Showing papers by "Enrico Bellotti published in 2013"

Auger recombination in InGaN/GaN quantum wells: A full-Brillouin-zone study

Abstract: 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.

Numerical Simulation of Third-Generation HgCdTe Detector Pixel Arrays

Abstract: In this paper, we present a physics-based full 3-D numerical simulation model of third-generation infrared (IR) detector pixel arrays. The approach avoids geometrical simplifications typical of 1-D and 2-D models that can introduce errors which are difficult to quantify. We have used a finite-difference time-domain technique to compute the optical characteristics including the reflectance and the carrier generation rate in the device. Subsequently, we employ the finite-element method to solve the drift-diffusion equations on a mixed-element grid to compute the electrical characteristics including the I(V) characteristics and quantum efficiency. Furthermore, we have used this model to study HgCdTe two-color detectors that operate in the medium-wave to long-wave IR and photovoltaic pixel arrays employing a photon-trapping structure realized with a periodic array of pillars that operate in the medium-wave IR.

Numerical simulation of crosstalk in reduced pitch HgCdTe photon-trapping structure pixel arrays.

Abstract: We have investigated crosstalk in HgCdTe photovoltaic pixel arrays employing a photon trapping (PT) structure realized with a periodic array of pillars intended to provide broadband operation. We have found that, compared to non-PT pixel arrays with similar geometry, the array employing the PT structure has a slightly higher optical crosstalk. However, when the total crosstalk is evaluated, the presence of the PT region drastically reduces the total crosstalk; making the use of the PT structure not only useful to obtain broadband operation, but also desirable for reducing crosstalk in small pitch detector arrays.

Theory of Carriers Transport in III-Nitride Materials: State of the Art and Future Outlook

Abstract: 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.

Numerical Simulation of Spatial and Spectral Crosstalk in Two-Color MWIR/LWIR HgCdTe Infrared Detector Arrays

Abstract: The sequential two-color Hg1−xCdxTe architecture has emerged as a key technology in the development of third-generation infrared detectors. Due to the expense required to manufacture these devices, it is imperative to create numerical models which can predict the electrical and optical behavior of the technology as well as evaluate design concepts prior to exhaustive development. We have developed a three-dimensional simulation model which fully accounts for the optical phenomena that become increasingly important in small pixels and uses a drift–diffusion approach to determine the electrical behavior of the device. In particular, we employ a finite-difference time- domain method to solve Maxwell’s equations and a finite-element method to evaluate the solutions of the coupled Poisson and carrier continuity equations. We apply our simulation model to simulate the dynamic resistance and current density versus voltage characteristics of this detector architecture. The quantum efficiency is then determined for both spectral bands while observing the effects of variable pixel pitch and detector geometry. Finally, we use a spatially finite Gaussian beam to analyze the crosstalk and perform a simulated spot scan.

Spectral Characteristics of Hot Electron Electroluminescence in Silicon Avalanching Junctions

Abstract: The emission spectra of avalanching n+p junctions manufactured in a standard 350-nm CMOS technology with no process modifications are measured over a broad spectral range and at different current levels. In contrast to the narrow-band forward-biased pn junction emission spectrum, the reverse biased avalanching emission spectrum extends from the ultraviolet 350 nm (3.6 eV) to the near infrared 1.7 μm(0.7 eV), covering the visual range. The photon emission energy spectrum is compared to the hot electron energy distribution within the conduction band, as determined from a full band Monte Carlo simulation. This allows the identification of phonon assisted indirect intraband (c-c) hot electron transitions as the dominant physical light emission processes within the high electric field avalanching junction. Device simulations are utilized to identify the device drift region as the source of the near infrared emissions.

Physics-based simulation of the modulation transfer function in HgCdTe infrared detector arrays

Abstract: We have developed a numerical technique for performing physics-based simulations of the modulation transfer function (MTF) of infrared detector focal plane arrays. The finite-difference time-domain and finite element methods are employed to determine the electromagnetic and electrical response, respectively. We show how the total MTF can be decomposed to analyze the effect of lateral diffusion of charge carriers and present several methods for mitigation of such effects. We employ our numerical technique to analyze the MTF of a HgCdTe two-color bias-selectable infrared detector array.

Properties of threading screw dislocation core in wurtzite GaN studied by Heyd-Scuseria-Ernzerhof hybrid functional

Abstract: We propose another structure as the most stable configuration for threading screw dislocation core of wurtzite GaN in N-rich conditions by first-principles calculations using Heyd-Scuseria-Ernzerhof hybrid functional. This configuration is fully consistent with recent experimental results observing electrical inactivity of GaN samples grown in N-rich conditions, in contrast with previously suggested dislocation core structures.

