High bandwidth Ge p - i - n photodetector integrated on Si

The authors present a germanium on silicon p - i - n photodiode for vertical light incidence. For a Ge p - i - n photodetector with a radius of 5 μ m a 3 dB bandwidth of 25 GHz is measured at an incident wavelength of 1.55 μ m and zero external bias. For a modest reverse bias of 2 V , the 3 dB bandwidth increases to 39 GHz . The monolithically integrated devices are grown on Si with solid source molecular beam epitaxy. The complete detector structure consisting of a highly p -doped Ge buried layer, an intrinsic absorption region, and a highly n -doped top contact layer of Ge ∕ Si is grown in one continuous epitaxial run. A low growth temperature sequence was needed to obtain abrupt doping transitions between the highly doped regions surrounding the intrinsic layer. A theoretical consideration of the 3 dB bandwidth of the Ge detector was used to optimize the layer structure. For a photodiode with 5 μ m mesa radius the maximum theoretical 3 dB frequency is 62 GHz with an intrinsic region thickness of 307 nm .