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@ARTICLE{Kindsmller:851662,
      author       = {Kindsmüller, A. and Schmitz, C. and Wiemann, C. and Skaja,
                      K. and Wouters, D. J. and Waser, R. and Schneider, C. M. and
                      Dittmann, R.},
      title        = {{V}alence change detection in memristive oxide based
                      heterostructure cells by hard {X}-ray photoelectron emission
                      spectroscopy},
      journal      = {APL materials},
      volume       = {6},
      number       = {4},
      issn         = {2166-532X},
      address      = {Melville, NY},
      publisher    = {AIP Publ.},
      reportid     = {FZJ-2018-05212},
      pages        = {046106 -},
      year         = {2018},
      abstract     = {The switching mechanism of valence change resistive memory
                      devices is widely accepted to be an ionic movement of oxygen
                      vacancies resulting in a valence change of the metal
                      cations. However, direct experimental proofs of valence
                      changes in memristive devices are scarce. In this work, we
                      have employed hard X-ray photoelectron emission microscopy
                      (PEEM) to probe local valence changes in Pt/ZrOx/Ta
                      memristive devices. The use of hard X-ray radiation
                      increases the information depth, thus providing chemical
                      information from buried layers. By extracting X-ray
                      photoelectron spectra from different locations in the PEEM
                      images, we show that zirconia in the active device area is
                      reduced compared to a neighbouring region, confirming the
                      valence change in the ZrOx film during electroforming.
                      Furthermore, we succeeded in measuring the Ta 4f spectrum
                      for two different resistance states on the same device. In
                      both states, as well as outside the device region, the Ta
                      electrode is composed of different suboxides without any
                      metallic contribution, hinting to the formation of TaOx
                      during the deposition of the Ta thin film. We observed a
                      reduction of the Ta oxidation state in the low resistance
                      state with respect to the high resistive state. This
                      observation is contradictory to the established model, as
                      the internal redistribution of oxygen between ZrOx and the
                      Ta electrode during switching would lead to an oxidation of
                      the Ta layer in the low resistance state. Instead, we have
                      to conclude that the Ta electrode takes an active part in
                      the switching process in our devices and that oxygen is
                      released and reincorporated in the ZrOx/TaOx bilayer during
                      switching. This is confirmed by the degradation of the high
                      resistance state during endurance measurements under
                      vacuum.},
      cin          = {PGI-7 / JARA-FIT / PGI-6},
      ddc          = {620},
      cid          = {I:(DE-Juel1)PGI-7-20110106 / $I:(DE-82)080009_20140620$ /
                      I:(DE-Juel1)PGI-6-20110106},
      pnm          = {521 - Controlling Electron Charge-Based Phenomena
                      (POF3-521)},
      pid          = {G:(DE-HGF)POF3-521},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000431141500006},
      doi          = {10.1063/1.5026063},
      url          = {https://juser.fz-juelich.de/record/851662},
}