Aaltodoc Repository :: Browsing by Author "Sadi, Toufik"

Browsing by Author "Sadi, Toufik"

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Absorption modeling with FMM, FEM and FDTD
(2019-07-01) Anttu, Nicklas; Mantynen, Henrik; Sadi, Toufik; Matikainen, Antti; Turunen, Jari; Lipsanen, Harri; Department of Electronics and Nanoengineering; Harri Lipsanen Group; Department of Neuroscience and Biomedical Engineering; University of Eastern Finland; Hinzer, Karin; Piprek, Joachim
Absorption modeling is at the core of the design process of nanostructured solar cells and photodetectors. We compare the performance of three of the most popular numerical modeling methods: the Fourier modal method (FMM), the finite element method (FEM) and the finite-difference time-domain (FDTD) method. We find that the numerically most efficient method depends on the geometry of the system, as well as on which physical quantities are needed for further analysis. From our study, we will highlight the optimum choice of method for various current nanostructures. With these guidelines, we enable design optimization that would otherwise be impossible with a suboptimal method choice.
Applied electromagnetic optics simulations for nanophotonics
(American Institute of Physics, 2021-04-07) Anttu, Nicklas; Mäntynen, Henrik; Sorokina, Anastasiia; Turunen, Jari; Sadi, Toufik; Lipsanen, Harri; Department of Electronics and Nanoengineering; Department of Neuroscience and Biomedical Engineering; Harri Lipsanen Group; University of Eastern Finland
Nanophotonics—the science and technology of confining, guiding, and making photons interact with matter at the nanoscale—is an active research field. By varying the geometry and constituent materials, nanostructures allow precise control of the scattering of incident light and tailoring of emitted light. In this Tutorial, we outline the use of the Maxwell equations to model the optical response of nanostructures. This electromagnetic optics approach uses the refractive indices of the constituent materials and the geometry of the nanostructures as input. For most nanostructure geometries, analytical solutions to the Maxwell equations are not available. Therefore, we discuss varying computational methods for solving the equations numerically. These methods allow us to simulate the optical response of nanostructures, as needed for design optimization and analysis of characterization results.
Comparison of absorption simulation in semiconductor nanowire and nanocone arrays with the Fourier modal method, the finite element method, and the finite-difference time-domain method
(IOP Publishing, 2020) Anttu, Nicklas; Mäntynen, Henrik; Sadi, Toufik; Matikainen, Antti; Turunen, Jari; Lipsanen, Harri; Harri Lipsanen Group; Department of Neuroscience and Biomedical Engineering; University of Eastern Finland; Department of Electronics and Nanoengineering
For the design of nanostructured semiconductor solar cells and photodetectors, optics modelling can be a useful tool that reduces the need of time-consuming and costly prototyping. We compare the performance of three of the most popular numerical simulation methods for nanostructure arrays: the Fourier modal method (FMM), the finite element method (FEM) and the finite-difference time-domain (FDTD) method. The difference between the methods in computational time can be three orders of magnitude or more for a given system. The preferential method depends on the geometry of the nanostructures, the accuracy needed from the simulations, whether we are interested in the total, volume-integrated absorption or spatially resolved absorption, and whether we are interested in broadband or narrowband response. Based on our benchmarking results, we provide guidance on how to choose the method.
Comparison of FMM, FEM and FDTD for absorption modeling of nanostructured solar cells and photodetectors
(2019-01-01) Anttu, Nicklas; Mantynen, Henrik; Sadi, Toufik; Matikainen, Antti; Turunen, Jari; Lipsanen, Harri; Department of Electronics and Nanoengineering; Harri Lipsanen Group; Department of Neuroscience and Biomedical Engineering; University of Eastern Finland
We compare FMM, FEM and FDTD for absorption modeling. We discuss optimum choice of modeling method for varying nanostructures, enabling solar cell and photodetector design optimization that would be impossible with a suboptimal method choice.
