We employed bias-voltage-application in hard x-ray photoelectron spectroscopy (BA-HXPES) to detect electronic states of materials in operating devices. To demonstrate the versatility of this method, we used a metal/SiO 2/Si(100) structure as an ideal platform and found that electronic states at the SiO2/Si(100) interface were changed depending on the bias-application to the structure. By analyzing the change as a function of bias-voltage, the interface electronic states in the whole Si gap have been directly obtained in which these states cannot be detected without the bias-application. BA-HXPES is a new method to characterize electronic states for advanced materials under device operation.
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