Phonon Knudsen flow in nanostructured semiconductor systems

We determine the size effect on the lattice thermal conductivity of nanoscale wire and multilayer structures formed in and by some typical semiconductor materials, using the Boltzmann transport equation and focusing on the Knudsen flow effect. For both types of nanostructured systems we find that the phonon transport is reduced significantly below the bulk value by boundary scattering off interface defects and/or interface modes. The Knudsen flow effects are important for almost all types of semiconductor nanostructures but we find them most pronounced in Si and SiC systems due to the very large phonon mean-free paths. We apply and test our wire thermal-transport results to recent measurements on Si nanowires. We further investigate and predict size effects in typical multilayered SiC nanostructures, for example, a doped-SiC/SiC/SiO$_2$ layered structure that could define the transport channel in a nanosize transistor. Here the phonon-interface scattering produces a heterostructure thermal conductivity smaller than what is predicted in a traditional heat-transport calculation, suggesting a breakdown of the traditional Fourier analysis even at room temperatures. Finally, we show that the effective thermal transport in a SiC/SiO$_2$ heterostructure is sensitive to the oxide depth and could thus be used as an in-situ probe of the SiC oxidation progress.Comment: 29 pages, 9 figures. (Submitted to Journal of Applied Physics

Theory for structure and bulk-modulus determination

A new method for direct evaluation of both crystalline structure, bulk modulus B_0, and bulk-modulus pressure derivative B'_0 of solid materials with complex crystal structures is presented. The explicit and exact results presented here permit a multidimensional polynomial fit of the total energy as a function of all relevant structure parameters to simultaneously determine the equilibrium configuration and the elastic properties. The method allows for inclusion of general (internal) structure parameters, e.g., bond lengths and angles within the unit cell, on an equal footing with the unit-cell lattice parameters. The method is illustrated by the calculation of B_0 and B'_0 for a few selected materials with multiple structure parameters for which data is obtained by using first-principles density functional theory.Comment: 7 pages, 2 figures, submitted to Phys. Rev.

Thermal transport in SiC nanostructures

SiC is a robust semiconductor material considered ideal for high-power application due to its material stability and large bulk thermal conductivity defined by the very fast phonons. In this paper, however, we show that both material-interface scattering and total-internal reflection significantly limit the SiC-nanostructure phonon transport and hence the heat dissipation in a typical device. For simplicity we focus on planar SiC nanostructures and calculate the thermal transport both parallel to the layers in a substrate/SiC/oxide heterostructure and across a SiC/metal gate or contact. We find that the phonon-interface scattering produces a heterostructure thermal conductivity significantly smaller than what is predicted in a traditional heat-transport calculation. We also document that the high-temperature heat flow across the metal/SiC interface is limited by total-internal reflection effects and maximizes with a small difference in the metal/SiC sound velocities.Comment: 15 pages, 4 figure

Structure, Bonding and Transport of SiC and Graphitic Systems

Today\u27s development of new electronic devices requires materials that can operate under harsh conditions. One such material is Silicon Carbide (SiC) which exhibits superior properties such as chemical inertness, high durability, high thermal stability, high thermal conductivity, and extreme hardness. Today SiC is recognized as a the most promising wide band-gap semiconductor. It is because of its characteristics. It is well known that the surfaces of SiC can produce well-ordered graphite overlayers at temperatures over 1400$^\circ$. The excellent quality of the resulting overlayer opens up exciting opportunities for growth and study of metallic nanostructures, and benefits studies of graphitic systems including alkali metal--graphite intercalation compounds. Other aspects that make SiC an interesting material is that it exists in more that 200 different forms, so--called polytypes. Thus the SiC materialrepresents a whole class of semiconductors.This thesis presents a broad theoretical study, which includes, among other things, studies and comparisonsof structure, cohesive/formation energies, and elastic response of the hard SiC and of the soft graphitic materials, which are characterized by sparseness in the electron-density distribution. It also contains studiesof the nature of bonding between SiC surfaces and graphene (a single layer of graphite). All studies use a recently developed density functional that includes van der Waals (vdW) forces (vdW-DF), as the main tool of the study, traditional density functional theory (DFT), lacks an account for van der Waals interactions significant in sparse materials. The nature of the bonds in the SiC/graphene system is revealed by analysis of the electronic structure. Size effect on the transport properties in nanostructured SiC and some related materials are investigated and calculated on the basis of the Boltzmann transport equation (BTE) approach. The phonon transport is found to be significantly reduced below the bulk transport value bythe phonon Knudsen effect that arise from boundary scattering of interface defects and/or interface modes

Structure, Bonding and Transport of SiC and Graphitic Systems

Today\u27s development of new electronic devices requires materials that can operate under harsh conditions. One such material is Silicon Carbide (SiC) which exhibits superior properties such as chemical inertness, high durability, high thermal stability, high thermal conductivity, and extreme hardness. Today SiC is recognized as a the most promising wide band-gap semiconductor. It is because of its characteristics. It is well known that the surfaces of SiC can produce well-ordered graphite overlayers at temperatures over 1400$^\circ$. The excellent quality of the resulting overlayer opens up exciting opportunities for growth and study of metallic nanostructures, and benefits studies of graphitic systems including alkali metal--graphite intercalation compounds. Other aspects that make SiC an interesting material is that it exists in more that 200 different forms, so--called polytypes. Thus the SiC materialrepresents a whole class of semiconductors.This thesis presents a broad theoretical study, which includes, among other things, studies and comparisonsof structure, cohesive/formation energies, and elastic response of the hard SiC and of the soft graphitic materials, which are characterized by sparseness in the electron-density distribution. It also contains studiesof the nature of bonding between SiC surfaces and graphene (a single layer of graphite). All studies use a recently developed density functional that includes van der Waals (vdW) forces (vdW-DF), as the main tool of the study, traditional density functional theory (DFT), lacks an account for van der Waals interactions significant in sparse materials. The nature of the bonds in the SiC/graphene system is revealed by analysis of the electronic structure. Size effect on the transport properties in nanostructured SiC and some related materials are investigated and calculated on the basis of the Boltzmann transport equation (BTE) approach. The phonon transport is found to be significantly reduced below the bulk transport value bythe phonon Knudsen effect that arise from boundary scattering of interface defects and/or interface modes