Thermoelectric transport in silicon nanofilms is investigated using a self-consistent electro-thermal Monte Carlo simulator that couples electron dynamics to a phonon bath with spatially varying temperature. A key novelty of this work is the explicit inclusion of the phonon-drag contribution, implemented by modifying the electron-phonon momentum exchange based on the local deviation of the phonon distribution from equilibrium. The method is validated against bulk silicon data and extended to incorporate rough boundary scattering for both electrons and phonons, yielding excellent agreement with experimental measurements on nanofilms. We also analyze the transient regime and show that a temperature bias produces a slower current response than a voltage bias, although the phonon-drag effect itself tends to accelerate the response. These results demonstrate that the proposed framework provides a powerful tool for predicting both steady-state and time-dependent thermoelectric behavior in semiconductor nanostructures.

Abstract We present a transient, self-consistent single-photon avalanche diodes (SPAD) model that couples an advection-diffusion Monte Carlo transport solver to a finite-element Poisson solver and a passive quench circuit. The method stochastically treats impact ionization via a random-path-length algorithm and captures the avalanche-induced space-charge effect that impacts quenching. Statistical analyses reveal the influence of stochastic fluctuations and device geometry on avalanche and quenching dynamics. We demonstrate the importance of space-charge effect effects in the quenching process and show that our method can simulate micron-scale SPAD devices with nanosecond RC time constants, yielding results consistent with experimental data.

A silicon based nanofilm is examined by using self-consistent electron-phonon transport model. An ensemble Monte Carlo solver for electrons is coupled with a phonon bath that can have a non-uniform temperature. In this simulator, the electron-phonon scattering rates depend on the local temperature and the boundary conditions are also temperature dependent. Using this simulation tool, the thermoelectric properties can be studied, at the microscopic level, in doped semiconductor nanostructures of different sizes and with different types of interfaces. In the present work, the only electronic so-called diffusive Seebeck coefficient of silicon-based nanofilms is investigated as the phonon drag effect is not considered. The influence of the average temperature, temperature gradient, device size, and carrier concentration are investigated.

Thermal conductivity typically decreases with increasing temperature along the three principal crystalline directions, primarily due to enhanced phonon anharmonicity. In this work, we conducted a comprehensive first-principles investigation of thermal transport in crystalline polyethylene by solving the Wigner transport equation, assisted with the stochastic self-consistent harmonic approximation. It is found that the thermal conductivity of crystalline polyethylene decreases along the chain direction, but increases nearly linearly in the out-of-chain directions. This anomalous contrasting behavior stems from the dominance of particle-like transport along the chain and wave-like transport in the out-of-chain directions. The strong anharmonicity facilitates phonon tunneling between high- and low-frequency modes in the out-of-plane directions. Therefore, further enhancement of thermal conductivity in those directions could benefit from increased anharmonicity and the introduction of additional disorder. These findings provide fundamental insights into the thermal transport mechanisms of anisotropic crystalline polymers, offering valuable guidance for rationally engineering their thermal properties.

Si- and Ge-based single-photon-avalanche-diodes (SPAD) are investigated by using self-consistent 3D Monte Carlo simulation, in a mixed-mode approach including the presence of a passive quenching circuit. This approach of transport allows us to capture all stochastic features of carrier transport and SPAD operation, not only for the avalanche triggering but also for the quenching process. Beyond the comparison of Si and Ge devices, we show in particular the strong inverse correlation between avalanche and quenching probabilities when tuning the bias voltage, which highlights the importance to find a tradeoff between these two probabilities for the optimization of SPAD operation.

We present an improved stochastic model for simulating avalanche quenching in Single-Photon Avalanche Diodes (SPADs), incorporating the build-up field effect caused by space-charge accumulation during avalanche events. Built upon a one-dimensional Monte Carlo framework, our model introduces a correction term that accounts for the transient electric field induced by the charge dipole, derived from self-consistent Advection-Diffusion Monte Carlo (ADMC) simulations. This hybrid approach preserves the computational efficiency of fast stochastic methods while capturing key physical effects often neglected in simplified models. The inclusion of the build-up field is shown to impact significantly the simulated voltage and current transients, enabling more accurate prediction of quenching behavior, especially in cases of delayed or failed quenching. Comparison with full ADMC simulations shows strong agreement, validating the accuracy and relevance of the correction scheme. The proposed model provides a practical and scalable solution for SPAD simulation and design, particularly in applications requiring precise timing and high-frequency operation.

Polyacene polymers are promising organic one-dimensional materials for optoelectronic and energy-conversion applications that can host nontrivial topologically insulating phases, without the need of spin-orbit coupling. In this work, by using ab initio methods, we show that in its multibridged configuration, it is possible to tune their topological phase by varying the bridges' number and relative position. Moreover, we show that these multibridged polymers can be described by a stacked Su-Schrieffer-Heeger (SSH) model, where interchain hoppings and next-nearest-neighbor interactions play a crucial role in stabilizing a multitude of nontrivial topologically insulating phases. The generality of this model is such that it can be used to describe any couple of interacting SSH chains.

