Design of wafer-bonded structures for near room temperature Geiger-mode operation of germanium on silicon single-photon avalanche photodiode

We investigate the effect of temperature on the single-photon properties of four germanium/silicon (Ge/Si) single-photon avalanche photodiodes (SPADs), which are fabricated by Ge-on-Si direct epitaxial growth, Ge-on-Si two-step epitaxial growth, Ge/Si direct wafer bonding, and Si/Si hydrophobic bonding, respectively. It is found that the wafer-bonded Ge/Si SPAD exhibits extremely low dark current and dark count rate (DCR) compared with the epitaxial ones at 250 and 300 K. This implies that the wafer-bonding technique is a possible candidate for the fabrication of Ge/Si SPAD, which can be operated at near room temperature. Additionally, due to the low DCR and high operation temperature, the wafer-bonded Ge/Si SPAD shows extremely high pulse repetition rate (∼28  MHz in theory for DCR=108  Hz). That is, the wafer-bonded Ge/Si SPAD can be used in a high-speed field. Finally, the effect of voltage pulse width, number of photons per pulse, and hold-off time on the performance of the wafer-bonded Ge/Si SPAD at different temperatures is also clarified.