Persistent metallic Sn-doped In2O3 epitaxial ultrathin films with enhanced infrared transmittance

Infrared transparent electrodes (IR-TEs) have recently attracted much attention for industrial and military applications. The simplest method to obtain high IR transmittance is to reduce the electrode film thickness. However, for films several tens of nanometres thick, this approach unintentionally suppresses conduction due to surface electron scattering. Here, we demonstrate low sheet resistance (<400 Ω □−1 at room temperature) and high IR transmittance (>65% at the 2.5-μm wavelength) in Sn-doped In2O3 (ITO) epitaxial films for the thickness range of 17−80 nm. A combination of X-ray spectroscopy and ellipsometry measurements reveals a persistent electronic bandstructure in the 8-nm-thick film compared to much thicker films. This indicates that the metallicity of the film is preserved, despite the ultrathin film configuration. The high carrier mobility in the ITO epitaxial films further confirms the film’s metallicity as a result of the improved crystallinity of the film and the resulting reduction in the scattering defect concentration. Thus, ITO shows great potential for IR-TE applications of transparent photovoltaic and optoelectronic devices. © 2020, The Author(s).1

Tunable resistivity of correlated VO2(A) and VO2(B) via tungsten doping

Applications of correlated vanadium dioxides VO2(A) and VO2(B) in electrical devices are limited due to the lack of effective methods for tuning their fundamental properties. We find that the resistivity of VO2(A) and VO2(B) is widely tunable by doping them with tungsten ions. When x < 0.1 in V1−xWxO2(A), the resistivity decreases drastically by four orders of magnitude with increasing x, while that of V1−xWxO2(B) shows the opposite behaviour. Using spectroscopic ellipsometry and X-ray photoemission spectroscopy, we propose that correlation effects are modulated by either chemical-strain-induced redistribution of V−V distances or electron-doping-induced band filling in V1−xWxO2(A), while electron scattering induced by disorder plays a more dominant role in V1−xWxO2(B). The tunable resistivity makes correlated VO2(A) and VO2(B) appealing for next-generation electronic devices. © 2020, The Author(s).1

Electronic Structure and Insulating Gap in Epitaxial VO\u3csub\u3e2\u3c/sub\u3e Polymorphs

Determining the origin of the insulating gap in the monoclinic VO2(M1) is a long-standing issue. The difficulty of this study arises from the simultaneous occurrence of structural and electronic transitions upon thermal cycling. Here, we compare the electronic structure of the M1 phase with that of single crystalline insulating VO2(A) and VO2(B) thin films to better understand the insulating phase of VO2. As these A and B phases do not undergo a structural transition upon thermal cycling, we comparatively study the origin of the gap opening in the insulating VO2 phases. By x-ray absorption and optical spectroscopy, we find that the shift of unoccupied t2g orbitals away from the Fermi level is a common feature, which plays an important role for the insulating behavior in VO2 polymorphs. The distinct splitting of the half-filled t2g orbital is observed only in the M1 phase, widening the bandgap up to ∼0.6 eV. Our approach of comparing all three insulating VO2 phases provides insight into a better understanding of the electronic structure and the origin of the insulating gap in VO2

Strongly enhanced oxygen ion transport through samarium-doped CeO2 nanopillars in nanocomposite films

Enhancement of oxygen ion conductivity in oxides is important for low-temperature (<500 °C) operation of solid oxide fuel cells, sensors and other ionotronic devices. While huge ion conductivity has been demonstrated in planar heterostructure films, there has been considerable debate over the origin of the conductivity enhancement, in part because of the difficulties of probing buried ion transport channels. Here we create a practical geometry for device miniaturization, consisting of highly crystalline micrometre-thick vertical nanocolumns of Sm-doped CeO2 embedded in supporting matrices of SrTiO3. The ionic conductivity is higher by one order of magnitude than plain Sm-doped CeO2 films. By using scanning probe microscopy, we show that the fast ion-conducting channels are not exclusively restricted to the interface but also are localized at the Sm-doped CeO2 nanopillars. This work offers a pathway to realize spatially localized fast ion transport in oxides of micrometre thickness. © 2015 Macmillan Publishers Limited. All rights reserved133431sciescopu

Self-assembled oxide films with tailored nanoscale ionic and electronic channels for controlled resistive switching.

