Germanium-on-SOI Infrared Detectors for Integrated Photonic Applications

An overview of recent results on high-speed Ge-on-silicon-on-insulator (Ge-on-SOI) photodetectors and their prospects for integrated optical interconnect applications are presented. The optical properties of Ge and SiGe alloys are described and a review of previous research on Si-on-insulator (SOI) and SiGe detectors is provided as a motivation for the Ge-on-SOI detector approach. The photodetector design is described, which consists of lateral alternating p- and n-type surface contacts on an epitaxial Ge absorbing layer grown on an ultra-thin-SOI substrate. When operated at a bias voltage of -0.5 V, 10 x 10 μm² devices have dark current, I_dark, of only ~ 10 nA, a value that is nearly independent of finger spacing, S, down to S = 0.3 μm. Detectors with S = 1.3 μm have external quantum efficiencies, η, of 52% (38%) at λ = 895 nm (850 nm) with corresponding responsivities of 0.38 A/W (0.26 A/W). The wavelength-dependence of η agrees fairly well with expectations, except at longer wavelengths, where Si up-diffusion into the Ge absorbing layer reduces the efficiency. Detectors with 10 x 10 μm² area and S = 0.6 μm have -3 dB bandwidths as high as 29 GHz, and can simultaneously achieve a bandwidth of 27 GHz, with I_dark = 24 nA, at a bias of only -1 V, while maintaining high efficiency of η = 46% (33%), at λ = 895 nm (850 nm). Analysis of the finger-spacing and area dependence of the device speed indicates that the performance at large finger spacing is transit-time limited, while at small finger spacing, RC delays limit the bandwidth. Methods to improve the device performance are presented, and it is shown that significant improvement in the speed and efficiency both at λ = 850 nm and 1300 nm can be expected by optimizing the layer structure design.