O-band membrane photodetector with InGaAsP-bulk absorption core using Franz-Keldysh effect

There has been an increasing need for small, low-cost, and low-power consumption optical transceivers for short-reach fiber links. Waveguide-integrated photodetectors (PDs) with wide bandwidth and high responsivity on Si photonics platforms are an essential element for these applications. We have fabricated an O-band membrane PD which is suitable for integration with high-performance III-V-based membrane devices such as lasers and modulators, and passive waveguide circuits on the Si photonics platforms. The membrane PD consists of an InGaAsP-bulk absorption core embedded with an InP-based lateral p-i-n junction. The width of the InGaAsP absorption core is designed to be 350 nm and the doped regions overlap with the edges of the core to reduce the carrier transit time. In the structure, however, photocarriers generated in the doped core appear to be a limiting factor of the operating speed since the photocarriers diffuse into the depletion region with a long time constant. To avoid this, we design the bandgap wavelength of the absorption core to be shorter than the operating wavelength. As the electric field is mainly applied to the undoped InGaAsP absorption core region when the reverse bias is applied to the PD, the absorption coefficient in the undoped core region increases due to Franz-Keldysh effect (FKE) while the doped regions maintain a low absorption coefficient due to the wavelength detuning. A membrane PD with an absorption length of 30 µm integrated with the SiO_x waveguide was fabricated by using a heterogeneous integration technique on a Si photonics platform. We verified that the FKE causes the photocurrent to increase. In addition, the OE bandwidth was observed to increase due to the wavelength detuning. The fabricated PD exhibited a fiber-to-PD responsivity of 0.6 A/W and a bandwidth over 67 GHz. Eye openings for 100-Gbit/s NRZ signals were demonstrated at a stage temperature of 25°C.