Resistive switches are non-volatile memory cells based on nano-ionic redox processes that offer energy efficient device architectures and open pathways to neuromorphics and cognitive computing. However, channel formation typically requires an irreversible, not well controlled electroforming process, giving difficulty to independently control ionic and electronic properties. The device performance is also limited by the incomplete understanding of the underlying mechanisms. Here, we report a novel memristive model material system based on self-assembled Sm-doped CeO2 and SrTiO3 films that allow the separate tailoring of nanoscale ionic and electronic channels at high density (∼10(12) inch(-2)). We systematically show that these devices allow precise engineering of the resistance states, thus enabling large on-off ratios and high reproducibility. The tunable structure presents an ideal platform to explore ionic and electronic mechanisms and we expect a wide potential impact also on other nascent technologies, ranging from ionic gating to micro-solid oxide fuel cells and neuromorphics.This work was supported by the European Research Council (ERC) (Advanced Investigator grant ERC-2009-AdG-247276-NOVOX) and the Cambridge Commonwealth, European & International Trust. We further acknowledge funding from ERC grant InsituNANO, 279342, (S.T. and S.H.) and the Engineering and Physical Sciences Research Council (EPSRC), EP/P005152/1 (S.H.). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. The work at Los Alamos was supported by the U.S. Department of Energy through the LDRD program and performed, in part, at the Center for Integrated Nanotechnologies (CINT), a U.S. Department of Energy, Office of Basic Energy Sciences user facility.This is the final version of the article. It first appeared from Nature Publishing Group via https://www.nature.com/articles/ncomms1237

Strongly enhanced oxygen ion transport through samarium-doped CeO2 nanopillars in nanocomposite films

Enhancement of oxygen ion conductivity in oxides is important for low-temperature (<500 °C) operation of solid oxide fuel cells, sensors and other ionotronic devices. While huge ion conductivity has been demonstrated in planar heterostructure films, there has been considerable debate over the origin of the conductivity enhancement, in part because of the difficulties of probing buried ion transport channels. Here we create a practical geometry for device miniaturization, consisting of highly crystalline micrometre-thick vertical nanocolumns of Sm-doped CeO(2) embedded in supporting matrices of SrTiO(3). The ionic conductivity is higher by one order of magnitude than plain Sm-doped CeO(2) films. By using scanning probe microscopy, we show that the fast ion-conducting channels are not exclusively restricted to the interface but also are localized at the Sm-doped CeO(2) nanopillars. This work offers a pathway to realize spatially localized fast ion transport in oxides of micrometre thickness

Turning antiferromagnetic Sm(0.34)Sr(0.66)MnO3 into a 140 K ferromagnet using a nanocomposite strain tuning approach.

Ferromagnetic insulating thin films of Sm(0.34)Sr(0.66)MnO3 (SSMO) on (001) SrTiO3 substrates with a T(C) of 140 K were formed in self-assembled epitaxial nanocomposite thin films. High T(C) ferromagnetism was enabled through vertical epitaxy of the SSMO matrix with embedded, stiff, ∼40 nm Sm2O3 nanopillars giving a c/a ratio close to 1 in the SSMO. In contrast, bulk and single phase SSMO films of the same composition have much stronger tetragonal distortion, the bulk having c/a >1 and the films having c/a <1, both of which give rise to antiferromagnetic coupling. The work demonstrates a unique and simple route to creating ferromagnetic insulators for spintronics applications where currently available ferromagnetic insulators are either hard to grow and/or have very low T(C).This work was supported by the European Research Council (ERC) (Advanced Investigator grant ERC-2009-AdG-247276-NOVOX). A. Suwardi would also like to acknowledge the Agency for Science, Technology and Research (A*STAR) Singapore for funding his graduate studies. M. E. Vickers is thanked for her help with the X-ray characterization work and A. Sangle for helping with the initial experimental works.This is the author accepted manuscript. The final version is available at http://pubs.rsc.org/en/Content/ArticleLanding/2016/NR/C6NR01037G#!divAbstract