Thermal Transfer in Semiconductor Nano Structures

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Title: Thermal Transfer in Semiconductor Nano Structures
Author: Kong, Byoung-Don
Advisors: Carlton M. Osburn, Committee Member
David E. Aspnes, Committee Member
Robert J. Trew, Committee Member
Ki Wook Kim, Committee Chair
Abstract: We present the results of theoretical investigation of thermal energy transfer in nanoscale semiconductors. The study mainly focuses on the newly discovered nano scale phenomena. First, we investigate near field thermal emission characteristics from semiconductors/vacuum interfaces with resonantly excited surface phonon polaritons and surface plasmon polaritons. All of the studied materials, InP, GaAs, GaN, SiC, and sapphire which support surface phonon polariton excitations, exhibit quasimonochromatic thermal emission symbolized by strong peaks of evanescent modes at well-defined frequencies in the near field that correspond to the appropriate peaks in the density of states for surface phonon polaritons. It is also found that the materials with lower polariton frequencies (e.g., InP and GaAs) generally demonstrate a higher peak spectral energy density compared to those with higher frequencies (e.g., SiC). This trend is maintained over the entire range of temperature (300-–600 K) and the distance from the surface (<10 um) considered in the calculation. The energy density stored in the evanescent peaks, when close to the surface, is estimated to be many orders of magnitude larger than that in the blackbody radiation. Surface plasmon polariton excitations are studied with n-doped GaAs, GaN, and Si. The study shows that the characteristic plasma and surface plasmon polariton resonant frequencies in the interval from 0.3 THz to 10 THz can be controlled with conventional doping densities. All considered demonstrate the spectral energy density in the near field that is several orders of magnitude larger than the blackbody radiation. The strongly resonant surface polariton excitations are also shown to enhance drastically the radiative heat transfer between two semi-infinite surfaces separated by nanometric distances. The possibility of extending spatially coherent emission through a 1-D binary grating is examined based on a rigorous coupled wave analysis. It is shown that spatially coherent thermal emission can be achieved using properly designed grating structures. Thermal emission properties are further investigated with more complex structures in one dimensional photonic crystals using Green's dyadic tensor and the concept of local electromagnetic density of states. The results show that high density near-field energy can be transfered via surface wave coupling across the one dimensional photonic crystals so it can be used as energy transfer mechanism without thermal and electric conduction. To explore future possibilities of active terahertz generation, the stimulated and spontaneous interactions were studied using photon-phonon interaction Hamiltonian and it is shown that there exist stimulated interactions. The energy transfer by the conduction mechanism is also studied with low dimensional crystal structures. We investigate the lattice thermal conductivity of ideal monolayer and bilayer graphene, using calculations from first principles. Our result estimates that the intrinsic thermal conductivity of both is around 2200 W/mK at 300 K, a value close to the one observed theoretically and experimentally in graphite along the basal plane. It also illustrates the expected 1/T dependence at higher temperatures. The minor variation between monolayer and bilayer thermal conductivity suggests that the number of layers may not affect significantly the in-plane thermal properties of these systems. The intrinsic thermal conductivity also appears to be nearly isotropic for graphene.
Date: 2010-04-28
Degree: PhD
Discipline: Electrical Engineering

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