Silicon surface passivation by atomic layer deposited Al₂O₃

Thin Al₂O₃ films with a thickness of 7–30 nm synthesized by plasma-assisted atomic layer deposition (ALD) were used for surface passivation of crystalline silicon (c-Si) of different doping concentrations. The level of surface passivation in this study was determined by techniques based on photoconductance, photoluminescence, and infrared emission. Effective surface recombination velocities of 2 and 6 cm/s were obtained on 1.9 Ω cm n-type and 2.0 Ω cm p-type c-Si, respectively. An effective surface recombination velocity below 1 cm/s was unambiguously obtained for nearly intrinsic c-Si passivated by Al₂O₃. A high density of negative fixed charges was detected in the Al₂O₃ films and its impact on the level of surface passivation was demonstrated experimentally. The negative fixed charge density results in a flat injection level dependence of the effective lifetime on p-type c-Si and explains the excellent passivation of highly B-doped c-Si by Al₂O₃. Furthermore, a brief comparison is presented between the surface passivations achieved for thermal and plasma-assisted ALD Al₂O₃ films prepared in the same ALD reactor.