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Guidelines for the fabrication of nanoscale light-emitting diode arrays (i.e., nanoLED arrays) based on patterned gallium nitride (GaN) with very small dimensions and pitches have been derived in this work. Several challenges during... more
Guidelines for the fabrication of nanoscale light-emitting diode arrays (i.e., nanoLED arrays) based on patterned gallium nitride (GaN) with very small dimensions and pitches have been derived in this work. Several challenges during top-down LED array processing have been tackled involving hybrid etching and polymer-based planarization to yield completely insulated highaspect-ratio LED fin structures and support the creation of p-GaN crossing line contacts, respectively. Furthermore, simulations of the light emission patterns were also performed providing hints for enhancing the device designs. As a result, regardless of the required device processing optimization, the developed nanoLED arrays are expected to offer high potential as novel illumination sources in biomedical imaging and sensing applications (e.g., mini compact microscopes and wearable biological/chemical nanoparticle counters)
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ABSTRACT The aim of this work is to present a consistent model for simulation of organic solar cells (OPV) with a correct description of mobility, density of state, organic-metal contacts, and exciton. We simulate the photoconversion by... more
ABSTRACT The aim of this work is to present a consistent model for simulation of organic solar cells (OPV) with a correct description of mobility, density of state, organic-metal contacts, and exciton. We simulate the photoconversion by means of an integration of the optical and electrical part: light absorption is calculated with a Transfer Matrix Model and the charge transport is computed using Drift Diffusion approach including the effect of energetically disorder materials. Most model parameters are directly taken from experiment. The model is used to study the effect of energetic disordered materials and cell thickness on the performance of the cell in terms of short circuit current, open circuit voltage, and fill factor. Based on the results of this model, it will be possible to design and predict the optimal thickness of OPV toward higher efficiencies.
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We present a study of blue III-nitride light-emitting diodes (LEDs) with multiple quantum well (MQW) and quantum dot (QD) active regions (ARs), comparing experimental and theoretical results. The LED samples were grown by metalorganic... more
We present a study of blue III-nitride light-emitting diodes (LEDs) with multiple quantum well (MQW) and quantum dot (QD) active regions (ARs), comparing experimental and theoretical results. The LED samples were grown by metalorganic vapor phase epitaxy, utilizing growth interruption in the hydrogen/nitrogen atmosphere and variable reactor pressure to control the AR microstructure. Realistic configuration of the QD AR implied in simulations was directly extracted from HRTEM characterization of the grown QD-based structures. Multi-scale 2D simulations of the carrier transport inside the multiple QD AR have revealed a non-trivial pathway for carrier injection into the dots. Electrons and holes are found to penetrate deep into the multi-layer AR through the gaps between individual QDs and get into the dots via their side edges rather than via top and bottom interfaces. This enables a more homogeneous carrier distribution among the dots situated in different layers than among the later...
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A model for exciton formation, dissociation and transport is proposed for the simulation of an electrically pumped polariton laser with a geometry similar to that of a VCSEL and resonant cavity LEDs. We demonstrate how the strain effects... more
A model for exciton formation, dissociation and transport is proposed for the simulation of an electrically pumped polariton laser with a geometry similar to that of a VCSEL and resonant cavity LEDs. We demonstrate how the strain effects and the geometry of the device influence the exciton distribution for a GaN/InGaN laser structure.
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In the last years GaN-based heterostructures have attracted much attention for their application as optoelectronic devices. The strain due to lattice mismatch of the constituent materials plays a crucial role in the behaviour of these... more
In the last years GaN-based heterostructures have attracted much attention for their application as optoelectronic devices. The strain due to lattice mismatch of the constituent materials plays a crucial role in the behaviour of these structures, especially if they are of reduced dimensions, as e.g. nanocolumns. We show an implementation of a new device simulator which accounts for strain-related effects and quantum mechanical properties and couples them with the transport of the quasi-particles in the system. Simulations of an AlGaN/GaN nanocolumn LED are reported as an example.
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Filament-type HfO2-based RRAM has been considered as one of the most promising candidates for future non-volatile memories. Further improvement of the stability, particularly at the "OFF" state, of such devices is mainly... more
Filament-type HfO2-based RRAM has been considered as one of the most promising candidates for future non-volatile memories. Further improvement of the stability, particularly at the "OFF" state, of such devices is mainly hindered by resistance variation induced by the uncontrolled oxygen vacancies distribution and filament growth in HfO2 films. We report highly stable endurance of TiN/Ti/HfO2/Si-tip RRAM devices using a CMOS compatible nanotip method. Simulations indicate that the nanotip bottom electrode provides a local confinement for the electrical field and ionic current density; thus a nano-confinement for the oxygen vacancy distribution and nano-filament location is created by this approach. Conductive atomic force microscopy measurements confirm that the filaments form only on the nanotip region. Resistance switching by using pulses shows highly stable endurance for both ON and OFF modes, thanks to the geometric confinement of the conductive path and filament only ...
