DX Centers and their mitigation in transition metal dichalcogenides

一个完美的半导体是无聊和无用的。不完美和缺陷让半导体变得有趣。了解和控制半导体缺陷的种类和数量是现代电子学的基础。有些缺陷是有害的，有些是有用的。然而，有一种特殊类型的缺陷，称为DX中心，它可以从“好”（使半导体导电性更好）切换到“坏”（使半导体导电性更差），由温度、光照、应力或成分变化触发。过渡金属二硫族化合物（TMDs）是开发下一代计算机芯片和太阳能电池的一种具有巨大技术潜力的新材料体系，对DX中心的研究尚处于起步阶段。如果不能很好地理解和管理，DX中心将成为释放tmd全部潜力的主要障碍。本课题的目标是将对tmd中DX中心的理解和控制从婴儿期推进到成熟期。探索了通过施加高压或改变材料成分来打开或关闭DX中心的方法。结合研究成果，首席研究员还提出了一个教育项目，以激励和准备大学预科学生从事与电子相关的材料工程工作。与大学开放日合作，为中学生的参观创建了一系列动手展览、小项目和实验。技术描述dx中心是1960-80年代在III-V族半导体中发现和解释的一种特殊类型的点缺陷。它们可以在浅供体态和深阱态之间进行光学切换。这两种状态被由局部晶格弛豫引起的能量势垒分开，因此与其他缺陷不同，DX中心的电子不会与宿主材料热平衡，它们的行为是独特的，动力学调节的。DX中心补偿浅层供体，严重降低后者的活化效率。它们还负责动态物理效应，例如可以找到潜在设备应用的持久光电导率。过渡金属二硫族化合物（TMDs）最近成为一类新的材料，提供了传统半导体中没有的互补功能。最近，首席研究小组报告了在MoS2、WS2及其合金中观察到DX中心的证据。这些DX中心被认为是导致tmd掺杂效率低下和不希望出现的滞后现象的原因。阐明原子起源和发现减轻DX中心有害影响的方法对于这些材料的最佳器件利用至关重要。针对这些关键需求，这个项目的目标是理解和控制tmd中的DX中心。在从头计算和合作者合成材料的支持下，研究小组使用了深能级瞬态光谱、光导、二次离子质谱等技术来确定DX中心的能量、密度、结构和行为。采用高压和材料合金化来控制tmd中DX中心的活化。所获得的知识为晶体管和发光器件等缺陷限制的应用以及催化和传感器等缺陷促进的应用提供了定量参考。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical descriptionA perfect semiconductor is boring and useless. It takes imperfections and defects to make semiconductors interesting. Understanding and controlling the types and numbers of defects in semiconductors lay the foundation for modern electronics. Some defects are harmful, and some are useful. However, there is one special type of defect, called DX centers, that can switch from being “good” (making the semiconductor conduct better) to “bad” (making the semiconductor conduct worse), triggered by temperature, light illumination, stress, or change in composition. In transition metal dichalcogenides (TMDs), a new material system of great technological potential for developing next-generation computer chips and solar cells, study of DX centers is at its infancy. If not understood and managed well, DX centers would become a major roadblock to unleashing the full potential of TMDs. The goal of this project is to advance from infancy to maturity the understanding and control of DX centers in TMDs. Methods are explored to turn on or off the DX centers by applying high pressure or altering the material composition. Integrated with the research effort, the principal investigator also proposes an educational project that stimulates and prepares pre-college students for careers in materials engineering pertaining to electronics. A series of hands-on exhibits, small projects, and experiments are created for visits of middle-high school students in partnership with the university open house day.Technical descriptionDX centers are a special type of point defects discovered and explained in group III-V semiconductors in the 1960-80s. They can be optically switched between a shallow donor state and a deep trap state. The two states are separated by an energy barrier arising from local lattice relaxation, hence unlike in other defects, electrons of DX centers do not thermally equilibrate with the host material, and their behavior is uniquely, kinetically regulated. DX centers compensate shallow donors and severely degrade activation efficiency of the latter. They are also responsible for kinetic physical effects such as persistent photoconductivity that could find potential device applications. Transition metal dichalcogenides (TMDs) have recently emerged as a new class of materials that offer complementary functionalities not found in traditional semiconductors. Recently the principal investigator’s group has reported evidence of DX centers observed in MoS2, WS2 and their alloys. These DX centers are expected to be responsible for poor doping efficiency and undesired hysteretic phenomena in TMDs. Elucidation of atomic origin and discovery of ways to mitigate harmful effects of DX centers are essential for optimal device utilization of these materials. Precisely focused on these critical needs, the goal of this project is to understand and control DX centers in TMDs. Supported with ab initio calculations and materials synthesis from collaborators, the research team uses Deep Level Transient Spectroscopy, photoconductivity, secondary ion mass spectrometry and other techniques to determine the energy, density, structure, and behavior of DX centers. High pressure and materials alloying are used to control the activation of DX centers in TMDs. The knowledge gained offers quantitative references for both applications that are limited by defects such as transistors and light emitting devices, as well as applications that are facilitated by defects such as catalysis and sensors.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.