高压下VO_2的电输运性质及其金属化相变研究
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摘要
VO_2是一种对温度变化很敏感的半导体材料，它在68℃时具有从单斜晶系到四方晶系的温致结构相变并伴随电学性质上的金属化相变。得益于其光学性质和电学性质在相变前后所表现出的明显差异，VO_2被广泛应用于太阳能温控材料、红外脉冲激光辐射保护膜、光电开关、锂电池电极等诸多领域，有关其物性的研究也普遍开展。但到目前为止，有关VO_2在高压这一极端条件下的物性研究工作还鲜有报导，因此在本论文中，我们特别针对高压下VO_2的电输运性质进行了系统的研究，具体研究内容包括高压下VO_2电阻率随压力的变化关系、VO_2的高压霍尔效应以及高压下VO_2能带结构的理论计算等，并得到如下结论：
（1）通过基于金刚石对顶砧技术的电阻率原位测量方法，我们发现在12GPa左右出现了电阻率随压力的不连续变化。电阻率随温度变化的测量结果表明：在12GPa之前VO_2的电阻率随温度的上升而减小，表现出半导体的传导特性；而在12GPa之后其电阻率随温度的上升而增加，表现出金属的传导特性。结合VO_2的载流子激活能随压力的变化关系我们认为在12GPa左右出现的VO_2电阻率的不连续变化是由压致金属化相变引起的。（2）利用基于金刚石对顶砧技术的高压原位霍尔效应测量方法，我们发现VO_2的载流子浓度、霍尔系数和霍尔迁移率在12GPa左右都发生了突变，这同样来源于VO_2的压致金属化相变。（3）此外，我们还利用第一性原理，对VO_2的能带结构进行了计算，发现高压下VO_2的能隙变窄。
VO2is a kind of semiconductor whose physical and chemical properties are very sensitive to temperatures. It undergoes a structural phase transition from monoclinic structure to tetragnal structure at68℃, which simultaneously is accompanied with a temperature-induced metallization. Taking advantage of the big difference in the optical and electrical properties between before and after phase transition, VO2has been widely applied in various technical fields including solar temperature-controlled materials,infrared pulsed laser radiation protective film, photoelectric switches, and lithium electrode. Relevant studies on the physical properties of VO2have also been conducted. But up till now, the study on the physical properties of VO2under the extreme condition of high pressure has seldom reported. So, in this thesis, we have put our main focus on the systematic research of electrical transport properties of VO2under high pressure, including the study of the pressure dependent resistivity, the Hall effect and the energy band structure of VO2under high pressure, and have obtained the results as follow:
(1) With the DAC based in-situ electrical resistivity measurement, we have found a discontinuous change in the resistivity at12GPa. It is shown by the temperature dependent resistivity measurement that below12GPa the electrical properties of VO2are of semiconductor as the resistivity decreases with temperature increase, however, above12GPa VO2becomes a metal because the resistivity increases with the temperature increase. Together with the information of activation energy curve, we consider that the discontinuity of VO2resistivity at12GPa results from the pressure induced metallization.(2) With the in-situ Hall effect measurement, we have investigated the pressure dependence of charge carrier concentration, Hall coeficient and mobility of VO2, and abnormal changes could be found at12GPa, which is also caused be the pressure induced metallization of VO2.(3) Meanwhile, we have conducted first-principle calculation on the energy band structure of VO2under high pressure. The theoretical calculation has suggested that the energy band gap width would become narrow as increasing the pressure.
（1）通过基于金刚石对顶砧技术的电阻率原位测量方法，我们发现在12GPa左右出现了电阻率随压力的不连续变化。电阻率随温度变化的测量结果表明：在12GPa之前VO_2的电阻率随温度的上升而减小，表现出半导体的传导特性；而在12GPa之后其电阻率随温度的上升而增加，表现出金属的传导特性。结合VO_2的载流子激活能随压力的变化关系我们认为在12GPa左右出现的VO_2电阻率的不连续变化是由压致金属化相变引起的。（2）利用基于金刚石对顶砧技术的高压原位霍尔效应测量方法，我们发现VO_2的载流子浓度、霍尔系数和霍尔迁移率在12GPa左右都发生了突变，这同样来源于VO_2的压致金属化相变。（3）此外，我们还利用第一性原理，对VO_2的能带结构进行了计算，发现高压下VO_2的能隙变窄。
VO2is a kind of semiconductor whose physical and chemical properties are very sensitive to temperatures. It undergoes a structural phase transition from monoclinic structure to tetragnal structure at68℃, which simultaneously is accompanied with a temperature-induced metallization. Taking advantage of the big difference in the optical and electrical properties between before and after phase transition, VO2has been widely applied in various technical fields including solar temperature-controlled materials,infrared pulsed laser radiation protective film, photoelectric switches, and lithium electrode. Relevant studies on the physical properties of VO2have also been conducted. But up till now, the study on the physical properties of VO2under the extreme condition of high pressure has seldom reported. So, in this thesis, we have put our main focus on the systematic research of electrical transport properties of VO2under high pressure, including the study of the pressure dependent resistivity, the Hall effect and the energy band structure of VO2under high pressure, and have obtained the results as follow:
(1) With the DAC based in-situ electrical resistivity measurement, we have found a discontinuous change in the resistivity at12GPa. It is shown by the temperature dependent resistivity measurement that below12GPa the electrical properties of VO2are of semiconductor as the resistivity decreases with temperature increase, however, above12GPa VO2becomes a metal because the resistivity increases with the temperature increase. Together with the information of activation energy curve, we consider that the discontinuity of VO2resistivity at12GPa results from the pressure induced metallization.(2) With the in-situ Hall effect measurement, we have investigated the pressure dependence of charge carrier concentration, Hall coeficient and mobility of VO2, and abnormal changes could be found at12GPa, which is also caused be the pressure induced metallization of VO2.(3) Meanwhile, we have conducted first-principle calculation on the energy band structure of VO2under high pressure. The theoretical calculation has suggested that the energy band gap width would become narrow as increasing the pressure.
引文
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