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Register a new userEfficient calculation of the Greens function in scattering region for electron-transport simulations
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We propose a first-principles method of efficiently evaluating electron-transport properties of very long systems. Implementing the recursive Greens function method and the shifted conjugate gradient method in the transport simulator based on real-space finite-difference formalism, we can suppress the increase in the computational cost, which is generally proportional to the cube of the system length to a linear order. This enables us to perform the transport calculations of double-walled carbon nanotubes~(DWCNTs) with 196,608 atoms. We find that the conductance spectra exhibit different properties depending on the periodicity of doped impurities in DWCNTs and they differ from the properties for systems with less than 1,000 atoms.
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The electronic transport behaviour of materials determines their suitability for technological applications. We develop an efficient method for calculating carrier scattering rates of solid-state semiconductors and insulators from first principles inputs. The present method extends existing polar and non-polar electron-phonon coupling, ionized impurity, and piezoelectric scattering mechanisms formulated for isotropic band structures to support highly anisotropic materials. We test the formalism by calculating the electronic transport properties of 16 semiconductors and comparing the results against experimental measurements. The present work is amenable for use in high-throughput computational workflows and enables accurate screening of carrier mobilities, lifetimes, and thermoelectric power.
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We investigate transport properties of gate-all-around Si nanowires using non-equilibrium Greens function technique. By taking into account of the ionized impurity scattering we calculate Greens functions self-consistently and examine the effects of ionized impurity scattering on electron densities and currents. For nano-scale Si wires, it is found that, due to the impurity scattering, the local density of state profiles loose its interference oscillations as well as is broaden and shifted. In addition, the impurity scattering gives rise to a different transconductance as functions of temperature and impurity scattering strength when compared with the transconductance without impurity scattering.
The wave-function-matching (WFM) technique for first-principles transport-property calculations was modified by So{}rensen {it et al.} so as to exclude rapidly decreasing evanescent waves [So{}rensen {it et al.}, Phys. Rev. B {bf 77}, 155301 (2008)]. However, this method lacks translational invariance of the transmission probability with respect to insertion of matching planes and consistency between the sum of the transmission and reflection probabilities and the number of channels in the transition region. We reformulate the WFM method since the original methods are formulated to include all the generalized Bloch waves. It is found that the translational invariance is destroyed by the overlap of the layers between the electrode and transition regions and by the pseudoinverses used to exclude the rapidly decreasing evanescent waves. We then devise a method that removes the overlap and calculates the transmission probability without the pseudoinverses. As a result, we find that the translational invariance of the transmission probability with respect to insertion of the extra layers is properly retained and the sum of the transmission and reflection probabilities exactly agrees with the number of channels. In addition, we prove that the accuracy in the transmission probability of this WFM technique is comparable with that obtained by the nonequilibrium Greens function method. Furthermore, we carry out the electron transport calculations on two-dimensional graphene sheets embedded with B--N line defects sandwiched between a pair of semi-infinite graphene electrodes and find the dependence of the electron transmission on the transverse momentum perpendicular to the direction of transport.
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