过渡金属硫化物/铁电隧道结的光致极化翻转特性及机理

Along with the accelerating demands for digital data storage in market, the conventional silicon-based data storage technology is approaching the limit when it shrinks to a few nanometers due to quantum effects. Ferroelectric tunnel junction (FTJ) is a promising candidate for the next generation nonvolatile digital memories. It has advantages of ferroelectric memories including non-volatility, rapid switching speed, and good endurance, as well as that of resistive memories such as nondestructive reading and low power consumption. The primary challenge of FTJ is still seeking a design that simultaneously provides high OFF/ON ratio, high data storage density, good retention and endurance, ease of operation, and possible other novel properties or functions. Recent researches reported that replacing the top metal electrode of FTJ by a semiconducting material dramatically increases the OFF/ON ratio by adjusting the ferroelectric film thickness and doping level of the semiconductor. Based on our preliminary observations, using semiconducting 2-dimensional (2D) transition metal dichalcogenides (TMDs) can improve the OFF/ON ratio and achieve multiple resistance states from a single device, i.e. synaptic memory devices. Besides tunable bandgap and good mechanical strength and flexibility, TMDs also exhibit intriguing photoelectronic properties. We found that by illuminating TMD/ferroelectric FTJ with UV light can lead to the polarization switching of the ferroelectric layer, which is a fascinating behavior indicates that FTJ or in general ferroelectric materials can be switched optically in a non-contact or remote way. However, the preliminary studies only demonstrated single direction polarization switching, and the involved mechanisms are not fully understood yet. Therefore, we propose this project to achieve optically-induced polarization switching in both up and down directions, and to increase the optical switching speed. To get an optimized design, it is necessary to investigate the physics and mechanisms of optical effects of both TMDs and ferroelectrics, as well as their interfacial interactions. These results of optical switching will bring more possibilities to the future applications of ferroelectrics and will also be important for the field of 2D materials using ferroelectrics as an electronic modulation tool.

铁电隧道结（FTJ）是一种新型存储概念。它具有铁电存储的非易失性、读写速度快和持久性，以及电阻式存储非破坏性读取和低功耗等优点。近期研究发现用半导体代替FTJ的金属顶电极可以大辐提高开关比。我们的研究表明使用半导体二维过渡金属硫化物（TMD）可以在提高开关比的同时实现单器件多阻态的功能。此外基于TMD丰富的光电特性，我们发现紫外光照可以改变此类FTJ铁电层的极化方向。此现象表明FTJ以及广义上的铁电材料可以不再需要外加电场，而是通过非接触式光照实现极化翻转。但前期的结果仅实现了单方向的极化翻转，而其中的机理也有待研究。因此本项目将通过对不同半导体类型的TMD与铁电界面的研究，一方面实现FTJ的光致双向极化翻转，另一方面提高此极化翻转的速度。当中涉及到的物理现象也是本项目的重要研究内容。研究成果会为未来铁电器件的操作模式提供更广阔的空间，同时对二维材料电子器件领域也具有重要意义。

铁电材料由于其独特的可调节的剩余极化场特性，被大量应用在非易失性存储、光学、感应、制动等众多领域。电场是最常用于调控铁电极化的手段。然而，由于铁电极化强烈受到晶体结构、屏蔽电荷等因素的影响，应力场、光场、化学环境都可以用于铁电畴的调控。本项目主要将铁电隧道结(FTJ)作为研究铁电薄膜光、电、应变多场调控的器件结构。基于过渡金属硫化物(TMD)丰富的半导体和光电特性，我们将其作为FTJ的其中一个电极，通过TMD与铁电薄膜(FE)的界面实现极化调控。本项目主要研究双极性TMD材料MoTe2与铁电薄膜形成的异质结体系。在该结构中，我们首先在顶层MoTe2施加电场，成功实现了底层铁电薄膜的双向极化翻转。在光场作用下，我们发现不同波段的单色可见光仅能激发MoTe2内部载流子并调控MoTe2/FE界面处的屏蔽电荷密度，并不能改变MoTe2/FE界面处的屏蔽电荷类型进而改变铁电极化方向。不同的是，紫外光可同时激发MoTe2以及FE。我们基于MoTe2/FE/SrRuO3异质结，在不同样品中实现了紫外光照下两个方向的极化翻转。由于铁电极化引起的电场，光生载流子可以迁移运动并改变MoTe2/FE界面处的屏蔽电荷浓度，屏蔽电荷类型很大程度上取决于铁电薄膜的内建电场方向。此外，我们也研究了柔性铁电薄膜在内部应变场作用下的畴结构演变，并且在具有一定作用力的探针扫描下可以形成铁电畴的超结构。本项目研究结果进一步加深了在电场、光场以及应变场作用下的铁电薄膜多场耦合效应的理解，为其在神经形态视觉传感以及柔性电子器件中的应用奠定了基础。