No description available.

No description available.

Measurements have been made of the X-ray scatter from an MgO single crystal at low temperatures irradiated by an infra-red laser beam of wavelength 10.6μm. The purpose of these measurements was to obtain information about the anharmonic coupling between the lattice vibrations. The modes investigated were the transverse acoustic phonons in the [100] direction, which it has been suggested have anomalously long lifetimes at very low temperatures. The above method was chosen after considering a variety of possible excitation methods including electron-phonon interactions via the piezoelectric effect and the deformation potential. These other processes had to be rejected because they either excited phonons which were unsuitable for X-ray measurement or required pulsed experiments. The dispersion curves for MgO, obtained from a deformable shell model for the lattice were analysed to provide information as to the most likely positions in reciprocal space for phonon population enhancement to occur. The decay routes for the primary phonons excited directly by the infra-red radiation were predicted from the dispersion curves. A major part of the work involved the design and construction of the helium cryostat on which the sample crystal was mounted and kept at as low a temperature as possible. The standard X-ray detection technique of a scintillating crystal-photomultiplier combination was employed and the infra-red beam was produced by a CO2 gas laser with an output power of up to lOW. Because of the nature of the measurements, the results obtained for the change in X-ray scatter by the MgO crystal when the infra-red beam was shone on the crystal required analysis by statistical methods in order to provide the maximum information. It was possible to set an upper limit to the effects found of a few percent, and to compare the orders of magnitude of the coupling for two different processes.

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This dissertation is focused on thermal transport at nanometer scale point and line constrictions and in nanowires with sawtooth surface roughness. To better understand thermal transport at a point contact such as that at the tip-sample junction of a scanning probe microscope, a Non Equilibrium Molecular Dynamics (NEMD) method is employed to calculate the temperature distribution and thermal resistance of a nanoscale point constriction formed between two silicon substrates. The simulation reveals surface reconstruction at the two free silicon surfaces and at the constriction. The radius of the heated zone in the cold substrate approaches a limit of about 20 times the average nearest-neighbor distance of boron doping atoms when the constriction radius (a) is reduced below the inter-dopant distance. The phonon mean free path at the constriction is suppressed by diffuse phonon-surface scattering and phonon-impurity scattering. The MD thermal resistance is close to the ballistic resistance when a is larger than 1 nm, suggesting that surface reconstruction does not reduce the phonon transmission coefficient significantly. When a is 0.5 nm and comparable to the dominant phonon wavelength, however, the NEMD result is considerably lower than the calculated ballistic resistance because bulk phonon dispersion and bulk potential are not longer accurate. The MD thermal resistance of the constriction increases slightly with increasing doping concentration due to the increase in the diffusive resistance. The NEMD method is further employed to calculate the temperature distribution and thermal resistance at nanoscale line constrictions formed between two silicone substrates. Similar to the nano point constriction, the thermal resistance at the nano line constriction is dominated by the ballistic resistance for constriction width in the range of 1 nm to 12 nm. An additional question that this dissertation seeks to answer is whether one can engineer the surface roughness on a nanowire to facilitate phonon backscattering so as to reduce the thermal conductivity below the diffuse surface limit. Monte Carlo simulation is used to show that phonon backscattering can occur at sawtooth surfaces of a silicon nanowire, suppressing the thermal conductivity below the diffuse surface limit. Asymmetric sawtooth nanowire surfaces can further cause phonon rectification, making the axial thermal conductance of the nanowire direction dependent. The phonon backscattering and rectification effects can be employed to enhance the thermoelectric figure of merit of nanowires. / text

A new quantum transport simulator -- Schrödinger Equation Monte Carlo in Three Dimensions (SEMC-3D) -- has been developed for simulating the carrier transport in nanoscale 3D MOSFET geometries. SEMC-3D self-consistently solves: (1) the 1D quantum transport equations derived from the SEMC method with open boundary conditions and rigorous treatment of various scattering processes including phonon and surface roughness scattering, (2) the 2D Schrödinger equations of the device cross sections with close boundary conditions to obtain the spatially varying subband structure along the conduction channel, and (3) the 3D Poisson equation of the whole device. Therefore, SEMC-3D can provide a physically accurate and electrostatically selfconsistent approach to the quantum transport in the subbands of 3D nanoscale MOSFETs. SEMC-3D has been used to simulate Si nanowire (NW) nMOSFETs to both demonstrate the capabilities of SEMC-3D, itself, and to provide new insight into transport phenomena in nanoscale MOSFETs, particularly with regards to interplay among scattering, quantum confinement and transport, and strain. / text