Toufik Sadi

On the correlation of the Auger generated hot electron emission and efficiency droop in III-N light-emitting diodes

Recent experiments presented in by Iveland et al. [Phys. Rev. Lett. 110, 177406 (2013)] demonstrated that hot electron emission from cesiated p-contacts of III-nitride quantum-well (QW) light-emitting diodes (LEDs) coincides with the onset of the efficiency droop. We have carried out Monte Carlo simulations of hot-electron transport in realistic III-N LEDs. The simulations account for the hole population and all relevant electron scattering and recombination processes. We show that Auger recombination generates a significant hot electron population, which is temporarily trapped in the conduction band side-valleys, without decaying completely before reaching the p-contact. The leakage current due to electron overflow and thermal escape from the QWs is shown to have a minimal impact on the droop. We conclude that the experimentally observed hot electrons are created by Auger recombination in QWs, and that the Auger effect as the origin of the droop is the only consistent explanation for the experimental findings of Iveland et al., [Phys. Rev. Lett. 110, 177406 (2013)].

Enhanced light extraction from InGaN/GaN quantum wells with silver gratings

We demonstrate that an extraction enhancement by a factor of 2.8 can be obtained for a GaN quantum well structure using metallic nanostructures, compared to a flat semiconductor. The InGaN/GaN quantum well is inserted into a dielectric waveguide, naturally formed in the structure, and a silver grating is deposited on the surface and covered with a polymer film. The polymer layer greatly improves the extraction compared to a single metallic grating. The comparison of the experiments with simulations gives strong indications on the key role of weakly guided modes in the polymer layer diffracted by the grating.

The Green’s Function Description of Emission Enhancement in Grated LED Structures

Using scattering to improve light extraction from semiconductors is a widely adopted method to increase the efficiency of modern light-emitting devices. Recently, there has also been much interest in the potential emission enhancement provided by the strong coupling between surface plasmons and semiconductor emitters. In this study, we develop a Greens function-based model to describe the emission enhancement and modification in optical properties obtained as a result of scattering and plasmon engineering. The Greens function method is used to answer fundamental questions regarding luminescence enhancement in periodically grated GaN light-emitting structures. The Greens function approach is a very attractive analytical method to studying the emission properties of grated multilayer structures, providing insight beyond numerical solutions. Modeling results from reflectometry measurements of silver-grated GaN structures allows to explain experimentally observed interference features. A discussion regarding the role of periodic grating in enhancing emission from the structures is included.

Effect of plasmonic losses on light emission enhancement in quantum-wells coupled to metallic gratings

Recent experimental work has shown significant luminescence enhancement from near-surface quantum-well (QW) structures using metallic grating to convert surface plasmon (SP) modes into radiative modes. This work introduces a detailed theoretical study of plasmonic losses and the role of SPs in improving light extraction from grated light-emitting QW structures, using the fluctuational electrodynamics method. The method explains experimental results demonstrating emission enhancement, light scattering, and plasmonic coupling in the structures. We study these effects in angle-resolved reflectometry and luminescence setups in InGaN QW structures with silver grating. In contrast to experiments, our model allows direct calculation of the optical losses. The model predicts that the plasmonic coupling and scattering increases light emission by a factor of up to three compared to a flat semiconductor structure. This corresponds to reducing the absorption losses from approximately 93% in the ungrated metallic stru...

Self-consistent electrothermal Monte Carlo simulation of single InAs nanowire channel metal-insulator field-effect transistors

Electron transport and self-heating effects are investigated in metal-insulator field-effect transistors with a single InAs nanowire channel, using a three-dimensional electrothermal Monte Carlo simulator based on finite-element meshing. The model, coupling an ensemble Monte Carlo simulation with the solution of the heat diffusion equation, is carefully calibrated with data from experimental work on these devices. This paper includes an electrothermal analysis of the device basic output characteristics as well the microscopic properties of transport, including current-voltage curves, heat generation and temperature distributions, and electron velocity profiles. Despite the low power dissipation, results predict significant peak temperatures, due to the high power density levels and the poor thermal management in these structures. The extent of device self-heating is shown to be strongly dependent on both device biasing configuration as well as geometry.

