F. Sacconi

Spontaneous and piezoelectric polarization effects on the output characteristics of AlGaN/GaN heterojunction modulation doped FETs

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 directions. The onset of a parasitic p-channel for particular growth directions and alloy concentrations is also shown.

The Multiscale Paradigm in Electronic Device Simulation

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 on a multiscale coupling between atomistic descriptions and continuous media models, illustrating the application of such a multiscale approach to electronic device simulation.

Enhancement of the effective tunnel mass in ultrathin silicon dioxide layers

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 thickness from 4 to 1 nm). At least five factors, usually neglected in effective-mass-based calculations can contribute to this effect: the nonparabolicity of the complex bands in the gap of SiO2, the gradual (rather than abrupt) change of the electrostatic potential across the Si/SiO2 interface, a possible image force correction, the presence of native defects in the oxide, and the effective-mass approximation itself. Very good quantitative agreement between the theoretical predictions for the thickness dependence of the mass enhancement and corresponding results from transfer matrix fits to experimental currents is obtained if defect densities smaller than 10...

Full band approach to tunneling in MOS structures

Using atomistic quantum mechanical tight-binding (TB) methods that include the full band structure, we study electron tunneling through three-dimensional models of n/sup +/-Si/SiO/sub 2//p-Si capacitors with thicknesses between 0.7 and 4.4 nm. We find that the microscopic oxide structure influences transmission coefficients and tunnel currents significantly. The best agreement with experimental current-thickness and current-voltage data is obtained for a model derived from the /spl beta/-cristobalite polytype of SiO/sub 2/ that has a fairly small conduction band mass of 0.34 m/sub 0/. Standard approximate effective mass-based methods reproduce the TB results only if an energy and oxide thickness dependence of the mass parameter is introduced.

Modeling of GaN-based resonant tunneling diodes: Influence of polarization fields

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 benefits and make the design of RTD quite complicated. In this work, we have applied an atomistic point of view to describe current flowing in GaN-based RTD and investigate polarization issues. We used the sp3d5s* tight-binding (TB) model and a transfer matrix approach to describe the proper scattering states of the RTD system. The TB model allows us to describe the whole Brillouin zone of the semiconductors and relax all the envelope function approximations usually made for treating tunneling problems in RTDs. We observe that the effect of the polarization fields is to shift the transmission coefficient peaks while keeping a very high PVR.

Full-Band Tunneling in High-

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 coefficients and tunneling currents have been calculated for Si/ZrO2/Si and Si/HfO2/Si MOS structures, showing a very low gate leakage current in comparison to SiO2-based structures with the same equivalent oxide thickness. The complex band structures of ZrO2 and HfO2 have been calculated and used to develop an energy-dependent effective tunneling mass model. We show that effective mass calculations based on this model yield tunneling currents in close agreement with full-band results.

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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 focused in the center of the QD and emission energy is little depending on the column size. Moreover, the effect of surface states on transport is largely reduced when quantum densities are taken into account, since current in the nanocolumn light emitting diode (LED) results to flow mainly in the QD region. Nanocolumn geometry appears to be quite robust against variation of lateral scale, while it is largely sensitive to the QD alloy composition.

Oxide MOS Structures

Due to the downscaling of semiconductor device dimensions and the emergence of new devices based on nanostructures, CNTs 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 multiscale and multiphysics simulation of electronic and optoelectronic devices with the final aim of coupling classical with atomistic simulation approaches.

Optoelectronic Properties of Nanocolumn InGaN/GaN LEDs

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.

TiberCAD: Towards multiscale simulation of optoelectronic devices

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 on In content have been studied.