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Abstract
Polycrystalline VO2 films were obtained through a vacuum annealing of sputtered V-rich films over quartz substrates and were characterized with x-ray diffraction, field-emission scanning electron microscopy, and x-ray photoelectron spectroscopy, respectively. The semiconductor–metal transition (SMT) was studied with the temperature-variable electric resistances. It was seen that the VO2 film crystalline orientation changes with the O2 partial pressure during the vacuum annealing. We observed a relation between the thermal hysteresis of the SMT and the crystalline orientation of the monoclinic VO2 films. The (011) oriented monoclinic film presents a narrower thermal hysteresis and a larger transition amplitude as compared to the (200) orientated films. In addition, a transition shoulder appears in the thermal hysteresis of the SMT for the (200) oriented VO2 films during the cooling process and becomes absent for the (011) orientated VO2 films.
Highlights
VO2 is one of the most interesting materials characterized by the reversible first order structural phase transition (SPT) from the monoclinic phase to tetragonal rutile phase.1–5 The SPT of a bulk VO2 occurs at the critical temperature (Tc) of ∼68 ○C within a ∼0.1 K and in ps timescale; a change in the electric resistance of approximatelyfive orders of magnitude is caused, known as the semiconductor–metal transition (SMT).6,7 The single-crystals are subject to a volume expansion (1%) over the SPT;8 this will lead to a material damage and a direct application is limited
The thermal hysteresis is an important feature of the SMT for the polycrystalline VO2 films; this can be modulated for different applications
The film crystalline orientation changed from the (200) orientation to the (011) orientation with the increase in PO2, which was found to connect with the transition shoulder on the SMT of the VO2 films
Summary
VO2 is one of the most interesting materials characterized by the reversible first order structural phase transition (SPT) from the monoclinic phase to tetragonal rutile phase. The SPT of a bulk VO2 occurs at the critical temperature (Tc) of ∼68 ○C within a ∼0.1 K and in ps timescale; a change in the electric resistance of approximatelyfive orders of magnitude is caused, known as the semiconductor–metal transition (SMT). The single-crystals are subject to a volume expansion (1%) over the SPT; this will lead to a material damage and a direct application is limited. VO2 polycrystalline films can stand many times of phase transitions and have been studied as the functional materials for smart windows, memory devices, thermal detectors, optical switches, thermal radiators, and thermal diodes.. The single crystalline bulk VO2 has a very narrow sharp thermal hysteresis, while that of the VO2 films is wider and slower.. The thermal hysteresis width (ΔTr) of VO2 films can vary largely, which is dependent on the grain size, boundaries, and defects. Some studies observed a transition shoulder in the thermal hysteresis of the SMT during the cooling process; this was thought to be related to the intermediate phase in the SMT and can be changed by preparation conditions It is seen that ΔTr decreases as the grain size increases and the number of grain boundaries decreases. It is seen that the transition amplitude and the sharpness are related to the crystallite size; the larger the grain size, the larger the transition sharpness and amplitude. ΔTr can be changed with dopants; for example, it has been seen that doping W and Nb can reduce ΔTr, and the hysteresis sharpness and amplitude were inevitably decreased. Some studies observed a transition shoulder in the thermal hysteresis of the SMT during the cooling process; this was thought to be related to the intermediate phase in the SMT and can be changed by preparation conditions
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