栅压调控单根Cd3As2纳米线的电声耦合

GATE-TUNED ELECTRON-PHONON COUPLING IN SINGLE Cd3As2 NANOWIRE

黄巧玲

11649122

硕士

物理电子学

黄明远

2018-05-30

2018-07-04

哈尔滨工业大学

深圳

拓扑狄拉克半金属具有两种特性：一是三维的体态狄拉克锥状的能带结构，二是费米弧状表面态。在动量空间的三维方向上，三维狄拉克半金属的电子能带结构都具有线性的色散关系。这种三维的狄拉克色散关系受到时间反演对称性和空间反对称性的保护。近年来，狄拉克半金属的研究受到了物理学界科研人员的广泛关注，迅速成为凝聚态物理及其相关领域的研究热点之一。在三维拓扑狄拉克半金属中，砷化镉（Cd3As2）材料备受关注。由于其良好的抗氧化性和极高的载流子迁移率，被认为是优异的狄拉克材料。Cd3As2 材料具有简单独特的能带结构和优异的性质，而且在空气中相对稳定，这更加有利于其实验测试与研究。本课题的研究内容主要是通过栅压调控单根Cd3As2 纳米线的电子-声子耦合效应，并采用拉曼光谱进行探测。固体中量子化的晶格振动与电子之间的相互作用一直是凝聚态物理中最基本的研究领域之一。在Cd3As2 纳米线研究中，观察到光电子发射光谱出现异常现象，电子-声子耦合效应可能是产生该异常现象的原因。传统上，一般利用化学掺杂来研究电子-声子相互作用，通过引入杂质来改变材料的载流子密度。在低维系统中，本课题选取的场效应晶体管结构，也是一种有效改变载流子浓度的方法。一直以来， Cd3As2 纳米线的拉曼光谱研究都处于狄拉克半金属材料研究的前沿。在本项工作中，主要采用拉曼光谱方法来探测Cd3As2 纳米线中的电场效应和与三维狄拉克费米子相关的声子动力学。通过调控Cd3As2 的栅压，可以调节其不同的拓扑相位，例如Weyl 半金属、拓扑超导体和拓扑绝缘体等。主要的研究内容如下:（1）Cd3As2 纳米线的结构表征。通过化学气相沉积法（CVD）合成Cd3As2纳米线，并利用拉曼光谱测试和扫描电子显微镜（SEM）对Cd3As2 纳米线进行成分和形貌表征。（2） 纳米材料的器件制备。利用场效应晶体管来调控SiO2/Si 衬底上的Cd3As2 纳米线，因其器件制备较容易和调谐费米能级的效率较高，高浓度的载流子便可以在表面上累积以诱导一个非常大的电场。在具有285 nm SiO2 层的Si 衬底上，首先使用电子束光刻技术（EBL）制备Cd3As2 纳米线器件，然后利用电子束蒸发镀膜技术制备铬（30 nm）/金（200 nm） 厚的电极，与Cd3As2纳米线形成欧姆接触。Si 衬底被用作场效应晶体管的背栅电极。利用光学显微镜和SEM 来观察测量器件电极的形态。最后，通过KEITHLEY4200 测试器件是否导通，并获取I-V 曲线。（3） Cd3As2 纳米线的拉曼光谱研究。以532nm 激光器作为激发源，以单根Cd3As2 纳米线为研究对象，分别研究Cd3As2 纳米线的低温拉曼光谱、不同纳米线直径对应的拉曼光谱、常温下栅压调控拉曼光谱。

Topological Dirac semimetals have two characteristics: one is a three-dimensional body-like Dirac cone-shaped band structure, and the other is a Fermi arc-like surface state. The electronic band structure of the three-dimensional Dirac semimetal has a linear dispersion relationship in the three-dimensional direction of the momentum space. This three-dimensional Dirac dispersion relationship is protected by time reversal symmetry and spatial antisymmetry. In recent years, the research on Dirac semimetals has received extensive attention from researchers in the physics community and has quickly become one of the research hotspots in condensed matter physics and related fields.Among the three-dimensional topography Dirac semimetals, cadmium arsenide (Cd3As2) material has attracted much attention. Because of its good oxidation resistance and extremely high carrier mobility, it is considered to be an excellent Dirac material. Cd3As2 material has simple and unique band structure and excellent properties and is relatively stable in the air, which is more conducive to its experimental testing and research.The research content of this topic is mainly to regulate the electron-phonon coupling effect of a single Cd3As2 nanowire by gate voltage and detect it by Raman spectroscopy. The interaction between lattice vibrations and electrons in quantized solids has always been one of the most basic areas of research in condensed matter physics. In the study of Cd3As2 nanowires, abnormal photoelectron emission spectra were observed, and the electron-phonon coupling effect may be the cause of this anomaly. Traditionally, chemical doping has been used to change the carrier density of materials to study electron-phonon interactions. In a low-dimensional system, the field effect transistor introduced in this topic is also an effective method to change the carrier density. Raman spectroscopy of Cd3As2 nanowires has been at the forefront of research on Dirac semimetal materials. In this work, Raman spectroscopy is used as a primary method to detect the electric field effect in the Cd3As2 nanowires and the phonon dynamics associated with the three-dimensional Dirac fermions. Cd3As2 also can be adjusted between different topological phases by gate voltage regulation, such as Weyl semimetals, topological superconductors, and topological insulators. The main content is as follows:1. Structure characterization of Cd3As2 nanowires. Cd3As2 nanowires were synthesized by chemical vapor deposition (CVD), and the composition and morphology of the Cd3As2 nanowires were characterized respectively by Raman spectroscopy and scanning electron microscopy (SEM).2. Preparation of nanomaterial device. Field-effect transistors can be used to IVmodulate Cd3As2 nanowires on SiO2/Si substrates, due to the easier device preparation and the higher efficiency of tuning Fermi levels. Therefore high-concentration carriers can accumulate on the surface to induce a very large electric field. On a Si substrate with a 285 nm SiO2 layer, a Cd3As2 nanowire device was first fabricated via electron beam lithography (EBL), and then a Cr (30 nm)/Au (200 nm) thick electrode was prepared by electron beam evaporation coating technique to form an ohmic contact with a Cd3As2 nanowire. The Si substrate was used as the back gate electrode of the field effect transistor. The morphology of the device was observed by optical microscope and SEM. Finally, we utilized the KEITHLEY 4200 to test whether the device was conductive and to obtain the I-V curve.3. Raman spectroscopy of Cd3As2 nanowires. Using a 532nm laser as the excitation source and a single Cd3As2 nanowire as the research object, the low-temperature Raman spectra of Cd3As2 nanowires, we respectively studied the the cryogenic Raman spectra, diameter-dependent Raman spectra, and gate-dependent Raman spectra at room temperature of Cd3As2 nanowires.

Cd3As2纳米线 场效应晶体管 拉曼光谱 电声耦合

Cd3As2 nanowires Raman spectroscopy electroacoustic coupling field effect transistor

中文

联合培养

哈尔滨工业大学

黄巧玲. 栅压调控单根Cd3As2纳米线的电声耦合[D]. 深圳. 哈尔滨工业大学,2018.