Vanadium dioxide (VO2) is a transition metal oxide of great
interest that exhibits a first-order metal-insulator phase transition
(MIT) around 341 K. In addition to the MIT, VO2 exhibits a transition
from a tetragonal structure to a monoclinic structure as the temperature
decreases [1–12]. In addition, such a MIT is accompanied by
a large change in the infrared optical transmittance [13–16]. These
intriguing electrical and optical properties, which can be easily
triggered by temperature change, have attracted much attention as
they enable potential applications to switching devices and sensors
working around room temperature (RT) [17–20].
Because the temperature difference between RT and the MIT
temperature (TMI) of VO2 is 50 K, many studies have aimed to
arbitrarily modulate the TMI for specific applications of VO2. Thus
far, it has been reported that either applying a stress to VO2 to
induce lattice strain or an element doping into VO2 to induce a
variation in the length of the V–V chain along the c-axis are
effective methods to modify the TMI [14,21–26]. Muraoka reported
that the c-lattice constant is closely related to a variation of TMI in
epitaxially grown VO2 films, in which lattice strain is caused by the
substrate [21,26,27]. For instance, the TMI was increased to 369 K
for a strained VO2 film with an elongated c-lattice constant on a
TiO2(1 1 0) substrate, while it was decreased to 300 K for one
with a shortened c-lattice constant on a TiO2(0 0 1) substrate.
However, information on how transition behaviors are influenced
by the orientation of the TiO2 substrate is lacking in the literature.
For better control of the TMI of VO2 thin films through epitaxial
lattice strain, the effects of substrate orientation on the phase
transition properties of VO2 must be investigated in detail. In this
work, we studied the characteristics of the MIT and the optical
transmittance of epitaxial VO2 films on TiO2 substrates for five
different crystal orientations. The MIT temperature (TMI) and the
infrared optical switching behavior were significantly modified by
changing the crystal orientation of the substrate and an intermixing
layer of 10 nm thickness at the interface between VO2
and TiO2 was observed by transmission electron microscopy