Hydrogen in aluminium oxide films grown for silicon surface passivation by plasma-enhanced atomic layer deposition

• Al 2 O 3 films grown by atomic layer deposition passivate solar cell wafer surfaces. • ToF-ERDA shows hydrogen level in film to vary with depth/ annealing temperature. • Al 2 O 3 as-deposited and annealed ≤450–500 °C contains ∼1.2 at.% hydrogen. • Passivation level and hydrogen concentration reduces with annealing >500 °C. Aluminium oxide (Al 2 O 3 ) films grown by atomic layer deposition (ALD) are widely used in surface passivation, including for silicon-based solar cells. Post-deposition annealing is used to tune interfacial properties by modifying field-effect and chemical passivation. We have used time-of-flight elastic recoil detection analysis (ToF-ERDA) to depth-profile hydrogen concentrations in silicon coated with Al 2 O 3 grown by plasma-enhanced ALD at 200 °C, with effective carrier lifetimes in the same sample set monitored by photoconductance decay measurements. Effective lifetime — which strongly relates to surface passivation — peaks with post-deposition annealing at 450–500 °C. Corona charging experiments demonstrate that chemical passivation improves with annealing from 400 to 550 °C, worsening with annealing ≥600 °C. ToF-ERDA reveals an average hydrogen concentration of ∼1.2 at.% for films as-deposited and annealed ≤450–500 °C. As passivation starts to reduce upon annealing >550 °C, the film’s average hydrogen concentration reduces to ∼0.5 at.%, with no detectable increase in hydrogen in the silicon. Hydrogen out-diffusion should be a consideration in understanding reduced passivation at higher annealing temperatures. If the Al 2 O 3 film is considered as two regions — near-surface and near-interface — then below 500 °C, hydrogen concentrations are greater near-surface than near-interface, a trend reversed when annealing >550 °C