Search

In state-of-the-art, large format, HgCdTe-based infrared focal plane arrays the typical pixel size (3–10 μ m ) is of the order of the operating wavelength and much smaller than the carrier diffusion length. This makes inter-pixel crosstalk a limiting factor, especially in planar structures. Employing three-dimensional electromagnetic and electrical simulations we show that, besides reducing the dark current through Auger suppression, majority carrier depletion of the detector absorber is also effective in curtailing the inter-pixel crosstalk due to carrier diffusion. In the case of a 5 μ m -pitch pixel, a proper design of the absorber composition and doping profile allows to reduce inter-pixel crosstalk by more than a factor of two when increasing the reverse bias from - 0.1 to - 0.5 V , keeping the contribution to crosstalk coming from carrier diffusion between 2 and 12% in the mid- and long-wavelength infrared spectrum. [ABSTRACT FROM AUTHOR]

A nonlocal semiempirical pseudopotential calculation of the electronic structure of wurtzite ZnO is proposed. The local and nonlocal components of the atomic effective potentials have been sequentially optimized and an excellent quantitative agreement has been achieved with a wide range of band features (energy gaps at high symmetry points, valence band ordering, in-plane and perpendicular components of the effective masses for electrons and holes at Γ), selected not only from available experimental and ab initio results, but also from new calculations performed with the code developed by the ABINIT project. The valence band description has been further improved through the inclusion of spin-orbit corrections. The complex dielectric function along the main crystallographic directions corresponding to the optimized electronic structure is also presented, along with extensive comparisons of all the computed quantities with the literature data. [ABSTRACT FROM AUTHOR]

This work presents nonlocal pseudopotential calculations based on realistic, effective atomic potentials of the wurtzite phase of GaN, InN, and AlN. A formulation formulation for the model effective atomic potentials has been introduced. For each of the constitutive atoms in these materials, the form of the effective potentials is optimized through an iterative scheme in which the band structures are recursively calculated and selected features are compared to experimental and/or ab initio results. The optimized forms of the effective atomic potentials are used to calculate the band structures of the binary compounds, GaN, InN, and AlN. The calculated band structures are in excellent overall agreement with the experimental/ab initio values, i.e., the energy gaps at high-symmetry points, valence-band ordering, and effective masses for electrons match to within 3%, with a few values within 5%. The values of the energy separation, effective masses, and nonparabolicity coefficients for several secondary valleys are tabulated as well in order to facilitate analytical Monte Carlo transport simulations. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Highlights • Simulation of HgCdTe infrared detectors with realistic alloy composition profiles essential for their optimization. • Realistic full-wave electromagnetic with compositionally-graded layers is difficult with commercial simulators. • We give practical hints for discretizing the grading into sublayers. • We test several different discretizations. • We assess when the grading may be neglected. Abstract The ability to simulate HgCdTe infrared detectors with realistic alloy composition profiles is essential for their optimization. We give practical guidelines for the realization of combined three-dimensional, realistic full-wave electromagnetic and electrical modeling for the description of detectors with compositionally-graded layers, suitable to most available numerical solvers. Following the known procedure to sample the compositionally-graded layers into a number N of sublayers, we assess the effects of different choices for N , both on calculation accuracy and computational cost. Quantum efficiency spectra calculated with the proposed approach are compared with those determined through two shortcuts: ray tracing, based on classical optics, and full-wave simulation where graded layers are replaced by constant, spatially-averaged optical properties. It is shown that the former is not generally a valid alternative, since it does not address interference effects due to back-reflections, while the latter can lead to incorrect estimates of the detector cutoff wavelength. [ABSTRACT FROM AUTHOR]

The electron capture time in single quantum wells is calculated by considering capture and escape as scattering events in Monte Carlo simulation. The calculation is performed for an AlGaAs/GaAs quantum well as a function of the well width at 300 K. The overall capture time of carriers is found to be controlled by the transition from the free state to the uppermost confined levels. Subsequent interband transitions cause rapid decay into lower levels. [ABSTRACT FROM AUTHOR]

