Deleterious electrostatic interaction in silicon passivation stack between thin ALD Al₂O₃ and its a-SiN_X:H capping layer: Numerical and experimental evidences

This study focuses on the electrostatic interaction between the ALD Al₂O₃ passivation layer and the PECVD a-SiN_X:H capping layer, while reducing ALD Al₂O₃ thickness. The first one embeds negative fixed charges while the second is well known to possess a fixed charge density with an opposite polarity. We reduced the Al₂O₃ thickness from 20 to 2 nm while keeping constant the a-SiN_X:H thickness. To increase the magnitude of the field effect passivation provided by Al₂O₃, we used an original light-induced field effect enhancement strategy. QSS-PC was used to quantify the initial passivation properties. We monitored the evolution of the passivation quality under low-intensity light-soaking until its stabilization. We report here a minority carrier lifetime enhancement up to 900% and surface recombination velocity reduction below 10 cm.s^-1. The evidence of field effect compensation at c-Si surface due to a-SiN_X:H positive charges has been supported by numerical simulations (SILVACO ATLAS). While keeping the parameters of the a-SiN_X:H layer constant, the configuration of the Al₂O₃ layer (thickness and fixed charge density) were varied. These results allow us to conclude that in order to use thinner Al₂O₃ passivation layer (2 nm), ideal capping layer has to be free of positive fixed charges and to release enough hydrogen at desired temperature to ensure the chemical passivation.