Low-temperature growth of VO2 films on transparent ZnO/glass and Al-doped ZnO/glass and their optical transition properties

Vanadium dioxide (VO2) is a promising compound for thermochromic applications because of its insulator-metal transition (IMT) characteristic with concomitant transmittance change for infrared-light. For practical applications, we deposited VO2 thin films on transparent ZnO films on glass substrate. Growth of (010)-oriented VO2 film was obtained on c-axis oriented hexagonal ZnO layer, however, zinc vanadate (Zn2V2O7) was formed at the interface region in conventional radio-frequency magnetron sputtering at substrate temperature of 350 °C. On the other hand, (010)-oriented VO2 films were successfully grown on ZnO/glass substrates even at lower temperature of 250 °C by introducing inductively coupled plasma (ICP)-assisted sputtering, realizing steep interface between VO2 and ZnO without undesirable compounds. The VO2 films showed resistivity drop of more than two orders of magnitude together with transmittance change for infrared-light from 45 to 10% at around 70 °C. Simultaneous achievement of steep changes for both resistivity and transmittance is a milestone for practical application of VO2 films into thermochromic devices, such as smart windows and optoelectrical devices. Further, we succeeded the low temperature growth of VO2 films on conductive Al-doped ZnO (AZO) by using ICP-assisted sputtering technique. The transmittance change of VO2 on AZO/glass from 50 to 10% was observed, realizing a basis for electrically controllable intelligent optical windows.