Current Spreading in Back-Contacted GaInP/GaAs Light-Emitting Diodes
(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2020-03-01) Myllynen, Antti; Sadi, Toufik; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering
The recently proposed diffusion-driven charge transport (DDCT) method can allow a paradigm shift in the design of optoelectronic devices, by changing both the current injection principle and the device structure. The DDCT injection technique is based on the bipolar electron and hole diffusion currents that are used to electrically inject charge carriers into an active region (AR) located outside the p-n junction. In this article, we study an interdigitated back-contacted DDCT-light-emitting diode (LED) based on a GaInP/GaAs double heterojunction (DHJ) structure consisting of lateral heterojunctions (LHJs) located above a uniform AR. The structure uses single-sided electrical injection and is suitable for large-area applications and thin-film devices with near-surface ARs. Our analysis, based on charge transport simulations, suggests that the structure permits more efficient current spreading and lower surface recombination than conventional structures, leading to a very high internal quantum efficiency (IQE) and injection efficiency exceeding 99%. Particularly, we investigate the implications of using the new structure for improving the efficiency of LEDs, bringing them closer to the threshold of electroluminescent cooling (ELC). The results predict an above-unity internal power conversion efficiency for the DDCT-LEDs, substantially exceeding the efficiency of conventional reference devices, highlighting the new possibilities that DDCT devices offer especially for high-power ELC at room temperature.
Diffusion-driven GaInP/GaAs light-emitting diodes enhanced by modulation doping
(Springer New York, 2019-03-01) Myllynen, Antti; Sadi, Toufik; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering
Diffusion-driven charge transport (DDCT) in III–V light-emitting diodes (LEDs) can enable unconventional optoelectronic devices and functionality by fundamentally changing device design and the current injection principle. In our recent study, an AlGaAs/GaAs DDCT–LED consisting of an array of lateral heterojunctions was studied for large-area applications at high powers. Here, we investigate the current spreading and recombination uniformity of a modulation doped GaInP/GaAs DDCT–LED. In particular, we analyze how the background doping of the lower GaInP cladding layer (CL) and the GaAs substrate changes the carrier distribution within the active region of the device. Our charge transport simulations based on the drift-diffusion current and continuity equations predict that modulation doping by a p-doped CL provides much higher recombination uniformity at high powers compared to an n-doped CL. Most importantly, improved current spreading is achieved while maintaining excellent device performance.
Effect of interface recombination on the efficiency of intracavity double diode structures
(SPRINGER, 2019-06-01) Sadi, Toufik; Radevici, Ivan; Kivisaari, Pyry; Casado, Alberto; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering
In the past ten years, there has been significant progress in solid-state optical refrigeration, causing a renewed interest in the possibility of electroluminescent cooling (ELC) in light emitting diodes (LEDs). More recently, our work on III-As based intracavity double diode structures (DDSs) indicates that the threshold for ELC can be reached, in practice, at high powers and 300K if certain non-radiative recombination mechanisms and photodetector (PD) losses are minimized. The studied DDSs consist of a LED, incorporating a high-quality GaAs active layer, optically coupled to a GaAs p-n homojunction PD. Both the LED and PD are integrated in a single device, offering a unique environment for studying ELC. In this paper, we provide a brief overview of the DDS characteristics and investigate the impact of non-radiative interface recombination on the LED, showing how the choice of the barrier layer materials can suppress this effect. We use experimental characterization techniques to calibrate numerical simulations, coupling the drift-diffusion model for charge transport to a photon transport model. To explore the interface effects, we compare DDSs with either GaInP/GaAs or AlGaAs/GaAs double heterojunctions. The results suggest that GaInP barriers allow interface recombination suppression that is sufficient to reach internal cooling in the LED.
