In this paper, the diffusivity in suspended monolayer graphene at low and high electric fields is investigated. The knowledge of this quantity and its dependence on the electric field is of primary importance not only for the investigation of the electronic transport properties of this material but also for the development of accurate drift-diffusion models. The results have been obtained by means of an ensemble Monte Carlo simulation. For the calculation of the diffusion coefficient, two different methods are considered, one based on the second central moment and the other one based on the Fourier analysis of velocity fluctuations, which are directly related to the noise behaviour at high frequencies. The diffusion coefficient is analyzed considering both parallel and transversal directions with regard to the applied field. Taking into account the importance of degeneracy in this material, the calculations are properly performed by considering an excess electron population obeying a linearized Boltzmann transport equation, which allows studying in an adequate fashion the diffusivity phenomena. The results show the importance of degeneracy effects at very low fields in which transport is mainly dominated by acoustic phonon scattering. Values of the diffusion coefficient larger than 40 000 cm2/Vs are obtained for a carrier concentration equal to 1012 cm−2. The correlation function of instantaneous velocity fluctuation is explained in terms of the wavevector distribution, and their power spectral density is evaluated in the THz range, showing an important dependence on the applied field and being strongly related to microscopic transport processes.

Knowing the influence of the substrate type on the diffusion coefficient and the momentum relaxation in graphene is of great importance for the development of new device models specifically adapted to the peculiarities of this material. In this work, the influence of surface polar phonons at low and high electric fields is evaluated by means of ensemble Monte Carlo simulations for several types of substrates. The results show that at low fields surface polar phonons have a major role on reducing the scattering time, breaking the correlation of velocity fluctuations and degrading the diffusion coefficient. At high fields the differences with regard to suspended samples in terms of diffusivity and momentum relaxation tend to reduce, providing at the same time larger saturation velocities, particularly for h-BN. PACS numbers: 72.80.Vp, 72.10.Di, 05.10.Ln a) Electronic mail: raulr@usal.es; The following article appeared in Appl. Phys. Lett. 104, 233107 (2014) and may be found at

Silicene is a two-dimensional buckled material with broken horizontal mirror symmetry and Dirac-like dispersion. Under such conditions, flexural acoustic (ZA) phonons play a dominant role. Consequently, it is necessary to consider some suppression mechanism for electron-phonon interactions with long wavelengths in order to reach mobilities useful for electronic applications.In this work, we analyze, by means of an ensemble Monte Carlo simulator, the influence of several possibilities for the description of the effect of ZA phonon damping on electronic transport in silicene. The results show that a hard cutoff situation (total suppression for phonons with a wavelength longer than a critical one), as it has been proposed in the literature, does not yield a realistic picture regarding the electronic distribution function, and it artificially induces a negative differential resistance at moderate and high fields. Sub-parabolic dispersions, on the other hand, may provide a more realistic description in terms of the behavior of the electron distribution in the momentum space, but need extremely short cutoff wavelengths to reach functional mobility and drift velocity values.

In this paper, an ensemble 2D bipolar Monte Carlo simulator is employed for the study of static characteristics, high-frequency response and noise behaviour in a 0.3 µm gate-length n-MOSFET in common source configuration. Short-channel effects, such as velocity overshoot in the pinch-off region, together with the appearance of hot electrons at the drain end of the channel are observed in the static characteristics. Admittance parameters and the small-signal equivalent circuit have been calculated in order to characterize the dynamic response of the device. The use of a bipolar simulator allows one to study the dynamics of both types of carriers simultaneously. While the static results are dominated by the electron transport, the contribution of holes mainly affects the drain-substrate capacitive coupling. The noise behaviour of the simulated MOSFET is also studied (up to 40 GHz) by means of different parameters, such as the spectral densities of the current fluctuations at the drain and gate terminals (and their cross-correlation), normalized α, β and C parameters and NF min . In the saturation regime, due to the presence of hot carriers, an increase in drain and gate noise with respect to the long-channel prediction has been found. Moreover, a stronger correlation between drain and gate noise is observed, especially at low drain current. Induced gate noise is found to play a crucial role in the determination of NF min at high drain currents.

We study, by means of a Monte Carlo simulator, the hot phonon effect on the relaxation dynamics in photoexcited graphene and its quantitative impact as compared to considering an equilibrium phonon distribution. Our multi-particle approach indicates that neglecting the hot phonon effect significantly underestimates the relaxation times in photoexcited graphene. The hot phonon effect is more important for a higher energy of the excitation pulse and photocarrier densities between 1 and 3 × 10 12 cm −2 . Acoustic intervalley phonons play a non-negligible role, and emitted phonons with wavelengths limited up by a maximum (determined by the carrier concentration) induce a slower carrier cooling rate. Intrinsic phonon heating is damped in graphene on a substrate due to additional cooling pathways, with the hot phonon effect showing a strong inverse dependence with the carrier density. * Electronic mail: raulr@usal.es; Copyright 2016 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Appl. Phys. Lett. 108, 043105 (2016) and may be found at http://dx. Graphene features, among other properties, large carrier mobility at room temperature along with gapless linear energy spectra for electrons and holes, that results in a linear optical absorption with virtually no photon wavelength restriction,[1] making it a promising material for the development of a wide range of highly efficient photonic and optoelectronic applications, including those operating in the terahertz range. [2-4] Consequently, an intense effort has been made in the recent years in order to get a good understanding of the carrier dynamics involved during and after photoexcitation [5][6][7][8][9][10][11][12][13][14] from a purely theoretical approach and also from an experimental point of view (pump-probe differential transmission spectroscopy) accompanied by means of various modelling techniques. Right after photoexcitation, an ultrafast thermalization of the carriers takes place driven by Coulomb dual carrier scattering. [13,15,16] Simultaneosly, the carriers partially cool by transferring their energy to the graphene and substrate lattices by means of phonon cascade emissions. [5,8,11,16] As a result, a hot thermal distribution of electrons and holes is achieved in tens of fs. [7,8,10] Later processes involve the cooling of the carriers as a consequence of scattering with phonons in the tail of the energy distribution, at the same time that recombination leads the system towards the full thermodynamic equilibrium. [5,11] Several authors remark the importance that the hot phonon (HP) effect would have on this dynamics. [6,7,17] The ensemble Monte Carlo (EMC) technique has been employed to study the influence of HP on the static transport properties of monolayer graphene under high-field conditions. [18] This method has been proven also to be worthy in the study of ultrafast carrier dynamics in a sub-ps time scale in other m...