Formation of VO2 by rapid thermal annealing and cooling of sputtered vanadium thin films

Abstract

Sputtered vanadium-rich films were subjected to rapid thermal annealing-cooling (RTAC) in air to produce vanadium dioxide (VO2) thin films with thermochromic switching behavior. High heating and cooling rates in the thermal oxidation process provided an increased ability to control the film's microstructure. X-ray diffraction patterns of the films revealed less intense VO2 peaks compared to traditional polycrystalline samples fabricated with a standard (slower) cooling time. Such films also exhibit a high optical switching reflectance contrast, unlike the traditional polycrystalline VO2 thin films, which show a more pronounced transmittance switching. The authors find that the RTAC process stabilizes the VO2 (M2) metastable phase, enabling a rutile-semiconductor phase transition (R-M2), followed by a semiconductor–semiconductor phase transition (M2-M1).

A Rational Fabrication Method for Low Switching-Temperature VO2

“…The significant lower transition temperature compared to the pure VO 2 (68 • C) can be attributed to either the non-stoichiometric composition [30], the stress due to the lattice mismatch with the SiO 2 substrate [13] or the small grain size [11,12]. The transition temperature of our VO x film is about 10 • C lower than layers which were oxidized in air by other groups [23,29,31]. This drop in the T c may be ascribed to the lower annealing temperature.…”

“…The magnitude of the transition is calculated by the resistance contrast between 30°C and 100 °C (see blue and red dots, respectively, in Figure 1 a on the heating branch) and its value for this particular sample is 68. This resistance switching ratio is in the same regime [ 23 , 29 , 31 ] or higher [ 12 , 28 ] than the other layers which were prepared by oxidation of metallic V under atmospheric pressure. However, all those samples were annealed at higher temperature and required a more thorough optimization process, because a few percent variation in the oxidation time leads a substantial change in the hysteresis curve.…”

Facile Fabrication of Composite Vanadium Oxide Thin Films with Enhanced Thermochromic Properties

“…This indicates that the oxidation happens to the films during rapid annealing. Different from the conventional rapid thermal annealing process which is performed at a protective atmosphere (such as N 2 ), 28 our modified rapid annealing process is carried out entirely in air. Generally, a low-valent vanadium oxide thin film is easily noticeably oxidized at high temperature in air.…”

“…2 Until recently, most studies on the thermochromic properties of VO 2 (M) have been focusing on obtaining high-purity monoclinic VO 2 as the functional layer of the coat- ings. 11,17,18,27,28 The vanadium−oxygen system displays high complexity due to the variety of oxidation states and the variability of V−O coordination geometries. , etc.).…”

Enhanced Transition-Temperature Reduction in a Half-Sphere Au/VO2 Core-Shell Structure: Local Plasmonics versus Induced Stress and Percolation Effects

“…(ii) The form of the thermal hysteresis between the heating branch and the cooling branch is asymmetric for all deposition times. This phenomenon is attributed to the difference in the metallic domains distribution variation in the heating cycle compared to the cooling cycle of the experiment [63]. VO 2 exists in two distinct phases, a low-temperature insulator phase where it possesses a monoclinic structure, and a high-temperature metallic phase with a tetragonal rutile structure.…”