| It is well known that vanadium oxides (V2O3, VO2 and V2O5, etc.) undergoes a first phase transition at a critical temperature (Tc), accompanied by an abrupt change in its resistivity and near-infrared transmission. Most notably, vanadium dioxide (VO2) is especially interesting as it undergoes a reversible semiconductor-to-metal transition (SMT) at a critical temperature of 341 K (more close to room temperature), i.e., transformation from a monoclinic (Monoclinic—P21/c) semiconducting state at low temperature to a tetragonal (rutile—P42/mnm) metallic state at high temperature. Due to these distinct properties, VO2 material shows high potential for various applications, such as electronic switches, thermal sensors, optical data storage disks, and thermochromic smart windows. However, the complexity of vanadium oxide system and the SMT critical temperature of 341 K was still rather higher for its practical applications. Currently doping is commonly adopted method to reduce the transition temperature. Nevertheless, the lowered transition temperature generally suffered from the remarkably weaken change in resistivity and infrared transmittance as well as increased hysteresis width, which was certainly undesirable for its practical applications as phase transition materials. Thus the most significant challenge for the real application of VO2 film materials was lowering the SMT temperature to near room temperature while maintaining its abrupt SMT transition. This paper is aimed at the growth of high-quality VO2 thin films and the modulation of phase transition properties of VO2 thin films with A1N stress layer. The main content in this thesis as follows:(1) The influences of O2 partial pressure on the structural, electrical and optical properties of VO2 thin films were investigated systemically. It has been demonstrated that the properties of VOX films are very sensitive to the O2 partial pressure during the PLD growth process. Vanadium oxide (VOX) thin films were grown on single crystal sapphire substrates by pulsed laser deposition (PLD) under 0.1、0.5、1.0、1.5、2.0Pa O2 partial pressures. Therefore, the optimized O2 partial pressure was first determined for high quality pure phase VO2 films.(2) The remarkable influences of AlN stress layer on the structural, electrical and optical properties of VO2 thin films were investigated systemically. The results indicated that the crystalline structure and SMT transition behaviors can be modified significantly by the introduction of AlN induced stress layer, which needs to be well controlled for reliable device performance. Moreover, in comparison with the pure VO2 films, the samples with AlN stress layer exhibit a lowered SMT transition temperature and higher optical transmittance in visible and near-infrared region, which would be especially favorable for various applications.(3) In-situ AlN induced valence state variation of V in vanadium oxide films investigated by XPS. To interpret the mechanism behind these phenomena, the valence state variation of vanadium (V) in VOx films induced by in-situ AlN layer was investigated by X-ray photoelectron spectroscopy (XPS) analyses. Our achievements will be helpful for understanding the physical mechanism behind the exotic characteristics of VOx films induced by stress layer. |
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