Wet-Chemical Treatment of Solar Grade CZ Silicon Prior to Surface Passivation

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1640-1644
We have performed a systematic variation of the wet chemical oxidation and the subsequent oxide etching steps during the cleaning of Czochralski (CZ) single crystalline silicon wafers prior to surface passivation. The optimization of these preconditioning steps was carried out on saw-damage etched or textured Si wafers subsequently passivated by amorphous silicon nitride (a-SiNx:H) or chemical passivation by an iodine ethanol (I/E) solution. Measuring the carrier lifetime using the spatially resolved microwave detected photoconductance decay, we monitored the impact of the wet-chemical surface conditioning on the surface morphology and wafer base doping type. For damage etched surfaces in alkaline potassium oxide solutions and passivated by iodine ethanol, an optimum surface passivation is obtained by omitting the last water rinse (as used in the standard clean) and adding a hot water treatment as final step in the cleaning procedure. While this results was found for both p-type and n-type wafers, suggesting that the passivation mechanism is based on the saturation of dangling bonds, a strong dependence on the doping type was observed for the passivation by a-SiNx:H. Prior to passivation by a-SiNx:H, the best preconditioning for n-type was achieved by adding a hot water treatment and subsequent etching of the oxide by hydrofluoric acid (HF). For p-type however, no improvement was achieved with respect to the standard cleaning step. This dependency on the base doping type could be attributed to the passivation mechanism of a-SiNx:H based on the field effect. By employing ammonium fluoride as final etching solution instead HF, followed by hot water treatment, compared to the standard treatment, an improvement of the lifetime can be observed for both surface structures, indicating that the surface micro-roughness can be reduced and the contamination can be removed.