Thermal transport by phonons across semiconductor interfaces, thin films, and superlattices.

摘要

Advanced technological devices often contain a high density of semiconductor interfaces. Phonon scattering at these interfaces impedes thermal transport through the device and can adversely affect performance and reliability. To improve device design, accurate phonon transport models are needed. Such models will also allow improvements in the design of semiconductor superlattices (periodic nanostructures containing thin films of alternating species) for thermoelectric energy conversion applications. In this thesis, thermal transport by phonons across silicon- and germanium-based interfaces, thin films, and superlattices is studied using molecular dynamics (MD) simulation and lattice dynamics (LD) calculations.;Insight into the phonon transport across interfaces and thin films is gained by comparing MD-predicted thermal resistances to values calculated theoretically using LD-predicted phonon properties. Using this approach, the phonon distributions on either side of an interface are inferred to deviate from their bulk values. For interfaces with large species mismatch, however, the phonon distributions are well-approximated by the equilibrium distribution. Two regimes for the thickness-dependence of the thin film thermal resistance are identified. For films with thicknesses less than ∼2 nm, the thermal resistance is affected by changes in the allowed vibrational states in the film. For thicker films, the thermal resistance is affected by the presence of phonon-phonon scattering in the film.;The effect of interfacial species mixing on the thermal conductivity of semiconductor superlattices and the link between the superlattice unit cell design and the thermal conductivity are then explored. Adding species mixing to an otherwise perfectly periodic superlattice removes the phonon coherence, which reduces the thermal conductivity and alters its dependence on period length. For a model superlattice system, a new class of unit cell design is found to yield significant reductions (17%) in the thermal conductivity compared to the minimum value predicted for traditional designs containing two layers in the unit cell. The low thermal conductivities are attributed to reductions in both the phonon group velocities and mean free paths in the regime where the phonon transport has both coherent and incoherent qualities.