High-speed germanium photodiodes monolithically integrated on silicon with MBE

Abstract

Germanium on silicon pin-photodiodes for vertical light incidence is presented. The monolithically integrated devices are grown on Si with solid source molecular beam epitaxy. The lattice mismatch between Si and Ge is adjusted with a special virtual substrate concept. This thin virtual substrate allows the integration of Ge photodiodes with Si-based circuits. For a Ge pin-photodetector with a diameter of 10 μm, a 3-dB bandwidth of 25.1 GHz is measured at an incident wavelength of 1552 nm and zero external bias. For a modest reverse bias of 2 V, the 3-dB bandwidth increases to 38.9 GHz. The whole device structure consisting of a highly p-doped Ge buried layer, an intrinsic absorption region, a highly n-doped top contact layer from Ge/Si is grown in one continuous epitaxial run. The intrinsic region of the Ge detector has a thickness of only 300 nm in order to facilitate device integration by a thin layer stack. A low growth temperature sequence was selected to obtain abrupt doping transitions between the highly doped regions surrounding the intrinsic layer. The abruptness of Ge and doping profiles was confirmed with SIMS.