Simulations of nanowire transistors: Atomistic vs. Effective Mass Models

As device sizes shrink towards the nanoscale, CMOS development investigates alternative structures and devices. Existing CMOS devices will evolve to 3D non-planar devices at nanometer sizes. They will operate under strong confinement and strain, regimes where atomistic effects are important. This work investigates atomistic effects in the transport properties of nanowire devices by using a nearest-neighbor tight binding (TB) model (sp3s*d5-SO) [1] for electronic structure calculation, coupled to a 2D Poisson solver for electrostatics. The 2D cross section of a 3D device is described with an arbitrary geometrical shape such as rectangular, cylindrical and tri-gate/FinFET type of structures (Fig. 1(a-d)) using a finite element mesh. Upon convergence, the ballistic transport characteristics are calculated with a semi-classical ballistic model [2]. Comparisons to the effective mass approach (EM) are discussed. Finally, the nonequilibrium Greens’ function (NEGF) approach is used to obtain the transmission coefficients for nanowires in different orientations. This approach will be deployed on nanoHUB.org as an enhancement of the existing Bandstructure Lab [3]

Numerical study of the thermoelectric power factor in ultra-thin Si nanowires

Low dimensional structures have demonstrated improved thermoelectric (TE) performance because of a drastic reduction in their thermal conductivity, {\kappa}l. This has been observed for a variety of materials, even for traditionally poor thermoelectrics such as silicon. Other than the reduction in {\kappa}l, further improvements in the TE figure of merit ZT could potentially originate from the thermoelectric power factor. In this work, we couple the ballistic (Landauer) and diffusive linearized Boltzmann electron transport theory to the atomistic sp3d5s*-spin-orbit-coupled tight-binding (TB) electronic structure model. We calculate the room temperature electrical conductivity, Seebeck coefficient, and power factor of narrow 1D Si nanowires (NWs). We describe the numerical formulation of coupling TB to those transport formalisms, the approximations involved, and explain the differences in the conclusions obtained from each model. We investigate the effects of cross section size, transport orientation and confinement orientation, and the influence of the different scattering mechanisms. We show that such methodology can provide robust results for structures including thousands of atoms in the simulation domain and extending to length scales beyond 10nm, and point towards insightful design directions using the length scale and geometry as a design degree of freedom. We find that the effect of low dimensionality on the thermoelectric power factor of Si NWs can be observed at diameters below ~7nm, and that quantum confinement and different transport orientations offer the possibility for power factor optimization.Comment: 42 pages, 14 figures; Journal of Computational Electronics, 201

Study of Thermal Properties of Graphene-Based Structures Using the Force Constant Method

The thermal properties of graphene-based materials are theoretically investigated. The fourth-nearest neighbor force constant method for phonon properties is used in conjunction with both the Landauer ballistic and the non-equilibrium Green's function techniques for transport. Ballistic phonon transport is investigated for different structures including graphene, graphene antidot lattices, and graphene nanoribbons. We demonstrate that this particular methodology is suitable for robust and efficient investigation of phonon transport in graphene-based devices. This methodology is especially useful for investigations of thermoelectric and heat transport applications.Comment: 23 pages, 9 figures, 1 tabl