PLD Growth And Properties Of Epitaxial VO₂ Thin Films

Vanadium dioxide?VO₂?is a metal oxide material which has a thermally-induced phase-transition.VO₂ undergoes a reversible transition from the high-temperature metal phase to the low-temperature insulator phase at 68°C.Across metal-to-insulator transition?MIT?,there are drastic reversible changes in its electrical,optical transmittance in the near-IR region and magnetic characteristics.These make VO₂ an attractive material in various applications,such as optical and/or electrical switches,infrared detectors and storage media.However,VO₂is easier to form V₂O₃,VO₂ and V₂O₅,so the growth of pure phase VO₂ is more difficult.MIT of VO₂ is very sensitive to oxygen vacancy,strain and so on.The growth of high quality VO₂is the basis for further study.In this work,a VO₂ ceramic disk was used as the ablation target and the reactive gas was high-purity O₂.The VO₂ films were deposited on c-plane sapphire and TiO₂?110?substrates by pulsed laser deposition?PLD?,respectively.We systematically studied the influence of oxygen pressure on the structure and MIT properties of VO₂ epitaxil thin films,and the thickness dependent properties of VO₂ thin films were studied as well.The main contents and results are as follows:1.We investigated the influence of oxygen pressure on the growth of VO₂ epitaxil thin films and got high quality epitaxial VO₂ films by adjusting oxygen pressure.MIT properties are not obvious at too low oxygen pressure and it is esay to produce V₂O₅ at too high oxygen pressure.As the substrate temperature was fixed at 600°C,the optimum oxygen pressure for preparing VO₂ films is 1.2 Pa.The XRD results revealed the epitaxial relationships between films and c-plane sapphire are VO₂?010?//Al₂O₃?0001?and VO₂[100]//Al₂O₃[10-10].AFM showed the surface of the films is very smooth.When the deposited oxygen pressure is between 0.3 Pa and 1.5 Pa,the films undergo obvious MIT characteristics.Temperature of MIT can be tuned from 52.1°C to 67.8°C by varying the O₂ pressure from 0.3 Pa to 1.5 Pa.The oxygen vacancy in the film can effectively adjust the VO₂ phase transition temperature.2.The substrate temperature was set at 600°C and the O₂ pressure was kept at 1.2 Pa for the film growth.We produced a series of different thickness VO₂ epitaxil films on c-plane sapphire substrates by changing the numbers of pluse.We studied the effects of film thickness on the crystal structure,surface morphology and MIT characteristics.When the film thickness is 210 nm,the FWHM of?020?plane rocking curve is 0.034°,indicating the film has good crystallinity.As the film thickness increases,the defect density in the films increases,resulting in an increase of RMS.When the film thickness increases from 14 nm to 210 nm,the phase transition temperature increased from 65.15°C to 70.6°C.The T_c decreases from70.6°C to 68.1°C when the films thickness increases from 210 nm to 525 nm.The hysteresis width??35?H?decreases monotonically with the increase of film thickness,and the film thickness can effectively tune the MIT properties of VO₂ thin films.3.VO₂ thin films were epitaxially grown on TiO₂?110?substrates with the thickness varied from 5.2 to 300 nm.The thickness dependent properties have been investigated.The defect density increases with the increasing films thickness,which makes the FWHM of the?110?plane increase from 0.078°to 0.567°,and the root mean square of film increases.When the film thickness is 9.5 nm,RMS is only 0.208 nm,indicating the film surface reaches to the atomic level smooth.The obvious structural transition was observed by in situ temperature-dependent high-resolution XRD-RSM with the film thickness 88 nm,and temperature of structural transition and resistivity change is the same point.The phase transition temperature T_c decreases from 75.8°C to 49.4°C when the film thickness increases from 5.2 nm to 300 nm,indicating the T_c decreases with increasing film thickness.With the increasing of the film thickness,the lattice constant a increases,c decreases,c/a decreases,and c/a can effectively modify the phase transition temperature.The intrinsic factor affecting the phase transition temperature is the lattice constant.