Controlling of Polarity on the Surface Passivation Mechanism of Al₂O₃ for Black Silicon by CLD

Thin Al₂O₃ films were deposited on p-type black silicon (b-Si) by using chemical liquid phase deposition (CLD) technique. The influence of annealing temperatures on the structural and optical properties of Al₂O₃ films was investigated. The b-Si with 80-nm Al₂O₃ films exhibits a low total reflectance of 5%. The sample annealed 300 °C exhibits negative fixed charge with the density of 1.5×10¹² cm^-2. With the increasing of annealing temperature, negative shift of C-V curve was observed, indicating the polarity of fixed charge changes to positive, with maximal the density of 8.7×10¹¹ cm^-2. The evolution of the polarity of fixed charge is assigned to the decreasing of O: Al ratio caused by the transition of the crystalline type of Al₂O₃. The change of fixed charge polarity in Al₂O₃ provides a feasible route for both p- and n-type Si passivation in Si solar cells by adjusting the thermal post-treatment.