(Invited) Si Waveguide-Integrated High-Speed Ge Photodetector

Silicon photonics has recently become a subject of intense interest because it offers an opportunity for low cost, low power consumption, and high bandwidth of optoelectronic solutions for applications ranging from telecommunications down to chip-to-chip interconnects. By the integration of germanium into silicon photonics circuit, very efficient photodetection has been demonstrated for the past several years. However, higher performance with low drive voltage and small footprint has not been achieved.

In this paper, we present a Si waveguide (WG)-integrated PIN-type Ge-PD (photodetector), which shows high speed of 50 GHz bandwidth with small footprint. We also compare Si waveguide-integrated PIN and Schottky Ge PDs, which showed very low dark current density with high efficiency.

We fabricated two types of PIN Ge-PDs, which were evanescently coupled and butt-joint coupled with the Si WGs. The optimized evanescently coupling type of a PIN Ge-PD showed good photoresponsivity of 0.8 to 1.0 A/W and low dark current density of 0.6 nA/um². It showed 50 GHz bandwidth at V_dc more than 3 V, and 30 GHz at 0 V_dc in case of 10 um optical coupling length. Furthermore, we developed a butt-joint type PIN Ge-PD and achieved 46 GHz bandwidth and high-efficiency with only 5 um optical coupling length.

We also studied PIN and Schottky type Ge PDs. Both PIN and Schottky type Ge PDs showed good photoresponsivity of about 1.0A/W, and a low enough dark current density of 0.4 nA/um² for the PIN Ge PD and 0.8 nA/mm² for the Schottky Ge PD. The frequency of the 3 dB bandwidth was 40 GHz for the PIN Ge PD and 30 GHz for the Schottky Ge PD in case of 1 um-thick Ge layer. At lower dc bias voltage (V_dc), a much larger bandwidth was obtained for the PIN Ge PD. From the equivalent circuit model fitting, the Schottky Ge PD showed much larger capacitance around the zero bias voltage.

The PIN Ge PD showed a larger bandwidth with decreasing Ge thickness. About 50 GHz bandwidth was obtained with a voltage more than 3 V_dc, and at even zero bias, 30 GHz bandwidth was obtained.

Finally, we studied a PIN Ge PD with a small footprint and low consumption CMOS TIA circuit. To achieve a smaller footprint and larger impedance gain, we used an inverter-type TIA circuit. We demonstrated large sensitivity and high-speed response of 12.5 Gbps.