An 850-nm Normal-Incidence Germanium Metal–Semiconductor–Metal Photodetector With 13-GHz Bandwidth and 8- $\mu$ A Dark Current

This letter presents a new germanium metal-semiconductor-metal photodetector (PD), in which an amorphous silicon (a-Si) layer is added on top of the undoped germanium substrate. The a-Si under the Ti-Au contacts serves as a barrier enhancement layer and mitigates Schottky barrier height lowering due to fermi-level pinning, hence reducing dark current. In the active region, the a-Si layer passivates surface states, thus preventing low-frequency gain due to charge accumulation and image force lowering, and improving the device bandwidth. A prototype PD with an area of 47 × 47 μm² with 2-μm contact spacing and 1.25-μm contact width achieved a small-signal 3-dB bandwidth over 10 GHz. In an ultrafast electrooptic sampling measurement, its impulse response exhibited 15-ps pulsewidth and 30-ps rise time, which corresponds to 13-GHz bandwidth. The measured PD capacitance was 40 fF. The responsivity and dark current were 0.23 A/W and 8 μA, respectively, at 850-nm wavelength and 7-V bias.