Emerging technologies utilizing vanadium dioxide films with insulator metal transition - Smart windows for energy savings and self-sustained electrical oscillations -

抄録

There is a growing interest for environmentally friendly metal-oxide materials. Vanadium dioxide (VO₂) is known as a phase transition compound which shows insulator-metal transition (IMT) with its structural phase transition (SPT) from low-temperature monoclinic phase (P2₁/c) to high-temperature tetragonal phase (P4₂/mnm) at 68 °C. The abrupt resistivity change over 4 orders of magnitude is realized even in thin films with thickness of less than 15 nm. The reversible and durable IMT has triggered a lot of studies on VO₂ thin films not only for unravelling the physics of phase transition but also for applications to various engineering field. In this presentation, we introduce applications of VO₂ films to energy-saving smart windows [1] and oscillation phenomenon of voltage-induced IMT.

We have been fabricating stoichiometric VO₂ thin films by using reactive magnetron sputtering method. By introducing radio-frequency substrate biasing, we succeeded low temperature growth of stoichiometric VO₂ films on various substrates. As one of promising application, we are trying to fabricate VO₂-coated glasses for smart windows where infrared-light (IR) incidence is automatically controlled to avoid room temperature rise in hot summer. Large change of transmittance for IR concomitant with the IMT enabled VO₂-coated glass as smart windows with automatic reduction of IR with temperature rise. As shown in Fig. 1, we introduced layered structure of VO₂ on ZnO-buffered glass. As performance index, we realized the solar modulation ability (ΔT_sol) higher than 15% which is approaching top-ranking value, although the luminous transmittance (T_lum) around 30% is still rather low.[2]

Electrically biased VO₂ films shows self-sustained electrical oscillation (SEO) phenomenon when current-voltage characteristics possess a negative resistance region. We fabricated planer VO₂ device where Ti/Au electrodes are metallized on the VO₂ film grown on Al₂O₃ (001) substrate. The width and the gap in the electrodes were 5000 μm and 10 μm, respectively, as shown in Fig. 2. In this device, we achieved SEO with frequency from 15 K to 80 kHz, by selecting parallel capacitance to VO₂ as shown in Fig. 3. Further, it is known that parallel circuit of SEO realizes coupled oscillation phenomenon, in which their phases are automatically synchronized [3]. The coupled oscillations are expected to be applied to various issues in mathematical engineering field, such as graph coloring problem. ⁴⁾ In the present study, coupled oscillation with 10 kHz was realized in the coupling of two oscillators. The anti-phase synchronization appeared at the certain supply voltage. We directly observed changes of coupling modes by using curve tracer. In the presentation, we will show minute results of SEO and coupled oscillations.

References

[1] J. Houska, J. Appl. Phys. 131, 110901 (2022).

[2] K. Okimura et al., Sol. Ener. Mater. and Sol. Cells, 251,112162 (2023).

[3] Tobe et al., J. Appl. Phys., 127, 195103 (2020).