Self-sustained electrical oscillations of VO₂-based planar devices and their coupled oscillation phenomena

抄録

Vanadium dioxide (VO₂) shows a sharp insulator-metal transition (IMT) with resistance change over 4 orders at around 68°C concomitant with a structural phase transition (SPT). The VO₂ film also shows rapid decrease in resistance with current jump when certain voltage is applied between facing electrodes on the film. This current jump possesses a negative resistance region, resulting in the phenomenon of self-sustained oscillation. Self-sustained oscillation is expected to be applied to neuromorphic devices¹⁾. Further, it is known that parallel connection of self-sustained oscillators realizes coupled oscillation, in which their phases are automatically synchronized ^2,3). The coupled oscillations are expected to be applied to various issues in mathematical engineering field, such as graph coloring problem. ⁴⁾ In this study, the coupled oscillation was successfully demonstrated in a planar device with two self-oscillation circuits. VO₂ film was reactively sputtered on Al₂O₃ (001) substrate with size of 20 × 20 mm² at 400℃. The VO₂ film with thickness of 100 nm showed IMT with resistance change over three orders. As electrodes, 10 nm-thick Ti was coated on the VO₂ film followed by 200 nm-thick Au deposition. In the planar electrodes, the width and the gap were 5000 μm and 10 μm, respectively, as shown in Fig. 1. In order to investigate the coupled oscillation, self-oscillation circuits were composed between 2 and 3, and between 3 and 4, respectively. The two self-oscillation circuits were connected with a 22 nF-capacitor as shown in Fig. 2. Through this capacitance, two circuits interact each other and adjust their phases automatically. An AC voltage was supplied through an internal resistance of 1 kΩ by using curve tracer (Kokusai Denki Co. Ltd.). In the experimental results, anti-phase synchronization of two oscillating waves, in which voltage across VO₂ (2 - 3) rose during off period of voltage (3-4), was found as shown in Fig. 3. Thus, cooperative oscillation was realized in the coupling of two oscillators. Coupled oscillation frequency was around 10 kHz at a capacitance of 22 nF. The anti-phase synchronization appeared at the supply voltage from 100 to 92 V, and the amplitude between minimum and maximum values was 12.8V. In the present study, we directly observed changes of coupling mode by using curve tracer. In the presentation, we will show minute results of synchronization of two oscillators. References 1)Liu et al., J. Appl. Phys., 120, 124102 (2016). 2) M. S. Mian et al., J. Appl. Phys., 117, 215305 (2015). 3) Tobe et al., J. Appl. Phys., 127, 195103 (2020). 4) A.Parihar et al. Sci.Rep. 7, 911 (2017).