DX Centers and their mitigation in transition metal dichalcogenides

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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.