Recent experiments of electron emission spectroscopy (EES) on III-nitride light-emitting diodes (LEDs) have shown a correlation between droop onset and hot electron emission at the cesiated surface of the LED p-cap. The observed hot electrons have been interpreted as a direct signature of Auger recombination in the LED active region, as highly energetic Auger-excited electrons would be collected in long-lived satellite valleys of the conduction band so that they would not decay on their journey to the surface across the highly doped p-contact layer. We discuss this interpretation by using a full-band Monte Carlo model based on first-principles electronic structure and lattice dynamics calculations. The results of our analysis suggest that Auger-excited electrons cannot be unambiguously detected in the LED structures used in the EES experiments. Additional experimental and simulative work are necessary to unravel the complex physics of GaN cesiated surfaces. [ABSTRACT FROM AUTHOR]

Electroluminescence (EL) characterization of InGaN/GaN light-emitting diodes (LEDs), coupled with numerical device models of different sophistication, is routinely adopted not only to establish correlations between device efficiency and structural features, but also to make inferences about the loss mechanisms responsible for LED efficiency droop at high driving currents. The limits of this investigative approach are discussed here in a case study based on a comprehensive set of currentand temperature-dependent EL data from blue LEDs with low and high densities of threading dislocations (TDs). First, the effects limiting the applicability of simpler (closed-form and/or one-dimensional) classes of models are addressed, like lateral current crowding, vertical carrier distribution nonuniformity, and interband transition broadening. Then, the major sources of uncertainty affecting state-ofthe- art numerical device simulation are reviewed and discussed, including (i) the approximations in the transport description through the multi-quantum-well active region, (ii) the alternative valence band parametrizations proposed to calculate the spontaneous emission rate, (iii) the difficulties in defining the Auger coefficients due to inadequacies in the microscopic quantum well description and the possible presence of extra, non-Auger high-current-density recombination mechanisms and/or Auger-induced leakage. In the case of the present LED structures, the application of three-dimensional numerical-simulation-based analysis to the EL data leads to an explanation of efficiency droop in terms of TD-related and Auger-like nonradiative losses, with a C coefficient in the 10-30 cm6/s range at room temperature, close to the larger theoretical calculations reported so far. However, a study of the combined effects of structural and model uncertainties suggests that the C values thus determined could be overestimated by about an order of magnitude. This preliminary attempt at uncertainty quantification confirms, beyond the present case, the need for an improved description of carrier transport and microscopic radiative and nonradiative recombination mechanisms in device-level LED numerical models. [ABSTRACT FROM AUTHOR]

2-D numerical simulation is employed to assess a number of possible design approaches aimed at optimizing the internal quantum efficiency (IQE) of ZnO-based light-emitting diodes (LEDs) grown along the c-axis. First, the relative performance of similar ZnO-based and GaN-based LED structures is compared and discussed. Second, the effects on IQE of thickness, doping, and alloy composition of the MgZnO electron blocking layer (EBL) is studied in order to maximize the carrier confinement in the active region. The optimum number of quantum wells is also addressed, and different strategies for barrier doping are considered, showing that, if the EBL is doped p-type, a similar doping in the barriers is not required to compensate for the spontaneous and piezoelectric interface charges and to enhance hole transport. Different choices of the geometrical and doping parameters of the n-type access region are considered, and the impact of different values of the electron mobility is determined. Finally, the analysis of a ZnO/BeZnO LED structure suggests that the incorporation of BeZnO layers does not provide significant advantages. [ABSTRACT FROM PUBLISHER]

A theoretical justification is presented of the partial-capacitance (PC) approach, widely exploited in the modeling of coplanar waveguides on finite-thickness and multilayered substrates. The analysis is based on the static spectral-domain approach to the computation of the capacitance of a set of planar conductors embedded into a multilayered substrate. It is shown that the PC method can be derived from approximating the static Green's function of a multilayered dielectric as the sum of partial contributions; such a decomposition can be applied to the Green's function either in parallel (admittance) or series (impedance) form. The resulting (parallel or series) PC approaches are shown to be accurate with substrates having layers of decreasing or increasing permittivity, respectively. Lines backed by magnetic or electric walls are introduced as limiting cases of multilayered structures. [ABSTRACT FROM AUTHOR]

Presents a class of coaxial broad-band directional couplers for high-power radio frequency testing. Drawbacks of classical stripline and coaxial couplers overcome by the devices; Features of the design technique for the longitudinally nonuniform transversally nonsymmetrical couplers.

