Raman spectroscopic analysis of the effect of annealing on hydrogen concentration and microstructure of thick hot wire grown a-Si:H films aimed as precursor layers for crystallized thin film silicon

摘要

For application as precursor layers for silicon solar cells fabricated by laser liquid phase crystallization, thick amorphous silicon films on glass are of interest. However, for hydrogenated amorphous silicon (a-Si:H) precursor layers containing about 10 at.% hydrogen, hydrogen needs to be removed prior to liquid phase crystallization to avoid bubble formation and peeling. For this purpose, an at least 12 hours annealing procedure up to 550 degrees C is considered necessary thus involving long process time and high costs. In this article, we investigate the use of thick hot wire grown a-Si:H films which turn out to need considerably less time for dehydrogenation than dense plasma-grown a-Si:H. The dehydrogenation process is studied by depth profiles of hydrogen concentration and medium range order (MRO) using Raman spectroscopy analysis at etch pits. The results show already at an annealing temperature of 450 degrees C the disappearance of all detectable H in the substrate-near part and the complete removal of H at 550 degrees C after about 4 hours annealing. We attribute this rather fast hydrogen removal to the formation of interconnected voids primarily in the substrate-near range. In the same range of the film, we find a correlation between hydrogen concentration and medium range order suggesting that a silicon network re-construction due to hydrogen out-diffusion causes an observed decrease of reciprocal MRO. The results stress the importance of void-related microstructure in the a-Si:H for hydrogen removal at a rather low annealing temperature and short annealing time. Our results suggest that hot wire a-Si:H films which can be grown with a high deposition rate and a rather pronounced void-related microstructure may be well suited as economic precursor layers.