Abstract
This article describes a superior passivation of p-type (p<sup>+</sup>) Si surface by an Al<sub>2</sub>O<sub>3</sub> thin film that is synthesized by plasma-assisted atomic layer deposition (ALD) for ultraviolet-sensitive silicon photomultipliers (SiPMs), compared to conventional SiO<sub>2</sub> and SiN<sub><i>x</i></sub> passivation schemes. The superiority of Al<sub>2</sub>O<sub>3</sub> 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 Al<sub>2</sub>O<sub>3</sub> film can yield an emitter saturation current density of ~8 fA/cm<sup>2</sup> on a high sheet resistance p<sup>+</sup> layer, compared to ~60 and ~1480 fA/cm<sup>2</sup> for thermal SiO<sub>2</sub> and SiN<sub><i>x</i></sub> 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 Al<sub>2</sub>O<sub>3</sub> 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 Al<sub>2</sub>O<sub>3</sub> thin film passivation to improve the quantum efficiency and thus photo-detection efficiency (PDE) of ultraviolet-sensitive SiPM with p<sup>+</sup>/n<sup>−</sup>/n/n<sup>+</sup> structure.
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