Thermal and optical properties of vanadium oxide thin films near the transition temperature

M. Kang1, M. Chu1, S. Kim1, J. Ryu2, H. Park3 and S. Lee4

1University of Ulsan, Korea
2Kongju National University, Korea
3Ulsan College, Korea
4Korea Research Institute of Standards and Science, Korea

Keywords: phase transition, transition temperature
property: thermal conductivity
material: VO2, thin film

Vanadium oxides undergo a transition from an insulating state to a metallic state due to change in crystalline structure, at their transition temperature Tc [1,2]. The transition temperature of vanadium oxides changes with chemical composition, and the resistance of vanadium oxides exhibits hysteresis loops under heating and cooling near Tc. Of all the vanadium oxides, vanadium dioxide (VO2) has a transition temperature (68℃) closest to room temperature. VO2 also exhibits a change in electrical resistivity on the order of 105 over infinitesimal temperature changes near 68℃. Vanadium pentoxide (V2O5) is the most stable of the vanadium oxides [3,4]. V2O5 film has a Tc of 257℃, and its transition accompanies great variation in electrical and optical properties near Tc. Owing to these features, VO2 and V2O5 films, have been seen much interest for applications such as thermal sensors, light modulators, infrared shutters, thin film secondary batteries, and thin film electrodes [5–7]. The optical transmittance of VO2 films in the infrared region decreases significantly above Tc. The infrared transmittance of VO2 films exhibits a typical hysteresis loop under heating and cooling near Tc, as well as optical switching [8]. Since electrical properties and optical transmittance of the vanadium oxides greatly vary around Tc, thermal properties such as thermal conductivity, thermal diffusivity and specific heat and optical constants of the oxides are expected to sensitively react to very small temperature change around their Tc. However, it has not yet been reported to the thermal properties and optical constants of the vanadium oxides. In this study, the optical properties of the V2O5 films prepared by RF sputtering method and VO2 film obtained from post-annealed V2O5 film are investigated. Also, the through-plane thermal conductivities of the V2O5 and VO2 films is measured by the thermo-reflectance method [10]. Variations of the optical and thermal properties of the films with change temperature are discussed. V2O5 thin films were prepared on an (0001) Al2O3 substrate using an RF magnetron sputtering system with a V2O5 (99.99%) disk target having diameter 10 cm. Sputtering was performed at an RF power of 200 W, at substrate temperatures of room temperature and 500℃, for 200 minutes. The distance between the target and the substrate was a constant 10 cm. The sputtering gas and reactive gas used were Ar and O2, respectively, each with 99.999% purity. The gases were injected into the chamber with at an O2 partial pressure of 10% and a total flow rate of 30 sccm. The base pressure of the chamber was less than 5.0 × 10-6 Torr, and the working pressure during the sputtering was approximately 1.0 × 10-3 Torr. The VO2 film was obtained as follows: the first, a V2O5 film was deposited at an RF power of 200 W, substrate temperature of 250℃, and O2 partial pressure of 0%, this film was then post-annealed at 600℃ for 90 minutes under an ambient O2 flow rate of 3 sccm, yielding the final VO2 film. The post-annealing was carried out in a chamber maintained at a low pressure of 3.0 × 10-3 Torr. The microstructures of the V2O5 and VO2 films were investigated using a scanning electron microscope (SEM; JEOL, JSM6335F) and an X-ray diffractometer (XRD; Rigaku, D/MAX-Rc) with Cu Kα radiation. The spectra of the elliptic constants (Ψ and ∆) of the V2O5 and VO2 films were measured using a phase modulated spectroscopic ellipsometer (Jobin-Yvon, Uvisel UV/NIR), in the photon energy region from 1.0 eV to 4.0 eV, at an incident angle of 65°. The elliptic constants of the films were fitted using the triple new amorphous (TNA) dispersion formula [10]. An optical model for analysis of the V2O5 and VO2 films was generated based on the Bruggeman effective medium approximation (BEMA) [10]. The optical model for the V2O5 film consists of a surface layer, a film layer, and an anisotropic substrate. The transmission spectra of the VO2 film were measured by UV-Vis-NIR spectrophotometry in the wavelength range 300 nm to 2500 nm, both below and above Tc. Through-plane thermal conductivities of the VO2 and V2O5 films were measured by thermo-reflectance method at room temperature. He-Ne laser with the wavelength of 632.8 nm was used as light source. Sample holder was set in vacuum chamber to prevent oxidation and convection. The sample temperature was uniformly controlled and maintained by the thermocouple and generator of heat. Bi films with the thickness of 50 nm, which had a large temperature coefficient of resistance was deposited by a thermal evaporation on the VO2 film. An alternating current was supplied to the film surface and an alternating voltage was measured using electrode connected by conducting epoxy resin. He-Ne laser was irradiated and reflected on the sample surface and reflectance variation of the sample as angular frequency (ω) change was measured by the lock-in amplifier. Figure 1 shows transmission spectra of the VO2 film below and above Tc. The transmittance of the VO2 film decreased significantly as the temperature increased from room temperature to 80℃ over wavelengths between 600 nm and 2500 nm. The transmittance at 72℃ was approximately 46% less than at room temperature at a wavelength of 2500 nm; at temperatures above 72℃, the transmittance of the VO2 film did not change. Fig. 1. Transmission spectra of the VO2 film below and above Tc. Figure 2(a) shows the n spectra of the VO2 film at temperatures of room temperature and 80℃. As the temperature increased to 80℃, the spectra changed remarkably. In particular, the largest difference of n spectra of the film showed in the near IR region (below approximately 1.5 eV). Figure 2(b) shows the change in k spectra of the film with respect to temperature. The changes in the spectra below and above Tc showed trends analogous to those for the n spectra. As the temperature increased to 80℃, the spectra increased greatly with decreasing photon energy below 1.5 eV. This result is an evidence for the optical transition in the VO2 film. (a) (b) Fig. 2. The spectra of (a) n and (b) k of the VO2 film below and above Tc. Figures 3(a) and (b) display T(0)/qd0 versus ω-1/2 plots for thermal conductivity measurement at room temperature of the V2O5 films deposited at substrate temperatures of room temperature and 500℃, respectively. Calibration factors of the V2O5 films with the amorphous and crystalline structures and VO2 film were determined , , and m2KW-1V-1, respectively. The plots for the Figs. 3 and 4 showed well linearity. The thermal conductivities of V2O5 films with the amorphous and crystalline structures are 0.209 Wm-1K-1 and 0.594 Wm-1K-1 at room temperature, respectively. The significant variation of thermal conductivity of V2O5 films is not observed with the crystalline structure induced by the substrate temperature. This result may be considered because thickness of the V2O5 films used in this study is too thin. Also, the thermal conductivity of VO2 film is found to 1.40 Wm-1K-1 and the value is lager about 0.806 Wm-1K-1 than that of the V2O5 film grown at the substrate temperature of 500℃. Fig. 3. T(0)/qd0 vs. ω-1/2 plots for thermal conductivity measurement at room temperature of the V2O5 films deposited at substrate temperatures of (a) room temperature and (b) 500℃. Fig. 4. T(0)/qd0 vs. ω-1/2 plot for thermal conductivity measurement of the VO2 film at room temperature.

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