Amorphous/microcrystalline phase control in sillicon film deposition for improved solar cell performance

摘要

Real time optical studies have provided insights into the growth of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (#mu#c-Si:H) thin films by plasma-enhanced chemical vapor deposition as a function of the H_2-dilution gas flow ratio R=[J_2]/[SiH_4], the accumulated film thickness d_b, and the substrate material. Results pertinent to the optimization of a-Si:H-based solar cells have been obtained in studies of Si film growth at moderate to high R on dense amorphous semiconductor film surfaces. For depositions with 15<=R<=80 on freshly-deposited a-Si:H, initial film growth occurs in the amorphous phase. Upon continued growth, however, a transition is observed as crystallites begin to nucleate from the amorphous film. The thickness at which this amorphous-to-microcrystalline (a->#mu#c) transition occurs is found to decrease with increasing R. Based on these results, a deposition phase diagram has been proposed that dexcribes the a->#mu#c transition as a continuous function of R and d_b. We find that the optimum stabilized a-Si:H p-i-n solar cell performance is obtained in an i-layer growth process that is maintained as close as possible to the phase boundary (but on the amorphous side) versus film thickness.