Enhanced Surface Passivation by Atomic Layer-Deposited Al2O3 for Ultraviolet-Sensitive Silicon Photomultipliers

This article describes a superior passivation of p-type (p⁺) Si surface by an Al₂O₃ thin film that is synthesized by plasma-assisted atomic layer deposition (ALD) for ultraviolet-sensitive silicon photomultipliers (SiPMs), compared to conventional SiO₂ and SiN_x passivation schemes. The superiority of Al₂O₃ passivation is due not only to a sufficiently low interface defect density, but also to a high density of built-in negative charges. A 7-nm thin Al₂O₃ film can yield an emitter saturation current density of ~8 fA/cm² on a high sheet resistance p⁺ layer, compared to ~60 and ~1480 fA/cm² for thermal SiO₂ and SiN_x passivation. This superior surface passivation allows the photon-generated carriers to have higher probabilities to reach the high-field region to trigger an avalanche event. In addition, the Al₂O₃ thin film provides very low values of effective surface recombination velocity on low resistivity n- and p-type Si surfaces, which can lead to well-passivated surface features on the guard ring and trench isolation regions of SiPM. These demonstrate the potential of the Al₂O₃ thin film passivation to improve the quantum efficiency and thus photo-detection efficiency (PDE) of ultraviolet-sensitive SiPM with p⁺/n⁻/n/n⁺ structure.