We have investigated the impacts of the gate metal on the chemical bonding state in the metal/Pr-oxide/Ge gate stack structure by the hard X-ray photoelectron spectroscopy method and have tried to explain about the obtained results focusing on the difference of oxygen chemical potential (mu(o)) among metal-oxide, Pr-oxide and Ge-oxide. The reductive character of the metals induces the reductive reaction of the Pr-oxide films. The reductive character changes the Pr valence state not only at the surface but also in the Pr-oxide films. The reductive character increases the ratio of Pr3+ to Pr4+ in the Pr-oxide films, suggesting the formation of the crystalline phase with high permittivity of h-Pr2O3. The reductive character of the metals also affects on the Pr-oxide/Ge interfacial structure. Besides it leads to the decrease in the amount of Ge atoms bonding with oxygen. These reductive reactions can be explained by mu(o) of the metals with reductive character lower than those of PrO2 formation and GeO2 formation. From the comparison of the results for the Ge and Si systems, it was found that the drawing reaction of oxygen, which means the pullout of oxygen in the Pr-oxide film, is facile for the Ge system, relating to mu(o) of GeO2 higher than that in SiO2. These results suggest that the selection of gate metals in the metal/Pr-oxide/Ge gate stack structure taking into account mu(o) is quite important to achieve the thin EOT, resulting in both the formation of the h-Pr2O3 crystalline phase of the Pr-oxide and the decrease in the amount of Ge-oxides. (c) 2013 Elsevier Ltd. All rights reserved.