In recent years, computational approaches which couple density functional theory (DFT)-based description of the electron–phonon and phonon–phonon scattering rates with the Boltzmann transport equation have been shown to obtain the electron and thermal transport characteristics of many 3D and 2D semiconductors in excellent agreement with experimental measurements. At the same time, progress in the DFT-based description of the electron–phonon scattering has also allowed to describe the non-equilibrium relaxation dynamics of hot or photo-excited electrons in several materials, in very good agreement with time-resolved spectroscopy experiments. In the latter case, as the time-resolved spectroscopy techniques provide the possibility to monitor transient material characteristics evolving on the femtosecond and attosecond time scales, the time evolution of photo-excited, nonthermal carrier distributions has to be described. Similarly, reliable theoretical approaches are needed to describe the transient transport properties of devices involving high energy carriers. In this review, we aim to discuss recent progress in coupling the ab initio description of materials, especially that of the electron–phonon scattering, with the time-dependent approaches describing the time evolution of the out-of-equilibrium carrier distributions, in the context of time-resolved spectroscopy experiments as well as in the context of transport simulations. We point out the computational limitations common to all numerical approaches, which describe time propagation of strongly out-of-equilibrium carrier distributions in 3D materials, and discuss the methods used to overcome them.

The progress in DFT-based description of the electron-phonon scattering allowed to describe the relaxation dynamics of hot or photoexcited electrons in several materials, in very good agreement with time-resolved spectroscopy experiments. As hot carriers also start to attract attention in the context of emerging concepts for energy conversion, here we present our first results related to the coupling of ab initio data with device-oriented Monte Carlo simulation methods. We show that DFT-based description of the electron-phonon intervalley scattering in GaAs, coupled with the stochastic Monte Carlo method, allows to describe the energy transfer from electrons to phonons in transient regime, in good agreement with previous time-resolved photoemission experiments.

We study the thermal transport properties of twisted-layer graphite nanofibers. We show that in the presence of a twisted layer, the phonon thermal conductance of a graphite nanofiber of rectangular cross-section varies remarkably with the twist angle and can reach minimum values either at two critical angles theta(1) and theta(2) that conform to the rule theta(1) + theta(2) = 180 degrees or exactly at the angle theta = 90 degrees. A reduction of roughly 50% of the phonon thermal conductance can be achieved in some structures. We unveil that the twisting effect mainly influences the optical modes, more precisely, the breathing modes ZO', and leaving almost unaltered the acoustic ones. The effect is also visible in the higher and more numerous van Hove singularities of the phonon density of states. We also point out that the behavior of the thermal conductance with the twist angle is associated with and domi-nated by the alteration in the overlap area between the twisted and non-twisted layers. The finite-size effect is demonstrated to play an essential role in defining the critical angles at the local minima, where these angles are dependent on the size of the investigated nanofibers, in particular on the proportion between the widths of zigzag and armchair edges. We also analyze the behavior of circular nanofibers where the correlation between the overlap area and the thermal conductance is much smaller.

We present a full-band quantum transport model based on the Empirical Pseudopotential Method (EPM) that includes electron-phonon scattering and Shockley-Read-Hall (SRH) recombination via non-radiative multiphonon relaxation. The model has been used to simulate the current-voltage characteristics of two different Silicon-based devices and it shows that a) in a p-i-n diode, SRH recombination gives the dominant contribution to the OFF-state current of the junction, while phonon scattering is responsible for the degradation of the current in the ON-state; b) the inclusion of SRH recombination in the simulation of an Esaki tunneling diode predicts a reduction of the peak-to-valley current ratio, while phonon scattering enhances indirect tunneling transitions from the valence band in Γ to the conduction band minima in the Δ valleys that are not located along the transport direction.

Diffusion of carriers under high electric field is studied using full-band Monte Carlo simulations. The diffusion of hot carriers is found to be strongly anisotropic with respect to the electric field direction and to be times higher than the one predicted by the Einstein’s law. A new calibration of analytical models is proposed to account for the anisotropy of the diffusion coefficient as well of the impact of the electric field. The effect of the new model is illustrated by studying its impact on the lateral and longitudinal spread of the electronic avalanches that occur in single-photon avalanche diodes, using advection-diffusion Monte Carlo.

We present a first-principles model to study tunnel transistors based on van der Waals heterojunctions of 2D materials in the presence of dissipative mechanisms due to the electron–phonon interaction. To this purpose, we employed a reduced basis set composed of unit-cell restricted Bloch functions computed with a plane wave ab-initio solver and performed self-consistent quantum transport simulations within the non-equilibrium Green’s functions formalism. Phonon scattering was included with specific self-energies making use of the deformation potential approximation for the electron–phonon coupling. Our simulations identify the van der Waals tunnel FET as a promising option to attain high on-state currents at low supply voltages, but also show a strong impact of the phonon scattering on the transport properties of such device in the sub-threshold regime.

