Reduction of atomically flat epitaxial WO₃ thin films to metallic WO_3-x suboxide phases

WO₃ exhibits rich polymorphic phases and suboxides, garnering interest as an electronic material and possible high-temperature superconductor [1]. For many applications, WO₃ single crystals are desired, and thin films are essential. To this end, pulsed laser deposition (PLD) is used to grow high crystalline quality, atomically flat epitaxial WO₃ thin films on SrLaAlO₄ (SLAO) substrates. This substrate is interesting, as it applies a significant tensile strain to the WO₃ film, the effects of which can be quantified using X-ray diffraction (XRD) measurements [2]. Soft chemistry approaches, including vacuum and hydrogen annealing, reduce the strained WO₃ films to suboxide phases, introducing ordered oxygen vacancies to access an even wider range of crystal structures. After synthesis, the resistivity of the strained and reduced films is measured as a function of temperature down to 2 K, as both strain and oxygen content have significant consequences for the electronic properties of WO₃ thin films and, therefore, on the devices into which they will eventually be integrated.

[1]

Shengelaya, A., K. Conder, and K. A. Müller. 2020. “Signatures of Filamentary Superconductivity up to 94 K in Tungsten Oxide WO2.90.” Journal of Superconductivity and Novel Magnetism 33 (2): 301–6. https://doi.org/10.1007/s10948-019-05329-9.

[2]

Leng, X., J. Pereiro, J. Strle, A. T. Bollinger, and I. Božović. 2015. “Epitaxial Growth of High Quality WO3 Thin Films.” APL Materials 3 (9). https://doi.org/10.1063/1.4930214.