Electrical Spectroscopy of Defect States in Synthetic 2D Materials

Two-dimensional (2D) materials such as graphene and transition metal dichalcogenide (TMDC) are considered as one of the most promising material platforms for future electronic devices, due to their ultra-thin thickness and fascinating electrical and optical properties. Although large-scale production of 2D materials has been realized by chemical vapor deposition (CVD), their applications are limited by the small grain sizes, nonuniform distributions, ubiquitous structural defects and poor control of the vapor rate. These defects can create complex electronic states that can significantly influence the performance of 2D semiconductor devices. This thesis focuses on the development of new solutions to these problems, by optimizing the growth process of metal-organic chemical vapor deposition (MOCVD) to produce large-area high-quality crystals and developing two electrical spectroscopy methods to comprehensively characterize the defect-induced trap states in 2D materials.To this end, the first part of the thesis explores the optimal growth conditions for monolayer TMDCs. By studying the influence of the precursor phase, temperature, types and concentrations of the catalyst, and the reactor dimensionality, we show precise control of the reaction kinetics and the production of desired crystals. First, temperature and NaCl catalyst are observed to jointly promote the lateral growth of single grains. Second, growth confined to space in narrow regions allows the efficient formation of nuclei and balanced reaction-diffusion conditions, which produce highly-aligned MoS2 crystals with lateral sizes up to 300 um. Subsequent studies using a commercial MOCVD system further extend the growth method to applicable scenarios, confirming its great potential for future industrial applications. The second and third part of the thesis examines the electrical response and characteristics of defect-induced band gap states. First, capacitance-voltage measurement is used to provide a rapid and quantitative analysis of the band-tail states induced by sulfur vacancies. The work then focuses on the development of a sensitive and powerful approach, deep-level transient spectroscopy, for comprehensive defect characterizations in 2D materials. Discrete defect states and their hybridization, respectively induced by isolated sulfur vacancy and their neighboring pairs, are revealed in a monolayer MOCVD-grown material deposited on CMOS-compatible substrates, which can induce an outsize effect on the off currents and switching slopes of field-effect transistors based on 2D semiconductors.