Nano-rod Array for Enhanced Single Photon Avalanche Diode in NIR regime

Abstract

Due to its high sensitivity, superior dark count rate, and cost-effectiveness, Single Photon Avalanche Diodes (SPADs) are becoming increasingly important in critical future applications such as Lidar sensors, 3D ToF sensors and quantum communications. In near-infrared(NIR) regime, which is a standard for Lidar sensors and fibe communications, InGaAs and Germanium material are usually adopted for NIR regime due to its high detection efficiency. However, InGaAs and Germanium SPAD has high dark count rate because of its low direct energy bandgap. Therefore, we adopted silicon material for SPAD that can enable integration with signal processing system through CMOS fabrication with low dark count rate. However, Si SPADs have shown poor detection efficiency because of the weak absorption of silicon in this spectral range. A thick absorption layer is usually adopted to increase detection efficiency, but this leads to the deterioration of the response frequency and difficulties in the process integration with the control circuits. Here, we show Nano-rod arrays that can enable a high detection efficiency in the NIR regime even with a shallow absorption layer by light trapping to enhance the performance of fully CMOS-based SPADs. To confirm the light trapping effect of the Nano-rod array structure, we adopt Rigorous-coupled-wave-analysis(RCWA) approach that can provide detailed information on the diffraction modes and their power distributions in a complex dielectric layer structure. The analysis on how each mode propagates after the reflection at Si-SiO2 interface and re-diffracted at the top grating interface using Fresnel equations as well as RCWA reveals that more than 26 % of light can be trapped using only 0.5um thick layer, which is 20 times thinner than the conventional Si SPAD. Furthermore, it achieved a high frequency response which is 7 times faster than conventional SPAD. Based on these characteristics, Nano-rod array SPAD can be used not only in lidar sensors and tof sensors, but also in fields such as color filters.