单层MoS2中的自旋极化输运与调控

With the continuous progress of the micro-nano electronics technology, further high-density integrated circuits and multi-functional devices become the inevitable trend of development, the two-dimensional material is undoubtedly the first choice for future preparation of nano devices. Single layer MoS2 is a direct band gap semiconductor with a band gap of 1.8 eV, the single layer MoS2 based transistor having a high mobility, high current switching ratio, and low power consumption characteristics, making it become one of the most promising materials of the preparation of the next generation two-dimensional electronic device. Control of the charge and spin degrees simultaneously in these devices is extremely important. The basis for controlling the spin degree of freedom requires a strong spin-orbital coupling in the materials. Experimental results showed that in single layer MoS2, the spin-orbital coupling-induced spin splitting in the top of the valence band reaches about 160 meV due to the missing inversion symmetry, which is much larger than typical values in graphene (4 meV) and in the two-dimensional electron gas of conventional III-V and II-VI group semiconductors (30 meV). The most interesting thing is that the electrons in this semiconductor are predicted to be fully out of plane due to the two-dimensional nature of the electron motion and the potential gradient asymmetry, and have a very long spin lifetime due to the suppression of the Dyakonov-Perel spin relaxation. Further theoretical study also pointed out that the spin Hall effect may be observed in this system. Whether can one realize the effective control of the spin degree of freedom in single layer MoS2 has become the key issue in this field. In this proposal, we will deeply investigate the spin-orbit coupling-induced spin-polarized transport phenomena, such as the injection of the spin polarized current, the detection of the spin coherence length, the observation of the spin Hall effect, and the control of the spin polarized transport, etc., in single layer MoS2. Relevant experimental results will provide important basis for the design of the MoS2-based spin polarized transport devices.

随着微纳电子技术的不断深化，集成电路的进一步高密度化和器件的多功能化成为发展的必然趋势，二维材料是未来制备纳米器件的首选。单层MoS2晶体管具有高迁移率、高电流开关比率和低能耗的特性，使得单层MoS2成为制备下一代二维电子器件的最具前景的材料之一。在该基器件中同时实现电荷与自旋的操控，具有重要的意义。操控自旋的基础是材料的自旋轨道耦合要比较强。单层MoS2中的自旋轨道耦合达到了160meV，理论预言其电子的自旋还具有面外极化的特性以及较长的自旋弛豫时间，进一步的理论研究还指出在该体系中可能观测到自旋霍尔效应。是否能够在MoS2实现自旋的操控，是目前亟待解决的问题。本课题中，我们将深入的研究MoS2中自旋轨道耦合诱导的相关极化输运现象，包括极化电流的注入、自旋相干长度的测定、自旋霍尔效应的观测以及自旋极化输运的调控等等，为MoS2基自旋极化输运器件的设计提供重要的实验依据。

过渡金属硫族化物材料MoS2是理论认为的最有可能在同一器件中同时实现电荷和自旋操控的材料之一，这对于电子器件的应用具有重要意义。本课题利用机械剥离单晶以及化学气相沉积的办法制备了大面积MoS2等过渡金属氧硫族化物薄膜。采用电子束曝光结合电子束蒸发等微纳米加工手段进行各种原型器件加工。同时，利用微区拉曼与荧光光谱、原子力显微镜、扫描电镜、多功能物性测试系统等分析与表征手段对器件的性能进行了表征。项目实施基本按照原计划进行并做了些微调，获得了如下研究成果：.1）实现了包括MoS2在内的多种过渡金属氧硫族化物层状材料薄层样品的机械剥离制备，以及MoS2的化学气相沉积制备。.2）通过施加应变的方法实现了对MoS2自旋轨道耦合性能的原位调控，最大可以达到17%，这对开发MoS2基器件的功能具有一定意义。.3）完成了多种MoS2基原型器件的制备与表征，包括顶栅场效应结构、底栅场效应结构、双极场效应以及以纳米线为栅极的场效应结构，均实现了对MoS2电学性能的调控。在以铁磁体为输运电极的MoS2基器件中，未观测到自旋极化输运的信号，暗示该材料可能不具备自旋极化的输运能力，这可能与该材料本身的化学稳定性有关。.4）在具有比MoS2更强自旋轨道耦合以及更好化学稳定性的Sr4Ru3O10材料中，通过尺寸受限，实现了对其自旋序的调控，并观测到了“自旋阀”行为。同时，借助研究MoS2的办法，通过改变尺度实现了对VSe2电荷密度波特性的调控，通过施加应变以及电场的办法，实现了对(Fe,Se)Te超导电性的调控。研究发现通过改变层状材料尺度、所处电场、所受应变等均能有效的调控材料的物性，这对于丰富包括MoS2基器件在内的各种器件的功能具有一定意义。.5）在本项目资助下，共发表SCI论文4篇，包括Applied physics Letters两篇，New Journal of Physics和Chinese Physics Letters各一篇。