Double Zinc Diffusion Optimization for Charge Persistence Reduction in InGaAs/InP SPADs

We present an in-depth characterization of the impact of the double zinc diffusion 2D profile on the charge persistence effect in InGaAs/InP Single-Photon Avalanche Diodes (SPADs). Charge persistence might be mitigated by increasing the operating temperature, so as to reduce minority carriers' lifetime, as well as by increasing the number of grading layers. However, the first approach leads to higher dark count rate due to stronger thermal generation of dark carriers, while the latter is technologically challenging. We show that, by adjusting the depth and radius of the shallow zinc diffusion, the electric field profile in the InGaAs layer outside the active area can be optimized in order to effectively mitigate charge persistence, achieving also its complete suppression in specific structures. Our study is based on both TCAD simulations and experimental measurements of different SPADs, whose only difference is in the geometrical shallow diffusion parameters.