Efficient Fully-Coupled Electro-Optical Simulation Framework for Large-Area Planar Device
(2019-07-01) Sadi, Toufik; Casado, Alberto; Radevici, Ivan; Kivisaari, Pyry; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering; Hinzer, Karin; Piprek, Joachim
Ongoing progress in optoelectronic devices necessitates computational tools that self-consistently account for both electronic charge carrier and photon dynamics and interactions. In this paper, we introduce an efficient simulation framework, using the concepts of nonlinear transmission lines, to study fully-coupled charge and photon transport in planar devices. Within the developed framework, the drift-diffusion equations for charge transport are self-consistently coupled with the radiative transfer equation for photon transport and a separate lateral transport model, to obtain a realistic picture of the electro-optical device behaviour. The model allows the detailed study of large-area devices with full access to the wavelength and angle dependent features. It also accounts for photon recycling, providing deeper insight into the complex nature of optical energy transfer and losses in planar multi-layer structures. The efficiency of the framework is illustrated by applying it to study intracavity diode structures, which are intended for exploring high-power electroluminescent cooling in III-V light-emitting diodes.
Electrically-injected III-V diodes for large-area optoelectronics
(2018-12-07) Myllynen, Antti; Sadi, Toufik; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering; Piprek, Joachim; Djurisic, Aleksandra B.
The growing demand for high-power light-emitting diodes (LEDs) for large-area optoelectronic applications has lead to the development of the diffusion driven charge transport (DDCT) technique that provides an unconventional current injection method for planar LED devices. In this work, we have performed the first numerical simulations of an electrically-injected laterally doped heterojunction (LHJ) LED based on the conventional III-As compound semiconductors, utilizing the DDCT method. Our device consists of a GaAs/AlGaAs double heterojunction (DHJ) on a n-GaAs substrate where the lateral pn-junction can be fabricated by a selective area doping process. We employ a numerical model based on the drift-diffusion current and the continuity equations to model charge transport, and also develop fabrication methods for the devices. Our simulation results suggest that the proposed device can work as an ultra-efficient LED that can be used for large-area applications by repeating the simulated unit to form a multi-finger structure.
Electro-Optical Coupling in Double Diode Structures
(American Physical Society, 2023-06) Anttu, Nicklas; Dagytė, Vilgailė; Behaghel, Benoît; Radevici, Ivan; Sadi, Toufik; Kivisaari, Pyry; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering
Alternative types of artificial cooling techniques are of large interest for multiple applications. Here, we develop a framework for studying the role of electro-optical coupling in the analysis of solid-state refrigerators based on electroluminescent cooling (ELC) by combining device measurements with optical simulations. The studied device consists of a light-emitting diode (LED) epitaxially connected to a photodetector (PD) in a double-diode structure (DDS). Previous results of the DDS have indicated that the LED side already operates at conditions corresponding to ELC, but Ohmic losses and imperfect photodetection of the LED light in the PD have prevented observing the effect directly. Here, to break down the detection losses of the DDS, we report on the electro-optical response of the LED and the PD in detail, as well as the role of the spectral coupling from the LED to the PD. We present a detailed framework for combining measurements and simulations of the DDS to gain quantitative insight of the electro-optical response of the LED and PD, as well as the coupling between them, including the analysis of effects that are not directly accessible by standard measurements. The developed approach allows identifying the different photodetection loss mechanisms from the current-voltage and electroluminescence measurements and thereby gives guidance for designs toward a direct demonstration of ELC at practically relevant cooling powers. Somewhat surprisingly, the results show that an imperfect spectral absorption efficiency of the PD, in addition to its below unity quantum efficiency, are together required to explain the previously observed low photodetection efficiency of the DDS even for several microns thick PD structures. In comparison, the LED top mirror introduces only a minuscule drop in photodetection efficiency. Put in plain numbers, our analysis reveals that in the current DDS designs, there is headroom by 14% in the spectral matching between the LED and the PD, 5% in the charge collection efficiency of the PD, and 4% in the efficiency at which photons emitted from the LED reach the PD.