Proposes a conformal-mapping design tools for coaxial couplers with complex cross section. Accurate evaluation of per-unit-length capacitances, impedances and coupling factors; Even- and odd-mode characteristic impedance of a dielectric-filled circular coaxial coupler.

Examines coplanar waveguides with arbitrary polygonal geometry using conformal mapping approach. Estimation of capacitance, inductance, impedance and skin-effect attenuation; Application in electrooptic modulators; Electrode geometry optimization; Comparison of conformal mapping with full-wave finite element method.

Focuses on microwave modeling and characterization of thick coplanar waveguides on oxide-coated lithium niobate substrates for electrooptical applications. Cross section of coplanar waveguide for electrooptic modulator applications; Extraction of the characteristics impedance.

Presents a study which demonstrates closed-form computer-aided design- (CAD) oriented conformal-mapping approximations, for the high-frequency (HF) attenuation of two-conductor symmetrical coupled coplanar waveguides (CCPW's). Development of the ohmic loss analysis of a symmetrical CCPW according to the classical skin-effect HF approach.

Direct interband and intraband Auger recombination due to electron-electron-hole and hole-hole-electron transitions in bulk InGaN is investigated by first-order perturbation theory including Fermi statistics, realistic electronic structures obtained by nonlocal empirical pseudopotential calculations, and their corresponding wavevector-dependent dielectric functions. Our results confirm that the intraband Auger coefficient is negligible in alloy compositions relevant for solid-state lighting and indicate that the resonant enhancement associated with interband transitions for wavelengths ranging from blue to green cannot account for the efficiency droop experimentally observed in GaN-based light emitting diodes. [ABSTRACT FROM AUTHOR]

We present a multiband envelope-function model for wurtzite nanostructures based on a rigorous numerical procedure to determine operator ordering and band parameters from nonlocal empirical pseudopotential calculations. The proposed approach, implemented within a finite-element scheme, leads to well-posed, numerically stable envelope equations that accurately reproduce full-Brillouinzone subband dispersions of quantum systems. Although demonstrated here for III-nitride nonlocal empirical pseudopotentials, the model provides a general theoretical framework applicable to ab initio electronic structures of wurtzite semiconductors. [ABSTRACT FROM AUTHOR]

We present a k·p model for wurtzite semiconductors that allows the accurate approximation of the electronic structure over the entire Brillouin zone. The inclusion of an additional expansion point besides Γ allows significant improvements over standard full-Brillouin-zone approaches while keeping a manageable number of model parameters. We provide complete information about the Hamiltonian matrices of both expansion points and discuss the details of the optimization process used to determine the matrix parameters. As a demonstration of our scheme, we propose an approximation of the electronic structure of wurtzite ZnO, optimized for application to full-band Monte Carlo electron transport simulation. (A MATLAB implementation of the k·p model for ZnO is available from the authors.) [ABSTRACT FROM AUTHOR]

Wave-vector-dependent rates of disorder-induced alloy scattering have been computed for wurtzite AlGaN and InGaN to determine the transport properties of III-nitride alloys through full band Monte Carlo simulation. Contrary to previous studies, the empirical selection of a constant alloy scattering potential has been replaced by a more fundamental approach based on detailed information about the electronic structure and the corresponding screened atomic potentials. Band structures and atomic potentials have been determined in the framework of the nonlocal empirical pseudopotential method; good agreement of the fundamental energy gap with available experimental information has been achieved over the entire composition range of the alloys with the inclusion of a disorder contribution in the pseudopotential. The calculated alloy scattering potential is in reasonable agreement with the few indirect measurements available for AlGaN. Calculations of electron steady-state velocity-field curves confirm that alloy scattering has significant effects on the transport properties of AlGaN and InGaN. [ABSTRACT FROM AUTHOR]

