Predicting intrinsic and interfacial thermal transport in two-dimensional materials
Keywords: Electrical Engineering.

The semiconductor industry has rapidly innovated to meet the increasing needs of computationally demanding applications such as artificial intelligence and high-performance computing. Recent efforts have focused on reducing memory access times and the energy-per-computation by implementing heterogeneous systems-on-a-chip or systems-in-a-package. The natural convergence of these technologies is a monolithic three-dimensional (M3D) integrated circuit (IC). Unfortunately, conventional silicon processes require high temperatures that are incompatible with M3D-IC fabrication. Semiconducting two-dimensional (2D) materials like molybdenum disulfide (MoS2), which have good transistor characteristics and low-temperature processing capabilities, may enable the realization of M3D-ICs. However, MoS2 transistors suffer from self-heating which reduces performance, and high temperatures in M3D-ICs may exacerbate this problem. As such, characterizing the thermal properties of MoS2 is essential to designing MoS2 transistors with optimal performance. This thesis focuses on accurate, atomistic calculations of MoS2 thermal properties with an emphasis on structures where MoS2 is in contact with electrical insulators which, though ubiquitous in practical applications, cause difficulties for thermal measurements. The thesis begins with an extensive review of thermal conductivity (TC) and thermal boundary conductance (TBC) measurements of 2D materials. I examine the structural properties of promising 2D materials, briefly review the physics of relevant thermal properties, and present a comprehensive set of thermal property measurements. For each property, I highlight trends within individual and between multiple 2D materials while also highlighting the weaknesses and gaps in literature. Motivated by MoS2 applications, I present my calculations of the TC of monolayer MoS2 when supported or encased by the common insulator SiO2. Such data are noticeably missing in literature. This work demonstrates how the TC of monolayer MoS2 is substantially degraded when MoS2 is in contact with surrounding materials, as it will be in applications. I also show that, when supported or encased, bilayer MoS2 carries three times more heat than monolayer, a factor that should be considered when designing MoS2 transistors. To gain deeper insights into TC calculations, I compare three methods that calculate the phonon-frequency-dependent TC, one of which I propose for the first time. I demonstrate that the spectral heat current (SHC) method is the most computationally efficient and is best-suited for arbitrary atomic structures. Subsequent frequency-dependent TC calculations of MoS2 on amorphous and crystalline SiO2, AlN, and Al2O3 substrates reveal that contributions from long-wavelength MoS2 phonons, which carry most of the heat in MoS2, are significantly reduced, especially when MoS2 is in contact with amorphous substrates. I demonstrate how inserting an h-BN layer between MoS2 and each substrate can minimize the TC degradation from the substrate. Next, I present my TBC calculations of MoS2, which indicate that the substrate interactions which control the TC and TBC of MoS2 are different, suggesting that both properties can be simultaneously optimized. I then use these application-relevant TCs and TBCs to determine the thermal resistance of an MoS¬2 transistor, showing that a transistor based on a crystalline Al2O3 substrate leads to the lowest thermal resistance and temperature rise. I also demonstrate how TBC is the thermal bottleneck for most MoS2 transistors, especially for those with channel lengths longer than ~150 nm. Finally, I conclude with my thoughts on this work and briefly discuss directions for future research