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We present the thermoelectric properties of TiN/MgO superlattices employing first-principles calculation techniques. The Seebeck coefficients, the electrical conductances, the thermal conductances, and the figure of merit are investigated employing electrical and thermal transport calculations based on density functional theory combined with the nonequilibrium Green's function and nonequilibrium molecular dynamics simulation methods. The TiN/MgO superlattices with a small lattice mismatch at the interfaces are ideal systems to study the way for an enhancement of thermoelectric properties in artificial nanostructures. We find that the interfacial scattering between the two materials in the metal/insulator superlattices causes the electrical conductance to change rapidly, which enhances the Seebeck coefficient significantly. We show that the figure of merit for the artificial superlattice nanostructures has a much larger value compared with that of the bulk material and changes drastically with the superlattice configurations at the atomistic level. [ABSTRACT FROM AUTHOR]

We present an efficient computation technique for ab-initio electron transport calculations based on density functional theory and the nonequilibrium Green's function formalism for application to heterostructures with two-dimensional (2D) interfaces. The computational load for constructing the Green's functions, which depends not only on the energy but also on the 2D Bloch wave vector along the interfaces and is thus catastrophically heavy, is circumvented by parallel computational techniques with the message passing interface, which divides the calculations of the Green's functions with respect to energy and wave vectors. To demonstrate the computational efficiency of the present code, we perform ab-initio electron transport calculations of Al(100)-Si(100)-Al(100) heterostructures, one of the most typicalmetal-semiconductor-metal systems, and show their transmission spectra, density of states (DOSs), and dependence on the thickness of the Si layers. [ABSTRACT FROM AUTHOR]

Nanostructured TiN(001) dot/MgO(001)-4 × 4 and 5 × 5 superlattices based on a repeated slab model have been investigated. The electronic states of TiN(001) dot/MgO(001)-4 × 4 and 5 × 5 superlattices were obtained by using the total-energy pseudopotential method. The internal atomic coordinates in a unit cell were fully relaxed. We consider two types of TiN dot structures as rectangular and rectangular parallelepiped. They all correspond to semiconductor, although relaxed rectangular parallelepiped TiN(001) dot/MgO(001)-2 × 2 and 3 × 3 superlattices correspond to metallicity. A bandgap value of the rectangular TiN(001) dot/MgO(001) superlattice increases with increasing the size of the MgO substrate, although the difference in the band gap between 4 × 4 and 5 × 5 is small at approximately 0.01 eV. Their electronic properties depend on the shape of the TiN dot and the size of the MgO substrate. The bandgap value is corrected by using a generalized density-functional theory. [ABSTRACT FROM AUTHOR]

Various nanostructured TiN(001)/MgO(001) superlattices based on a repeated slab model with a vacuum region have been investigated by the total energy pseudopotential method. They are rectangular and rectangular parallelepiped TiN(001) dot structures on MgO(001)-2×2 and 3×3 substrates. A rectangular TiN(001) structure on a MgO(001)-2×1 substrate has also been calculated. Their detailed electronic and internal lattice properties were investigated systematically. The internal atomic coordinates in a unit cell were fully relaxed. The rectangular TiN(001) structure on the MgO(001)-2×1 superlattice, which is not a dot owing to its periodicity, corresponds to metallicity. The electronic states of relaxed rectangular TiN(001) dot/MgO(001)-2×2 and MgO(001)-3×3 superlattices are semiconducting. All relaxed rectangular parallelepiped TiN(001) dot/MgO(001)-2×2 and MgO(001)-3×3 superlattices correspond to metallicity. The electronic properties depend on the shape of the TiN dot and the size of the MgO substrate. [ABSTRACT FROM AUTHOR]