Metropolis Monte Carlo simulations are used to construct minimal energy configurations by electrostatic coupling of rotating dipoles associated with each unit cell of a perovskite CH3NH3PbI3 crystal. Short-range antiferroelectric order is... more
Metropolis Monte Carlo simulations are used to construct minimal energy configurations by electrostatic coupling of rotating dipoles associated with each unit cell of a perovskite CH3NH3PbI3 crystal. Short-range antiferroelectric order is found, whereas at scales of 8-10 nm, we observe the formation of nanodomains, strongly influencing the electrostatics of the device. The models are coupled to drift-diffusion simulations to study the actual role of nanodomains in the I-V characteristics, especially focusing on charge separation and recombination losses. We demonstrate that holes and electrons separate into different nanodomains following different current pathways. From our analysis we can conclude that even antiferroelectric ordering can ultimately lead to an increase of photoconversion efficiencies thanks to a decrease of trap-assisted recombination losses and the formation of good current percolation patterns along domain edges.
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Research Interests: Computer Aided Design, Quantum Mechanics, Atomistic Simulation, Carbon Nanotubes, Carbon Nanotube, and 14 moreSemiconductor Devices, Quantum Dots, Multiphysics, Semiconductor Lasers, Multiscale Simulation, Nanostructures, Nanostructure, Optical physics, Device Simulation, Quantum Wires, Optoelectronic Devices, Electrical And Electronic Engineering, Drift-diffusion, and optoelectronic device
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ABSTRACT We report on numerical simulations of InP surface lateral quantum-dot molecules on In0.48Ga0.52 P buffer, using a model strictly derived by experimental results by extrapolation of the molecules shape from atomic force microscopy... more
ABSTRACT We report on numerical simulations of InP surface lateral quantum-dot molecules on In0.48Ga0.52 P buffer, using a model strictly derived by experimental results by extrapolation of the molecules shape from atomic force microscopy images. Our study has been inspired by the comparison of a photoluminescence spectrum of a high-density InP surface quantum dot sample with a numerical ensemble average given by a weighted sum of simulated single quantum-dot spectra. A lack of experimental optical response from the smaller dots of the sample is found to be due to strong inter-dot strain fields, which influence the optoelectronic properties of lateral quantum-dot molecules. Continuum electromechanical, k → · p → bandstructure, and optical calculations are presented for two different molecules, the first composed of two dots of nearly identical dimensions (homonuclear), the second of two dots with rather different sizes (heteronuclear). We show that in the homonuclear molecule the hydrostatic strain raises a potential barrier for the electrons in the connection zone between the dots, while conversely the holes do not experience any barrier, which considerably increases the coupling. Results for the heteronuclear molecule show instead that its dots do not appear as two separate and distinguishable structures, but as a single large dot, and no optical emission is observed in the range of higher energies where the smaller dot is supposed to emit. We believe that in samples of such a high density the smaller dots result as practically incorporated into bigger molecular structures, an effect strongly enforced by the inter-dot strain fields, and consequently it is not possible to experimentally obtain a separate optical emission from the smaller dots.
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ABSTRACT Material layers at electrode/semiconductor interfaces are fundamental for the photovoltaic properties of polymer solar cells. The relationship between open-circuit voltage (VOC) and the work function (φ) of these interface layers... more
ABSTRACT Material layers at electrode/semiconductor interfaces are fundamental for the photovoltaic properties of polymer solar cells. The relationship between open-circuit voltage (VOC) and the work function (φ) of these interface layers is still a matter of debate. Simulations, together with experiments on over more than 20 cell architectures based on P3HT:PC60BM, enabled us to analyze the physical dependence of VOC on φ. In particular, when the work function of the contacts is well inside the gap we observe that performance depends strongly on even small variations of φ. On the other hand, when it approaches the energy levels of the semiconducting polymers, device operation becomes the most efficient and less sensitive to variations in φ. Furthermore, by varying the Gaussian density of states (DOS) of the active blend we were able to show that VOC performance depends significantly also on the DOS. Our study based on the simultaneous variation of transport layers represents a promising method to optimize the design and performance of polymer solar cells. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015.
