三维拓扑绝缘体的自旋光电流研究

There is strong spin-orbit coupling (SOC) in the two-dimensional electron gas (2DEG) of three dimensional topological insulators (3D TIs), and the surface states of 3D TIs are spin polarized Dirac-like electron states. They have great potential applications in opto-spintronic devices. Spin photocurrent is a powerfull tool to investigate the SOC of materials as well as to explore new kinds of opto-spintronic devices. However, it is difficult to distinguish the spin photocurrent induced by the inner 2DEG from that induced by surface Dirac fermions (SDF) in experiments, and their characteristics and tuning mechanisms are still unclear. In this project, we would like to separate the spin photocurrent induced by inner 2DEG and SDF by comparing the spin photocurrent excited by visible light and by terahertz radiation, respectively, and then futher investigate the generation mechanisms and the tuning methods of these two kinds of spin photocurrent as well as their SOC and the spin-related transport properties. Besides, we also want to explore the influence of strain and electric field on the spin photocurrent, SOC and spin-related transport properties of inner 2DEG and SDF. By changing the thickness, substrates of the 3D TIs and by measuring the spin photocurrent under different temperatures, we would like to study the effects of these three factors on the spin photocurrent of the inner 2DEG and SDF, and to pave the way for the application of the 3D TIs in opto-spintronic devices.

三维拓扑绝缘体中，体内的二维电子气具有巨大的自旋轨道耦合，表面为完全极化的狄拉克型自旋电子态，它们在自旋光电子器件领域有很好的应用前景。自旋光电流在室温零磁场条件下即可观测到，是研究材料自旋轨道耦合和新型自旋光电子器件的有力工具。然而，在实验上很难将体内二维电子气和表面态的自旋光电流进行区分，而且它们各自自旋光电流以及自旋轨道耦合的特点和调控机制仍不清楚。本项目拟通过比较可见光波段和太赫兹波段的自旋光电流，将三维拓扑绝缘体体内二维电子气和表面态产生的自旋光电流进行分离，研究这两种不同起源的自旋光电流的产生机制与调控方法，揭示其自旋轨道耦合和自旋相关电学输运性质，探索应变、电场对其自旋光电流以及自旋轨道耦合的影响规律。通过改变三维拓扑绝缘体的厚度、衬底以及测试温度，阐明这些因素对拓扑绝缘体内部二维电子气和表面狄拉克电子自旋光电流的影响规律，为三维拓扑绝缘体在自旋光电子器件领域的应用奠定基础。

三维拓扑绝缘体中，体内的二维电子气和表面狄拉电子在自旋光电子器件领域有很好的应用前景。自旋光电流在室温零磁场条件下即可观测到，是研究材料自旋轨道耦合和新型自旋光电子器件的有力工具。然而，在实验上很难将体内二维电子气和表面态的自旋光电流进行区分，而且它们各自自旋光电流以及自旋轨道耦合的特点和调控机制仍不清楚。本项目通过测量不同光斑位置处圆偏振光激发的自旋极化光电流以及模型拟合，将三维拓扑绝缘体Bi2Se3上表面态和二维电子点的自旋光电流分离了出来，并研究了它们随温度的变化规律。同时，本项目研究了三维拓扑绝缘体中由线偏振光引起的自旋光电流，阐明了其内在机理。本项目还通过比较近红外波段和中红外波段的自旋光电流，表明Bi2Se3薄膜的自旋光电流是由上表面态信号占主导，分析了中红外波段下的自旋光电流信号增强的物理机制。系统研究了应力、电场、薄膜厚度、衬底材料、测试温度对三维拓扑绝缘体表面态和体内二维电子气的自旋光电流的影响规律。研究了厚度分别为7、20和30nm的Sb2Te3薄膜的圆偏振光致电流。研究表明，厚度为7nm的样品的圆偏振光致电流与厚度为20和30nm的样品的符号相反，这是由于前者为上表面态占主导，而后者是下表面态占主导。研究结果表明，在厚的样品中热电效应较强，热电效应与逆自旋霍尔效应的协同作用将会引起自旋光电流在低温下反号。系统研究了不同衬底材料的Bi2Te3薄膜的圆偏振光致电流随厚度的变化规律。我们发现随厚度的增加，圆偏振光致电流先增加后减小。通过正背入射的自旋光电流测试以及变激发波长的测试，我们发现测得的圆偏振光致电流是由上表面态主导的。研究还发现，Si衬底样品的圆偏振光致电流比钛酸锶衬底样品的大了约三个数量级，这是由Si衬底的自旋注入效应以及Si衬底样品较大的光吸收系数导致的。本项目几乎全部完成了预期的目标，成功将三维拓扑绝缘体表面态和二维电子气的自旋光电流分离出来，并构建了通过厚度、温度、衬底、激发波长、应变、电场对自旋光电流进行有效调控的机制。