This tutorial paper focuses on the physical origin of thermal droop, i.e., the decrease in the luminescence of light-emitting diodes (LEDs) induced by increasing temperature. III-nitride-based LEDs are becoming a pervasive technology, covering several fields from lighting to displays, from automotive to portable electronics, and from horticulture to sensing. In all these environments, high efficiency is a fundamental requirement, for reducing power consumption and system cost. Over the last decade, a great deal of effort has been put in the analysis of the efficiency droop, the decrease in LED internal quantum efficiency (IQE) induced by high current density. On the other hand, an IQE decrease is observed also for increasing temperature, a phenomenon usually referred to as thermal droop. For commercial LEDs, the IQE decrease related to thermal droop can be comparable to that of efficiency droop: for this reason, understanding thermal droop is a fundamental step for making LEDs capable of operating at high temperature levels. In several fields (including street lighting, automotive, photochemical treatments, projection, entertainment lighting, etc.), compact and high-flux light sources are required: typically, to reduce the size, weight, and cost of the systems, LEDs are mounted in compact arrays, and heat sinks are reduced to a minimum. As a consequence, LEDs can easily reach junction temperatures above 85–100 °C and are rated for junction temperatures up to 150–175 °C (figures from commercially available LED datasheets: Cree XHP70, Osram LUW HWQP, Nichia NVSL219CT, Samsung LH351B, and LedEngin LZP-00CW0R) and this motivates a careful analysis of thermal droop. This paper discusses the possible physical causes of thermal droop. After an introduction on the loss mechanisms in junctions, we will individually focus on the following processes: (i) Shockley–Read–Hall (SRH) recombination and properties of the related defects; (ii) Auger recombination and its temperature dependence, including the discussion of trap-assisted Auger recombination; (iii) impact of carrier transport on the thermal droop, including a discussion on carrier delocalization, escape, and freeze out; (iv) non-SRH defect-related droop mechanisms. In addition, (v) we discuss the processes that contribute to light emission at extremely low current levels and (vi) the thermal droop in deep ultraviolet LEDs, also with reference to the main parasitic emission bands. The results presented within this paper give a tutorial perspective on thermal droop; in addition, they suggest a pathway for the mitigation of this process and for the development of LEDs with stable optical output over a broad temperature range. [ABSTRACT FROM AUTHOR]

Indirect phonon-assisted Auger recombination mechanisms in bulk InGaN are investigated in the framework of perturbation theory, using first-principles phonon spectral density functions and electronic structures obtained by nonlocal empirical pseudopotential calculations. Nonpolar carrier-phonon interactions are treated within the rigid pseudoion framework, thus avoiding the introduction of empirical deformation potentials. The calculated indirect Auger coefficients exhibit a weak temperature dependence and dominate over direct processes for alloy compositions corresponding to the entire visible spectrum. The present results suggest that indirect Auger processes may be relevant in the operation of InGaN-based light-emitting diodes and lasers, at least in the yellow-green spectral region. [ABSTRACT FROM AUTHOR]

A conformal-mapping approach to model the skin-effect conductor losses of thick multiconductor coplanar waveguides and multiconductor coplanar strips is proposed. The model allows for arbitrary strip or slot number and widths and is accurate for conductor thicknesses up to around 40% of the minimum strip and slot width. Examples are presented to demonstrate the accuracy of the approach when compared to the results from a finite-element method numerical code. [ABSTRACT FROM PUBLISHER]

