Silicon surface passivation by ultrathin Al₂O₃ films synthesized by thermal and plasma atomic layer deposition

Abstract In this Letter, we report that both thermal atomic layer deposition (ALD) with H 2 O, and plasma ALD with an O 2 plasma, can be used to deposit Al 2 O 3 for a high level of surface passivation of crystalline silicon (c‐Si). For 3.5 Ω cm n‐type c‐Si, plasma ALD Al 2 O 3 resulted in ultralow surface recombination velocities of S eff < 0.8 cm/s. Thermal ALD Al 2 O 3 also showed an excellent passivation level, with S eff < 2.5 cm/s. In contrast to plasma ALD Al 2 O 3 , thermal ALD Al 2 O 3 provides some surface passivation in the as‐deposited state, although annealing is required to activate it to the full extent. For thermal ALD, the optimal temperature for this anneal was found to be slightly lower, ∼375 °C, than for plasma ALD Al 2 O 3 , ∼425 °C. The minimal Al 2 O 3 thickness without compromising the passivation properties was 5 nm for plasma ALD Al 2 O 3 , whereas for thermal ALD, films >10 nm were required. Thermal stability against a high temperature firing step was demonstrated for ultrathin thermal and plasma ALD Al 2 O 3 films of 5 nm by S eff < 9.2 and < 6.5 cm/s, respectively. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)