Toward an Insulator-to-Metal Transition (IMT) in VOx Deposited by ALD: Huge Effect on Large IR Transmission Range

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