We propose an original method to quantum mechanically treat anharmonic interactions in the atomistic non-equilibrium Green's function (NEGF) simulation of phonon transport. We demonstrate that the so-called lowest order approximation (LOA), implemented through a rescaling technique and analytically continued by means of the Padé approximants, can be used to accurately model a third-order anharmonicity. Although the paper focuses on a specific self-energy, the method is applicable to a very wide class of physical interactions. We apply this approach to the simulation of the anharmonic phonon transport in realistic Si and Ge nanowires with uniform or discontinuous cross-section. The effect of increasing the temperature above 300 K is also investigated. In all the considered cases, we were able to obtain a good agreement with the routinely adopted self-consistent Born approximation (SCBA), at a remarkably lower computational cost. In the more complicated case of temperatures much higher than the room temperature, we found that the first-order Richardson extrapolation applied to the sequence of the Padé approximants N-1/N results in a significant acceleration of the convergence.

We combine the linearized Boltzmann Transport Equation (LBTE) and quantum transport by means of the Nonequilibrium Green's Functions (NEGF) to simulate single-layer MoS2 and WS2 ultra-scaled transistors with carrier mobilities extracted from experiments. Electron-phonon, charged impurity, and surface optical phonon scattering are taken into account with all necessary parameters derived from ab initio calculations or measurements, except for the impurity concentration. The LBTE method is used to scale the scattering self-energies of NEGF, which only include local interactions. This ensures an accurate reproduction of the measured mobilities by NEGF. We then perform device simulations and demonstrate that the considered transistors operate far from their performance limit (from 50% for MoS2 to 60% for WS2). Higher quality materials and substrate engineering will be needed to improve the situation.

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We propose and numerically investigate, through a multi-scale approach, a tandem solar cell based on a van der Waals heterostructure composed of two monolayers of transition metal dichalcogenides. The electronic connection between the two subcells is obtained via tunneling through the van der Waals heterojunction which is electrostatically controlled by means of a dual-gate. Furthermore, by adjusting the dual-gate voltages, the photocurrents in the two subcells can be matched and the tandem cell performances optimized. Assuming an optimal absorptance, as expected in light-trapping systems, we predict that a power conversion efficiency of 30.7%, largely exceeding that of the single subcells, can be achieved. The proposed design being suitable for other 1 van der Waals heterojunctions, this result shows that it represents a viable option for future high efficiency photovoltaic.

Two-dimensional materials with strong bandstructure anisotropy such as black phosphorus (BP) have been identified as attractive candidates for logic application due to their potential high carrier velocity and large density-of-states. However, perfectly aligning the source-to-drain axis with the desired crystal orientation remains an experimental challenge. In this paper, we use an advanced quantum transport approach from first-principle to shed light on the influence of orientation misalignments on the performance of BP-based field-effect transistors. Both n-and p-type configurations are investigated for six alignment angles, in the ballistic limit of transport and in the presence of electron-phonon and charged impurity scattering. It is found that up to deviations of 50 • from the optimal angle, the ON-state current only decreases by 30%. This behavior is explained by considering a single bandstructure parameter, the effective mass along transport direction.