Direct observation of electronic structure change by resistance random access memory effect in amorphous alumina

We measured X-ray absorption spectra of amorphous alumina with vacancy-type oxygen defects (AlOx) which exhibits the resistance random access memory (ReRAM) effect. We were able to detect changes in the electronic structure owing to the ReRAM effect. A major difference in the spectra near the O K-absorption edge was observed between a low resistance state (LRS) and a high resistance state (HRS). The subpeak profile within the band gap appeared in the LRS, while it was suppressed in the HRS. By contrast, the spectra near the Al K-absorption edge in the LRS and HRS appeared almost identical, indicating that no byproducts are generated. These findings imply that the distribution of charged electrons primarily changes near oxygen sites from the HRS to the LRS. The features of the subpeak profile are analogous to those of the mid-gap profile, as speculated by the first-principles calculation [Momida et al., Appl. Phys. Lett. 98, 042102 (2011)]. The LRS was mainly detected near the surface of the thin film.