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Zeitschrift für Kristallographie - Crystalline Materials

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Phase formation and stability in TiOx and ZrOx thin films: Extremely sub-stoichiometric functional oxides for electrical and TCO applications

Ralph A. Henning
  • Institute of Physical Chemistry, Justus Liebig University Giessen, Heinrich Buff Ring 17, 35392 Giessen, Germany
/ Thomas Leichtweiss
  • Institute of Physical Chemistry, Justus Liebig University Giessen, Heinrich Buff Ring 17, 35392 Giessen, Germany
/ Daniel Dorow-Gerspach
  • Institute of Physics (IA), RWTH Aachen University, Sommerfeldstraße 14, 52056 Aachen, Germany
/ Rüdiger Schmidt
  • Institute of Physics (IA), RWTH Aachen University, Sommerfeldstraße 14, 52056 Aachen, Germany
/ Niklas Wolff
  • Faculty of Engineering, Christian-Albrechts-Universität zu Kiel, Kaiserstrasse 2, 24143 Kiel, Germany
/ Ulrich Schürmann
  • Faculty of Engineering, Christian-Albrechts-Universität zu Kiel, Kaiserstrasse 2, 24143 Kiel, Germany
/ Yannic Decker
  • Institute of Physical Chemistry, Justus Liebig University Giessen, Heinrich Buff Ring 17, 35392 Giessen, Germany
/ Lorenz Kienle
  • Faculty of Engineering, Christian-Albrechts-Universität zu Kiel, Kaiserstrasse 2, 24143 Kiel, Germany
/ Matthias Wuttig
  • Corresponding author
  • Institute of Physics (IA), RWTH Aachen University, Sommerfeldstraße 14, 52056 Aachen, Germany
  • Email:
/ Jürgen Janek
  • Corresponding author
  • Institute of Physical Chemistry, Justus Liebig University Giessen, Heinrich Buff Ring 17, 35392 Giessen, Germany
  • Email:
Published Online: 2017-02-03 | DOI: https://doi.org/10.1515/zkri-2016-1981

Abstract

Most functional materials are thermodynamic equilibrium phases representing minima in the thermodynamic phase space. However, it is expected that many metastable phases with highly interesting properties also exist. Here, we report on a systematic approach to prepare thin-films of such non-equilibrium phases based on the gas phase deposition methods sputtering and pulsed laser deposition (PLD). Our synthetic strategy is to deposit a “precursor phase” which is amorphous or already a crystalline non-equilibrium phase. Subsequent heat treatment leads to the nucleation of crystalline phases which again may be metastable or stable compounds. In the present paper we focus on the binary systems Ti–O and Zr–O, both systems being widely applied and technologically relevant. Highly oxygen-deficient titanium oxide (TiO1.6) and zirconium oxide (ZrO) films prepared by pulsed laser deposition at room temperature are optically absorbing and possess electronic conductivities in the range of 10 S/cm. Both materials are metastable in respect to both composition and structure. For TiO1.6 we find an amorphous matrix with embedded grains of cubic titanium monoxide (γ-TiO) directly after deposition. Upon annealing nanocrystalline grains of metallic Ti are formed in the amorphous matrix due to an internal solid-state disproportionation whereas the electrical conductivity of the films increases and comes close to metal-like conductivity (1000 S/cm) at about 450 °C. Congruently, room temperature deposited ZrO films with an average composition of Zr:O= 1:1 contain small ZrO nanocrystals within an amorphous matrix. Heat treatment again leads to an internal disproportionation reaction whereas small crystals of Zr2O and ZrO2 precipitate at temperatures as low as 75 °C. Increasing the temperature then results in the crystallization of metastable tetragonal ZrO2 at about 400 °C. Sputter deposition allows a subtler control of the oxygen partial pressure. Slightly non-stoichiometric TiO2−x films form a degenerate semiconductor with room temperature conductivities as high as 170 S/cm. Moreover, controlling both, the doping level and the vacancy concentration of these films allows to control the phase formation and the transition temperature between the rutile and anatase TiO2 polymorphs. Niobium doping of sputter deposited TiO2 can lead to films with very high electrical conductivities while maintaining a high optical transmittance demonstrating the potential of the material as an alternative transparent conducting oxide (TCO) with extraordinary properties.

Keywords: anatase; hafnia; memristors; resistive switches; TiO; titania; transparent conducting oxide; zirconia; ZrO

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About the article

Received: 2016-06-20

Accepted: 2016-08-22

Published Online: 2017-02-03

Published in Print: 2017-02-01


Citation Information: Zeitschrift für Kristallographie - Crystalline Materials, ISSN (Online) 2196-7105, ISSN (Print) 2194-4946, DOI: https://doi.org/10.1515/zkri-2016-1981. Export Citation

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