Toward an Insulator-to-Metal Transition (IMT) in VOx Deposited by ALD: Huge Effect on Large IR Transmission Range
Authors: Jolivet Aline , Cardin Julien , Frilay Cédric , Debieu Olivier , Marie Philippe , Duprey Sylvain , Lemarié Franck , Portier Xavier , Bazin Philippe , More-Chevalier Joris , Fitl Premysl , Cichoň Stalislav , Lančok Ján , Jiříček Petr , Yuan Weiqian , Jadwisienczak Wojciech , Ingram David , Labbé Christophe 
Published: February 2024

Vanadium dioxide (VO2) exhibits an insulating-to-metallic transition (IMT) from an insulating monoclinic to a metallic rutile phase at a IMT temperature (TIMT) of about 68 °C. This transition is accompanied by a decrease in electrical resistivity of 5 orders of magnitude and a drastic change in optical properties. Until recently, several works described the fundamental nature of the phase transition in a contradictory way, attributing it either to a structure-induced electron-phonon interaction: the Peierls transition [1], or to a strong electron-electron correlation: the Mott transition [2]. Nowadays, theoretical and experimental results [3-6] tend to show that the phase transition is affected by the crystal and electronic structures according to a mechanism combining the two opposed mechanisms and thus called collaborative Mott-Peierls transition.
In order to achieve the growth of high-quality and conformal VO2 thin films and with a low thermal budget, Atomic Layer Deposition (ALD) is one of the most suitable ones, among physical and chemical vapor deposition techniques [7]. In this aim, vanadium oxide (VOx) films were deposited on both silicon (100) and glass substrates (figure with and without VOx) at 240°C using vanadium tri-isopropoxide (VTIP) asV-precursor and water as oxydant reagent. Deposited films, with thicknesses ranging from 30 nm to 120 nm depending on the number of ALD cycles, were annealed during one hour under a reducing atmosphere (forming gas) at four temperatures ranging from 400°C to 550°C with 50°C steps. Thanks to X-ray diffraction (XRD), and transmission electron microscopy (TEM), unannealed films were found amorphous independently of the substrate. Annealed films were found polycrystalline with a mixture of both VO2 and V2O5 (or V6O13) phases with crystallite lateral size up to 300 nm. RBS analysis showed how the chemical stoichiometry evolves with annealing temperature. The V content remains constant with annealing temperature while the O content and the film density were both decreasing under reducing conditions. By means of XPS measurements, at a temperature below TIMT, different valence states of V element were identified with a majority of V4+ ions and a minority of V3+ and V5+ ions. At a temperature above TIMT, a valence state modification occurs with a reduction of the V4+ ion concentration in favor of the V3+ and V5+ones. Cooling back under TIMT led almost to the same valence states initial reorganization with a majority of V4+.
The variation of electrical and optical properties of VOx films occurring during IMT was also investigated. It was found that the resistivity r decreased down to 10-1 Ω.cm with a rising temperature above TIMT, and the r rose again with decreasing temperature with a hysteresis by contrast to the first drop. Dramatically, the optical transmittances of VOx films on Si wafer, measured with FTIR, displayed an important reduction during IMT of almost 70% in a broad spectral range from 2 to 20 µm wavelength. The optical properties of VOx on a glass substrate in UV-visible spectral range at IMT evidenced a modification of dielectric constant at IMT. Raman spectroscopy of VOx films around IMT showed the disappearance of low-temperature VO2 peaks, characteristic for this transition.
In conclusion, crystalline VOx films were successfully produced by an ALD process and subsequent annealing at 500°C leading to films with reversible and reproducible IMT occurring at ~70°C. The important variation of some electrical or optical properties in infrared range paves the way for the light and heat monitoring in many applications including thermal stealth, heat management, tunable thermochromic or electrochromic windows.
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