Bonded semiconductor structures having bonding contacts made of indiffusible conductive materials and methods for forming the same

Embodiments of bonded semiconductor structures and fabrication methods thereof are disclosed. In an example, a semiconductor device includes a first semiconductor structure, a second semiconductor structure, and a bonding interface between the first semiconductor structure and the second semiconductor structure. The first semiconductor structure includes a substrate, a first device layer disposed on the substrate, and a first bonding layer disposed above the first device layer and including a first bonding contact. The second semiconductor structure includes a second device layer and a second bonding layer disposed below the second device layer and including a second bonding contact. The first bonding contact is in contact with the second bonding contact at the bonding interface. At least one of the first bonding contact or the second bonding contact is made of an indiffusible conductive material.