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Self-heating effects can significantly degrade the performance in nanoscale devices. We investigate self-heating effects in such devices based on two-dimensional materials using ab-initio techniques. A new algorithm was developed to allow for efficient self-energy computations, achieving a $\sim$500 times speedup. It is found that for the simple case of free-standing MoS$_2$ without explicit metal leads, the self-heating effects do not result in significant performance degradation.

We discuss the effect of the dielectric environment (insulators and metal gates) on electronic transport in two-dimensional (2D) transition metal dichalcogenides (TMD) monolayers. We employ well-known ab initio methods to calculate the low-field carrier mobility in free-standing layers and use the dielectric continuum approximation to extend our study to layers in double-gate structures, including the effects of dielectric screening of the electron-phonon interaction caused by the bottom oxide and the gate insulator, and of scattering with hybrid interface optical-phonon/plasmon excitations (`remote phonon scattering'). We find that the presence of insulators with a high dielectric constant may improve significantly the carrier mobility. However, scattering with the interface hybrid excitations negates this gain and degrades the mobility significantly below its free-standing value. In a double-gate geometry with SiO$_{2}$ as bottom-oxide and various top-gate insulators, we find that the mobility decreases as the top-insulator dielectric constant increases, as expected. However, a high mobility is predicted in the case of the weakly polar hBN, and a mobility much lower than expected is calculated in the case of gate-insulator/TMD/bottom-oxide stacks in which two or more polar materials have optical-phonon with similar resonating frequencies. We also find that the effect of screening by metal gates is noticeable but not particularly strong. Finally, we discuss the effect of the TMD dielectric constant, of the free-carrier density, and of temperature on the transport properties of TMD monolayers., Comment: 22 pages, 13 figures

We present, here, advanced DFT-NEGF techniques that we have implemented in our ATOmistic MOdelling Solver, ATOMOS, to explore transport in novel materials and devices and in particular in van-der-Waals heterojunction transistors. We describe our methodologies using plane-wave DFT, followed by a Wannierization step, and linear combination of atomic orbital DFT, that leads to an orthogonal and non-orthogonal NEGF model, respectively. We then describe in detail our non-orthogonal NEGF implementation including the Sancho-Rubio and electron-phonon scattering within a non-orthogonal framework. We also present our methodology to extract electron-phonon coupling from first principle and include them in our transport simulations. Finally, we apply our methods towards the exploration of novel 2D materials and devices. This includes 2D material selection and the Dynamically-Doped FET for ultimately scaled MOSFETS, the exploration of vdW TFETs, in particular the HfS2/WSe2 TFET that could achieve high on-current levels, and the study of Schottky-barrier height and transport through a metal-semiconducting WTe2/WS2 VDW junction transistor.

Experimental studies on two-dimensional (2D) materials are still in the early stages, and most of the theoretical studies performed to screen these materials are limited to the room-temperature carrier-mobility in the free standing 2D layers. With the dimensions of devices moving towards nanometer-scale lengths, the room-temperature carrier-mobility -- an equilibrium concept -- may not be the main quantity that controls the performance of devices based on these 2D materials, since electronic transport occurs under strong off--equilibrium conditions. Here we account for these non-equilibrium conditions and, for the case of monolayer phosphorene (monolayer black phosphorus), show the results of device simulations for a short channel n-MOSFET, using the Monte Carlo method coupled with the Poisson equation, including full bands and full electron-phonon matrix elements obtained from density functional theory. Our simulations reveal significant intrinsic limitations to the performance of phosphorene as a channel material in nanotransistors.

Over the last few years, $ab~initio$ methods have become an increasingly popular tool to evaluate intrinsic carrier transport properties in 2D materials. The lack of experimental information, and the progress made in the development of DFT tools to evaluate electronic band structures, phonon dispersions, and electron-phonon scattering matrix-elements, have made them a favored choice. However, a large discrepancy is observed in the literature among the $ab~initio$ calculated carrier mobility in 2D materials. Some of the discrepancies are a result of the physical approximations made in calculating the electron-phonon coupling constants and the carrier mobility. These approximations can be avoided by using a sophisticated transport model. However, despite using appropriate transport models, the uncertainty in the reported carrier mobility is still quite large in some materials. The major differences observed between these refined model calculations are the `flavors' of DFT (exchange-correlation functional, pseudopotential, and the effect of spin-orbit coupling) used. Here, considering several monolayer 2H-TMDs as examples, we calculate the low- and high-field transport properties using different `flavors' of DFT, and calculate a range for the electron mobility values. We observe that in some materials the values differ by orders of magnitude (For example, in monolayer WS$_{2}$ the electron low-field mobility varies between 37 cm$^{2}$/(V$\cdot$s) and 767 cm$^{2}$/(V$\cdot$s)). We analyze critically these discrepancies, and try to understand the limitations of the current $ab~initio$ methods in calculating carrier transport properties.

Recent $\textit{ab initio}$ theoretical calculations of the electrical performance of several two-dimensional materials predict a low-field carrier mobility that spans several orders of magnitude (from 26,000 to 35 cm$^{2}$ V$^{-1}$ s$^{-1}$, for example, for the hole mobility in monolayer phosphorene) depending on the physical approximations used. Given this state of uncertainty, we review critically the physical models employed, considering phosphorene, a group V material, as a specific example. We argue that the use of the most accurate models results in a calculated performance that is at the disappointing lower-end of the predicted range. We also employ first-principles methods to study high-field transport characteristics in mono- and bi-layer phosphorene. For thin multi-layer phosphorene we confirm the most disappointing results, with a strongly anisotropic carrier mobility that does not exceed $\sim$ 30 cm$^{2}$ V$^{-1}$ s$^{-1}$ at 300 K for electrons along the armchair direction.