Vanadium dioxide (VO2) transits to a tetragonal crystal structure from a monoclinic crystal structure at a temperature of about 68ºC concomitant with a resistance change over four orders of magnitude, so-called insulator-metal transition (IMT) 1). It is also known that VO2 changes not only its resistance but also transparency abruptly due to the IMT. Thus, VO2 is expected to have a wide range of applications such as sensors, actuators, and smart windows. If VO2 can be grown on a flexible substrate, applicability of VO2 will be highly enhanced. In this study, polyimide films, which have high heat resistance and chemical resistance, were employed as flexible substrates. In addition, zinc oxide (ZnO), which has a lattice matching with VO2, was employed as a buffer layer to promote VO2 crystal growth2). Further, ZnO nanorod (NR) was employed in order to enhance crystallinity of VO2 films. Because NR’s crystalline grain size is larger than that of zinc oxide seed film prepared by the sputtering method, it allows large in-plane crystalline growth in VO2 film.
In sample preparations, polyimide films with a thickness of about 10 µm were prepared by spin coating with polyimide varnish followed by heat treatment. Then, ZnO_seed with a thickness of 460 nm was deposited on the polyimide by sputtering method. Next, ZnO_seed substrates were placed in a solution of HMTA (C6H12N4) and zinc nitrate 6-hydrate (Zn(NO3)2・6H2O) at 90°C for 120 min to fabricate ZnO_NR layer with a height of about 400 nm. The VO2 films were deposited on the ZnO_seed or ZnO_NR buffered polyimide films by reactive sputtering with substrate biasing at low temperature of 250 °C. Low temperature growth of 250 °C was effective for preventing the diffusion of ZnO during VO2 deposition3,4). VO2 deposition conditions were Ar and O2 total pressure of 0.5 Pa, O2 flow rate of 1.0 sccm, RF power of 200 W, bias power of 20 W, and deposition time of 40 min.
Fig. 1 shows the SEM image of ZnO_NR from the top, in which ZnO_NR growth with a hexagonal crystal system can be seen. The grain sizes of the nanorods were 150 - 200 nm. Fig. 2 shows the XRD patterns of VO2/ZnO_NR/polyimide (a) and VO2/ZnO_seed/polyimide (b). The (020) and (040) peaks of VO2 were confirmed in both samples. In addition, values of FWHM of the rocking curve were 5.95° for VO2/ZnO_NR/polyimide and 6.53° for VO2/ZnO_seed/polyimide, suggesting that the sample using ZnO_NR as the buffer layer had higher crystallinity of VO2. Fig. 3 is a comparison of the resistivity-temperature characteristics of VO2/ZnO_NR/Polyimide/quartz (a) and VO2/ZnO_seed/Polyimide/quartz (b). Both the ZnO_NR buffer sample and the ZnO_seed buffer sample showed a resistivity change of about three orders of magnitude with temperature, but the ZnO_NR buffer sample showed a steeper transition than the ZnO_seed buffer sample. In the presentation, we will also report more detailed surface morphology images and transmittance-temperature measurement results.
1) F. J. Morin, Phys. Rev.Lett 3, 34 (1959). 2) K. Kato et al., Jpn. J. Appl. Phys. 42, 6523 (2003). 3) N. H. Azhan et al., J. Appl. Phys. 117, 185307 (2015). 4) Y. Miyatake et al., J. Vac. Sci. Technol A, 40, 043406 (2022).
