High-field transport in two-dimensional graphene and molybdenum disulfide

My research focuses on the study of nanoscale transistor physics, particularly that of atomically-thin two-dimensional (2D) crystals such as graphene and molybdenum disulfide (MoS2). The excellent electrical and thermal properties of 2D materials like graphene have attracted much attention for potential applications in integrated-circuit technology. Understanding high-field transport in a semiconductor material is crucial not only from the perspective of fundamental device physics, but also for achieving practical device applications. Unfortunately, many of the early measurements on these materials, especially graphene, were focused on low-temperature and low-field physics. Motivated by the above, we investigate electrical transport in graphene across a wide range of temperatures (including near and above room temperature) up to high electric fields (> 1 V/µm) typical of modern transistors. For our measurements, we carefully engineered test structures to obtain uniform potential and heating along the channel, and we developed simple yet practical models for heating and high-field drift velocity in graphene, including the roles of both temperature and carrier density. We find that transport in supported-graphene devices does not resemble that of ideal graphene, indicating that interactions with the underlying SiO2 play a role in limiting high-field transport. We sought to understand the intrinsic electrical and thermal properties of graphene, by examining devices freely suspended across microscale trenches. We study the coupled electrical and thermal transport in suspended graphene at high-fields, extracting both high-field drift velocity and thermal conductivity at breakdown of graphene up to higher temperatures than previously possible (300-2200 K). We also directly measure the temperature rise due to self-heating in an electrically biased suspended graphene device via Raman thermometry. Lastly, we investigate the electron transport properties of few-layer MoS2 transistors. We observe a strong temperature dependence of low-field mobility as well as strong self-heating effects during high-field operation. Interestingly, we observe high-field negative differential conductance (NDC) at low temperature and high bias. Our high-field electrical measurements, combined with detailed modeling and simulations, allow us to provide insight into the high-energy band structure of MoS2. As the scaling of transistor lengths approaches 10 nm, it becomes necessary to investigate novel materials for future nanoscale electronics. The atomically-thin body of 2D materials makes them robust against short-channel effects, which could enable scaling down to sub-10 nm MOSFET channel lengths. Additionally, these materials may prove useful for new applications that take advantage of their inherent 2D nature