Fabio Sacconi

In this work, we present a review of quantum dot (QD) material systems that allow to obtain light emission in the telecom C-band at 1.55 µm. The material systems considered are InAs QDs grown on InP, metamorphic InAs/InGaAs QDs grown on GaAs, InAs/GaSb QDs grown on Si, and InAsN QDs grown on GaAs. In order to provide a quantitative comparison of th...

In the experimental electroluminescence (EL) spectra of light-emitting diodes (LEDs) based on N-polar (In,Ga)N/GaN nanowires (NWs), we observed a double peak structure. The relative intensity of the two peaks evolves in a peculiar way with injected current. Spatially and spectrally resolved EL maps confirmed the presence of two main transitions in...

In the experimental electroluminescence (EL) spectra of light-emitting diodes (LEDs) based on N-polar (In,Ga)N/GaN nanowires (NWs), we observed a double peak structure. The relative intensity of the two peaks evolves in a peculiar way with injected current. Spatially and spectrally resolved EL maps confirmed the presence of two main transitions in...

We propose a design for a semiconductor structure emitting broadband light in the infrared, based on InAsquantum dots(QDs) embedded into a metamorphic step-graded InxGa1−xAs buffer. We developed a model to calculate the metamorphic QD energy levels based on the realistic QD parameters and on the strain-dependent material properties; we validated th...

In this work we present a study of electronic and optoelectronic properties of InGaN/GaN nanowire (NW) light-emitting diodes (LEDs) with a multiscale parametric approach. InGaN alloy bandgap bowing parameters extracted from empirical tight-binding (ETB) calculations are used in continuous level 3D simulation models. Strain and transport calculation...

In this work we present the effect of compositional fluctuations in InGaN/GaN quantum wells (QWs) on their spontaneous emission properties. We show that random alloy fluctuations lead to fluctuations of both the optical matrix elements and the emission energy and that the two quantities are correlated. A qualitatively different behaviour between fl...

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 distrib...

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...

In this article we highlight the necessity of atomistic based, fully quantum mechanical simulation approaches for modern electronic devices and their coupling with classical models. We review different ways of such couplings and provide application examples.

We present atomistic simulations of InGaN quantum disk and quantum well structures considering randomly distributed In atoms. It is shown that the random alloy fluctuations lead to an intrinsic broadening of the optical emission lines with an asymmetric tail towards long wavelengths. The amount of broadening is found to be dependent on In content.

Top-down fabricated GaN nanowires, 250 nm in diameter and with various heights, have been used to experimentally determine the evolution of strain along the vertical direction of 1-dimensional objects. X-ray diffraction and photoluminescence techniques have been used to obtain the strain profile inside the nanowires from their base to their top fac...

In this paper, we present simulation results on the optical and transport properties of InGaN/GaN core-shell nanorod light-emitting diodes. The influence of contact position, surface recombination, and doping configuration on internal quantum efficiency is examined. The qualitative behavior when adding an electron blocking layer and the dependence...

In this work, we use the multiscale software tool TiberCAD to study the electronic and optical properties of InGaN-quantum-disk (QD)-based GaN nanocolumn p-i-n diode structures. Strain maps show a clear relaxation effect close to the column boundaries; however, results from full self-consistent 3-D quantum calculations indicate that emission is foc...

InGaN/GaN nanorod core-shell LEDs have shown to be very promising candidates for high efficiency lighting devices. Such nanorods can be grown in different ways, leading to different device geometry and in particular to different structures near the polar Ga- and N-face nanorod surfaces. In this work the influence of the properties of the polar surf...

In this work we investigate electronic and optoelectronics properties of InGaN/GaN nanocolumn quantum disk LEDs. Calculations have been performed with an atomistic tight-binding model. Results show that emission energies have a minor dependence on the nanocolumn dimension.

In this paper, we present a framework for the simulation of electronic devices based on a multiscale and multiphysics approach. A formal description is provided that includes both multiscale and multiphysics problems and which can be linked to already established multiscale methods. We present a set of simulations of an AlGaN/GaN nanocolumn based o...

