Fabrication of bonded Germanium on insulator (GeOI) substrates with thin Al2O3/SiO2 buried oxide layers was demonstrated for the first time. Thin Al2O3 layers grown by an atomic-layer deposition method, which have hydroxyl groups on their surfaces, were inserted to Ge/SiO2 interfaces to strengthen the adhesion between them. The interface bonded in the atmosphere at room temperature without post-anneal processes is flat and exhibits no additional interface. After the wafer bonding, thick GeOI layers were thinned by mechanical polishing, chemical mechanical polishing and O-3 water etching. The flatness of the surface is equivalent to that of commercially available Ge substrates. The interface state density (D-it) of the Ge/Al2O3 interface is estimated with low-temperature conductance technique and the D-it of as low as 3.9 x 10(11) eV(-1) cm(-2) at the interface is obtained. This value is almost one order of magnitude lower than previously reported D-it of Ge/SiO2 interfaces. These results suggest that negligible negative impact on carrier mobility and sub-threshold characteristics is expected for the back-side interface in the GeOI substrate with thin Al2O3/SiO2 BOX layers. (C) 2013 Elsevier Ltd. All rights reserved.