Room temperature GaN-Si direct wafer bonding was done by surface activated bonding (SAB). At first, a feasibility study using GaN template has been done. Then, crystal-face dependence of the bonding results for freestanding GaN substrate has been investigated between Ga-face and N-face. The results of Ga-face to Si bonding are better than that of N-face to Si bonding such as higher bonding energy and larger bonded area. This difference should be caused by their different surface roughnesses after chemical-mechanical polishing (CMP). Besides, both of the structure and composition of the two kinds of interfaces were investigated to understand the bonding mechanisms. The phenomenon of Ga-enrichment during surface activation and Ga-diffusion into Si at room temperature for both Ga-face bonding and N-face bonding has been confirmed. (C) 2017 Elsevier B.V. All rights reserved.