Electroluminescent Cooling in III-V Intracavity Diodes: Efficiency Bottlenecks
(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2019-06-01) Sadi, Toufik; Radevici, Ivan; Kivisaari, Pyry; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering
Recent advances in the photoluminescent cooling of doped glasses provoke the question of whether similar progress is possible in electroluminescent cooling (ELC), and if so, what are the conditions for observing it at high powers. Here, we establish a simulation framework for III-V intracavity double-diode structures (DDSs) intended for studying ELC and introduce and analyze the most relevant figures of merit for the recently measured devices exhibiting the highest reported quantum efficiency of 70%. In essence, the DDSs optically couple a GaInP/GaAs double heterojunction light-emitting diode (LED) and a GaAs p-n homojunction photodetector (PD), integrated as a single device. The modeling framework couples the drift-diffusion charge transport model with a photon transport model and uses our recent experimental measurements for validation and the extraction of important material parameters. Results show that the model can accurately describe the experimental behavior over many orders of magnitude and suggest that the internal efficiency of the LED already exceeds the cooling threshold. Directly observing cooling in the presently studied devices, however, is still hindered by bottlenecks arising from the surface recombination at the LED walls and recombination losses in the PD.
Electroluminescent Cooling in III-V Intracavity Diodes: Practical Requirements
(2018-12) Sadi, Toufik; Radevici, Ivan; Kivisaari, Pyry; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering
Recent studies of electroluminescent cooling (ELC) in III-V structures demonstrate the need to better understand the factors affecting the efficiency of light emission and energy transport in light-emitting diodes (LEDs). In this paper, we establish the physical and operational requirements for reaching the efficiencies needed for observing ELC in the III-V intracavity double-diode structures at high powers. The experimentally validated modeling framework used in this paper, coupling the drift-diffusion charge transport model with a photon transport model, indicates that the bulk properties of the III-V materials are already sufficient for ELC. Furthermore, the results suggest that the bulk power conversion efficiency of the LED in the devices, which allowed the experimentally measured record high coupling quantum efficiency of 70%, already exceeds 115%. However, as shown here, direct observation of ELC by electrical measurements still requires a combination of a more efficient suppression of the nonradiative surface recombination at the LED walls and the reduction of the detection losses in the photodetector of the intracavity structures.
Electroluminescent cooling in intracavity light emitters
(2018-01-01) Sadi, Toufik; Kivisaari, Pyry; Tiira, Jonna; Radevici, Ivan; Haggren, Tuomas; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering
We develop a coupled electronic charge and photon transport simulation model to allow for deeper analysis of our recent experimental studies of intracavity double diode structures (DDSs). The studied structures consist of optically coupled AlGaAs/GaAs double heterojunction light emitting diode (LED) and GaAs p–n-homojunction photodiode (PD) structure, integrated as a single semiconductor device. The drift–diffusion formalism for charge transport and an optical model, coupling the LED and the PD, are self-consistently applied to complement our experimental work on the evaluation of the efficiency of these DDSs. This is to understand better their suitability for electroluminescent cooling (ELC) demonstration, and shed further light on electroluminescence and optical energy transfer in the structures. The presented results emphasize the adverse effect of non-radiative recombination on device efficiency, which is the main obstacle for achieving ELC in III-V semiconductors.
