Gabriele Penazzi

Efficiency Drop in Green InGaN/GaN Light Emitting Diodes: The Role of Random Alloy Fluctuations

White light emitting diodes (LEDs) based on III-nitride InGaN/GaN quantum wells currently offer the highest overall efficiency for solid state lighting applications. Although current phosphor-converted white LEDs have high electricity-to-light conversion efficiencies, it has been recently pointed out that the full potential of solid state lighting could be exploited only by color mixing approaches without employing phosphor-based wavelength conversion. Such an approach requires direct emitting LEDs of different colors, including, in particular, the green-yellow range of the visible spectrum. This range, however, suffers from a systematic drop in efficiency, known as the green gap, whose physical origin has not been understood completely so far. In this work, we show by atomistic simulations that a consistent part of the green gap in c-plane InGaN/GaN-based light emitting diodes may be attributed to a decrease in the radiative recombination coefficient with increasing indium content due to random fluctuations of the indium concentration naturally present in any InGaN alloy.White light emitting diodes based on III-nitride InGaN/GaN quantum wells currently offer the highest overall efficiency for solid state lighting applications. Although current phosphor-converted white LEDs have high electricity-to-light conversion efficiencies, it has been recently pointed out that the full potential of solid state lighting could be exploited only by color mixing approaches without employing phosphor-based wavelength conversion. Such an approach requires direct emitting LEDs of different colors, in particular in the green/yellow range ov the visible spectrum. This range, however, suffers from a systematic drop in efficiency, known as the “green gap”, whose physical origin has not been understood completely so far. In this work we show by atomistic simulations that a consistent part of the “green gap” in c-plane InGaN/GaN based light emitting diodes may be attributed to a decrease in the radiative recombination coefficient with increasing Indium content due to random fluctuations of the Indium concentration naturally present in any InGaN alloy.

Towards atomic level simulation of electron devices including the semiconductor-oxide interface

We report a milestone in device modeling whereby a planar MOSFET with extremely thin silicon on insulator channel is simulated at the atomic level, including significant parts of the gate and buried oxides explicitly in the simulation domain, in ab initio fashion, i.e without material or geometrical parameters. We use the density-functional-based tight-binding formalism for constructing the device Hamiltonian, and non-equilibrium Greens functions formalism for calculating electron current. Simulations of Si/SiO2 super-cells agree very well with experimentally observed band-structure phenomena in SiO2-confined sub-6 nm thick Si films. Device simulations of ETSOI MOSFET with 3 nm channel length and sub-nm channel thickness also agree well with reported measurements of the transfer characteristics of a similar transistor.

Possibility of a field effect transistor based on Dirac particles in semiconducting anatase-TiO2 nanowires.

In Dirac materials, like graphene or topological insulators, massless pseudorelativistic electrons promise new, very fast electronic devices by utilizing the partial suppression of backscattering. However, the semimetal nature of graphene makes the realization of practical field effect transistors difficult, due to small on-off current ratios. Here, we propose a new concept, based on Dirac states inside the conduction (or valence) band of a lightly doped wide band gap semiconductor. With the application of a gate voltage, the Dirac states become populated; that is, the Fermi level is switched between the classical high-resistivity semiconducting and the relativistic high-mobility metallic range. We demonstrate by theoretical calculations that such a transition can be realized, for example, in thin anatase nanowires, which have been synthesized before. Ta-doped anatase nanowires offer an excellent possibility to build field effect transistors with high speed and good on-off ratio. Guidelines for finding similar Dirac semiconductors are provided.

Permittivity of Oxidized Ultra-Thin Silicon Films From Atomistic Simulations

We establish the dependence of the permittivity of oxidized ultra-thin silicon films on the film thickness by means of atomistic simulations within the density-functional-based tight-binding (DFTB) theory. This is of utmost importance for modeling ultra-thin and extremely thin silicon-on-insulator MOSFETs, and for evaluating their scaling potential. We demonstrate that electronic contribution to the dielectric response naturally emerges from the DFTB Hamiltonian when coupled to Poisson equation solved in vacuum, without phenomenological parameters, and obtain good agreement with the available experimental data. Comparison with calculations of H-passivated Si films reveals much weaker dependence of permittivity on film thickness for the SiO2-passivated Si, with less than 18% reduction in the case of 0.9-nm silicon-on-insulator.

Atomic level simulation of permittivity of oxidized ultra-thin Si channels

We use density-functional-based tight binding theory, coupled to a Poisson solver to investigate the dielectric response in oxidized ultra-thin Si films with thickness in the range of 0.8 to 10.0 nm. Building on our recent work on the electronic structure of such Si films using the same formalism, we demonstrate that the electronic contribution to the permittivity of Si and of SiO2 is modeled with good accuracy. The simulations of oxidized Si films agree well with available experimental data and show appreciable degradation of permittivity by nearly 18% at 0.8 nm. Notable is however that simulations with hydrogenated Si substantially overestimate the degradation of permittivity. Beyond clarifying the quantitative trend of permittivity versus Si thickness, which is very relevant e.g. for fully-depleted Si-on-insulator MOSFETs, the present work is a cornerstone towards delivering an atomistic modelling approach that is free of material- or device-related phenomenological parameters.

Simulation of random alloy effects in InGaN/GaN LEDs

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.