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 O2 plasma, can be used to deposit Al2O3 for a high level of surface passivation of crystalline silicon (c-Si). For 3.5 Ocm n-type c-Si, plasma ALD Al2O3 resulted in ultralow surface recombination velocities of Seff <0.8 cm/s. Thermal ALD Al2O3 also showed an excellent passivation level, with Seff <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, 375 °C, than for plasma ALD Al2O3, 425 °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 Seff <9.2 and <6.5 cm/s, respectively.