Design and research of Ge/Si avalanche photodiode with a specific lateral carrier collection structure

In order to achieve infrared photodetectors with high gain, low noise, and high 3 dB-bandwidth(3 dB-BW) for low-loss and high-efficiency fiber communication, many researchers have paid attention to the Ge/Si avalanche photodiode(APD) which is regarded as a potential photoelectric device for the detection of infrared light. In this paper, we propose and theoretically study a potential structure of a Ge/Si APD with a specific lateral carrier collection structure. The influence on the electric field distribution of the doping concentration of the top Si layer, the size of the gap between heavily-doped n-Si and the Ge/Si mesa, and the thickness of the top Si layer thickness have been considered. It was found that the doping concentration of Si multiplication significantly affects the junction effect of the vertical p⁺-i-p^--n^- junction and lateral n⁺-n^- junction, which in turn affects the distribution of the electric field. Furthermore, the reason for the high 3 dB-BW is also clarified by studying the carrier transportation. This feature is explained by the high carrier velocity in the direction of carrier collection induced by the fringing electric field formed by the lateral n⁺-n^- and vertical p⁺-i-p^--n^- junction structure. It was found that the migration path of carriers in our lateral collection APD is along the edge of the Ge mesa and turns to the lateral direction in the Si layer. This is very different from the vertical migration path in a traditional SACM Ge/Si APD. We researched and found that an extremely low dark current can be obtained using an oxide-free Ge/Si direct wafer bond. A high 3 dB-bandwidth of~20 GHz was achieved under an optical input power of -30 dBm at 1 310 nm. Such a high 3 dB-bandwidth is demonstrated using the specific lateral carrier collection structure of this APD.