Integrated Growth Of Functional Vanadium Oxide Thin Films And Basic Research On The Related Devices

In recent years, vanadium oxide with unique metal- insulator transition（MIT） properties have attracted a lot of attention. Especially for vanadium dioxide（VO₂）, the transition can be undergone near room temperature. It has shown great potential applications in microelectronics, optoelectronics, mechanics and sensors. However, the chemical valences of vanadium can be + 2, + 3, + 4 or + 5, and the phase digram of their oxides are very complicated. As a result, it is still a big chanllege to grow high quality epitaxial thin films of vanadium oxide with pure phase. For the purpose of developing new functional devices, it is actively demanded to find a simple thin film fabrication technique for vanadium oxides with controllable chemical valence as well as high quality epitaxial nature.In physics, all functional devices based on vanadium oxides rely on their MIT properties. For device applications, it is an important route to modulate the MIT by controlling the microstructure evolution and strain status in the thin films. In addition, it is a new idea to develop interfacial coupling devices through the heterostructure by integrating other types of functional materials on VO₂ thin films. In this case, the control of the interface between VO₂ and other functional materials is critical to achieve the coupling effects.To solve the above problems, we have successfully deposited high quality epitaxial functional vanadium oxide films by a newly developed chemical solution technique named polymer assisted deposition（PAD） and a physical deposition technique of pulsed laser deposition（PLD）. According to the differences in crystal symmetry of vanadium oxide and the selected substrates, the MIT in vanadium oxides are modulated by 4 kinds of epitaxial matching ways. The correlation between strain relaxation and physical properties have been analysed. Based on this, the magnetostrictive CoFe₂O₄ thin films were integrated on the epitaxial VO₂ thin films to form the heterostructures. The coupling properties of such devices were characterized.Using the PAD technique, the vanadium oxide films were deposited with controllable chemical valences by adjusting atmosphere in the sintering process based on the theoretical analysis of thermodynamic principle. And pure phases of V2O3, V4O7, VO₂ and V2O5 thin films were successfully preprareed. Among them, V2O3 and VO₂ films were verified to be epitaxially grown on sapphire. The films have shown good metal-insulator transitions: for V2O3, the critical transition temperature in the heating process is 158 K, and transition point in the cooling process is 130 K; for VO₂ these values are 341 K and 336 K, respectively. The structural characterization demonstrated the close relationship between strain effects and the MIT properties.To further investigate the relationship between microstructures and structural phase transition（SPT）, in-situ Raman characterization in the heating process was taken in epitaxial VO₂ thin films with different thicknesses prepared by the PAD technique. The formation and evolution of the intermediate M2 phase in SPT can be clearly observed in the sample with the thickness about 150 nm. However, this phenomenon was no obvious in the 90-nm-thick sample. The results of X-ray diffraction showed that there was no significant difference in strain status between the two samples. However, in the thicker film, more dislocations were introduced during the growth, which suggests the dislocations can promote the formation of the intermediate phase during SPT.On the other hand, smooth epitaxial VO₂ thin films on sapphire（0001） were prepared by pulsed laser deposition（PLD）. The deposition conditions were optimized by the structure zone model（SZM）. Finally, thin films with the root mean square（RMS） surface roughness of 6.81 nm were prepared. This result provides great opportunity for further integration of other functional materials on VO₂ thin films.Using the same PLD process, the VO₂ films were epitaxially grown on sapphire（11-20） substrates which pocess anisotropic in-plane lattices constants.Electrical transport results showed that an anisotropic MIT process can be observed in the film with the thickness of 185 nm, i.e. two MIT critical points along the in-plane orientation of sapphire [0001] can be observed, whereas only one can be clearly obsvered along the [10-10] orientation. This effect would gradually disappeared with the decrease of the VO₂ thickness. In the sample of 30 nm, this anisotropic behavior could no longer be observed i.e., only one critical transition point exhibits in both in-plane orientations. A possible model was proposed which suggests that this anisotropic phenomena are caused by different strain relaxation induced by different lattice matching ways between thin films.Based on the successful realization of epitaxial growth of VO₂ thin films on sapphire（0001）, integration of the magnetostrictive CoFe₂O₄ film on VO₂ were performed to form the heterostructure of Co Fe2O4/VO₂. Preliminary characterizations of physical properties were conducted in the original device. It is found that the width of the MIT loop in the VO₂ became narrow when the magnetic field was applied, whereas the magnetic susceptibility in this heterostructure was changed during the SPT of VO₂. These results indicate that the magnetic-thermal coupling effects exist in the CoFe₂O₄/VO₂ heterostructure, which provides a new route for magnetic refrigeration functional devices.