Thermal Transport and Power Dissipation in Two-Dimensional (2D) Materials and Interfaces
thesisposted on 08.02.2018, 00:00 by Poya Yasaei
Thermal and electrical transport characterization in devices based on two-dimensional (2D) materials have great implications in many areas such as nano- and optoelectronics, and energy generation, conversion, and storage systems. In many of these applications, the performance of the 2D-based devices is hindered by the heterogeneities such as grain boundaries (GBs) and interfaces. In the case of graphene, the effects of GBs on its electrical properties are well investigated. One of the goals of this dissertation is to establish an understanding on how individual graphene GBs affect the overall thermal properties of graphene films. Measurements on individual GBs are performed to identify the correlations between the thermal resistance imposed by the GBs and the crystallographic mismatch across the GB. Benchmarking the experimental results against theoretical predictions allows us to identify the governing mechanisms of the phonon scattering across GBs with different mismatch angles and morphological details. Next, the thermal transport in the through-plane direction is investigated which accounts for a major fraction of power dissipation from hot-spots in 2D-based devices. First, the interfacial thermal transport in graphene and MoS2 monolayers is characterized which may serve as the bottle-neck of dissipation in the through-plane direction. The effects of interface coupling and metal encapsulation are explored on thermal boundary conductance (TBC) across MoS2 and graphene monolayers. A system-level analysis of heat transport in the through-plane direction is also carried out to quantify the thermal dissipation limits in 2D-material-based structures on different technologically-viable substrates, e.g., diamond, aluminum nitride (AlN), sapphire, and silicon with different oxide types/thicknesses. The results highlight the importance of simultaneous optimization of the interfaces and the substrate and provides a route to maximize the heat removal capability of 2D-material-based devices. Another goal of this thesis was to develop a method for reliable fabrication of high-quality lateral interfaces between graphene and MoS2 monolayers for all-2D circuitry applications. The results show that the MoS2-graphene devices exhibit an order of magnitude higher mobility and lower noise metrics compared to the conventional MoS2-metal devices as a result of energy band rearrangement and smaller Schottky barrier height at the contacts.