In this paper we present a multiscale framework for the simulation of electronic devices allowing the coupling of continuum and atomistic models in a transparent way. We introduce the basic features of the TiberCAD simulation software which is based on the multiscale simulation concept, and we show a simulation example to illustrate the basic aspec...

In this work we use the multi-scale software tool TiberCAD to study the transport and optical properties of InGaN quantum disk (QD) - based GaN nanocolumn p-i-n diode structures. IV characteristics have been calculated for several values of In concentration in the QD and of nanocolumn width. Strain maps show a clear relaxation effect close to the c...

The TiberCAD simulation tool for calculation of optical and electronic properties of nanostructured devices has been used to study spontaneous emission of a GaN quantum dot embedded in an AlGaN nanocolumn. Macroscopic calculations provide corrections to the quantum calculation, showing the role of strain and the polarization field in spectra and th...

Multiscale methods coupling quantum mechanical/atomistic models such as envelope function and tight binding approaches with continuous media models e.g. for strain or electronic transport are very useful for an accurate simulation of modern and emerging electronic and optoelectronic devices based on nanostructured active regions. We present simulat...

Calculations of optoelectronic properties of a GaN quantum dot embedded in an AlGaN nanocolumn are presented, using the TiberCAD simulator. The calculations emphasize the role of the growth direction in determining the quantum efficiency of such light emitting devices. Multiband kldrp is used, with corrections from drift diffusion and strain calcul...

We report the observation of well-resolved A and B excitonic resonances in the reflectivity spectra of a wedged GaN/AlGaN quantum well (QW) with 5% Al content in the barriers for different well thicknesses. For the thicker well cases, the energy splitting between the A and B excitons is larger than the one found for bulk GaN. However, the A-B excit...

In this paper we review the progress made in the development of the new multiscale/multiphysics simulator TIBERCAD, able to combine on equal footing macroscopic and microscopic scales of device models. The tool is not limited to conventional devices but has been also applied to systems were transport of excitons or ions are important for the correc...

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We report on a multiscale simulation approach that includes both macroscopic drift-diffusion current model and quantum tunneling model. The models are solved together in a self-consistent way inside a single simulation package. As an example, we study the subthreshold transfer characteristics of MOS transistors based on high-κ oxides. We compare th...

Due to the downscaling of semiconductor device dimensions and the emergence of new devices based on nanostructures, carbon nanotubes and molecules, the classical device simulation approach based on semi-classical transport theories needs to be extended towards a quantum mechanical description. We present a simulation environment designed for multis...

Quantum dot (QD) systems based on III-nitride have recently shown to be very promising nanostructures for high-quality light emitters. In this work, electronic and transport properties of AlN/GaN QDs are investigated by means of the TIBERCAD software tool, which allows both a macroscopic and an atomistic approach, with the final aim to couple them...

In this paper, we investigate the tunneling properties of ZrO₂ and HfO₂ high-k oxides, by applying quantum mechanical methods that include the full-band structure of Si and oxide materials. Semiempirical sp³s*d tight-binding parameters have been determined to reproduce ab-initio band dispersions. Transmission coeffi...

In this work, we show full-band calculations of the tunneling properties of ZrO2 and HfO2 high-κ oxides. First, we have determined semiempirical sp 3 s*d tight-binding (TB) parameters which reproduce ab-initio band dispersions of the high-κ oxides; then we have calculated transmission coefficients and tunneling currents for Si/ZrO2/Si and Si/HfO2/S...

The electronic, structural and transport properties of silicon nanowires have been investigated with different approaches. The Empirical Tight-Binding model (ETB) and Linear Combination of Bulk Bands (LCBB) method are used to calculate effect of quantum confinement on electronic energies, bandgap and effective masses in silicon nanowires in functio...

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 d...

High-κ oxides such as ZrO2 and HfO2 have attracted great interest, due to their physical properties, suitable to replacement of SiO2 as gate dielectric materials. In this work, we investigate the tunneling properties of ZrO2 and HfO2 high-κ oxides, by applying quantum mechanical methods that include the full-band structure of Si and oxide materials...

AlGaN/GaN and AlGaAs/InGaAs/GaAs HEMT structures are investigated theoretically to calculate the dependence of the 2DEG sheet resistance on strain. The inhomogeneous strain pattern induced by an external force is computed numerically using a continuous media model, assuming that the structures are grown on a thick substrate which remains unstrained...

