Crystalline silicon surface passivation investigated by thermal atomic-layer-deposited aluminum oxide

Atomic-layer-deposited (ALD) aluminum oxide (Al₂O₃) has demonstrated an excellent surface passivation for crystalline silicon (c-Si) surfaces, as well as for highly boron-doped c-Si surfaces. In this paper, water-based thermal atomic layer deposition of Al₂O₃ films are fabricated for c-Si surface passivation. The influence of deposition conditions on the passivation quality is investigated. The results show that the excellent passivation on n-type c-Si can be achieved at a low thermal budget of 250 ℃given a gas pressure of 0.15 Torr. The thickness-dependence of surface passivation indicates that the effective minority carrier lifetime increases drastically when the thickness of Al₂O₃ is larger than 10 nm. The influence of thermal post annealing treatments is also studied. Comparable carrier lifetime is achieved when Al₂O₃ sample is annealed for 15 min in forming gas in a temperature range from 400 ℃to 450 °C. In addition, the passivation quality can be further improved when a thin PECVD-SiN_x cap layer is prepared on Al₂O₃, and an effective minority carrier lifetime of 2.8 ms and implied V_oc of 721 mV are obtained. In addition, several novel methods are proposed to restrain blistering.