Improving Light Extraction From GaN Light-Emitting Diodes by Buried Nano-Gratings

Recent experimental work has demonstrated that the light extraction enhancement due to scattering by a metallic nano-grating in an InGaN/GaN quantum well (QW) structure can be improved significantly by burying the grating in a dielectric, such as polyvinyl alcohol (PVA). In this paper, we employ the fluctuational electrodynamics method to investigate the origin of this improvement and to provide guidelines on how to optimize emission efficiency in these structures. Our results show that metallic grating diffracts efficiently the high-intensity PVA resonances trapped in the structure, because of the large permittivity contrast between the metal and semiconductor, providing the reported exceptional enhancement for s-polarization. We also study the effect of two important physical factors in the enhancement: 1) the thickness of the GaN barrier separating the QW from the grating and 2) the thickness of the InGaN QW. Results reveal that the enhancement efficiency can be maintained even when the QW is not in the near field of the grating, for a QW-grating separation of up to 1 μm. This is in contrast to plasmonic structures, where enhancement strongly decreases as the separation is increased. However, the enhancement factor can also vary strongly at the local level in smaller spatial intervals. Results also show that the enhancement significantly decreases with the QW thickness due to the losses in the QW.

A Survey of Carbon Nanotube Interconnects for Energy Efficient Integrated Circuits

This article is a review of the state-of-art carbon nanotube interconnects for Silicon application with respect to the recent literature. Amongst all the research on carbon nanotube interconnects, those discussed here cover 1) challenges with current copper interconnects, 2) process & growth of carbon nanotube interconnects compatible with back-end-of-line integration, and 3) modeling and simulation for circuit-level benchmarking and performance prediction. The focus is on the evolution of carbon nanotube interconnects from the process, theoretical modeling, and experimental characterization to on-chip interconnect applications. We provide an overview of the current advancements on carbon nanotube interconnects and also regarding the prospects for designing energy efficient integrated circuits. Each selected category is presented in an accessible manner aiming to serve as a survey and informative cornerstone on carbon nanotube interconnects relevant to students and scientists belonging to a range of fields from physics, processing to circuit design.

Bipolar Monte Carlo simulation of electrons and holes in III-N LEDs

Recent measurements have generated a need to better understand the physics of hot carriers in III-Nitride (III-N) lightemitting diodes (LEDs) and in particular their relation to the efficiency droop and current transport. In this article we present fully self-consistent bipolar Monte Carlo (MC) simulations of carrier transport for detailed modeling of charge transport in III-N LEDs. The simulations are performed for a prototype LED structure to study the effects of hot holes and to compare predictions given by the bipolar MC model, the previously introduced hybrid Monte Carlo–drift-diffusion (MCDD) model, and the conventional drift-diffusion (DD) model. The predictions given by the bipolar MC model and the MCDD model are observed to be almost equivalent for the studied LED. Therefore our simulations suggest that hot holes do not significantly contribute to the basic operation of multi-quantum well LEDs, at least within the presently simulated range of material parameters. With the added hole transport simulation capabilities and fully self-constistent simulations, the bipolar Monte Carlo model provides a state-of-the-art tool to study the fine details of electron and hole dynamics in realistic LED structures. Further analysis of the results for a variety of LED structures will therefore be very useful in studying and optimizing the efficiency and current transport in next-generation LEDs.

Advanced physical modeling of SiO x resistive random access memories

We apply a three-dimensional (3D) physical simulator, coupling self-consistently stochastic kinetic Monte Carlo descriptions of ion and electron transport, to investigate switching in silicon-rich silica (SiOx) redox-based resistive random-access memory (RRAM) devices. We explain the intrinsic nature of resistance switching of the SiOx layer, and demonstrate the impact of self-heating effects and the initial vacancy distributions on switching. We also highlight the necessity of using 3D physical modelling to predict correctly the switching behavior. The simulation framework is useful for exploring the little-known physics of SiOx RRAMs and RRAM devices in general. This proves useful in achieving efficient device and circuit designs, in terms of performance, variability and reliability.

Impact of strain on the performance of Si nanowires transistors at the scaling limit: A 3D Monte Carlo/2D poisson schrodinger simulation study

In this work we investigate the correlation between channel strain and device performance in various n-type Si-NWTs. We establish a correlation between strain, gate length and cross-section dimension of the transistors. For the purpose of this paper we simulate Si NWTs with a <;110> channel orientation, four different ellipsoidal channel cross-sections and five gate lengths: 4nm, 6nm, 8nm, 10nm and 12nm. We have also analyzed the impact of strain on drain-induced barrier lowering (DIBL) and the subthreshold slope (SS). All simulations are based on a quantum mechanical description of the mobile charge distribution in the channel obtained from a 2D solution of the Schrödinger equation in multiple cross sections along the current path, which is mandatory for nanowires with such ultra-scale dimensions. The current transport along the channel is simulated using 3D Monte Carlo (MC) and drift-diffusion (DD) approaches.