基于SrTiO_3氧化物界面的光伏与自旋轨道耦合效应

发布时间：2018-05-05 10:55

本文选题：复杂氧化物 + LaAlO_3/SrTiO_3　；参考：《中国科学技术大学》2015年博士论文

【摘要】：在过去的十多年里,ABO3型的钙钛矿复杂氧化物得到了广泛的研究,它们在温度,磁场,掺杂等外界条件下出现丰富的性质,具有复杂的相图。随着薄膜生长技术的发展,特别是脉冲激光沉积技术(PLD)的长足发展与反射高能电子衍射仪(RHEED)在PLD上使用,使得高质量外延生长钙钛矿氧化物薄膜成为可能,研究者在这些氧化物界面甚至发现了与块体材料全然不同的性质。所以界面体系一直是氧化物研究的热点。本人博士工作重点关注以SrTiO3为衬底材料的极性/非极性界面的能带结构,探索了可能的应用研究,以及Ti的d电子的多带结构。主要工作包括下面三方面。我们用NH4F-HF在适当的参数下腐蚀处理SrTiO3(001)表面,得到TiO2单一终结面,原子级平整的SrTiO3(001)表面。在这样的表面下,我们用PLD-RHEED系统在合适的条件下实现了LaA103, LaVO3薄膜的层状生长,能很好地控制生长速率与厚度。不仅原位的RHEED衍射斑与振荡能很好地表明样品的高质量,XRD, TEM, CTR的测量也表明薄膜具有很好的晶体结构。高质量的界面样品的制备为我们探索界面的性质与物理提供了很好的条件。在LaA103/SrTi03(5uc-15uc)的表面镀上金属电极,用wire-bonder将铝线欧姆接触到界面,测量界面与表面之间的I-V曲线,发现185nm(6.7eV)的紫外光照射能在表面与界面之间产生光伏效应,256nm (3.4eV)与模拟太阳光却无法激发光伏效应。通过不同功函数电极和控制样品氧空位的对比实验,排除了可能导致光伏的原因,最终我们将这一现象归结于内建场与金属电极功函数的共同作用使得光激发的LaA103的电子-空穴对分离,产生光伏效应。这一发现不仅证明了LaA103/SrTi03界面“极性崩溃”下电子重构的物理图像,同时也是探索原子层厚度的光伏材料的一次尝试。我们用光刻等工艺将LaA103/SrTi03, LaVO3/SrTiO3做成场效应管的器件研究它们在低温磁场下的输运性质。在两种界面的磁阻都表现为栅压可调的反弱局域化的现象(WAL)。WAL的定量分析能给出能带中自旋分裂能等一系列与能带相关的重要信息,是研究自旋轨道耦合(SOC)的重要手段。测量发现即使LaAlO3/SrTiO3, LaVO3/SrTiO3的边界条件不一样,但是栅压对它们的磁阻却有相同的调节作用。我们用ILP理论拟合了WAL,发现界面的SOC表现为k的三次方的Rashba效应。进一步的定量分析得出自旋分裂能随载流子浓度的变化是非单调的,在某一浓度下出现最大值,与传统半导体的单调变化形成鲜明对比。在这一变化过程中,界面的载流子发生由单一到多种的转变,使得霍尔效应出现非线性。WAL与非线性霍尔可用界面SrTiO3一则Ti的3d电子的t2g轨道的自旋分裂与多子带结构解释。在这里,我们从实验上建立了输运性质与电子能带结构的对应关系。
[Abstract]:In the past decade or so, the complex perovskite oxides of ABO3 type have been widely studied. They have rich properties and complex phase diagrams under the conditions of temperature, magnetic field, doping and so on. With the development of thin film growth technology, especially the rapid development of pulsed laser deposition (PLD) and the use of reflective high-energy electron diffractometer (RHEED) in PLD, it is possible to grow perovskite oxide thin films with high quality epitaxial growth. At these oxide interfaces, the researchers even discovered properties completely different from those of bulk materials. Therefore, the interface system has been the focus of oxide research. My doctor focuses on the energy band structure of polar / nonpolar interface based on SrTiO3 substrate, and explores the possible application studies, as well as the multiband structure of d electrons in Ti. The main work includes the following three aspects. The surface of SrTiO3C001) is corroded by NH4F-HF with appropriate parameters, and the TiO2 surface with a single terminus and a flat atomic level SrTiO3C001) surface is obtained. On this surface, the layered growth of LaA103, LaVO3 thin films is realized by using PLD-RHEED system under suitable conditions, and the growth rate and thickness can be well controlled. Not only in situ RHEED diffraction patterns and oscillations can well indicate the high quality of the samples, but also the measurements of TM and CTR show that the thin films have good crystal structure. The preparation of high quality interface samples provides a good condition for us to explore the properties and physics of the interface. Metal electrodes were plated on the surface of LaA103 / SrTi035uc-15uc. the ohmic aluminum wire was contacted to the interface by wire-bonder, and the I-V curve between the interface and the surface was measured. It is found that the photovoltaic effect between the surface and the interface can be produced by the irradiation of 1855 nm ~ 6.7 EV) and the simulated solar light can not excite the photovoltaic effect. Through the comparative experiments of different work function electrodes and controlled oxygen vacancy of the sample, the possible cause of photovoltaic is excluded. Finally, we attribute this phenomenon to the interaction between the built-in field and the work function of the metal electrode, which results in the photovoltaic effect caused by the separation of electron-hole pairs of photoexcited LaA103. This discovery not only proves the physical image of electron reconstruction under the "polar collapse" of LaA103/SrTi03 interface, but also is an attempt to explore photovoltaic materials with atomic layer thickness. The transport properties of LaA103 / SrTi03and LaVO3/SrTiO3 FET are studied by photolithography. The magnetoresistive phenomena of the two interfaces are the anti-weak localization of gate voltage adjustable. The quantitative analysis of WALL. WAL can give a series of important information related to the band, such as spin splitting energy in the energy band, which is an important means to study the spin-orbit coupling SOC. It is found that even though the boundary conditions of LaAlO _ 3 / SrTiO _ 3 are different, the gate voltage has the same effect on their magnetoresistance. We use the ILP theory to fit Wall and find that the SOC of the interface is the Rashba effect of the cubic power of k. Further quantitative analysis shows that the variation of spin splitting energy with carrier concentration is non-monotone, and the maximum value appears at a certain concentration, which is in sharp contrast with the monotone variation of traditional semiconductor. In this process, the carriers at the interface change from single to multiple, which makes the Hall effect appear nonlinear. WAL and nonlinear Hall can be used to interface a Ti 3D electron t2g orbital spin splitting and the interpretation of the multi-subband structure. Here, we have experimentally established the corresponding relationship between the transport properties and the electron band structure.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O469

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