Marc Bescond

A Self-Consistent Full 3-D Real-Space NEGF Simulator for Studying Nonperturbative Effects in Nano-MOSFETs

In this paper, we present a full 3-D real-space quantum-transport simulator based on the Greens function formalism developed to study nonperturbative effects in ballistic nanotransistors. The nonequilibrium Green function (NEGF) equations in the effective mass approximation are discretized using the control-volume approach and solved self-consistently with the Poisson equation in order to obtain the electron and current densities. An efficient recursive algorithm is used in order to avoid the computation of the full Green function matrix. This algorithm, and the parallelization scheme used for the energy cycle, allow us to compute very efficiently the current-voltage characteristic without the simplifying assumptions often used in other quantum-transport simulations. We have applied our simulator to study the effect of surface roughness and stray charge on the ID-VG characteristic of a 6-nm Si-nanowire transistor. The results highlight the distinctly 3-D character of the electron transport, which cannot be accurately captured by using 1-D and 2-D NEGF simulations, or 3-D mode-space approximations.

Full-band study of current across silicon nanowire transistors

The authors report an atomistic study of the ballistic current through silicon nanowire metal-oxide-semiconductor transistors. A self-consistent quantum ballistic transport model is used to calculate the current in gate-all-around nanowire transistors, taking into account the full-band structure of the quantum wire with a sp3 tight-binding approach. The authors demonstrate the occurence of an optimal wire cross section for which the on-state/off-state current ratio is maximum, a result which cannot be obtained in a standard bulk effective mass description.

Three-Dimensional Real-Space Simulation of Surface Roughness in Silicon Nanowire FETs

We address the transport properties of narrow gate-all-around silicon nanowires in the presence of surface-roughness (SR) scattering at the Si/SiO2 interface, considering nanowire transistors with a cross section of 3 times 3 nm2 and gate length of 15 nm. We present transfer characteristics and effective-mobility calculations based on a full 3-D real-space self-consistent Poisson-Schrodinger solver within the nonequilibrium Greens function formalism. The effect of SR is included via a geometrical method consisting in a random realization of potential fluctuations described via an exponential autocorrelation law. The influence on transfer characteristics and on low-field mobility is evaluated by comparison with the clean case and for different values of the root mean square of potential fluctuations. The method allows us to exactly account for mode-mixing and subband fluctuations and to evaluate the effect of SR up to all orders of the interaction. We find that SR scattering is mainly responsible for positive threshold-voltage shift in the low-field regime, whereas SR-limited mobility slowly depends on the linear charge density, showing the inefficiency of mode-mixing scattering mechanism for very narrow wires.

Single donor induced negative differential resistance in silicon n-type nanowire metal-oxide-semiconductor transistors

This work presents a theoretical study of the influence of a single donor on the transport properties of silicon nanowire transistors. Using a three-dimensional self-consistent nonequilibrium Green’s function approach we find that the donor states induce transitions from resonant to antiresonant Breit–Wigner interferences when increasing the gate or drain voltages. Numerical and analytical calculations demonstrate that these interferences strongly degrade the transistor performances but can also generate a remarkable negative differential resistance behavior. The robustness of this phenomenon with respect to a change of the defect position in the channel is an opportunity to develop novel device properties.

Modeling of nanoscale solar cells: The Green's function formalism

Solar cells incorporating nano-structures represent a promising solution to overtake the Schockley-Queisser limit. On the other hand, the non-equilibrium Greens function formalism provides a sound conceptual basis for the development of quantum simulators that are needed for nanoscale devices. While this approach has already been applied to solar cells, it remains unfamiliar to most photovoltaic physicists. In this paper we show the main concepts of this formalism and illustrate it with a simple 1D model of solar cell. This model is applied to a thin film GaAs solar cell. Our investigations permit to show and analyze current flowing in the solar cell at the nanometer scale.