A self-energy-based model for the calculation of the electron capture time through phonon emission in presence of carrier population is applied to nitride-based quantum wells (QWs). The role of plasma dynamical screening on the Frölich interaction is investigated, highlighting its central role in the dependence of capture time on carrier density. The proposed approach yields a closed-form expression for capture time as a function of the carrier densities in QW and barrier states. Its application to simple two-population rate equations allows to reproduce available experimental data with excellent agreement, offering an accurate yet practical alternative to the usual approximation of a constant capture time in modeling light-emitting diodes and lasers. [ABSTRACT FROM AUTHOR]

In a combined experimental and numerical investigation, we present the effects of trap-assisted tunneling on the sub-threshold forward bias characteristics of a blue InGaN/GaN single-quantum-well LED test structure grown on a SiC substrate. The different role of donor- and acceptor-like traps has been studied, for the information it can provide on the role played by point defects. Using the energy Et and trap density Nt as the only tunneling-related fitting parameters, the behavior of the measured I( V) curves is well reproduced by our model over a wide current and temperature range. The very good agreement between simulations and experiments suggests that trap-assisted forward tunneling is one of the most relevant contributions to the current flow below the optical turn-on of the diode. [ABSTRACT FROM AUTHOR]

A microscopic model, based on a full-Brillouin-zone description of the electronic structure, is used to investigate Auger transitions in InGaN/GaN quantum wells. The lack of momentum conservation along the confining direction enhances direct (i.e., phononless) Auger transitions, leading to Auger coefficients in the range of those predicted for phonon-dressed processes in bulk InGaN. The dependence of Auger coefficients on temperature and quantum well thickness is analyzed. The limitations of conventional multiband models, based on zone-center approximations of the band structure, are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Analytic-band Monte Carlo simulation of electron transport in bulk wurtzite ZnO, validated against the results of a full-band code and available experimental data, is used to determine the main parameters required by drift-diffusion and hydrodynamic models. Steady-state drift velocity, carrier temperature, energy and momentum relaxation time, noise diffusivity, and electron thermal conductivity are calculated, and their dependence on temperature and electric field or average electron energy is approximated by means of a complete set of fitting models suitable for inclusion in commercial device simulation tools. [Copyright &y& Elsevier]

Herein, an overview on the physics‐based and system‐oriented modeling of organic electro‐optic Mach–Zehnder modulators for optical communication systems is presented. State‐of‐the‐art solutions in electro‐optic organic materials and modulator designs are reviewed, with particular stress on silicon organic hybrid (SOH) and plasmonic organic hybrid (POH) solutions. Then, the physics‐based simulation of concentrated and traveling‐wave modulators is discussed both through 3D optical simulations and a combination of 2D and 3D models, where the modulator is partitioned into convenient subdomains. Neural network behavioral models are finally discussed and case studies are proposed on the physics‐based and system‐level simulation of SOH and POH Mach–Zehnder modulators. [ABSTRACT FROM AUTHOR]

An efficient finite-element reduced-order quasi-TEM model for the frequency-dependent characteristics of lossy multiconductor transmission lines is presented. Conductor losses are evaluated as functions of frequency through a magneto-quasi-static model. Numerically generated problem-matched basis functions reduce the problem size and, therefore, the CPU time required by frequency sweeps without appreciable loss of accuracy. The proposed approach is applied to complex coplanar waveguides and to multiconductor interconnects; its results are compared with quasianalytical techniques and with the full-wave finite-element method. [ABSTRACT FROM AUTHOR]

The evaluation of the frequency response of waveguiding structures by means of the full-wave finite-element method requires solving a large generalized eigenvalue problem for each frequency. This paper describes a novel approach, based on the singular-value decomposition, which drastically reduces the order of the eigenvalue problem. By inspection of the singular values, the accuracy level of the procedure may be controlled. The technique is applied to the analysis of open and closed waveguides with arbitrary cross section, lossy conductors, and anisotropic dielectric layers, by means of vector elements of generic order; higher order elements are shown to allow the accurate evaluation of fields inside lossy conductors with fewer unknowns, besides exactly modeling normal field discontinuities at material interfaces. Examples of application of the reduced-order technique are shown concerning both non-TEM and quasi-TEM structures. [ABSTRACT FROM AUTHOR]

