Browsing by Author "Ki Wook Kim, Committee Chair"

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    Nonvolatile Spin Memory based on Diluted Magnetic Semiconductor and Hybrid Semiconductor Ferromagnetic Nanostructures
    (2008-06-27) Enaya, Hani; Salah Bedair, Committee Member; Ki Wook Kim, Committee Chair; Jacqueline Krim, Committee Member; Venna Misra, Committee Member; John Zavada, Committee Member
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    Terahertz Generation in Submicron Nitride-based Semiconductor Devices
    (2009-04-27) Barry, Edwin Allen; Ki Wook Kim, Committee Chair; Ralph C. Smith, Committee Member; Douglas W. Barlage, Committee Member; Robert J. Trew, Committee Member
    In this thesis, the electron dynamics and transport properties of III-nitride semiconductors materials and devices are studied, with an emphasis on their application to the generation high-frequency electromagnetic radiation. Numerical simulation models, including Monte Carlo, drift-diffusion, and thermal diffusion are utilized to model transport in the hot-electron and moderate-field regimes. The Monte Carlo method is first applied to the study of the distribution function and the basic characteristics of hot electrons in III-nitrides under moderate electric fields. It is found that in relatively low fields (of the order of kV/cm) polar-optical phonon emission dominates the electron kinetics giving rise to a spindle-shaped distribution function and an extended portion of a quasisaturation of the current-voltage (I-V) characteristics. The Monte Carlo program developed for the study of the III-nitrides is then extended to include the quantum mechanical spin evolution of electrons in bulk GaAs at room temperature. The spin relaxation time and characteristic decay lengths of spin polarized electrons are determined. Next, the conditions for microwave power generation in a submicrometer GaN diode are investigated. By applying a high-field electron transport model based on the local quasistatic approximation, it is shown that oscillations in GaN diodes can be supported in the terahertz-frequency range near the LSA regime. The shape of the diode voltage and electronic current waveforms are examined in terms of the circuit parameters and operating frequencies over the bandwidth of active generation. Based on a Fourier series analysis of the diode voltage and current, the generated power and dc-to-RF conversion efficiency at the fundamental and the lowest higher harmonic frequencies are estimated. The calculation results clearly indicate that submicrometer GaN diodes (channel doping of $1 imes 10^{17}$ cm$^{-3}$) can achieve large output powers ($>$ 1 W) in the absence of Gunn domain formation, over a wide range of frequencies, near 0.5 terahertz. Finally, conditions for pulsed dc regimes of terahertz power generation are theoretically investigated in a vertical nanoscale $n^+nn^+$ GaN-based diode coupled to an external resonant circuit. A combined electrothermal model is adopted allowing for a detailed analysis of the dynamical local distributions of the electric field, drift velocity, and lattice temperature via self-consistent simulation of the high-field electron transport in the active channel and the thermal transport in the device structure. The main performance parameters including, generation power, efficiency, and operation frequency are determined for stable generation with short pulses of a few ns and a few tens of ns of duty cycle. The presented results can be used for optimization and design of two-terminal GaN-based high-power THz generators for pulsed regime operation.
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    Thermal Transfer in Semiconductor Nano Structures
    (2010-04-28) Kong, Byoung-Don; Carlton M. Osburn, Committee Member; David E. Aspnes, Committee Member; Robert J. Trew, Committee Member; Ki Wook Kim, Committee Chair
    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.