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

In this Letter, we report that both thermal atomic layer deposition (ALD) with H2O, and plasma ALD with an O-2 plasma, can be used to deposit Al2O3 for a high level of surface passivation of crystalline silicon (c-Si). For 3.5 Omega cm n-type c-Si, plasma ALD Al2O3 resulted in ultralow surface recombination velocities of S-eff < 0.8 cm/s. Thermal ALD Al2O3 also showed an excellent passivation level, with S-eff < 2.5 cm/s. In contrast to plasma ALD Al2O3, thermal ALD Al2O3 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, similar to 375 degrees C, than for plasma ALD Al2O3, similar to 425 degrees C. The minimal Al2O3 thickness without compromising the passivation properties was 5 nm for plasma ALD Al2O3, 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 Al2O3 films of 5 nm by S-eff < 9.2 and < 6.5 cm/s, respectively. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim