Paper TF+EN+PS-TuA11
Study of the Surface Passivation Mechanism of c-Si by Al₂O₃ using In Situ infrared spectroscopy

Tuesday, November 11, 2014, 5:40 pm, Room 307

The quality of Si surface passivation plays an integral role in the performance of c‑Si‑based solar cells. Recently, Al₂O₃ films grown by atomic layer deposition (ALD) have been shown to be an effective passivant for c-Si surfaces with surface recombination velocities (S_eff) that are <5 cm/s. The chemical passivation of the c-Si surface via Al₂O₃ is achieved by a reduction in the defect density at the interface, while field-effect passivation is attributed to the fixed negative charge associated with the Al₂O₃ films. However, a post-deposition annealing step is required to achieve this high level of passivation. We have investigated the mechanism of chemical passivation during the annealing step using in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Specifically, we have studied the role of residual H- and O-atom migration from the ALD Al₂O₃ films to the c-Si/Al₂O₃ interface. Using Al(CH₃)₃ and O₃ as the ALD precursors, Al₂O₃ films were deposited directly onto high-lifetime float-zone c-Si internal reflection crystals (IRCs) followed by thermal annealing at 400 °C in different atmospheres. Specifically, we have used D-terminated c-Si IRCs to differentiate the residual H atoms that may migrate from ALD Al₂O₃ films versus the residual D atoms present at the Al₂O₃/c-Si interface after ALD. Within the sensitivity of the ATR-FTIR spectroscopy setup of ~10¹² cm^-2 for Si-H bonds, we do not detect any migration of H from Al₂O₃ to the c-Si interface. Therefore, we conclude that the migration of O, and the subsequent restructuring of the interface during the annealing step, primarily contributes towards the chemical passivation of the Al₂O₃/c-Si interface. The ATR-FTIR spectroscopy measurements are complemented by the minority carrier lifetime, interface defect density, and built-in charge density measurements on SiO₂/Al₂O₃ stacks on c-Si, which enable us to isolate chemical passivation from field-effect passivation. The stacks were annealed in different atmospheres to better understand the role of O versus H atoms in the chemical passivation mechanism.

We gratefully acknowledge the support from the NCPV Fellowship Program and U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory.