We have elucidated the electronic states for Ru/SiON/Si(100) and Pt/HfO
2/Pt structures under device operation by means of bias-application in hard x-ray photoelectron spectroscopy (BA-HXPES). For the Ru/SiON/Si(100) structure, interface electronic states are increased and new states near the conduction band minimum and valence band maximum are formed compared to the case of the Ru/SiO
2/Si(100) structure. This is because nitrogen atoms at the SiO
2/Si interface cause bond breaking and weakened the Si-O and Si-Si bonds at the interface. For the Pt/HfO
2/Pt structure, oxygen atoms migrate to the top electrode under forward bias, forming Pt-O bond at the Pt/HfO
2 interface. Under reverse bias, on the other hand, oxygen and Hf atoms are migrated to the top electrode, resulting in the formation of Pt-O and Pt-Hf bonds at the interface. The migration and bond formation at the interface should form vacancy in the HfO
2 film, which might induce resistance switching behavior in this system. Here, it is emphasized that we present our approach using BA-HXPES, which successfully detects the electronic states under device operation. Therefore BA-HXPES could be applicable to various kinds of advanced materials and will be indispensable for evaluating their physical properties in detail.
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