Carlo Garetto

Bio: Carlo Garetto is an academic researcher from Instituto Politécnico Nacional. The author has contributed to research in topics: Pseudopotential & Electronic band structure. The author has an hindex of 4, co-authored 5 publications receiving 467 citations.

Monte Carlo simulation of electron transport in the III-nitride wurtzite phase materials system: binaries and ternaries

Abstract: We present a comprehensive study of the transport dynamics of electrons in the ternary compounds, Al/sub x/Ga/sub 1-x/N and In/sub x/Ga/sub 1-x/N. Calculations are made using a nonparabolic effective mass energy band model. Monte Carlo simulation that includes all of the major scattering mechanisms. The band parameters used in the simulation are extracted from optimized pseudopotential band calculations to ensure excellent agreement with experimental information and ab initio band models. The effects of alloy scattering on the electron transport physics are examined. The steady state velocity field curves and low field mobilities are calculated for representative compositions of these alloys at different temperatures and ionized impurity concentrations. A field dependent mobility model is provided for both ternary compounds AlGaN and InGaN. The parameters for the low and high field mobility models for these ternary compounds are extracted and presented. The mobility models can be employed in simulations of devices that incorporate the ternary III-nitrides.

Band structure nonlocal pseudopotential calculation of the III-nitride wurtzite phase materials system. Part II. Ternary alloys AlxGa1−xN, InxGa1−xN, and InxAl1−xN

Abstract: This work presents detailed information on the band structures of the III-nitride wurtzite ternary alloys, computed through the virtual crystal approximation approach. The key ingredient of this study is the set of realistic atomic effective potentials described in Part I of the present work, dedicated to the constituent binary compounds. The model relies on the linear interpolation of the structural parameters and of the local and nonlocal effective potentials: no further empirical corrections are included. The dependence on the mole fraction is computed for the energy gaps at all the high-symmetry points, the valence-band width, and the electron effective masses in the valleys relevant for carrier-transport simulation.

Simplex Algorithm for Band Structure Calculation of Noncubic Symmetry Semiconductors: Application to III-nitride Binaries and Alloys

Abstract: A set of software tools for the determination of the band structure of zinc-blende, wurtzite, 4H, and 6H semiconductors is presented. A state of the art implementation of the nonlocal empirical pseudopotential method has been coupled with a robust simplex algorithm for the optimization of the adjustable parameters of the model potentials. This computational core has been integrated with an array of Matlab functions, providing interactive functionalities for defining the initial guess of the atomic pseudopotentials, checking the convergence of the optimization process, plotting the resulting band structure, and computing detailed information about any local minimum. The results obtained for wurtzite-phase III-nitrides (ALN, GaN, InN) are presented as a relevant case study.

Simplex algorithm for band structure calculation of noncubic symmetry semiconductors: application to III-nitride binaries and alloys

Abstract: The simulation of transport and optical properties of semiconductors is based on the availability of an accurate description of the band structures for the materials under investigation. Ab initio techniques are invaluable for predicting the electronic band structure particularly in the absence of reliable experimental information. Nevertheless, the Empirical Pseudopotential Method (EPM) remains the method of choice for transport studies because of its optimum compromise between complexity, accuracy, and computational efficiency. We have developed a set of integrated, interactive software tools which may greatly help in the delicate process of finding the best EPM band structure of a semiconductor compound. At the core of our computational method is an efficient yet general f90 implementation of nonlocal EPM for zinc-blende and wurtzite crystals. A robust simplex algorithm allows the optimization of the EPM adjustable parameters in order to match a set of reference values, i.e. energy gaps in several points of the Brillouin zone, and effective masses along all the principal directions, both for direct and indirect-gap materials. This computational procedure has been integrated with an array of Matlab functions, providing interactive functionalities for defining the local part of the atomic pseudopotentials, checking the convergence of the optimization process, plotting the resulting band structure, and computing detailed information about. e.g. the position and value of minima not located at symmetry points, the nonparabolicity coefficients of secondary valleys, etc.

Monte Carlo simulation of electron transport in the III-nitride wurtzite phase materials system: binaries and ternaries

Abstract: We present a comprehensive study of the transport dynamics of electrons in the ternary compounds, Al/sub x/Ga/sub 1-x/N and In/sub x/Ga/sub 1-x/N. Calculations are made using a nonparabolic effective mass energy band model. Monte Carlo simulation that includes all of the major scattering mechanisms. The band parameters used in the simulation are extracted from optimized pseudopotential band calculations to ensure excellent agreement with experimental information and ab initio band models. The effects of alloy scattering on the electron transport physics are examined. The steady state velocity field curves and low field mobilities are calculated for representative compositions of these alloys at different temperatures and ionized impurity concentrations. A field dependent mobility model is provided for both ternary compounds AlGaN and InGaN. The parameters for the low and high field mobility models for these ternary compounds are extracted and presented. The mobility models can be employed in simulations of devices that incorporate the ternary III-nitrides.

Valence band splittings and band offsets of AlN, GaN and InN.

