Wafer-scale High-bandwidth Germanium on Silicon Photodetectors for Communications Applications

Thesis (Master's)--University of Washington, 2012The Silicon-on-Insulator (SOI) material system has emerged as a potentially attractive platform for integrated optics, due to the intrinsic low-cost of silicon manufacturing. An especially attractive application is that of telecommunications. One key component for telecommunications applications is high-performance photodetectors, which convert an optical signal to an electrical signal. The key performance metrics for waveguide-coupled photodetectors include low dark current, high responsivity and high bandwidth. In addition, the cross-wafer performance and defectivity is also of great importance, on which the yield of eventual systems will rely. Here I report the first cross-wafer data for waveguide-coupled Ge-on-Si photodetectors based on vertical p-i-n configuration. The performance across the whole wafer is relatively uniform and exhibits low defectivity. Detectors working at speeds up to 20 GHz with 4V reverse bias are achieved, with high responsivities of 0.5 A/W. I describe the testing and characterization methods used to measure the performance of these devices, and identify the source of the bandwidth limitations, and show how these photodetectors are optimal for use at telecom wavelengths, consisting of light with a free-space wavelength near 1550 nm. Finally, a path forward for optimizing the devices in this process is presented. Based on our measurements, using only modest changes in device geometry, it should be possible to improve bandwidths to 70 GHz or more, and responsivities to 1 A/W or more. I propose specific device geometries to implement this improvement. This level of performance is sufficient for SOI based Ge photodetectors to implement even the most high-bandwidth optical telecommunications systems, and should help to make SOI an important platform for integrated optics