Study on VO2 thin film prepared by atomic layer deposition and its characteristics

Atomic layer deposition (ALD) technology has attracted wide attention in various fields because of its precise controllable thickness, good three-dimensional uniformity, large area film formation and low film formation temperature. In this paper, ALD technology is used, with VO(acac)2 and O2 as the source of vanadium and oxygen, respectively, using different deposition temperature (420～480℃) and annealing conditions (natural cooling, 4 and 8h program cooling) to prepare on the surface of the glass substrate VO2 film. The valence state, crystalline state and surface microstructure of the film were characterized by X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy; the semiconductor-metal phase transition characteristics of the prepared film were measured by a four-probe tester. the study. The experimental results show that the phase transition characteristics of VO2 film are directly related to its microstructure and crystal orientation. The ALD pulse sequence is selected as [10s-20s-20s-20s], the number of cycles is 300, the VO2 film deposited at 450°C and natural cooling is in good crystalline state, and the sheet resistance of the film before and after the phase change has a large amount of change and good performance The thermally induced phase change characteristics. Therefore, the ALD technology can prepare VO2 thin films with better phase change characteristics.

VO2 crystal undergoes a semiconductor-metal (SM) phase transition at about 68°C under light and electrical induction, and its structure changes from a low-temperature monoclinic tetragonal structure (M) to a high-temperature tetragonal rutile structure (R). Grade-level phase transition; along with the transformation of the structure, its physical properties, especially the resistivity and infrared transmittance, undergo abrupt changes before and after the phase transition. The thermally induced phase change properties of VO2 make it widely used in thermal switching, optical storage, smart windows and laser protection. In order to avoid the deterioration of the performance of the VO2 bulk material due to aging and cracks during the repeated phase transition process, the film VO2 has a greater advantage. Although the S-M of VO2 is only 68°C, it is necessary to further reduce the phase transition temperature to near room temperature if it is to be applied to smart windows, thermal switches, etc. At this stage, reducing the S-M phase transition temperature of VO2 can be achieved by doping with appropriate ions, such as W5+, Mo5+, Nb4+ and Ta4+, etc. It can also be achieved by changing the film forming method and process, such as annealing conditions, to reduce the S-M phase transition temperature.

At present, the preparation methods of VO2 thin films mainly include sputtering, chemical vapor deposition, pulsed laser deposition, sol-gel and atomic layer deposition (ALD). The ALD method, also known as atomic layer epitaxy, is a thin film preparation method in which two or more vaporized precursors are alternately fed into the reaction chamber in the form of pulses, and a thin film is formed by chemical adsorption or chemical reaction on the surface of the substrate . The self-limiting characteristic of the saturated adsorption of the precursor on the substrate surface makes the film prepared by ALD technology have the characteristics of precise controllable film thickness, excellent step coverage and shape retention. In addition, ALD also It has the advantages of low reaction temperature and little damage to the substrate.

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Initially ALD technology was applied to the preparation of polycrystalline fluorescent material ZnS: Mn and electroluminescent flat panel display amorphous Al2O3 insulating film materials. With the rapid development of the microelectronics industry, higher requirements for the development of integrated nanotechnology have been put forward, making ALD technology The theory of surface chemistry is developing rapidly. Today, ALD technology is not only used in the field of microelectronics, but also in the preparation of catalysts, optical and electrical thin films, and nanomaterials.

Vanadium oxide can exist in complex phases, and any small changes during the preparation of its thin film will cause changes in phase and structure. The premise of applications such as VO2 thin film smart windows is to reduce the SM phase transition temperature and achieve large-area film formation. In this regard, ALD technology can not only achieve large-area film formation, but also can finely control the film formation process and doping ion process, so ALD technology is expected to solve the problems of this application. This article aims to use ALD technology, use VO(acac)2 and O2 as the source of vanadium and oxygen, respectively, and use glass as the substrate. Under the premise of achieving saturated adsorption, the different reaction temperatures and annealing can be studied by changing the deposition conditions and annealing conditions. The influence of conditions on the structure and performance of VO2 films is a theoretical study for the further research and application of VO2 films.

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Powder VO(acac)2 (purity of 99.99%, Saen Chemical Technology Co., Ltd.); porous anodic aluminum oxide (AAO) film (pore size 220nm, specific surface area 5.9m2/g, British Whatman company); four probe tester ( RTS-1345, Guangzhou Four Probe Technology Co., Ltd.); glass (25mm× 30mm); absolute ethanol; acetone; high-purity argon (99.999%); high-purity oxygen (99.995%); ALD system by Xi’an Modern Chemical Research The material surface engineering and nano-modification laboratory developed spontaneously according to the design plan published by Elam et al. [18-20].

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According to the thermal analysis data of the reaction precursor VO(acac)2, the film experiment procedure determines that its vaporization temperature is about 140°C, and then sets the reaction precursor storage tank temperature to 150°C to ensure that the precursor VO(acac)2 can be vaporized . Before the experiment, the reaction chamber was evacuated and filled with high-purity argon gas, the pressure in the reaction chamber was adjusted to 110 Pa through the outlet pressure regulating valve of the reaction chamber, and the temperature controller was adjusted to make the temperature in the reaction chamber the actual required reaction temperature (420～ 480°C). During the experiment, the reactants VO(acac)2 and O2 were brought into the reaction chamber alternately by carrier gas through the switch of the pulse valve in pulse form. The pulse sequence is represented by [t1-t2-t3-t4], where t1 and t3 are respectively It is the injection time of VO(acac)2 and O2 precursors, and t2 and t4 are the flushing time of high-purity argon. During this experiment, in order to satisfy the saturation adsorption of the substrate surface and the AAO pore size and the thorough flushing of by-products, the pulse sequence used is [10s-20s-20s-20s], that is, 10s carries the precursor VO(acac)2 argon pulse (60mL/min), 20s argon flushing pulse (60mL/min), 20s reaction gas O2 pulse (50mL/min), 20s argon flushing pulse (60mL/min), cycle number is 300.

Using ALD technology, choosing VO(acac)2 and O2 as precursors, and choosing different temperatures to prepare VO2 crystal films. The results show that the VOx films prepared at 450℃ have good crystalline characteristics and contain higher composition (011) crystal orientation. VO2 crystal, and the experiment has good repeatability. The VOx film was prepared at 450℃ and the annealing conditions were changed. The results showed that as the cooling rate slowed down, the high-valence vanadium oxide content in the film gradually decreased, and the low-valence vanadium oxide content gradually increased. The content of VO2 crystals with (011) crystal orientation is the highest in the prepared film. The temperature-resistance characteristics of the films prepared under different annealing conditions further prove that the VO2 of the (011) crystal orientation in the vanadium oxide is the fundamental factor that determines its phase transition. In summary, this ALD technology can prepare VO2 thin films with better phase change characteristics.

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Founded in 2015,Zunhua Baorui Titanium Equipment Co.,Ltd. is a manufacturer specializing in pvd vacuum ion coating equipment. The company’s products mainly include large plate coating machine, large tube collating machine, tool coating machine and LOW-E glass production line. Mr.Wang baijiang ,general manager of the company ,has been engaged in vacuum coating industry for more than 30 years. He continuously improve production technology, improve product performance and devote himself to provide customers with better product experience and higher production efficiency.