Abstract: First‐principles electronic structure calculations on wurtzite AlN, GaN, and InN reveal crystal‐field splitting parameters ΔCF of −217, 42, and 41 meV, respectively, and spin–orbit splitting parameters Δ0 of 19, 13, and 1 meV, respectively. In the zinc blende structure ΔCF≡0 and Δ0 are 19, 15, and 6 meV, respectively. The unstrained AlN/GaN, GaN/InN, and AlN/InN valence band offsets for the wurtzite (zinc blende) materials are 0.81 (0.84), 0.48 (0.26), and 1.25 (1.04) eV, respectively. The trends in these spectroscopic quantities are discussed and recent experimental findings are analyzed in light of these predictions.

Review—Ionizing Radiation Damage Effects on GaN Devices

Abstract: GalliumNitridebasedhighelectronmobilitytransistors(HEMTs)areattractiveforuseinhighpowerandhighfrequencyapplications, with higher breakdown voltages and two dimensional electron gas (2DEG) density compared to their GaAs counterparts. Specific applications for nitride HEMTs include air, land and satellite based communications and phased array radar. Highly efficient GaNbased blue light emitting diodes (LEDs) employ AlGaN and InGaN alloys with different compositions integrated into heterojunctions and quantum wells. The realization of these blue LEDs has led to white light sources, in which a blue LED is used to excite a phosphor material; light is then emitted in the yellow spectral range, which, combined with the blue light, appears as white. Alternatively, multiple LEDs of red, green and blue can be used together. Both of these technologies are used in high-efficiency white electroluminescent light sources. These light sources are efficient and long-lived and are therefore replacing incandescent and fluorescent lamps for general lighting purposes. Since lighting represents 20‐30% of electrical energy consumption, and because GaN white light LEDs require ten times less energy than ordinary light bulbs, the use of efficient blue LEDs leads to significant energy savings. GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. The response of GaN to radiation damage is a function of radiation type, dose and energy, as well as the carrier density, impurity content and dislocation density in the GaN. The latter can act as sinks for created defects and parameters such as the carrier removal rate due to trapping of carriers into radiation-induced defects depends on the crystal growth method used to grow the GaN layers. The growth method has a clear effect on radiation response beyond the carrier type and radiation source. We review data on the radiation resistance of AlGaN/GaN and InAlN/GaN HEMTs and GaN‐based LEDs to different types of ionizing radiation, and discuss ion stopping mechanisms. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects in GaN are discussed. The carrier removal rates are a function of initial carrier concentration and dose but not of dose rate or hydrogen concentration in the nitride material grown by Metal Organic Chemical Vapor Deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. State-of-art simulators now provide accurate predictions for the observed changes in radiation-damaged HEMT performance. Neutron irradiation creates more extended damage regions and at high doses leads to Fermi level pinning while 60 Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation. In InGaN/GaN blue LEDs irradiated with protons at fluences near 10 14 cm −2 or electrons at fluences near 10 16 cm −2 , both current-voltage and light output-current characteristics are degraded with increasing proton dose. The optical performance of the LEDs is more sensitive to the proton or electron irradiation than that of the corresponding electrical performances. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any

Trapping Effects in the Transient Response of AlGaN/GaN HEMT Devices

Abstract: In this paper, the transient analysis of an AlGaN/GaN high-electron mobility transistor (HEMT) device is presented. Drain-current dispersion effects are investigated when gate or drain voltages are pulsed. Gate-lag and drain-lag turn-on measurements are analyzed, revealing clear mechanisms of current collapse and related dispersion effects. Numerical 2-D transient simulations considering surface traps effects in a physical HEMT model have also been carried out. A comparison between experimental and theoretical results is shown. The presence of donor-type traps acting as hole traps, due to their low energy level of 0.25 eV relative to the valence band, with densities >1e20 cm-3 (>5e12 cm-2), uniformly distributed at the HEMT surface, and interacting with the free holes that accumulated at the top surface due to piezoelectric fields, accounts for the experimentally observed effects. Time constants next to 10 ms are deduced. Some additional features in the measured transient currents, with faster time constants, could not be associated with surface states

Current injection efficiency induced efficiency-droop in InGaN quantum well light-emitting diodes

Abstract: Current injection efficiency and its impact on efficiency-droop in InGaN single quantum well (QW) based light-emitting diodes (LEDs) are investigated. The analysis is based on current continuity relation for drift and diffusion carrier transport across the QW-barrier system. A self-consistent 6-band k · p method is used to calculate the band structure for InGaN QW. The analysis indicates that the internal quantum efficiency in the conventional 24-A In 0.28 Ga 0.72 N–GaN QW structure reaches its peak at low injection current density and reduces gradually with further increase in current due to the large carrier thermionic emission. Structures combining 24-A In 0.28 Ga 0.72 N QW with 15-A Al 0.1 Ga 0.9 N barriers show slight reduction in quenching of the injection efficiency as current density increases. The use of 15-A Al 0.83 In 0.17 N barriers shows significant reduction in efficiency-droop (10% reduction of the internal quantum efficiency at current density of 620 A/cm 2 ). Thus, InGaN QWs employing thin layers of larger bandgap AlInN barriers suppress the efficiency-droop phenomenon significantly.