Thermal Treatment Effects on Flat-band Voltage Shift in Atomic-layer-deposited Alumina or Aluminum Oxide/Silicon Nitride Passivation Stacks

We studied the effects of thermal annealing on the interfacial properties of atomic-layer-deposited alumina (Al2O3) films and aluminum oxide/silicon nitride (AlOx/SiNx) stacks on silicon. Thermal treatment was found to have a significant effect on the lifetime improvement, owing to full activation of field-effect passivation, which can be realized with an increased flat-band voltage (Vfb). The presence of a capping SiNx layer itself and/or during deposition is suspected to cause distribution of unstable negatively charged traps in the AlOx/SiNx stacks, resulting in a slight reduction in lifetime with inversely improved Vfb. The interface trap density, an indicator of chemical passivation, remains at the same order of magnitude after thermal treatment and SiNx deposition, suggesting that atomic-layer deposition induces a high passivation quality. These results suggest that consideration of thermal-annealing effects and SiNx deposition conditions are crucial for improving the quality of passivation stacks at the rear of passivated emitter rear contact (PERC) cells and, consequently, the high performance of PERC cells.