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Interface characteristics of different bonded structures fabricated by low-temperature a-Ge wafer bonding and the application of wafer-bonded Ge/Si photoelectric device

Ke, Shaoying, Ye, Yujie, Wu, Jinyong, Ruan, Yujiao, Zhang, Xiaoying, Huang, Wei, Wang, Jianyuan, Xu, Jianfang, Li, Cheng, Chen, Songyan
Journal of materials science 2019 v.54 no.3 pp. 2406-2416
byproducts, crystallization, germanium, hydrogen, hydrophilicity, silica, silicon, thermal stress, wafers
We report the interface characteristics of Si/Si, Si/SiO₂, SiO₂/SiO₂, Ge/Si, Ge/Ge, and Ge/SiO₂ bonded wafers based on an amorphous germanium (a-Ge) intermediate layer. The crystallization of a-Ge and the atom migration mechanism at different bonded structures are very different. The a-Ge turns into polycrystalline Ge (poly-Ge) at Si/Si bonded interface, while it exhibits amorphous phase at Si/SiO₂ and SiO₂/SiO₂ interfaces after post-annealing. This is due to the change of the stress field when SiO₂ is introduced. Thanks to the crystallization of a-Ge, serious atom migration appears at Si/Si bonded interface, leading to the decomposition of the interface oxide layer formed by the hydrophilic reaction. Interestingly, the a-Ge at Ge/Si, Ge/Ge, and Ge/SiO₂ interface becomes single-crystal Ge after post-annealing. The a-Ge crystallization starts from a-Ge/Ge interface. Similarly, the interface Ge oxide layer also decomposes after the crystallization of a-Ge. This results from the atom redistribution triggered by Ge-induced crystallization under high thermal stress. More importantly, the threading dislocations are not observed at Ge/Si and Ge/SiO₂ interface. The Si/Si, Si/SiO₂, SiO₂/SiO₂, and Ge/SiO₂ bonded interface is demonstrated to be bubble-free. The transferring of the interface by-products (H₂O and H₂) by SiO₂ and poly-Ge can be responsible for this phenomenon. Finally, a wafer-bonded Ge/Si heterojunction photodiode is fabricated to verify the application of a-Ge wafer bonding technique in photoelectric devices.