Investigation of thermal conductivity in silicon nanostructures and gallium nitride films

This dissertation investigates thermal conductivity in nanostructures made of conventional semiconductor materials such as Si and in wide-band gap semiconductors such as GaN. The motivations for this research are the continuous downscaling in conventional electronic devices and applications of GaN-related compounds in high-power density devices. Thermal management of scaled-down electronic devices presents significant difficulties due to increase in power dissipation per unit area and a variety of size effects that complicate thermal transport at nanoscale. Proposed applications of GaN-based devices rely heavily on the possibility of removing the high density of excess heat from the device active area.; A model is developed for phonon heat conduction in a semiconductor nanowire and thin film with lateral dimensions much smaller than the phonon mean free path and approaching the phonon thermal wavelength. The model is based on Callaway's phenomenological theory and Klemens' second-order perturbation theory for phonon scattering rates. The novel addition is the explicit account for the modification of the acoustic phonon dispersion in low-dimensional structures and change in the nonequilibrium phonon distribution due to partially diffuse boundary scattering. Phonon confinement and boundary scattering lead to a significant reduction in the in-plane lattice thermal conductivity in both nanowires and thin films. Inclusion of phonon confinement effects leads to deviation of the thermal conductivity from its bulk value even in the case of purely specular boundary scattering. The observed change in thermal resistance has to be taken into consideration in simulation of deep-sub micron and nanometer-scale devices.; A detailed calculation procedure for the lattice thermal conductivity in wurtzite GaN is also developed. The proposed model is material specific, which explicitly considers the effects of impurities, dopants, and dislocations on thermal conductivity of GaN layers. Dislocations play an important role in limiting room-temperature thermal conductivity when their density is high, e.g., N_D > 1010cm−2. The experimentally observed linear decrease of the thermal conductivity with logarithm of the carrier concentration can be explained by strongly enhanced phonon relaxation on Si dopants. The developed calculation procedure can be used for accurate simulation of self-heating effects in GaN-based devices.