Germanium waveguide photodetectors with 4μm widths and various lengths are fabricated on silicon-on-insulator substrates by selective epitaxial growth.The dependence of the germanium layer length on the responsivit.y and bandwidth of the photodetectors is studied.The optimal length of the germanium layer to achieve high bandwidth is found to be approximately 8μm.For the 4×8μm^2 photodetector,the dark current density is as low as 5 mA/cm^2 at^-1 V.At a bias of-1 V,the 1550 nm optical responsivity is as high as 0.82 A/W.Bandwidth as high as 29 GHz is obtained at-4 V.Clear opened eye diagrams at 50 Gbits/s are demonstrated at 1550 nm.
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