The Time Scale of Metal-Insulator Transitions by Electrostatic Gating in a Solid-State Three-Terminal VO₂ Device

Vanadium dioxide (VO₂) exhibits a metal-insulator transition (MIT) above room temperature and the electrical resistivity of VO₂abruptly changes by three orders of magnitude. At room temperature, this transition can be easily triggered by Joule heating, and therefore, various VO₂-based applications have been proposed including analog crossbar switches, oscillator networks, and neuromorphic computing [1]. The transition is induced not only by temperature change but also by electron doping, and its static characteristics have been studied in detail over many years [2]. On the other hand, its transient characteristics are not yet fully understood. The major challenge is the difficulty to control fast and precisely the temperature that causes the transition at the transition temperature (<i>T</i>_MIT). Although the transient characteristics were partially elucidated in laser-induced transition [3] and Joule heat-induced transition [4], the high nonequilibrium nature of laser-induced electronic excitation or the thermal runaway in conductive filaments makes the situation more complicated. Our research group has recently developed a solid-state electrostatic gating device that causes MIT by electron accumulation via high-permittivity titanium dioxide (TiO₂) gate dielectric [5,6]. Although the modulation is relatively small, this device has the potential to control the MIT with high speed and precision, being ideal for investigating the transient characteristics. In this study, we exploited this solid-state gating device to systematically investigate the transient characteristics of the MIT in the VO₂ channel. We found that the transition occurs in the time scale of several ten milliseconds, and this time scale is exponentially shortened by increasing the gate voltage. When the influence of the gate voltage is converted to the equivalent temperature effect, we found the transition speed is accelerated by three orders of magnitude per 1 K. These results provide the underlying physics for the high-speed operations of VO₂-based devices and enable further applications of this material to neuromorphic and other functionalities.<br/>Epitaxial VO₂films (7 nm) were grown on the Nb (0.05 wt.%) doped TiO₂ substrate and were patterned by photolithography. The fabricated device is a three-terminal device consisting of the substrate as the gate electrode, VO₂ film as the channel (35 μm × 50 μm), and the depletion layer formed inside the doped TiO₂ substrate as the gate insulation layer [5]. The VO₂ channel of the device showed an abrupt resistance change around 309 K. Next, the drain voltage of the three-terminal device was fixed at 0.1 V, a pulse voltage was applied to the gate terminal at the temperature slightly below <i>T</i>_MIT, and the time variation of the source current was measured. The measurement results showed that the transition was induced by the applied gate voltage and the speed of the transition can be enhanced up to three orders of magnitude by changing the gate voltage from 1 V to 5 V. When the electron accumulation effect due to gate voltage converted to the equivalent temperature effect, the speed of the transition can be enhanced up to three orders of magnitude per 1 K. The origin of this high-speed transition is attributed to the collective nature of the domain growth during MIT. These results provide the underlying physics for the high-speed operations of VO₂-based devices and enable further applications of this material to neuromorphic and other functionalities. [1] B. Y. ZHOU <i>et al</i>., PROCEEDINGS OF THE IEEE 103, 8, August 2015 [2] K. Shibuya <i>et al</i>., <i>Appl. Phys</i>. <i>Lett</i>. <b>96</b>, 022102 (2010) [3] A Cavalleri <i>et al</i>., Physical Rev. B 70 1161102 (2004) [4] J Valle <i>et al.</i>, Nature <b>569</b>, 388-392 (2019) [5] T. Yajima <i>et al</i>., <i>Nature Commun</i>. <b>6</b>, 10104 (2015) [6] T. Yajima et al., Adv. Elec. Mater. 8, 2100842 (2021)