Optical characterization of PureGaB germanium-on-silicon (Ge-on-Si) photodiodes was performed for wavelengths between 255 nm and 1550 nm. In PureGaB technology, chemical vapor deposition is used to grow germanium islands in oxide windows to the silicon substrate and then cap them in-situ with nm-thin layers of first gallium and then boron, thus forming nm-shallow p+n diodes. These PureGaB Ge-on-Si photodiodes are CMOS compatible and characterized by low leakage currents. Here they are shown to have high responsivity in the whole ultraviolet (UV) to near infrared (NIR) wavelength range. Particularly, two sets of diodes were studied with respect to possible detrimental effects of the Al metallization/alloying process steps on the responsivity. Al-mediated transport of the Ge and underlying Si was observed if the PureGaB layer, which forms a barrier to metal layers, did not cover all surfaces of the Ge islands. A simulation study was performed confirming that the presence of acceptor traps at the Ge/Si interface could decrease the otherwise high theoretically attainable responsivity of PureGaB Ge-on-Si photodiodes in the whole UV to NIR range. A modification of the device structure is proposed where the PureGaB layer covers not only the top surface of the Ge-islands, but also the sidewalls. It was found that to mitigate premature breakdown, it would be necessary to add p-doped guard rings in Si around the perimeter of Ge islands, but this PureGaB-all-around structure would not compromise the optical performance.
The light emission from silicon PureB photodiodes was investigated in both forward- and avalanchemode operation and correlated to the presence of process-dependent defects that influence the reverse IV characteristics. As opposed to “defect-free” diodes with low dark currents and abrupt breakdown behavior, the diodes with defects had higher current levels and light-emitting spots appearing at voltages far below the breakdown voltage otherwise set by the implemented doping profiles. The role of such defect-related behavior for the application of the photodiodes as single-photon avalanche diodes (SPADs) and avalanche-mode light-emitting diodes (AMLEDs) is assessed in connection with the recent demonstration of these basic devices as both the light-emitting and light-detecting elements in optocoupler circuits integrated in CMOS for data transmission purposes.