Flexible Photodiodes Based on Nitride Core/Shell p–n Junction Nanowires

A flexible nitride p-n photodiode is demonstrated. The device consists of a composite nanowire/polymer membrane transferred onto a flexible substrate. The active element for light sensing is a vertical array of core/shell p–n junction nanowires containing InGaN/GaN quantum wells grown by MOVPE. Electron/hole generation and transport in core/shell nanowires are modeled within nonequilibrium Green function formalism showing a good agreement with experimental results. Fully flexible transparent contacts based on a silver nanowire network are used for device fabrication, which allows bending the detector to a few millimeter curvature radius without damage. The detector shows a photoresponse at wavelengths shorter than 430 nm with a peak responsivity of 0.096 A/W at 370 nm under zero bias. The operation speed for a 0.3 × 0.3 cm2 detector patch was tested between 4 Hz and 2 kHz. The −3 dB cutoff was found to be ∼35 Hz, which is faster than the operation speed for typical photoconductive detectors and which is compatible with UV monitoring applications.

Modeling of phonon scattering in n-type nanowire transistors using one-shot analytic continuation technique

We present a one-shot current-conserving approach to model the influence of electron-phonon scattering in nano-transistors using the non-equilibrium Greens function formalism. The approach is based on the lowest order approximation (LOA) to the current and its simplest analytic continuation (LOA+AC). By means of a scaling argument, we show how both LOA and LOA+AC can be easily obtained from the first iteration of the usual self-consistent Born approximation (SCBA) algorithm. Both LOA and LOA+AC are then applied to model n-type silicon nanowire field-effect-transistors and are compared to SCBA current characteristics. In this system, the LOA fails to describe electron-phonon scattering, mainly because of the interactions with acoustic phonons at the band edges. In contrast, the LOA+AC still well approximates the SCBA current characteristics, thus demonstrating the power of analytic continuation techniques. The limits of validity of LOA+AC are also discussed, and more sophisticated and general analytic con...

Multiband quantum transport simulations of ultimate p-type double-gate transistors: Effects of hole-phonon scattering

A two-dimensional six-band k·p transport simulator has been developed within the nonequilibrium Green function formalism including hole-phonon interactions in the self-consistent Born approximation. Scattering mechanisms are studied in Si versus Ge double-gate p-type metal-oxide-semiconductor transistors. Although the hole-phonon interaction is larger in Ge, its impact on current characteristics is more important in Si. Indeed we obtain a ON current reduction of, respectively, 22% (Si) and 14% (Ge), due to scattering in 7-nm-long transistors. This result is explained by a higher hole-group velocity in Ge that increases the mean-free-path.

Multiband quantum transport simulations of ultimate p-type double-gate transistors: Influence of the channel orientation

We present a ballistic real-space six-band k.p transport model to study the influence of the channel orientation in double-gate p-type metal-oxide-semiconductor (pMOS) transistors. The six-band k.p Hamiltonian is integrated into a self-consistent two-dimensional ballistic transport simulator based on the nonequilibrium Green’s function formalism. The impact of the transport direction is analyzed as a function of the Si transistor channel length. We show that direct source-drain tunneling strongly degrades the subthreshold behavior in short [110]-oriented transistors. This result contradicts the commonly accepted idea that [110] channel orientation provides the best performances for pMOS devices.

Theoretical comparison of multiple quantum wells and thick-layer designs in InGaN/GaN solar cells

This theoretical work analyzes the photovoltaic effect in non-polar InGaN/GaN solar cells. Our electronic transport model considers quantum behaviors related to confinement, tunneling, electron-phonon, and electron-photon scatterings. Based on this model, we compare a multiple quantum wells cell with its thick-layer counterpart. We show that the structure of multiple quantum wells is a promising design providing better compromise between photon-absorption and electronic transport. This balance is necessary since these two phenomena are shown to be antagonist in nanostructure based solar cells. In these devices, we also show that phonon absorption increases the short-circuit current, while phonon emission reduces the open-circuit voltage.