slightly under-stoichiometric. Film thickness was typically 400 nm for the a-Si:N:H films an