Elsevier

Vacuum

Volume 192, October 2021, 110407
Vacuum

Flexible VO2/Mica thin films with excellent phase transition properties fabricated by RF magnetron sputtering

https://doi.org/10.1016/j.vacuum.2021.110407Get rights and content

Highlights

  • Growth of flexible VO2 films by RF reactive magnetron sputtering was investigated.

  • The film exhibits superior electrical switching and terahertz modulation property.

  • The film exhibits stable flexibility even after 10,000 bending cycles.

Abstract

Integration of vanadium dioxide (VO2) on flexible substrates can realize flexible devices which can modulate the photoelectric response triggered by temperature, electric field, laser, etc. But the preparation of high quality flexible films is still blocked due to the high-temperature condition for VO2 growth. In this paper, a one-step procedure was proposed to prepare VO2 thin films on low-priced natural mica substrate by radio frequency (RF) reactive magnetron sputtering with a pure V target in an atmosphere of Ar/O2 gas mixture. Through optimizing the deposition temperature and thickness systematically, a high performance flexible VO2 film was obtained. It demonstrated large resistance change of around 4.8 orders of magnitude, and giant terahertz (THz) switching ratio of around 86.1% across the phase transition. Moreover, the VO2/Mica flexible film remained stable phase transition characteristic during the bending flexibility measurement. This work provides an alternative way to fabricate flexible VO2 film and makes it a candidate for the applications in flexible oxide electronics.

Introduction

A typical strongly correlated oxide named vanadium dioxide (VO2) has attracted great attention, due to its metal-to-insulator transition (MIT) from monoclinic (M) phase to rutile (R) phase triggered by temperature (at around 68 °C), electric field, laser, etc. [1,2]. This phase transition accompanies with significant changes in the structural and physical properties, such as lattice constant, electrical conductivity and optical constant. Owing to these charming characters, VO2 has great application potential in smart optical and electrical devices, such as thermochromic coatings [3], optical and electrical switching [4,5], photoconductive infrared detectors [6], energy applications [7], thermal/gas sensors [8,9], microactuators [10], memory devices [11], etc.

The fabrication of high quality VO2 films are extremely important for obtaining ideal phase transition properties. And plenty of methods have been developed, such as sol-gel method [12], thermal evaporation [13], ion beam sputtering [14], chemical vapor deposition (CVD) [12], pulsed laser deposition (PLD) [12,15], molecular beam epitaxy (MBE) [16] and magnetron sputtering deposition [17]. These methods are generally operated at high temperature during the deposition or the post-annealing process of films. So the substrates are usually rigid crystals, such as glass, quartz and sapphire [18]. It means that the VO2 film is difficult to realize a flexible characteristic.

It should be noted that the flexible optoelectronic devices have attracted great attention, which can meet the wearable or flexible application demands [[19], [20], [21], [22], [23]]. Therefore, it is necessary to find a suitable method that can realize the flexible VO2 films. Recently, a lift-off method has been developed to prepare VO2 on flexible substrates [24,25]. In this method, the VO2 film was fabricated on rigid substrate using a sacrificial layer, and then the VO2 film can be released from the substrate by removing the sacrificial layer. The freestanding film can be transferred to the other substrates including flexible target. Nevertheless, the lift-off process is complex and less controllable than direct synthesis technology.

Considering the current research needs, we select natural mica pieces as an ideal flexible substrates because of its flexibility, thermostability and excellent optical properties. These advantages make mica a suitable candidate substrate for synthesizing flexible VO2 thin films. In this work, VO2 films were deposited on mica using RF reactive magnetron sputtering without post-annealing. A high quality VO2/Mica film was obtained, which showed excellent phase transition properties, including high resistance switching and teraherz (THz) modulation. This strategy to synthesize high quality film provides significance for the development of VO2-based flexible electronics.

Section snippets

Preparation of VO2 films

A metallic vanadium target (99.9% purity, 2 inches diameter, 5 mm thickness, Zhongnuo new material (Beijing) Technology Co., Ltd) was used to deposit the VO2 films on natural transparent mica (20 mm × 20 mm, TO-3II MICA), using RF reactive magnetron sputtering deposition.

The distance from target to substrate was about 15 cm. The working gas Ar (99.999% purity) and the reactive gas O2 (99.999% purity) were introduced separately into the chamber using two mass flow controllers. In the process of

Results and discussion

The XRD patterns of the VO2 films prepared at different deposition temperatures are shown in Fig. 1(a). The characteristic peaks of Mica substrate can be observed, which correspond to the standard PDF card of mica (PDF # 46–1311). And all the films exhibit the same diffraction peaks, which are located at 27.79°, 55.55° and 57.42° respectively. They are indexed to diffractions from the (011), (211) and (022) planes of single phase monoclinic VO2 (PDF#43–1051) [3,26]. This indicates that VO2

Conclusions

In conclusion, VO2/Mica films were prepared by RF magnetron sputtering. The growth of the films at different deposition temperatures were investigated. And the phase transition properties were evaluated by recording the resistance variation during heating and cooling. The variance of the sheet resistance of the films deposited at 500 °C, 540 °C and 600 °C can reach around 3.3, 3.8 and 4.7 orders of magnitude, respectively. Then the films with different thicknesses were fabricated. Due to

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

This work was financially supported by NSAF (Nos. U1730138 and U1930123), Science and Technology project of Sichuan Province (No. 2018GZ0328) and the Fundamental Research Funds for Central Universities.

Acknowledgments

This work was financially supported by NSFC (No. U20A20212), NSAF (Nos. U1730138 and U1930123), Science and Technology project of Sichuan Province (No. 2018GZ0328) and the Fundamental Research Funds for Central Universities.

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