Excellent c-Si surface passivation by thermal atomic layer deposited aluminum oxide after industrial firing activation

Abstract

We demonstrate that by using a water (H2O)-based thermal atomic layer deposited (ALD) aluminum oxide (Al2O3) film, excellent surface passivation can be attained on planar low-resistivity silicon wafers. Effective carrier lifetime values of up to 12 ms and surface recombination velocities as low as 0.33 cm s−1 are achieved on float-zone wafers after a post-deposition thermal activation of the Al2O3 passivation layer. This post-deposition activation is achieved using an industrial high-temperature firing process which is commonly used for contact formation of standard screen-printed silicon solar cells. Neither a low-temperature post-deposition anneal nor a silicon nitride capping layer is required in this case. Deposition temperatures in the 100–400 °C range and peak firing temperatures of about 800 °C (set temperature) are investigated. Photoluminescence imaging shows that the surface passivation is laterally uniform. Corona charging and capacitance–voltage measurements reveal that the negative fixed charge density near the AlOx/c-Si interface increases from 1.4 × 1012 to 3.3 × 1012 cm−2 due to firing, while the midgap interface defect density reduces from 3.3 × 1011 to 0.8 × 1011 cm−2 eV−1. This work demonstrates that direct firing activation of thermal ALD Al2O3 is feasible, which could be beneficial for solar cell manufacturing.