Investigation of radiation damage to the Al₂O₃/Si wafer interface during electron beam evaporation by means of C-V and lifetime measurements

Abstract

For the purpose of reducing recombination activity in crystalline silicon solar cells, atomic layer deposited aluminum oxide (Al2O3) has proven a promising candidate. Its excellent surface passivation is effected by an exceptionally high density of negative fixed charges Qf generating a strong field-effect along with a chemical passivation reducing the density of interface traps Dit. The dependence of these two measures on the temperature and the duration of the post-deposition anneal activating the passivation of Al2O3 is investigated by measuring the capacitance-voltage (C-V) characteristics. To directly correlate Qf and Dit with the effective minority carrier lifetime eff, a new kind of sample structure is developed, whereby both measurement types can be conducted on the same test sample. The interface properties of samples with thermal and electron beam evaporated metal contacts are compared and a correlation with the obtained passivation quality quantified by eff is identified in order to investigate the influence of the radiation damage. It is found that Qf, Dit and eff of Al2O3 passivated p-doped Si wafers exhibit a correlation when annealing parameters are varied and that an electron beam evaporation of Al damages the Al2O3/Si interface and significantly reduces eff. Finally, a method to restore the effective lifetime is developed and investigated which yields a recovery rate of 65% corresponding to a reduction of Dit and an increase of Qf.