Defects can significantly modify the electro-optical characteristics of InGaN light-emitting diodes (LEDs); however, modeling the impact of defects on the electrical characteristics of LEDs is not straightforward. In this paper, we present an extensive investigation and modeling of the impact of defects on the electrical characteristics of InGaN-based LEDs, as a function of the thickness of the quantum well (QW). First, we demonstrate that the density of defects in the active region of III-N LEDs scales with increasing thickness of the InGaN QW. Since device layers with high indium content tend to incorporate more defects, we ascribed this experimental evidence to the increased volume of defects-rich InGaN associated to thicker InGaN layers. Second, we demonstrate that the current–voltage characteristics of the devices are significantly influenced by the presence of defects, especially in the sub turn-on region. Specifically, we show that the electrical characteristics can be effectively modeled in a wide current range (from pA to mA), by considering the existence of trap-assisted tunneling processes. A good correspondence is obtained between the experimental and simulated electrical characteristics (I–V), by using—in the simulation—the actual defect concentrations/activation energies extracted from steady-state photocapacitance, instead of generic fitting parameters. [ABSTRACT FROM AUTHOR]

Presents a study on nonlinear characterization and simulation of single and multiple quantum barrier varactors which are obtained by using a multiparticle Monte Carlo simulator. Methodology used in the study; Results of the study; Conclusions.

In this article we discuss the effect of temperature on the impact ionization coefficients in wide band-gap semiconductors and compare it to that of bulk GaAs. The impact ionization coefficients as a function of temperature are examined for three semiconductors: gallium arsenide, cubic phase silicon carbide, and zinc-blende phase gallium nitride. It is found that the magnitude of the phonon energy is principally responsible for changes in the impact ionization coefficients as a result of temperature change. While the energy band gap of a material does have a temperature dependence that directly affects the impact ionization transition rate, that change is much smaller than the relative change in the magnitude of the phonon-scattering rates in all of the materials studied here. The phonon energies are found to play a vital role in the magnitude of the change in the scattering rates as a function of temperature. Materials with relatively small phonon energies have phonon scattering rates that change considerably with temperature, and therefore have impact ionization coefficients that also change considerably with changes in temperature. Conversely, the phonon scattering rate in materials with a large phonon energy is less affected and thus these materials have impact ionization coefficients that are relatively insensitive to changes in the temperature. [ABSTRACT FROM AUTHOR]

The role of deep defects and their physical origin in InGaN/GaN LED are still widely investigated and debated, in particular their impact on the electrical and optical characteristics and on the reliability of the device. In this paper we evaluate the electrical and optical behavior of a single quantum well InGaN/GaN 420 nm LED during a medium-term accelerated ageing test. From these measurements, an increase in the sub-turn on current was observed: this process was ascribed to the stress-induced generation of defects in proximity of the active region and thus related to an increase of the correlated trap assisted tunneling. Subsequently we modelled the device electrical characteristics, evidencing the fundamental role of deep defects in determining the shape of the I-V curve in low forward bias. We reproduced with high accuracy the experimental current-voltage curves acquired during the ageing test, demonstrating that the increase in forward leakage can be entirely ascribed to the increase in concentration of a sole defect deep level located near midgap. These results provide a better understanding about the defect formation mechanism, that can be used to model and understand the behavior of the devices during ageing. • SQW InGaN/GaN LED under constant current stress test • Deep defects • Increase in trap assisted tunneling-related current observed • A model capable of emulating the electrical behavior of the devices • Simulations highlight the role of deep levels in the quite early ageing of the device [ABSTRACT FROM AUTHOR]

The potential applications of germanium and its alloys in infrared silicon-based photonics have led to a renewed interest in their optical properties. In this letter, we report on the numerical determination of Auger coefficients at T=300K for relaxed and biaxially strained germanium. We use a Green's function based model that takes into account all relevant direct and phononassisted processes and perform calculations up to a strain level corresponding to the transition from indirect to direct energy gap. We have considered excess carrier concentrations ranging from 1016 cm-3 to 5×1019 cm-3. For use in device level simulations, we also provide fitting formulas for the calculated electron and hole Auger coefficients as functions of carrier density. [ABSTRACT FROM AUTHOR]

