Journal of Crystal Growth, Vol.364, 74-80, 2013
Epitaxial variants of VO2 thin films on complex oxide single crystal substrates with 3m surface symmetry
Rutile-based monoclinic VO2 films were grown on (1 1 1) LaAlO3, (0 0 0 1) Al2O3, (1 1 1) MgAl2O4, (1 1 1) MgO, and Zn-faced (0 0 0 1) and O-faced (0 0 0 (1) over bar) ZnO substrates. The out-of-plane alignment of the two-fold screw [0 1 0] axis of monoclinic VO2 with three-fold rotational axes of the substrates is made possible by the formation of epitaxial domains. We find that the in-plane [1 0 0] axis of VO2 is always oriented with one of the three in-plane eutactic axes of the all of the substrates, despite significant differences in lattice mismatch from substrate to substrate. From diffractometry analysis and upon considering the in-plane symmetry of the substrates with respect to the (0 1 0) VO2 film, we conclude the presence of 120 degrees rotational variants and 60 degrees twins. The electrical transport characteristics of all of the films except for those on the ZnO substrates are comparable, showing metal-insulator transitions with bulk-like transition temperatures. The growth of different stoichiometric phases and film orientations on ZnO is attributed to large lattice mismatch, and also leads to the apparent in-plane conductance transition being smeared out in temperature. We find that it is more challenging to stabilize (0 1 0) VO2 on O-faced ZnO compared to Zn-faced ZnO and discuss the differences in tetrahedra stacking between the two polarities. (C) 2012 Elsevier B.V. All rights reserved.
Keywords:Crystal structure;X-ray diffraction;Epitaxial;Physical vapor deposition processes;Rutile oxides;Vanadium dioxide