Low-temperature oxide-free silicon and germanium wafer bonding based on a sputtered amorphous Ge

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.