Creation and Ordering of Oxygen Vacancies at WO_3-δ and Perovskite Interfaces

Changes in the structure and composition resulting from oxygen deficiency can strongly impact the physical and chemical properties of transition-metal oxides, which may lead to new functionalities for novel electronic devices. Oxygen vacancies (V_O) can be readily formed to accommodate the lattice mismatch during epitaxial thin film growth. In this paper, the effects of substrate strain and oxidizing power on the creation and distribution of V_O in WO_3-δ thin films are investigated in detail. An ¹⁸O₂ isotope-labeled time-of-flight secondary-ion mass spectrometry study reveals that WO_3-δ films grown on SrTiO₃ substrates display a significantly larger oxygen vacancy gradient along the growth direction compared to those grown on LaAlO₃ substrates. This result is corroborated by scanning transmission electron microscopy imaging, which reveals a large number of defects close to the interface to accommodate interfacial tensile strain, leading to the ordering of V_O and the formation of semi-aligned Magnéli phases. The strain is gradually released and a tetragonal phase with much better crystallinity is observed at the film/vacuum interface. The changes in the structure resulting from oxygen defect creation are shown to have a direct impact on the electronic and optical properties of the films.