%0 Journal Article
%A Schmidt, Niclas
%A Rushchanskii, Konstantin Z.
%A Trstenjak, Urška
%A Dittmann, Regina
%A Karthäuser, Silvia
%T In-Gap States of HfO 2 Nanoislands Driven by Crystal Nucleation: Implications for Resistive Random-Access Memory Devices
%J ACS applied nano materials
%V 6
%N 1
%@ 2574-0970
%C Washington, DC
%I ACS Publications
%M FZJ-2022-03592
%P 148-159
%D 2023
%X 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.
%F PUB:(DE-HGF)16
%9 Journal Article
%U <Go to ISI:>//WOS:000877146000001
%R 10.1021/acsanm.2c04165
%U https://juser.fz-juelich.de/record/910064