Activation of Al2O3 surface passivation of silicon: separating bulk and surface effects

Understanding surface passivation arising from aluminium oxide (Al₂O₃) films is of significant relevance for silicon-based solar cells and devices that require negligible surface recombination.

In the paper 'Activation of Al2O3 surface passivation of silicon: separating bulk and surface effects' the authors aimed to understand the competing bulk and surface lifetime effects which occur during the activation of atomic layer deposited Al₂O₃.  They demonstrate that maximum passivation is achieved on n- and p-type silicon with activation at ~450^oC, irrespective of annealing ambient.

On stripping the Al₂O₃ films and re-passivating the surface using a superacid-based technique, they found that the bulk lifetime of float-zone and Czochralski silicon wafers degraded at annealing temperatures of >450^oC.  By accounting for that bulk lifetime degradation, the authors demonstrate that the chemical passivation component associated with Al₂O₃ remained stable at activation temperatures of 450-500^oC, achieving an SRV of <1cm/s on m- and p-type silicon.

In conjunction with the thermal stability, the authors show that films in the range of 3-30nm maintain an SRV of <1cm/s when annealed at 450^oC.  From this atomic-level energy dispersive X-ray analysis they were able to demonstrate that, post deposition, the interface has a structure of Si/SiO₂/Al₂O₃.  After activation at >300^oC, the interface became Si/Si_xAl_yO₂/Al₂O₃ due to diffusion of aluminium into the thin silicon oxide layer.