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We present a multiscale simulation of charge transport in a solid-state dye-sensitized solar cell, where the real morphology between TiO2 and the hole transport material is included.
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ABSTRACT We discuss three different models of switching between the high conductivity and low conductivity state in organic bistable devices (OBD) with embedded nanoparticles. All models assume the same basic mechanism: charge trapping... more
ABSTRACT We discuss three different models of switching between the high conductivity and low conductivity state in organic bistable devices (OBD) with embedded nanoparticles. All models assume the same basic mechanism: charge trapping and de-trapping in metal nanoparticles. We show trapped charges can both induce an increase or a reduction of the total current depending on device configurations. The influence of energy disorder is investigated.
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Research Interests: Finite Element Methods, Computer Aided Design, Modeling, Solar Cell, Dye Sensitized Solar Cells, and 12 moreFinite element method, Comparative Study, Coding, Dye Sensitized Solar cell, Steady state, Implementation, Computational, Circuit Design, Spectrum, Computational electronics, Collection Efficiency, and Drift-diffusion
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ABSTRACT In this work we present a theoretical study of the effect of random alloy fluctuations in a InGaN inclusion embedded in a GaN nanowire (NW) LED on the electronic and optoelectronic properties. The calculations are based on an... more
ABSTRACT In this work we present a theoretical study of the effect of random alloy fluctuations in a InGaN inclusion embedded in a GaN nanowire (NW) LED on the electronic and optoelectronic properties. The calculations are based on an empirical tight-binding (ETB) model, while strain is calculated with a valence force field (VFF) method. Energy gaps distributions are obtained and an optical spectral broadening of the cumulative spectra is found, due to alloy fluctuations. A correlation between ground state transition energies and optical strengths has been found, with Virtual Crystal Approximation (VCA) clearly overestimating random mean results.
ABSTRACT In this work we present a multiscale simulation of a solid state dye sensitized solar cell including the real morphology of the active layer. In order to include the real morphology the device domain is split into two different... more
ABSTRACT In this work we present a multiscale simulation of a solid state dye sensitized solar cell including the real morphology of the active layer. In order to include the real morphology the device domain is split into two different regions: one treated using an effective material approximation and another one using the real structure of the blend. The real morphology has been measured using electron tomography to reconstruct the mesoporous TiO2. The geometry was inserted into a mesher and used to solve a drift-diffusion model using finite element method. The simulation is used to cast light over morphology effects in solid state dye solar cells.
In the last years GaN-based heterostructures have attracted much attention for their application as optoelectronic devices. The strain due to lattice mismatch of the constituent materials plays a crucial role in the behaviour of these... more
In the last years GaN-based heterostructures have attracted much attention for their application as optoelectronic devices. The strain due to lattice mismatch of the constituent materials plays a crucial role in the behaviour of these structures, especially if they are of reduced dimensions, as e.g. nanocolumns. We show an implementation of a new device simulator which accounts for strain-related effects and quantum mechanical properties and couples them with the transport of the quasi-particles in the system. Simulations of an AlGaN/GaN nanocolumn LED are reported as an example.
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... 12:15 - 12~30 TuAI -5 4 - 12 GHz InP HEMT-BASED MMIC LOW-NOISE AMPLIFIER R. Limacher, M. Auf der Maur, H. Meier, A. Megej, A. Orzati, W. Bachtold ... Both, RF and dc probes are thermally anchored to the cold chuck by copper braids. ...
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ABSTRACT Electronic properties of nanoscale materials requires the calculation of eigenvalues and eigenvectors of large matrices. This bottleneck can be overcome by parallel computing techniques or the introduction of faster algorithms.... more
ABSTRACT Electronic properties of nanoscale materials requires the calculation of eigenvalues and eigenvectors of large matrices. This bottleneck can be overcome by parallel computing techniques or the introduction of faster algorithms. In this paper we report a custom implementation of the Lanczos algorithm with simple restart, optimized for graphical processing units (GPU). The whole algorithm has been developed using CUDA and runs entirely on the GPU, with a specialized implementation that spares memory and reduces at most machine-to-device data transfers. Furthermore parallel distribution over several GPUs has been attained using standard message passing interface (MPI). Benchmark calculations performed on a GaN/AlGaN wurtzite quantum dot with up to 600,000 atoms are presented. Empirical tight binding (ETB) model with an sp3d5s∗sp3d5s∗+spin–orbit parametrization has been used to build the system Hamiltonian (H).
