The　room-temperature　(RT)　bonding　mechanisms　of　GaAs/SiO2/Si　and　GaAs/Si　heterointerfaces　fabricated　by　surface-activated　bonding　(SAB)　are　investigated　using　a　focused　ion　beam　(FIB)　system,　cross-sectional　scanning　transmission　electron　microscopy　(TEM),　energy　dispersive　x-ray　spectroscopy　(EDX)　and　scanning　acoustic　microscopy　(SAM).　According　to　the　element　distribution　detected　by　TEM　and　EDX,　it　is　found　that　an　intermixing　process　occurs　among　different　atoms　at　the　heterointerface　during　the　RT　bonding　process　following　the　surface-activation　treatment.　The　diffusion　of　atoms　at　the　interface　is　enhanced　by　the　point　defects　introduced　by　the　process　of　surface　activation.　We　can　confirm　that　through　the　point　defects,　a　strong　heterointerface　can　be　created　at　RT.　The　measured　bonding　energies　of　GaAs/SiO2/Si　and　GaAs/Si　wafers　are　0.7　J/m(2)　and　0.6　J/m(2).　The　surface-activation　process　can　not　only　remove　surface　oxides　and　generate　dangling　bonds,　but　also　enhance　the　atomic　diffusivity　at　the　interface.