What has turned into highly complex and somewhat misunderstood efficiency loss mechanisms occurring in light-emitting diodes (LEDs) based on the InN-GaN material system at high injection levels are discussed. Suggestions are made as to the dominant mechanism (s) in an open forum format as well as pointing out some of the shortcomings of the methodologies used and premises forwarded. It is unequivocally known that increased junction temperature would cause a reduction in radiative power due to mainly the reduction in radiative recombination efficiency. Another obvious mechanism is the asymmetry in doping in wide bandgap semiconductors, such as GaN, wherein the hole concentration lags well behind that of electrons in the active region. Because an electron and a hole are required for radiative recombination, the radiative efficiency cannot keep up with increasing carrier injection due to progressively lagging hole population. This results in either electron escape without radiative recombination or electron accumulation, which in turn changes the internal bias of the device, manifested as reduced internal forward bias, which reduces the rate of increase in light intensity. Some of the reports ascribe the efficiency loss at high injection levels to Auger recombination (mainly through indirect and recently reportedly direct deductions) as the main and or the only source of efficiency loss by in many cases simply relying on the temperature and injection independent (not well taken) A, B, C coefficients to fit a third order polynomial to the efficiency vs. injection current. As for the direct deduction, the spectroscopic analysis of Auger kicked hot electrons as they traverse through the G and L bands before being emitted into the vacuum by means of cesiated surface challenges the existing theories and some experiments regarding carrier scattering and G-L separation. Despite just a few reports to the contrary, the bulk of the resonant optical emission experiments do not support the Auger argument as being the main cause. In parallel, there exists a body of theoretical and experimental reports for electron overflow of ballistic/quasi-ballistic electrons traversing the active region to p-GaN, escaping recombination altogether in the active region. In fact and from the get go, the LED industry ubiquitously employed, and continues to do so, an (Al,In)GaN electron-blocking layer (EBL) to prevent electron escape for improved light output that in and of itself would more than suggest that the electron escape (overflow) does indeed occur. The only adverse effect of EBL is that it impedes hole injection due to the valence band offset between the p-type (Al,In)GaN EBL and p-GaN and also generates piezoelectric (if not lattice matched) and differential spontaneous polarization induced fields that pull down the conduction band edge at the interface reducing EBL's effectiveness. To at least reduce the aforementioned aggravating factors to some extent, the electron overflow and the associated efficiency loss can be reduced substantially (particulars of which depend on the active layer design) by inserting a stair-case electron injector (SEI) with a step-like indium composition to act as an "electron cooler" or by linearly graded cooler in some form or another prior to the active region. Use of multiple layered heterostructures for the active layers also plays the role of electron cooler, albeit not necessarily in the most optimum fashion. As if oblivious to the raging issues alluded to above, the LED industry has been moving along very successfully with a 2-prong approach. In one, dubbed the "high voltage LED", a set of LEDs (most likely configured in the form of a full-wave bridge rectifier) operating at low currents, where the efficiency is at its maximum, is used, which also has the added benefit of much reduced power supply complexity and weight. The other is the continual improvement of layer quality and optimum active layer design, taking technological parameters into consideration, which at the time of writing sported 63% wall-plug efficiency at an injection current of 350 mA for ~1.1 mm x 1.1 mm LEDs, which translates to approximately 75% efficiency once the voltage (about 3 V) and phosphor conversion efficiencies are taken into account. Assuming the same extraction and external quantum efficiencies, one obtains about 86% for each. It would not be an exaggeration to conclude that mid 90% internal quantum efficiencies are very likely in play here, which means that the purported inherent problems dominating the discussion are practically reduced to an academic exercise. Despite aggravating factors involving holes and hetero-barriers, commercial LEDs almost invariably use multilayer heterostructure designs for the active layers, loosely termed as multiple quantum wells, presumably to circumvent technological challenges and the ramifications of the polarization induced field even though double heterojunction varieties could be preferable from the point of view of hole transport. Eventually, the real limiting factors are the extent of hole supply (can be mitigated by increased hole concentration, which is well known and continually explored) and the proverbial thermal wall (can be mitigated by increased efficiency and efficient heat removal) of course. [ABSTRACT FROM AUTHOR]

