High temperature oxidation pre-treatment of textured c-Si wafers passivated by a-Si:H

This work shows an alternative surface cleaning method for c-Si wafers to replace the standard chemical procedures as RCA or HNO ₃ which involve hazardous chemicals or unstable processes. The method consists in a high-temperature oxidation treatment (HTO) performed in a classical tube furnace that incorporates organic and metal particles present on the c-Si surfaces in the growing SiO ₂ layer. The result is as a reliable pre-treatment method for obtaining less defective c-Si surfaces ready for solar cell fabrication after SiO ₂ removal. To test the surface passivation quality obtained with our alternative cleaning method, we grow amorphous silicon (a-Si:H) layers by plasma enhanced chemical vapor deposition on both sides of the c-Si wafer and systematically compare the effective carrier lifetime (τ _eff ) and implied V _OC (iV _oc ) to the wafer treated with the standard cleaning in our laboratory. We optimize HTO treatment time reaching τ _eff of ∼6 ms and iV _oc of 721 mV for the best sample. We ascribe the improved passivation quality using HTO to two concurrent factors. Firstly, the encapsulation of defects into SiO ₂ layer that is then etched prior a-Si:H deposition and secondly, to modification of the pyramids’ morphology that facilitates the surface passivation. SEM pictures and reflection measurements support the latter hypothesis.