Electroluminescent cooling in intracavity light emitters: modeling and experiments
(Springer Nature, 2017) Sadi, Toufik; Kivisaari, Pyry; Tiira, Jonna; Radevici, Ivan; Haggren, Tuomas; Oksanen, Jani; Neurotieteen ja lääketieteellisen tekniikan laitos; Department of Neuroscience and Biomedical Engineering; Engineered Nanosystems; Perustieteiden korkeakoulu; School of Science
We develop a coupled electronic charge and photon transport simulation model to allow for deeper analysis of our recent experimental studies of intracavity double diode structures (DDSs). The studied structures consist of optically coupled AlGaAs/GaAs double heterojunction light emitting diode (LED) and GaAs p–n-homojunction photodiode (PD) structure, integrated as a single semiconductor device. The drift–diffusion formalism for charge transport and an optical model, coupling the LED and the PD, are self-consistently applied to complement our experimental work on the evaluation of the efficiency of these DDSs. This is to understand better their suitability for electroluminescent cooling (ELC) demonstration, and shed further light on electroluminescence and optical energy transfer in the structures. The presented results emphasize the adverse effect of non-radiative recombination on device efficiency, which is the main obstacle for achieving ELC in III-V semiconductors.
Electroluminescent cooling using double diode structures
(2018-12-07) Sadi, Toufik; Radevici, Ivan; Kivisaari, Pyry; Casado, Alberto; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering; Piprek, Joachim; Djurisic, Aleksandra B.
The progress in optical cooling in recent years is resulting in a renewed interest in electroluminescent (EL) cooling using conventional III-V semiconductor light emitting diodes (LEDs). In this work, we address the limiting factors for observing EL cooling in III-As intracavity double diode structures (DDSs), at high powers at and close to 300K, by using a combination of experimental characterization and physical device models. The studied DDSs incorporate optically-coupled III-As LED and p-n homojunction photodiode (PD) structures, integrated in a single device and providing a favourable environment for EL cooling observation. We employ a modelling framework coupling the drift-diffusion charge transport model to a photon transport model calibrated using measurements on real devices at different temperatures. Results suggest that the bulk properties of the III-V materials are already sufficient for EL cooling.
Geometry Tailoring of Emission from Semiconductor Nanowires and Nanocones
(MDPI AG, 2020-06-01) Anttu, Nicklas; Mäntynen, Henrik; Sorokina, Anastasiia; Kivisaari, Pyry; Sadi, Toufik; Lipsanen, Harri; Harri Lipsanen Group; Department of Neuroscience and Biomedical Engineering; Department of Electronics and Nanoengineering
Semiconductor nanowires are of interest as light emitters in applications such as light-emitting diodes and single-photon sources. Due to the three-dimensional geometry in combination with a size comparable to the wavelength of the emitted light, nanowires have shown strong scattering effects for the emitted light. Here, we demonstrate with electromagnetic modeling that the emission properties of nanowires/nanocones show a complicated dependence on the geometry of the nanowire/nanocone, the shape and position of the emitter region, and the polarization of the emitter. We show that with proper design, the extraction efficiency can close in on 80% for as-grown single nanowires/nanocones. Importantly, we demonstrate how the internal quantum efficiency of the emitter plays a large role in the design process. A considerably different geometry design approach should be undertaken at low and high internal quantum efficiency. Due to the complicated design optimization, we strongly recommend the use of electromagneticmodeling of the emission to give guidance for suitable designs before starting the fabrication and processing of nanowire/nanocone-based light emitters.
Impact of the Effective Mass on the Mobility in Si Nanowire Transistors
(2018-11-28) Medina-Bailon, Cristina; Sadi, Toufik; Nedjalkov, Mihail; Lee, Jaehyun; Berrada, Salim; Carrillo-Nunez, Hamilton; Georgiev, Vihar P.; Selberherr, Siegfried; Asenov, Asen; University of Glasgow; Department of Neuroscience and Biomedical Engineering; Vienna University of Technology; Helsinki School of Economics
In the simulation based research of aggressively scaled CMOS transistors, it is mandatoryto combine advanced transport simulators and quantum confinement effects with atomistic simulations which accurately reproduce the electronic structure at the nanometer scale. This work investigates the impact of cross-section dependent effective masses, obtained from atomistic simulations, on the mobility in Si nanowire transistors (NWTs). For the transport simulations, weuse the Kubo-Greenwood formalism with a set of multisubband phonon, surface roughness, and impurity scattering mechanisms.
