Computers and other current microelectronic devices based on silicon technology contain central processing units that produce excess heat while performing calculations and other tasks. This thermal energy migrates to the surface of the silicon chip and is dissipated by fans or other means (e.g., fluid cooling). However, as engineers seek to increase the transistor count per chip for greater computer power, silicon technology reaches its limit, and a different base material with a different hardware architecture will be needed. Recently, sheets of material have been developed which are only one atomic layer thick and which may function as semiconductors (transition metal dichalcogenides), conductors (graphene), or insulators (boron nitride)?the main elements of any electronic device. Transistors have already been developed by stacking these different atomic layers. However, the power density and tendency for such devices to overheat increase dramatically at such a small scale. Therefore, no practical devices from these materials will be possible until the physics of heat transfer at this scale is better understood and the heat dissipation problem is solved. This proposal seeks to comprehensively study the thermal transport dynamics of stacks of atomic layers of semiconductors, conductors, and insulators representative of future nano-electronic devices. The multidisciplinary team consists of a mechanical engineer, an electrical engineer, a chemist, and two physicists from three different universities. Each university has special programs for involving undergraduate and underrepresented minority students in research. The investigators will engage PhD students and undergraduate minority students as well as high school students in this combined experimental and theoretical study aimed at benefiting society through the development of a comprehensive picture of the dominant contributions and limitations of heat transport in future 2D devices.
The primary goal of this proposal is to provide the community with a deeper understanding of the limits set by the kinetics of dissipation and heat removal through various junctions and interfaces in two-dimensional heterogeneous materials. The team aims to establish a multiscale thermal transport study through a closely coupled combination of material synthesis and device-level experiments, atomic-level characterization, thermal transport and phonon spectroscopy, theoretical modelling, and computational studies. Transformative synthesis/fabrication methods, such as chemical vapor deposition and atomic layer deposition, will be used to produce hetero-structures of interest as fundamental components of any electronic devices. The in-plane and out-of-plane thermal conductivity of the synthesized structures will be measured using a custom-designed electrical thermometry platform. The length scale dependence of thermal conductivity and the mean free path (MFP) distribution of heat-carrying phonons will also be measured by heterodyne transient grating technique. In-situ scanning thermal microscopy characterization will be performed to study self-heating of selected semiconducting 2D materials at their contact metal electrodes under high-power operational conditions. Atomic-resolution scanning transmission electron microscopy will be used to characterize 2D material interfaces and surfaces. Characterization results will guide molecular dynamics simulations to predict the atomic structures of interfaces and grain boundaries and to quantify the effects of small-scale structural variations on thermal transport properties of the materials. Thermal transport, including the full spectrum of phonon MFPs, will be calculated using the phonon Boltzmann transport equations including intrinsic phonon scattering as well as scattering at interfaces. The simulation results and Boltzmann transport calculations of phonon MFP distributions will be benchmarked against the transient grating measurements and device level data to develop a predictive model for thermal dissipation in 2D heterogeneous materials. The proposed research has a transformative potential and is of high relevance across many research fields. The broader impacts of this work will be significant in enabling the design of new classes of 2D heterogeneous materials for future electronics/optoelectronics from a thermal management perspective.