A semi-analytical model for studying thermal transport at the nanoscale, able to accurately describe both the effect of out of equilibrium transport and the thermal transfer at interfaces, is presented. Our approach is based on the definition of pseudo local temperatures distinguishing the phonon populations according to the direction of their velocity. This formalism leads to a complete set of equations capturing the heat transfer in nanostructures even in the case of hetero-structures. This model only requires introducing a new intrinsic thermal parameter called ballistic thermal conductance and a geometric one called the effective thermal conductivity. Finally, this model is able to reproduce accurately advanced numerical results of Monte Carlo simulation for phonons in all phonon transport regime: diffusive (as the Fourier heat transport regime is included), ballistic, and intermediate ones even if thermal interface are involved. This formalism should provide new insights in the interpretation of experimental measurements.

A new methodology to accurately simulate the Photon Detection Efficiency and the Jitter tail of SPAD devices is presented. This method first relies on the use of the electric field lines to mimic the carriers' trajectories. A model for impact ionization and avalanche probability is then used on the obtained lines to simulate the probability of avalanche, coupled with the optical absorption, the PDE is then extracted. Finally, an advection-diffusion model is used to simulate the drift and diffusion of carriers within the device, which leads to the timing jitter due to the transport time from the photogeneration spot to the avalanche region. The results obtained numerically are compared with an extensive series of measurements and show a good agreement on a wide variety of device designs.

We present an efficient simulation method for electronic transport and avalanche in single-photon avalanche diodes (SPAD). Carrier transport is simulated in the real space using a particle Monte Carlo approach based on the Fokker–Planck point of view on an advection-diffusion equation, that enables us to reproduce mobility models, including electric fields and doping dependencies. The avalanche process is computed thanks to impact ionization rates implemented using a modified Random Path Length algorithm. Both transport and impact ionization mechanisms are computed concurrently from a statistical point of view, which allows us to achieve a full multi-particle simulation. This method provides accurate simulation of transport and avalanche process suitable for realistic three-dimensional SPADs, including all relevant stochastic aspects of these devices, together with a huge reduction of the computational time required, compared to standard Monte Carlo methods for charge carrier transport. The efficiency of our method empowers the possibility to precisely evaluate SPADs figures of merit and to explore new features that were untrackable by conventional methods. An extensive series of comparisons with experimental data on state-of-the art SPADs shows a very good accuracy of the proposed approach.

Polytype nanowires fabricated in both silicon and germanium are particularly attractive for thermoelectric engineering. In this work, the transport of phonons across polytype heterojunctions such as Si 3C/Si 2H and Ge 3C/Ge 2H is theoretically studied by using a particle Monte Carlo simulation for phonons. Full‐Band dispersions and phonon‐phonon scattering rates are calculated by using the density‐functional theory. Phonon transmission across interfaces is implemented by using a Full‐Band version of the Diffusive Mismatch Model. First, the different transport regimes (diffusive, ballistic, and intermediate) for homogenous 3C and 2H Si and Ge bars are investigated by using the Knudsen number as well as the spectral contributions of the thermal flux. Then, single and double polytype Si and Ge heterostructures are studied. The variation of the interface thermal conductance as a function of the geometric dimension as well as the effects of the spectral distribution of the flux are investigated. This local indicator of the phonon transport regime can be used as a local indicator of the occurrence of the out of equilibrium transport regime. Finally, it is shown that the polytype interfaces exhibit significant thermal resistances and generate an out of equilibrium phonon transport regime around the interface over several nanometers.

A Ge-based single-photon-avalanche-diode (SPAD) is investigated by using self-consistent 3-D Monte Carlo simulation including the presence of a passive quenching circuit. This approach of transport allows us to capture all stochastic features of carrier transport and SPAD operation. We analyze particularly the probabilistic character of the quenching mechanism and its dependence on the parameters (resistance and capacitance) of the passive quenching circuit.

A Full Band Monte Carlo simulator has been developed to consider phonon transmission across interfaces disposed perpendicularly to the heat flux. This solver of the Boltzmann transport equation does not require any assumption on the shape the phonon distribution and can naturally consider all phonon transport regimes from the diffusive to the fully ballistic regime. This simulator is used to study single and double Si/Ge heterostructures from the micrometer scale down to the nanometer scale, i.e. in all phonon transport regime from fully diffusive to ballistic. A methodology to determine the thermal conductivity at thermal interfaces is presented. It is also shown that the different transport regimes are correlated to different spectral contributions of the phonon modes to the heat flux along the devices. This local indicator of the transport regime gives new insights into the out-of-equilibrium phonon transport near the interfaces.

In this work, we present a first-principles study of quantum transport in tunnel FETs based on van der Waals (vdW) heterostructures of transition metal dichalcogenides (TMDs). We focus on 1T-HfSe2 and 1T-SnS2 monolayers to construct a vertical heterostructure with a type-II band alignment. By including dissipative effects due to the electron-phonon interaction, we show that vdW tunnel FETs are highly sensible to the phonon coupling due to polar optical phonons present in TMDs which results in an increased sub-threshold swing (SS) and reduced ON-current. However, vdW TFETs are still able to provide high ON-current values due to the inversion of CB and VB at high V-GS and high inter-valley tunneling.