Although gate stack structures with high-k materials have been extensively investigated, there are some issues to be solved for the formation of high quality gate stack structures. In the present study, we employed hard x-ray photoelectron spectroscopy in operating devices. This method allows us to investigate bias dependent electronic states while keeping device structures intact. Using this method we have investigated electronic states and potential distribution in a Pt gate metal/high-k gate stack structure under device operation. We have found that a potential gradient was formed at the Pt/HfO 2 interface by analyzing the shifts of the core levels as a function of the applied bias voltage. Angle resolved photoelectron spectroscopy revealed that a SiO 2 layer was formed at the Pt/HfO 2 interface. The formation of the SiO 2 layer at the interface might concern the Fermi level pinning, which is observed in metal/high-k gate stack structures.