The dominant key to achieve superior Ge surface passivation by GeOx interfacial layer is investigated based on ozone oxidation. The interface state density (D-it) measured from low temperature conduction method is found to decrease with increasing the GeOx, thickness (0.26-1.06 nm). The X-ray photoelectron spectroscopy (XPS) is employed to demonstrate the interfacial structure of GeOx/Ge with different GeOx thicknesses. And the XPS results show that GeOx oxide component is responsible to the decrease of the Dit due to the effective passivation of Ge dangling bonds. Therefore, the formation of GeOx component is the dominant key to achieve low Da for Ge gate stacks. Our work confirms that the same physical mechanism determines the Ge surface passivation by the GeOx regardless of the oxidation methods to grow the GeOx interfacial layer. As a result, to explore a growth process that can realize sufficient GeOx component in the GeOx interlayer as thin as possible is important to achieve both equivalent oxide thickness scaling and superior interfacial property simultaneously. This conclusion is helpful to engineer the optimization of the Ge gate stacks. (C) 2015 Elsevier B.V. All rights reserved.