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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=10 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.

Simulations of current-voltage characteristics of ion-implanted n-on-p photodiodes have been performed using SemiCad Device. In order to accurately simulate this device structure, several modifications to the simulator were implemented. These include the modified carrier statistics to account for the nonparabolic band structure of HgCdTe, the correct physics parameters for Shockley-Read-Hall, optical, and Auger recombination, and the Burstein-Moss shift for optical absorption important for heavily doped n-type HgCdTe. With these and other improvements, SemiCad Device is calibrated with the measured ideal dark current of an ion implanted diode and is used to simulate a source of non-ideal dark current from surface-charge induced band-to-band tunneling.

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