Deposition of Vanadium dioxide thin films by reactive High-Power Pulsed Magnetron Sputtering and improvement of crystallinity by adjusting the duty cycle

抄録

Vanadium dioxide (VO₂) has attracted attention due to its reversible insulator-to-metal transition (IMT) characteristics. VO₂ exhibits an abrupt resistance change of more than 3–4 orders of magnitude around 68 °C, accompanied by a change in crystal structure from a low-temperature monoclinic phase to a high-temperature tetragonal phase [1]. However, vanadium is a multivalent metal that forms various oxide phases [2]. Therefore, the selective growth of VO₂ film by reactive sputtering is difficult. When the amount of oxygen is excessive even slightly, the deposited films easily become V₂O₅. In our previous study, VO₂ thin films were prepared by reactive high-power pulsed magnetron sputtering (r-HPPMS), controlling the oxygen flow rate alternately. As a result, we confirmed a resistance change of 1 order of magnitude during heating and cooling cycles with one of the samples [3]. In this study, we attempted to improve the crystallinity of VO₂ and the resistance vs temperature (R-T) characteristics by changing the deposition parameter of HPPMS.

VO₂ was deposited on c-Al₂O₃ substrates using the r-HPPMS system. A vanadium metal (φ50 mm, 99.9%) was used for the target. The Ar gas flow rate and pressure were 5.0 sccm and 1.0 Pa, respectively. The O₂ flow rate was initially set to 1.5 sccm, and a fixed voltage was applied to the HPPMS power source, to achieve the time-averaged HPPMS discharge power of 10 W in oxide mode. During deposition, the voltage was adjusted to keep the 10 W power. Then the O₂ gas flow rate was controlled by software in the following manner: The higher O₂ flow rate of 1.5 sccm was set to achieve oxide mode at r HPPMS discharge power of 10 W. After the oxide mode was stabilized, VO₂ deposition was started. After holding the rate for 28 seconds, it was decreased at a rate of 0.50 sccm/sec, until it became zero. After waiting for 4 seconds, the O₂ flow rate was increased again at the rate of 0.50 sccm/sec until 1.50 sccm. This O₂ flow rate cycle was repeated for 20 min of the deposition run. The substrate temperature was 400 °C. The distance between the target and the substrate was 60 mm. The repetition frequency of the HPPMS discharge was XXX Hz, and the duty ratio was changed between 8.5% and 6.5%. The crystallinity of the deposited films was evaluated by the X-ray diffraction (XRD) in θ-2θ scan mode. Two- or four-probe methods were applied to investigate the R-T characteristics. The sample film temperature was changed by the Peltier device. The temperature range was between RT (almost 25 °C) and 100 °C in this study.

The R-T characteristics of the films are shown in Figure 1 (Left). The resistance change in three orders of magnitude against temperature was confirmed with the first 7% sample. The 8 % sample exhibited the resistance change in two orders of magnitude. The 8.5% and the second 7% samples revealed the resistance change in one order of magnitude. Figure 1(Right) shows the XRD patterns of the samples. The diffraction peak from the VO₂ (020) plane was confirmed in all samples except the one with the 6.5% duty cycle. The peak was located at 2θ=39.74° (8.5%), 39.86° (8.0%), 39.88° (7.5%), 39.94° (first 7.0%) and 39.87° (second 7.0%). The VO₂ (020) diffraction peak of the first 7.0% duty ratio sample showed the largest diffraction intensity and the largest diffraction angle. The peak shift suggested that the film with the 7.0% duty ratio sample was under compressive stress. These results indicated that the crystallinity and the electrical properties were improved by adjusting the duty ratio.

References:

[1] J. Morin, Appl. Phys. Lett., 3 (1959) 34.

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