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

Schematic of performance plotted against effective lifetime and annealing temperature

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 ~450oC, 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 >450oC.  By accounting for that bulk lifetime degradation, the authors demonstrate that the chemical passivation component associated with Al2O3 remained stable at activation temperatures of 450-500oC, 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 450oC.  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 >300oC, the interface became Si/SixAlyO2/Al2O3 due to diffusion of aluminium into the thin silicon oxide layer.