Electric field induced surface passivation of Si by atomic layer deposited Al2O3 studied by optical second-harmonic generation

Recently, we have demonstrated that ultrathin (<30 nm) films of Al₂O₃ synthesized by (plasma-assisted) atomic layer deposition (ALD) provide an excellent level of surface passivation of c-Si which may find important applications in (high-efficiency) solar cells. In this contribution, the Al₂O₃ passivation mechanism has been further elucidated by the contactless characterization of the c-Si/Al₂O₃ interface by optical second-harmonic generation (SHG). SHG has revealed effective field-effect passivation of the c-Si surface caused by a negative fixed charge density of 5×10¹² cm^-2 in an annealed, 11 nm thick Al₂O₃ film while it is on the order of 10¹¹ cm^¿2 in the as-deposited film which shows negligible passivation. A comparison with SHG measurements on a 84 nm thick a-SiN_x:H film treated in a conventional firing furnace has revealed the presence of a positive fixed charge density of 2×10¹² cm^-2 which further corroborates the SHG analysis and results.