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We report on a multiscale simulation approach that includes both macroscopic drift-diffusion current model and atomistic quantum tunneling model. The models are solved together in a self-consistent way inside a single simulation package.... more
We report on a multiscale simulation approach that includes both macroscopic drift-diffusion current model and atomistic quantum tunneling model. The models are solved together in a self-consistent way inside a single simulation package. We compare the high-K gates based on HfO2 and ZrO2 with a SiO2 gate of the same equivalent thickness and show the effect of the tunneling current
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Using solar power is one of the most important challenge of today technology. A big effort is devoted in going beyond traditional semiconductor, especially silicon based, solar cells. A well established and promising technology is... more
Using solar power is one of the most important challenge of today technology. A big effort is devoted in going beyond traditional semiconductor, especially silicon based, solar cells. A well established and promising technology is represented by electrochemical dye solar cells (DSC). Their functioning is a complicated interplay of different parts deeply interconnected which requires a model able to catch
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... Daniele Barettin ∗ , Alessandro Pecchia ∗ , Gabriele Penazzi ∗,‡ , Matthias Auf der Maur ∗ ,Benny Lassen † , Morten Willatzen † , and Aldo di Carlo ∗ ∗ Dept. of Electron. Eng., Univ. ... Rev. B 51, 4940 (1995). [24] LC Lew Yan Voon... more
... Daniele Barettin ∗ , Alessandro Pecchia ∗ , Gabriele Penazzi ∗,‡ , Matthias Auf der Maur ∗ ,Benny Lassen † , Morten Willatzen † , and Aldo di Carlo ∗ ∗ Dept. of Electron. Eng., Univ. ... Rev. B 51, 4940 (1995). [24] LC Lew Yan Voon and LR Ram-Mohan, Phys. Rev. ...
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In this work we present a multiscale method to model self-heating effects in nanostructured devices. While the heating is modeled within the drift-diffusion approximation, the heat dissipation is computed by means of a concurrent coupling... more
In this work we present a multiscale method to model self-heating effects in nanostructured devices. While the heating is modeled within the drift-diffusion approximation, the heat dissipation is computed by means of a concurrent coupling between a Phonon Boltzmann Transport Equation (PBTE) based method and the Fourier model. We develop the way to connect the two models to each other
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ABSTRACT The purpose of this work is to present a complete drift-diffusion model for a dye solar cells (DSCs) and to correlate numerical simulation with experimental efficiency of the cell, stressing the influence of the active layer... more
ABSTRACT The purpose of this work is to present a complete drift-diffusion model for a dye solar cells (DSCs) and to correlate numerical simulation with experimental efficiency of the cell, stressing the influence of the active layer thickness. We focus on two fundamental microscopic parameters, namely, the electron diffusion coefficient and the recombination rate constant, which are extracted by a proper simulation fitting of the experimental IV curves of four different sets of DSCs. Both the conduction band model and the multiple trapping model are considered in the fitting procedure. We show that a given set of parameters is able to fit the behavior of the cell under different illumination conditions. Conversely, parameters need to be varied to fit IV curves of cells with different TiO2 thicknesses. The calculated effective diffusion length show a dependence on the working point and on the model used to simulate the cell. This work, moreover, gives a solid numerical ground for neglecting the electronic drift component of the current.
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ABSTRACT In this work, a random distribution of Indium in a quantum well has been considered to study the effect on the energy gap of a GaN/InGaN/GaN LED device. Monte Carlo sampling technique has been used to generate hundreds of... more
ABSTRACT In this work, a random distribution of Indium in a quantum well has been considered to study the effect on the energy gap of a GaN/InGaN/GaN LED device. Monte Carlo sampling technique has been used to generate hundreds of atomistic model structures of the device active region. In order to calculate pseudomorphic strain and internal deformations of the alloy, a multiscale method combining continuous media elasticity and atomistic valence force field models has been used. A multiphysic quantum/classical simulation coupling driftdiffusion with empirical tight-binding in order to compute the electron and hole states of the system has been performed. The reliable sp3d5s* parametrization has been used in the calculations of the eigenstates. We have found an energy gap difference of 50.9 meV for x (In) = 0.1 molar fraction between using virtual crystal approximation and a random distribution of In which increase with increasing Indium concentration. Moreover, electrons wave function seems to be more sensitive than holes due to Indium fluctuations