Impact of PECVD SiON stoichiometry and post-annealing on the silicon surface passivation

Hydrogenated silicon oxynitride (SiON) could be used in combination with silicon nitride (SiN) to create multi-layer antireflection coatings for silicon solar cells. It could also be used as a passivation layer, especially on the back side of the cell. This work deals with the passivation effect obtained on silicon surface by SiON layer deposited by Low Frequency Plasma Enhanced Chemical Vapour Deposition (LF-PECVD). SiON layers of different compositions have been deposited by varying the gas flow mixture (NH3, SiH4 and N2O) in the reactor. Infrared and X-ray photon-electron spectroscopy were made to determine the chemical structure of SiON layer. Minority carrier lifetimes were measured by the photoconductance decay method (PCD) before and after a rapid thermal anneal. Effective lifetime, measured on 5 Omega cm FZ-silicon wafers, can reach up to several hundreds microseconds, depending on the stoichiometry of the SiON layer. Low oxygen content samples (close to SiN layer) exhibit a good surface passivation of 250 mu s but after annealing, this value is critically reduced to 6 mu s. The opposite situation is observed for oxygen-rich layers: the effective lifetime increases from 10 mu s to 150 mu s. These behaviours could be partly explained by the composition of SiON, the evolution of the main peaks values of the FTIR spectra and the disappearance of Si-H bonds with anneal. (C) 2007 Elsevier B.V. All rights reserved.