Role of annealing conditions on surface passivation properties of ALD Al2O3 films

The surface passivation performance of Al2O3 films attracted attention in the field of solar cells and semiconductor devices and depends on the conditions of the applied post-deposition annealing step. The effect of annealing temperature and different annealing atmospheres on the surface passivation quality of atomic layer deposited Al2O3 films was investigated on n-type float-zone Si wafers. Photoconductance decay measurements were carried out to characterize recombination velocities and carrier lifetimes. The chemical and field-effect passivation mechanism, i. e. the interface trap density and the fixed charge density, respectively, were studied by capacitance-voltage experiments. Low surface recombination velocities of S-eff,S- max similar to 1 cm/s corresponding to a carrier lifetime of 9.0 ms were achieved for samples annealed in O-2 atmosphere whereas annealing in H-2 and N-2 led to slightly higher S-eff max-values similar to 2 cm/s. The annealing temperature was found to affect both the fixed charge density and the interface trap density whereas in contrast the annealing atmosphere affected only the interface trap density, i. e. the chemical passivation. According to the expectations the highest surface passivation quality is based on a high fixed charge density and a low interface trap density. (C) 2013 The Authors. Published by Elsevier Ltd.