Impact of the trap attributes on the gate leakage mechanisms in a 2D MS-EMC nanodevice simulator
(2019-01-01) Medina-Bailon, Cristina; Sadi, Toufik; Sampedro, Carlos; Padilla, Jose Luis; Donetti, Luca; Georgiev, Vihar; Gamiz, Francisco; Asenov, Asen; University of Glasgow; Department of Neuroscience and Biomedical Engineering; University of Granada; Nikolov, Geno; Kolkovska, Natalia; Georgiev, Krassimir
From a modeling point of view, the inclusion of adequate physical phenomena is mandatory when analyzing the behavior of new transistor architectures. In particular, the high electric field across the ultra-thin insulator in aggressively scaled transistors leads to the possibility for the charge carriers in the channel to tunnel through the gate oxide via various gate leakage mechanisms (GLMs). In this work, we study the impact of trap number on gate leakage using the GLM model, which is included in a Multi-Subband Ensemble Monte Carlo (MS-EMC) simulator for Fully-Depleted Silicon-On-Insulator (FDSOI) field effect transistors (FETs). The GLM code described herein considers both direct and trap-assisted tunneling. This work shows that trap attributes and dynamics can modify the device electrostatic characteristics and even play a significant role in determining the extent of GLMs.
Improving the Efficiency of GaInP/GaAs Light Emitters Using Surface Passivation
(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2020-09) Tripathi, Tripurari S.; Radevici, Ivan; Dagyte, Vilgaile; Sadi, Toufik; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering
The quantum efficiency and reliability of III-V semiconductor-based light emitters can be significantly degraded by nonradiative recombination resulting from the presence of harmful surface states at the mesa sidewalls of the devices. Specifically, surface states are expected to substantially affect the possibility to observe electroluminescent cooling (ELC) in III-As light-emitting diodes (LEDs). Here, we confirm the existence of significant surface currents by exploring the effect of ammonium polysulfide [(NH4)(2)S-x] surface passivation on a GaInP/GaAs heterojunction double diode structure (DDS) designed to study the feasibility of ELC. The DDS consists of an LED and a photodiode (PD) within a single device structure, enabling easy monitoring of the photon-mediated thermal energy transport between the LED and the PD, and eliminating challenges associated with light extraction. Our results show that the surface passivation can improve the internal quantum efficiency of the LEDs by more than 10% points under optimal bias conditions.
Influence of photo-generated carriers on current spreading in double diode structures for electroluminescent cooling
(IOP Publishing, 2018) Radevici, Ivan; Tiira, Jonna; Sadi, Toufik; Oksanen, Jani; Neurotieteen ja lääketieteellisen tekniikan laitos; Department of Neuroscience and Biomedical Engineering; Engineered Nanosystems Group; Engineered Nanosystems Group; Perustieteiden korkeakoulu; School of Science
Current crowding close to electrical contacts is a common challenge in all optoelectronic devices containing thin current spreading layers (CSLs). We analyze the effects of current spreading on the operation of the so-called double diode structure (DDS), consisting of a light emitting diode (LED) and a photodiode (PD) fabricated within the same epitaxial growth process, and providing an attractive platform for studying electroluminescent (EL) cooling under high bias conditions. We show that current spreading in the common n-type layer between the LED and the PD can be dramatically improved by the strong optical coupling between the diodes, as the coupling enables a photo-generated current through the PD. This reduces the current in the DDS CSL and enables studying EL cooling using structures that are not limited by the conventional light extraction challenges encountered in normal LEDs. The current spreading in the structures is studied using optical imaging techniques, electrical measurements, simulations, as well as simple equivalent circuit models developed for this purpose. The improved current spreading leads further to a mutual dependence with the coupling efficiency, which is expected to facilitate the process of optimizing the DDS. We also report a new improved value of 63% for the DDS coupling quantum efficiency (CQE).

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