Study of Optoelectronic Properties of Two-Dimensional Materials and Mixed-Dimensional Heterostructures through Dielectric Engineering

Abstract

Layered two-dimensional materials (2D) have aroused huge research interest since the breakthrough of graphene. Prominent among them are the transition metal dichalcogenides (TMDs), with distinct optical, electrical, and mechanical properties. While considered as promising candidates for future device applications, they also form the playground for exploring novel physical phenomena. In the 2D limit, TMDs experience reduced dielectric screening, making their electrical and optical properties sensitive to the surrounding environment. In this thesis, we first discuss the synthesis of TMDs using Chemical Vapor deposition (CVD). By choosing MoS2 as a model system, the role of different growth parameters in the synthesis was systematically investigated and the growth conditions were optimized to obtain monolayer MoS2 with centimeter-scale large-area coverage. The optoelectronic transport properties of monolayer MoS2 were then improved by exploiting the sensitivity of TMDs to their surrounding environment. Engineering the local dielectric medium has been demonstrated to be an efficient method of modifying their optoelectronic properties. A two-order enhancement in mobility and improvement in photoresponse times were attained by modulating their surrounding dielectric medium. The strong optical absorption and excellent light-matter interaction in MoS2 are compelling enough to employ them for various optoelectronic applications. But their atomic thickness and the associated reduced dielectric screening results in large exciton binding energy causing an inefficient separation of photogenerated carriers. Forming a p-n junction can solve this problem. The inherent electric field generated at the junction helps separate the photogenerated charge carriers. We, therefore, made a p-n junction between silicon and MoS2, studied the optoelectronic properties, and aimed to enhance their photoresponse. A nearly three-order enhancement in photocurrent was successfully achieved by tuning their local dielectric medium. The photoresponse in these devices can often be limited by non-radiative recombinations, diminishing their quantum efficiency. In the last chapter, through temperature-dependent studies, deep-level defects were identified as a major cause of recombinations limiting the photoresponse. We devised a method to reduce these recombinations by screening the deep-level defects and enhancing the device photoresponse. This thesis demonstrates the importance of the surrounding dielectric medium in determining the performance of TMD devices and illustrates a pathway of using dielectric engineering for the development of 2D materials-based high-efficiency optoelectronic devices.