Growth and Characterization of N-Doped VO2 Thin Films for the Thermochromic and Smart Windows by the Mist Chemical Vapor Deposition

Vanadium dioxide (VO₂) has attracted considerable attention after F. J. Morin reported that it exhibits a metal-insulator transition(MIT) at around 67^○C [1]. The MITs of VO₂ have been demonstrated to occur by various stimuli, such as heating, applying voltage, and light, and to be used for smart windows, to absorb and reflect infrared(IR)-region light at a high temperature to prevent it from entering inside the room. To apply the smart windows using the MIT derived from the heat, the transition temperature is higher than the expected temperature operated around 30^○C. Therefore, experiments and theoretical calculations have been conducted to decrease the MIT temperature of VO₂ thin films by incorporating other elements, such as N, F, and Mo. In previous report, N-doped VO₂ thin films were deposited by reactive pulsed laser deposition (PLD) and the MIT temperature decreased at 50^○C [2]; however, further investigations are necessary to decrease at 30^○C. On the other hand, J.G. Lu et al. reported that mist CVD allowed to dope N into oxide thin films [3]. Furthermore, our group demonstrated that VO₂ thin films were deposited by mist CVD on synthetic mica substrates and showed a tunable MIT temperature through bending stress. In this study, we doped nitrogen(N) into VO₂ thin films to decrease the transition temperature by mist CVD and investigated the impact of N into thin films on the MIT temperatures.<br/>First, N-dope VO₂ thin films were grown on a synthetic mica substrate with a SnO₂ buffer layer to control the VO₂ orientations. X-ray diffraction 2θ-ω analysis revealed peaks of the VO₂(010) plane and the buffer layer SnO₂(100) plane in the undoped and N-doped samples in the temperature range from 425^○C to 500^○C. Furthermore, SIMS revealed that N concentration in the VO₂ thin film was approximately 10²⁰cm^-3 at the growth temperature of 425^○C. Thus, we successfully grew N-doped VO₂ epitaxial thin films by mist CVD.<br/>Temperature-resistance characteristics were measured of the VO₂ thin films to investigate their electrical properties and MIT temperatures. For undoped samples, the transition temperature slightly decreases with decreasing growth temperature, consistent with the results by Matamura et al. [4]. On the other hand, N-doped VO₂ thin films exhibited a decrease of the transition temperature, and the MIT temperature of VO₂ thin films at the growth temperature of 425^○C was 30^○C. This indicates that the VO₂ thin film blocks the IR light from entering the room above the 30^○C using as the thermochromic and smart windows. This result is lower than the transition temperature of 50^○C reported using PLD.<br/>The transmittance of the N-doped VO₂ thin films was investigated as varying the measurement temperatures. Similarly to the electrical properties, the N-doped VO₂ thin film grown at 425^○C showed the transition at around 30^○C. These results pave the way for the thermochromic and the smart windows applications that can help address future energy problems.<br/><br/>References<br/>[1]F. J. Morin, “Oxides Which Show a Metal-to-Insulator Transition at the Neel Temperature”, Phys. Rev. Lett. 3, 34-36 (1959).<br/>[2] Simon Chouteau et al., “Investigation of the metal-to-insulator transition of N-doped VO₂(M1) thin films”, Applied Surface Science Volume 554, 149661 (2021).<br/>[3] J.G. Lu et al., “Roles of hydrogen and nitrogen in p-type doping of ZnO”, Chemical Physics Letters 441 68–71 (2007).<br/>[4] Yuya Matamura et al., “Mist CVD of vanadium dioxide thin films with excellent thermochromic properties using a water-based precursor solution”, Solar Energy Materials & Solar Cells 230 111287 (2021).