The negative charge formation, the charge-trapping mechanisms and the interface defect passivation of aluminum oxide/silicon nitride (AlOx/SiNx) stacks deposited by plasma-enhanced chemical vapor deposition on p-type crystalline silicon (c-Si) are investigated. Constant voltage stress (CVS) investigations combined with capacitance-voltage (C-V) hysteresis analysis indicate the influence of different thermal treatments on the negative charge formation and allow discerning between fixed and trapped charges in the AlOx/SiNx system. The thermal budget during SiNx deposition activates negatively charged traps. An annealing step leads to the formation of a stable, fixed negative charge and reduces the defect state density (Dit) at the c-Si/AlOx interface. A wet-chemical silicon oxidation (SiOx) of the c-Si surface reduces Dit even further, but introduces additional traps at the wet-chemical SiOx/AlOx interface. These traps lead to instabilities of the negative charge density and have a detrimental effect on the passivation quality. However, a firing step leads to the formation of a higher negative charge density due to charged traps. Combined with the enhanced chemical passivation, this results in a higher passivation quality than upon annealing. The trap-related negative charge upon firing is unstable due to electron detrapping. However, a positive CVS can recharge traps in the wet-chemical SiOx/AlOx/SiNx system negatively through electron injection from the c-Si.