Nanoscale heat conduction with applications in nanoelectronics and thermoelectrics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references.When the device or structure characteristic length scales are comparable to the mean free path and wavelength of energy carriers (electrons, photons, phonons, and molecules) or the time of interest is on the same order as the carrier relaxation time, conventional heat transfer theory is no longer valid. Tremendous progress has been made in the past two decades to understand and characterize heat transfer in nanostructures. However most work in the last decade has focused on heat transfer in simple nanostructures, such as thin films, superlattices and nanowires. In reality, there is a demand to study transport process in complex nanostructures for engineering applications, such as heat transfer in nanoelectronic devices and the thermal conductivity in nanocomposites which consists of nanowires or nanoparticles embedded in a matrix material. Another class of problems which are rich in physics and might be explored for better design of both nanoelectronic devices and energy conversion materials and devices are coupled electron and phonon transport. Experimentally, most past work has been focused on thermal conductivity characterization of various nanostructures and very little has been done on the fundamental transport properties of energy carriers.(cont.) This thesis work contributes to the following aspects of heat transfer, nanoelectronics, and thermoelectrics. 1) Simulation tools are developed for transient phonon transport in multidimensional nanostructures and used to predict the size effect on the temperature rise surrounding a nanoscale heat source, which mimics the heating issue in nano-MOSFETs. 2) Semiconductor nanocomposites are proposed for highly efficient thermoelectric materials development where low thermal conductivity is a blessing for efficiency enhancement. Both the deterministic solution and Monte Carlo simulation of the phonon Boltzmann equation are established to study the size effect on the thermal conductivity of nanocomposites where nanoparticles and nanowires are embedded in a host material. 3) Explored the possibility of creating nonequilibrium conditions between electrons and phonons in thermoelectric materials using high energy flux coupling to electrons through surface plasmons, and thus to develop highly efficient thermoelectric devices.(cont.) 4) Established a sub-pico second optical pump-probe measurement system where a femtosecond laser is employed and explored the possibility of extracting phonon reflectivity at interfaces and the phonon relaxation time in a material, which are the two most fundamental phonon properties for nanoscale energy transport from the pump-probe measurements.by Ronggui Yang.Ph.D