Ion Irradiation Effects on Two-Dimensional Layered Materials

Abstract

Two-dimensional materials (2D materials) refer to atomically thin layered crystalline materials with strong intralayer bonding and weak interlayer van der Waals bonding. Since the discovery of graphene, a single layer of graphite, in 2004, 2D materials have attracted great attention due to their attractive mechanical, chemical, and electronic properties. Among all 2D materials, semiconducting transition metal dichalcogenides (TMDs), such as MoS2 and WSe2, have shown great promise for next-generation semiconductor devices. As the feature size in the state-of-the-art CMOS technology is approaching its fundamental limit, TMD-based transistors offer a prospect for transistor downsizing and exhibit favorable electronic properties including high on-and-off current ratio, low subthreshold slope, and high mobility. However, for these atomically thin materials, where the ultimate limit of their thickness is a monolayer, it is not fully understood how ion irradiation affects their properties. First, ion radiation can be used as an effective tool to provide controlled modifications to 2D materials so that defect engineering and material functionalization can be realized. Second, due to their reduced size and power consumption, 2D material-based electronics holds promise for space applications, where the intensity of cosmic rays is high. Therefore, understanding radiation effects on 2D materials is crucial for their use in radiation-harsh environments. In this dissertation, for different 2D materials (graphene, WSe2, and MoS2), the impact of ion irradiation on material structure, surface chemistry, and electronic properties was studied by molecular dynamics (MD) simulations and ion irradiation experiments.

First, a comprehensive study of graphene defects induced by proton irradiation was performed by MD simulations. The defect generation probabilities at different proton energies were determined by classical MD simulations and validated by ab-initio MD simulations. It is shown that with increasing proton energy, defect structures transition from single vacancies to a rich variety of defect configurations. The defect probabilities also show large dependence on the proton incident angle, which, combined with the proton energy, can be used to effectively tune the ratios of generation probabilities of different defect structures. Next, X-ray photoelectron spectroscopy was used to study the impact of ion irradiation on WSe2 chemical and electronic properties. With 2-MeV protons, no detectable oxidation was observed even at a high fluence level of 10^17 protons/cm^2. Starting from a fluence of 10^16 protons/cm^2, charge transfer between WSe2 and SiC substrate was observed due to a combination of radiation-induced defects and charge trapping in pre-existing defects. Lastly, the degradation of MoS2 field-effect transistor (FET) electrical performance induced by high-energy protons and helium ions was studied at different fluences. By irradiating individual FET components, the damage to MoS2 and SiO2 dielectric was decoupled. The nuclear stopping power was shown to play an important role in the generation of interface states and structural defects. With 390-keV He ions, degradation of I-V characteristics started to become statistically significant at a fluence level of 10^15 ions/cm^2. Nevertheless, high on-state current and high on-and-off current ratio were still maintained at this high fluence level, indicating a strong radiation resilience of MoS2. MD simulations were also performed to study the defect generation probability and sputtering yields within monolayer MoS2 as a function of the proton and helium ion energy. With increasing ion energy, larger defect size can be produced; however, point defects such as S vacancies are still the dominant defect structures.