The tunneling effect and interface state in the p-Ge/GeO2p-Si structure of a wafer-bonding Ge/Si avalanche photodiode (APD) are investigated. It is found that the thin interfacial GeO2 layer (1-2 nm) formed by the hydrophilic reaction at the wafer-bonding interface significantly affects the performance of the Ge/Si APD. With the increase of the GeO2 thickness, the dark current of the Ge/Si APD decreases enormously due to the blocking effect of this GeO2 layer. Owing to the carrier accumulation in Ge layer under illumination condition, the voltage sharing effect of the GeO2 layer (thicker) becomes serious, leading to the absence of the electric field in Ge layer. The photon-generated electrons at Ge/GeO2 interface can be captured and released by the interface states at certain reverse bias. This can adjust the avalanche current of the Ge/Si APD. The stronger interface recombination induced by the larger interface state density (ISD) results in the decrease of the electric field in Ge layer. This increases the transit time of carriers, which in turn decreases the 3dB-bandwidth. Due to the drastic increase of the dark current (larger ISD), the gain of the Ge/Si APD decreases.

We report a potential low-cost method for low-temperature silicon (Si) and germanium (Ge) wafer bonding based on an intermediate amorphous Ge (a-Ge). The sputtered a-Ge is demonstrated to be extremely flat (RMS = ∼0.28 nm) and hydrophilic (contact angle = ∼3°). The a-Ge turns to be the polycrystalline phase at the Si/Ge/Si bonded interface, whereas it fully turns to be single-crystal phase at the Ge/Ge/Si bonded interface after annealing. The simulated stress distribution reveals that the maximum thermal stress in a-Ge dominates the crystallization process and the crystalline phase of the intermediate Ge layer depends on the induction of seed crystals. More importantly, the threading dislocation and oxide layer are not observed at the bonded interface. This finding may be applied to fabricate high-performance Si-based Ge photoelectric devices.

We report a promising method for oxide-layer-free germanium (Ge)/silicon (Si) wafer bonding based on an amorphous Ge (a-Ge) intermediate layer between Si and Ge wafers. The effect of the exposure time (te), during which the a-Ge is exposed to the air after sputtering and being taken out of the chamber on the bubble density at the bonded interface, is identified and a near-bubble-free Ge/Si bonded interface is achieved for the te of 3 s. The crystallization of a-Ge at Ge/Si bonded interface starts from a-Ge/Ge interface and it fully turns to be single-crystal Ge after post-annealing. The oxide layer at a-Ge/a-Ge bonded interface formed by the interface hydrophilic reaction disappears due to the atom redistribution triggered by the crystallization of a-Ge. As expected, the performance of the Ge/Si heterojunction diode is significantly improved by this oxide-layer-free Ge/Si bonded interface. A low dark current of 1.6 µA, high on/off current ratio of 3.4  ×  105, and low ideality factor of 1.02 (150 K) is achieved at  −0.5 V for the bonded Ge/Si diode. Finally, the carrier transport mechanisms at Ge/Si bonded interface annealed at different temperatures are also clearly clarified.

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.

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