In this paper, we describe the state of the art in the numerical simulation of the carrier transport properties of GaN and its ternary alloys. We outline the characteristics of our state of the art full-band Monte Carlo model that includes a fitting parameter free approach to compute the carrier-phonon interaction and a full quantum mechanical model for multiband transport which is critical to understand the high-field transport properties of these materials. Finally, we provide several examples of applications of the model to the calculation of the low-field electron mobility of GaN and In0.18Al0.82N, drift velocity in GaN and the impact ionization coefficients in AlxGa1-xN alloys. [ABSTRACT FROM AUTHOR]

Ab initio computations of the structural and electronic properties of wurtzite BeO have been performed with the codes of the abinit project, and the resulting band structure has been approximated with the nonlocal empirical pseudopotential method and a new full-zone $${k \cdot p}$$ model using two expansion points (Γ and L). The very good overall quality demonstrated by both empirical approaches should allow their application to the accurate yet numerically efficient description of the electronic structure of the BeZnO materials system. [ABSTRACT FROM AUTHOR]

Physical mechanisms causing the efficiency droop in InGaN/GaN blue light-emitting diodes and remedies proposed for droop mitigation are classified and reviewed. Droop mechanisms taken into consideration are Auger recombination, reduced active volume effects, carrier delocalization, and carrier leakage. The latter can in turn be promoted by polarization charges, inefficient hole injection, asymmetry between electron and hole densities and transport properties, lateral current crowding, quantum-well overfly by ballistic electrons, defect-related tunneling, and saturation of radiative recombination. Reviewed droop remedies include increasing the thickness or number of the quantum wells, improving the lateral current uniformity, engineering the quantum barriers (including multi-layer and graded quantum barriers), using insertion or injection layers, engineering the electron-blocking layer (EBL) (including InAlN, graded, polarization-doped, and superlattice EBL), exploiting reversed polarization (by either inverted epitaxy or N-polar growth), and growing along semi- or non-polar orientations. Numerical device simulations of a reference device are used through the paper as a proof of concept for selected mechanisms and remedies. [ABSTRACT FROM AUTHOR]

On our way to develop a comprehensive simulator of vertical-cavity surface-emitting lasers including carrier transport, optical modes, light-matter interaction and heat conduction, we address some critical multiscale aspects of the adopted coupling strategy. The quantum corrections to the semiclassical carrier transport framework are the bridge from nanometer to micrometer scales. In this paper, such corrections are shown to be fundamental in view of predicting the mode competition ruling VCSEL operation. Nevertheless, they can generate unforeseen features such as swirling electron flows in the active region, which are discussed here in detail. The simulation approach has been finally tested through a successful comparison with a large set of experimental results. [ABSTRACT FROM AUTHOR]

The computer-aided design and optimization of Ge-on-Si pin waveguide photodetectors is usually performed by a multiphysics analysis, where carrier transport is described with the drift-diffusion model. This paper addresses the shortcomings of this approach when applied to the study of the n-on-p detector configuration, ascribes them to the overly simplified model of the Si/Ge heterojunction, and proposes an enhanced description of the interfacial region. Starting from technological considerations, a thin buffer layer is introduced between Si and Ge, with a graded composition profile that mimics the initial low-temperature epitaxial growth process and mitigates the sharp discontinuity of the abrupt Si/Ge heterojunction in the band diagram. The benefits of this more realistic model on the determination of the electrooptic response are demonstrated and discussed. [ABSTRACT FROM AUTHOR]