The use of a passivating layer can reduce or even eliminate surface effects responsible for limiting both the RF current and breakdown voltage of AlGaN/GaN HEMTs. To study the effect of passivation on electrical characteristics of GaN-based devices, we have developed a macroscopic model of strain in SiN/AlGaN/GaN heterostructure, considering the sy...

Using atomistic quantum mechanical tight-binding (TB) methods that include the full band structure, we study electron tunneling through three-dimensional models of n⁺-Si/SiO₂/p-Si capacitors with thicknesses between 0.7 and 4.4 nm. We find that the microscopic oxide structure influences transmission coefficients and tunnel cur...

Using quantum mechanical methods that include the full-band structure of Si and SiO2, we study two non-classical phenomena that occur in MOS transistors at the nanometer-scale: tunneling through ultrathin oxides and quantum confinement in Si layers. SiO2 models based on β-cristobalite, β-quartz and tridymite polymorphs have been implemented for the...

This paper reviews the basic methodology and highlights advantages and recent applications of atomistic tight-binding calculations for the investigation of carrier transport in extremely scaled SOI transistors. The calculations yield numerous insights into direct and defect-assisted gate oxide tunneling, source-drain transport and tunneling, subban...

Using quantum mechanical methods that include the full band structure of Si and SiO2 and a self-consistent potential, we study tunneling through ultrathin oxides. Limitations of the effective-mass approximation (EMA) are investigated. In particular, we obtain good agreement between calculated and measured tunneling current densities for a n-poly-Si...

Based on the results of three-dimensional atomistic tight-binding calculations, we argue that the effective tunnel mass of SiO2 employed as a fitting parameter in standard transfer-matrix multiple-scattering theory calculations increases strongly as the oxide thickness is decreased (we find more than 50% mass enhancement upon reduction of the oxide...

Using quantum mechanical methods that include the full-band structure, we study two quantum mechanical phenomena that occur in MOS transistors: ultrathin oxide tunneling and inversion layer quantization. We obtain good agreement between calculated and measured tunneling current densities for a n-poly Si/SiO 2 /p-Si capacitor under negative gate bia...

Resonant tunneling diodes (RTD) based on GaN/AlGaN heterojunctions should in principle show high values of peak/valley ratio due to the large conduction band discontinuities between GaN and AlGaN. Moreover, such structures have been studied to be used in quantum cascade lasers for near infrared emission. However, polarization fields can mask such b...

Self-consistent quantum models of GaN-based nanostructures are presented. We report on the calculation of electrical characteristics of AlGaN/GaN heterojunction field effect transistors through an optimized effective mass approach based on the self-consistent solution of the Schrödinger and Poisson equations coupled to a quasi-2D model for the curr...

We report on the calculation of electrical characteristics of AlGaN/GaN heterojunction field effect transistors (HFETs). The model is based on the self-consistent solution of the Schrodinger and Poisson equations coupled to a quasi-2D model for the current flow. Both single and double heterojunction devices are analyzed for [0001] or [000-1] growth...

Self-consistent quantum modeling of GaN-based nanostructure are presented. The tight-binding approach is used to calculate optical properties while optimized effective mass approaches are used to obtain the output characteristics of GaN HEMT.

The realization of blue lasers and power HEMTs has brought a lot of attention to nitride-based heterostructures. When grown in the wurtzite structures (which is actually the most interesting one), nitrides display a non-zero macroscopic polarization, comprising both a spontaneous and a piezoelectric component. Such polarization induces an internal...

We present the TiberCAD multiscale device simulation software. The scope of the project is a full description of charge transport and optoelectronic properties of devices with embedded active regions of nanometer-scale. We show simulations of a GaN LED that requires modeling of strain, transport of electrons, holes and excitons and device heating.

Self-consistent quantum models of GaN-based nanostructures are presented. We report on the calculation of electrical characteristics of AlGaN/GaN heterojunction field effect transistors through an optimized effective mass approach based on the self-consistent solution of the Schrödinger and Poisson equations coupled to a quasi-2D model for the curr...