Si Waveguide-Integrated Metal–Semiconductor–Metal and p–i–n-Type Ge Photodiodes Using Si-Capping Layer

Abstract

We studied Si waveguide-integrated metal–semiconductor–metal (MSM) and p–i–n-type Ge photodiodes (Ge-PDs), using a Si-capping layer. As for an MSM Ge-PD, the Schottky barrier height was increased up to 0.44 V by applying a 8–20 nm Si-capping layer, and a very low dark current density of approximately 0.4 nA/µm 2 was achieved with a high responsivity of 0.8 A/W. In addition, a small electrode spacing of 1 µm realized high-speed photodetection of 20 Gbps. As for a p–i–n-type Ge-PD, by applying a 10–20 nm Si capping layer, the contact resistance between a metal electrode of Ti/TiN/Al and n + -Si capping layer was successfully reduced to 1×10 -5 Ω·cm 2 . A 45 GHz bandwidth was obtained with a low dark current density of 0.8 nA/µm 2 . Moreover, a more than 20 GHz bandwidth was achieved with zero-bias voltage. In the case of zero-bias voltage operation, a 3 dB bandwidth was a little affected by input power, which would originate from the photocarrier screening effect on the built-in electric field.