“…Direct wafer bonding, in which two wafers are bonded to each other without adhesive or solder, has been widely applied for fabricating a variety of heterogeneous structures, such as Si/Si, 1 Si/SiO 2 , 2,3 GaAs/Si, 4 InP/Si, 5 GaAs/GaAs, 6 and GaAs/InP 7,8 because it enables us to form junctions of semiconductor materials with different crystal structures or lattice constants. In almost all direct bonding methods-hydrophilic, hydrophobic, plasma-assisted bonding, and others [9][10][11] -annealing after the bonding has been reported to be essential in achieving sufficient bonding strengths. [6][7][8]12 However, the fact that the need for annealing may limit the area of application of the direct wafer bonding, because the resultant thermal stress could cause the diffusion of doped impurities across the bonding interfaces.…”