【作者】 樊乐乐；

【导师】 吴自玉； 邹崇文；

【作者基本信息】 中国科学技术大学 ， 同步辐射及应用， 2014， 博士

【摘要】 二氧化钒(V02)由于接近室温的金属绝缘转变温度,同时伴随着光学、电学以及磁学性质的巨大突变而吸引了人们极大的兴趣。这些典型的特点使得V02在节能材料、记忆存储,光电转换开关等领域有着非常广阔的应用前景。然而钒的氧化物有着多重价态(如V2+, V3+,V4+,V5+)以及由于点缺陷而存在Magneli相如VnO2n-1(3<n<9)。所以,在钒的氧化物制备中氧环境对V02相的形成非常重要。目前,基于V02材料的光电器件应用主要以薄膜状态为主。高质量VO2外延薄膜的制备对于提高以及优化器件应用具有重要意义,理解VO2生长特性能够更好的制备高质量外延薄膜。限制V02实际应用的一个重要瓶颈问题是相对较高的相变温度,因而探索有效的相变调控方法,实现连续调控V02相变温度必将大大推动V02相变材料在能源材料中的实际应用。基于以上,本论文主要研究内容以及取得的成果如下：(1)、系统研究了薄膜生长过程中氧分压对VO2薄膜相组分的影响。首次利用氧源射频分子束外延(MBE)技术制备出2英寸大小的V02外延单晶薄膜,系统研究了氧空位在VO2相变过程中所起的作用,提出了一种新的观点来解释氧空位降低相变温度的机制。(2)、利用φ-scan x射线衍射研究了VO2/Al203外延薄膜面内晶格匹配情况并提出了区域匹配的外延生长模型。首次利用高分辨同步辐射x射线衍射开展V02/Al2O3外延薄膜生长特性的研究,发现了φ-scan过程中的复杂精细衍射结构。根据界面多区域生长模型,我们很好的解释了这种复杂精细衍射结构的来源。同时掠入射φ-scan x射线衍射实验进一步证实了我们所提出的界面外延生长模型。这种晶体薄膜的外延生长模型对六重旋转对称性的其它衬底上生长V02薄膜具有普适性。(3)、开展VO2相变温度调控的研究：首先制备了不同厚度的V02/TiO2外延薄膜,发现相变温度与薄膜的厚度密切相关。相变温度随着薄膜变薄逐步降低。利用同步辐射倒易空间成像方法研究了不同厚度的V02薄膜中应力演变的动力学过程,基于VASP软件计算了不同应力下VO2费米面附近态密度分布情况。根据应力演变以及计算结果,我们指出界面应力改变了VO2薄膜中导带电子占据状态,继而影响了相变温度。另一项工作是通过电压来调控相变温度,首先我们在压电材料PMN-PT衬底上制备了V02薄膜,通过外加电压调控PMN-PT材料的应力,继而可以连续调控VO2相变温度。最后制备了VO2/GaN pn结结构,并且通过外加电压调控pn结电压,在某种程度上也可以调控V02相变温度。更多还原

【Abstract】 Vanadium dioxide (VO2) is a system that still attracts a tremendous interest because of its metal-insulator transition near the room temperature, accompanied by abrupt and large changes in optical, electrical and magnetic properties. The particular characteristics of the VO2indicate many applications in different fields such as energy-saving material, memory devices and optical switchs. Vanadium oxides contain multivalent states such as V2+, V3+, V4+, V5+, and exists in a Magneli phase VnO2n-1(3<n<9) due to ordered point defects. The oxygen environment in the film preparation process is a key parameter to compose the rich VO2phase diagram. At present, the form of VO2that applys in toelectronic devices is in film condition. As a consequence the preparation of a high quality VO2epitaxial film is very meaningful for improving and optimizing the applications. Understanding the growth behavior of a VO2epitaxial film is mandatory to synthesize a high quality film. At present the bottleneck that limits VO2application is its higher critical temperature, and a continuous control of its phase transition temperature is mandatory. Based on the above consideration, the researches performed and the results achieved are summarized as follow:(1) We systematically studied the role of the oxygen pressure in the VO2film growth. For the first time we successfully prepared a2-inch VO2epitaxial film using oxide plasma Molecular Beam Epitaxy (MBE). The role of oxygen defect in the VO2phase transition process was also analyzed. We proposed a new model to explain how oxygen defects decrease the critical temperature.(2) The in-plane lattice matching relation of a VO2/Al2O3epitaxial film was studied by j-scan XRD and the epitaxial property was explained in the domain lattice matching. We also performed high resolution synchrotron radiation XRD experiment on the VO2/Al2O3epitaxial film discovering a fine diffraction structures in j-scan XRD patterns. The growth behavior was explained according to a new interfacial model, which was validated by the results of the grazing incidence φ-scan XRD. The result of the VO2film deposition can be extended also to other six-fold rotation symmetry substrates.(3) We proceeded on the research on VO2phase transition temperature control via interfacial strain and voltage. We prepared VO3/TiO2films with different thickness and correlated it with the critical temperature. The critical temperature decreases as the film becomes thinner. The correlation with the interfacial strain was also demonstrated thanks to the Reciprocal Space mapping (RSM) method and the density of state near the Fermi surface of these different thickness films was calculated by using the VASP software. From the analysis of the interfacial strain data and theoretical calculations we discovered that the electronic orbital occupancy is strongly affected by the interfacial strain, which changes the electron-electron correlation and shifts the phase transition temperature. We also tuned the critical temperature by applying an external voltage on VO2films growth on a piezoelectric material (PMN-PT). The lattice constant of the substrate PMN-PT was changed by applying an external voltage, which also changes the lattice of the growth VO2film through an interfacial strain. The critical temperature can be the controlled almost continuously with this method.Besides, we also prepared VO2/GaN p-n junction films that change the critical temperature in a similar way using the same procedure.更多还原