PECVD-AlO_x/SiN_x passivation stacks on silicon: Effective charge dynamics and interface defect state spectroscopy

The charge dynamics and the interface defect state density of AlO_x/SiN_x passivation stacks deposited by plasma-enhanced chemical vapor deposition (PECVD) on crystalline silicon (c-Si) wafers are investigated. High frequency (1 MHz) capacitance voltage (C-V) measurements were performed on stacks in the as deposited state and after an annealing step. C-V sweeps reveal an initially high negative charge density for the as deposited sample, activated by the thermal budget during SiN_x deposition. However, this charge state is unstable and reduced owning to electron detrapping and emission into the c-Si upon applying moderate voltages. In the annealed sample, the AlO_x/SiN_x stack has a stable negative fixed charge. Both for as deposited and for annealed samples, applying a positive or negative constant gate voltage stress (Vstress) enhances or reduces the negative effective charge density (Qox,eff), respectively. Injection of charges from the c-Si into traps in the AlO_x/SiN_x stack is identified as the mechanism responsible for this behavior. We conclude that in addition to fixed negative charges trapping of negative charges near the interface is a crucial mechanism contributing to the total effective negative charge of the stack. Their contribution depends on the temperature and duration of the thermal treatment. Additionally, a large Vstress leads to generation of additional Si dangling bond defects over the entire c-Si bang gap at the c-Si/AlO_x interface.