The impact of the post thermal oxidation of the Al2O3/ Ge structures on metal-oxide-semiconductor interface properties has been systematically investigated. Also, the reaction mechanisms at the Al2O3/ Ge interfaces with thermal oxidation through the thin Al(2)O3 layer have been investigated. Interface state density around the midgap of Ge is reduced with the formation of AlGeO layer near the Al2O3/Ge interface. Also, through the detail analysis of chemical bonding state of Ge oxide, it is found that the AlGeO layer which has a stable phase, in other words Al6Ge2O13, is formed near the Al2O3/Ge interface, and a GeO2 layer is formed on the Al2O3 surface, suggesting Ge or GeO diffusion from the Ge surface. Furthermore, it is clarified that the mechanism of formation of AlGeO layer is different from that of GeO2 on the Al2O3 surface which is limited by oxygen diffusion in Al2O3 layer. Therefore, Al6Ge2O13 formation near the Al2O3/Ge interface without the GeO2 formation on the Al2O3 surface is a key to achieve a low interface state density and a thin equivalent SiO2 thickness. (c) 2013 Published by Elsevier B.V.