DX Centers and their mitigation in transition metal dichalcogenides

一个完美的半导体是无聊和无用的。半导体之所以有趣，是因为它有缺陷。了解和控制半导体中缺陷的类型和数量为现代电子学奠定了基础。有些缺陷是有害的，有些是有益的。然而，有一种特殊类型的缺陷，称为DX中心，可以从“好”（使半导体导电更好）切换到“坏”（使半导体导电更差），由温度，光照，应力或成分变化触发。在过渡金属二硫属化物（TMD）中，DX中心的研究处于起步阶段，这是一种具有开发下一代计算机芯片和太阳能电池的巨大技术潜力的新材料系统。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。这个项目的目标是推进从婴儿期到成熟的理解和控制的DX中心在TMD。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。与研究工作相结合，首席研究员还提出了一个教育项目，刺激和准备大学预科学生的职业生涯在材料工程有关的电子。与大学开放日合作，为中学生参观创造了一系列实践展览、小项目和实验。技术知识DX中心是20世纪60 - 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。它们可以在浅施主态和深陷阱态之间进行光学切换。这两种状态被局部晶格弛豫产生的能垒分开，因此与其他缺陷不同，DX中心的电子不与主体材料热平衡，并且它们的行为是独特的动力学调节。DX中心补偿浅施主并严重降低后者的激活效率。它们还负责动力学物理效应，如持久的光电导性，可以找到潜在的器件应用。过渡金属二硫属化物（TMD）最近作为一类新材料出现，其提供传统半导体中没有的互补功能。最近，首席研究员小组报告了在MoS 2、WS 2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中掺杂效率差和不期望的滞后现象的原因。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设备利用率至关重要。正是针对这些关键需求，该项目的目标是了解和控制TMD中的DX中心。在合作者的从头计算和材料合成的支持下，研究团队使用深能级瞬态光谱学，光电导，二次离子质谱和其他技术来确定DX中心的能量，密度，结构和行为。高压和材料合金化是用来控制DX中心在TMD中的激活。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剂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NSF的法定使命，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.