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@ARTICLE{Schmidt:910064,
      author       = {Schmidt, Niclas and Rushchanskii, Konstantin Z. and
                      Trstenjak, Urška and Dittmann, Regina and Karthäuser,
                      Silvia},
      title        = {{I}n-{G}ap {S}tates of {H}f{O} 2 {N}anoislands {D}riven by
                      {C}rystal {N}ucleation: {I}mplications for {R}esistive
                      {R}andom-{A}ccess {M}emory {D}evices},
      journal      = {ACS applied nano materials},
      volume       = {6},
      number       = {1},
      issn         = {2574-0970},
      address      = {Washington, DC},
      publisher    = {ACS Publications},
      reportid     = {FZJ-2022-03592},
      pages        = {148-159},
      year         = {2023},
      abstract     = {Envisioned extremely scaled, high-performance memory
                      devices request to conduct the step from thin semiconductor
                      films to nanoscale structures and the use of promising
                      high-k materials such as hafnium oxide (HfO2). HfO2 is well
                      suited for use in resistive random-access memory (ReRAM)
                      devices based on the valence change mechanism. Here, we
                      provide a decidedly scaled system, namely, HfO2 nanoislands
                      that are grown by van der Waals epitaxy on highly oriented
                      pyrolytic graphite (HOPG). The electronic and structural
                      properties of these well-separated, crystalline HfO2
                      nanoislands are investigated by scanning probe methods as
                      well as ab initio methods. The topography reveals
                      homogeneously formed HfO2 nanoislands with areas down to 7
                      nm2 and a thickness of one unit cell. They exhibit several
                      acceptor- and donor-like in-gap states in addition to the
                      bulk band gap, implying bulk properties. X-ray photoelectron
                      spectroscopy indicates hafnium carbide formation as one
                      possible origin for defect states. Going further to the
                      crystal nucleation of HfO2, nanocrystals with a diameter of
                      2.7–4.5 Å are identified next to carbon vacancies in the
                      topmost HOPG layer, indicating that carbon is incorporated
                      into the islands at early nucleation stages. A precise
                      description of these nuclei is accomplished by the
                      simulation of small HfmOn(:C) clusters (m = 3 to 10; n = 3
                      to 22) with and without carbon incorporation using ab initio
                      methods. The comparison of the theoretically determined
                      lowest-energy clusters and electronic states with the
                      experimental results allows us to identify the structure of
                      the most relevant HfO2 sub-nanometer crystals formed during
                      the first nucleation steps and the nature of the in-gap
                      states found at the surfaces of HfO2 nanoislands. That way,
                      a model system is derived that consists of distinct
                      structural units, related to surface states or defect
                      states, respectively, that will promote the tailoring of
                      in-gap states of smallest HfO2 structures and thus the
                      scalability of memory devices.},
      cin          = {PGI-7 / IAS-1 / PGI-1 / JARA-FIT},
      ddc          = {540},
      cid          = {I:(DE-Juel1)PGI-7-20110106 / I:(DE-Juel1)IAS-1-20090406 /
                      I:(DE-Juel1)PGI-1-20110106 / $I:(DE-82)080009_20140620$},
      pnm          = {5233 - Memristive Materials and Devices (POF4-523) / DFG
                      project 167917811 - SFB 917: Resistiv schaltende
                      Chalkogenide für zukünftige Elektronikanwendungen:
                      Struktur, Kinetik und Bauelementskalierung "Nanoswitches"
                      (167917811)},
      pid          = {G:(DE-HGF)POF4-5233 / G:(GEPRIS)167917811},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000877146000001},
      doi          = {10.1021/acsanm.2c04165},
      url          = {https://juser.fz-juelich.de/record/910064},
}