Numerical Simulation and Analytical Modeling of InAs nBn Infrared Detectors with p-Type Barriers

Abstract: This paper presents finite-element one-dimensional numerical simulations and analytical modeling for ideal (diffusion current only) nBn detectors with p-type barrier layers. The simulations show that the current–voltage J(V) and the dynamic resistance versus voltage RD(V) relations, both dark and illuminated, are in excellent agreement with the equations for ideal back-to-back photodiodes. We present a depletion approximation model for the nBn detector, analogous to that for the conventional p–n junction photodiode, based on new boundary conditions on the hole concentrations versus voltage at the edges of the nBn barrier layer. We show that these nBn boundary conditions are identical to those for ideal back-to-back photodiodes, justifying the applicability of back-to-back photodiode equations to describe the ideal nBn detector. The simulations for the space-charge regions show a low-bias-voltage regime and a high-bias-voltage regime. The integrated space-charge densities in the layers adjacent to the barrier layer vary linearly with bias voltage. Negative dynamic resistance occurs because the bias voltage changes the effective thickness of the thin-base layers that generate diffusion current. We present a new formulation of the model for ideal back-to-back photodiodes with a more elegant and transparent set of equations for J(V) and RD(V).

Multiscale modeling of photon detectors from the infrared to the ultraviolet

Abstract: Due to the ever increasing complexity of novel semiconductor systems, it is essential to possess design tools and simulation strategies that include in the macroscopic device models the details of the microscopic physics and their dependence on the macroscopic (continuum) variables. Towards this end, we have developed robust multi-scale modeling capabilities that begin with modeling the intrinsic semiconductor properties. The models are fully capable of incorporating effects of substrate driven stress/strain and the material quality (dislocations and defects) on microscopic quantities such as the local transport coefficients and non-radiative recombination rate. Using this modeling approach we have extensively studied UV APD detectors and infrared focal plane arrays. Particular emphasis was placed on HgCdTe and InAsSb arrays incorporating photon trapping structures as well as two-color HgCdTe detectors arrays.

Electroluminescence Analysis and Simulation of the Effects of Injection and Temperature on Carrier Distribution in InGaN-Based Light-Emitting Diodes with Color-Coded Quantum Wells

Abstract: This paper reports on an extensive analysis of the electroluminescence characteristics of InGaN-based LEDs with color-coded structure, i.e., with a triple quantum well structure in which each quantum well has a different indium content. The analysis is based on combined electroluminescence measurements and two-dimensional simulations, carried out at different current and temperature levels. Results indicate that (i) the efficiency of each of the quantum wells strongly depends on device operating conditions (current and temperature); (ii) at low current and temperature levels, only the quantum well closer to the p-side has a significant emission; (iii) emission from the other quantum wells is favored at high current levels. The role of carrier injection, hole mobility, carrier density and non-radiative recombination in determining the relative intensity of the quantum wells is discussed in the text.

Comment on "Direct Measurement of Auger Electrons Emitted from a Semiconductor Light-Emitting Diode under Electrical Injection: Identification of the Dominant Mechanism for Efficiency Droop" (Phys. Rev. Lett. 110, 177406 (2013))

Abstract: In a recent letter [Phys. Rev. Lett. 110, 177406 (2013)], presenting a spectroscopic study of the electrons emitted from the GaN p-cap of a forward-biased InGaN/GaN light-emitting diode (LED), the authors observed at least two distinct peaks in the electron energy distribution curves (EDCs), separated by about 1.5 eV, and concluded that the only viable explanation for the higher-energy peak was Auger recombination in the LED active region. We present full-band Monte Carlo simulations suggesting that the higher-energy peaks in the measured EDCs are probably uncorrelated with the carrier distribution in the active region. This would not imply that Auger recombination, and possibily Auger-induced leakage, play a negligible role in LED droop, but that an Auger signature cannot be recovered from the experiment performed on the LED structure under study. We discuss, as an alternative explanation for the observed EDCs, carrier heating by the electric field in the band-bending region.

A full band Monte-Carlo study of carrier transport properties of InAlN lattice matched to GaN

Abstract: The growing importance of In 0:18 Al 0:82 N stems from the fact that it can be grown lattice matched to GaN and for its potential applications in a large number of electronics and optoelectronics devices. In this work we employed a full band Monte-Carlo approach to study the carrier transport properties of this alloy. We have computed the temperature and doping dependent electron and hole mobilities and drift velocities. Furthermore, for both sets of transport coefficients we have developed a number of analytical expressions that can be easily incorporated in drift-diffusion type simulation codes.

A numerical study of low- and high-field carrier transport properties in In0.18Al0.82N lattice-matched to GaN

Abstract: This work presents a numerical study of the carrier transport properties of In0.18Al0.82N lattice-matched to GaN. Using a full-band Monte-Carlo model, we have evaluated the low- and high-field transport coefficients of this alloy. We have computed the carrier mobilities as a function of temperature for different doping concentrations and areal dislocation densities. Furthermore, we have evaluated the electron and hole drift velocities with and without considering the effect of the alloy scattering. Finally, we have computed the carrier impact ionization coefficients for transport along both the Γ−A and Γ−M crystallographic directions.

Large-scale numerical simulation of reduced-pitch HgCdTe infrared detector arrays

Abstract: Numerical simulations play an important role in the development of large-format infrared detector array tech- nologies, especially when considering devices whose sizes are comparable to the wavelength of the radiation they are detecting. Computational models can be used to predict the optical and electrical response of such devices and evaluate designs prior to fabrication. We have developed a simulation framework which solves Maxwell’s equations to determine the electromagnetic properties of a detector and subsequently uses a drift-diffusion ap- proach to asses the electrical response. We apply these techniques to gauge the effects of cathode placement on the inter- and intra-pixel attributes of compositionally graded and constant Hg1−xCdxTe mid-wavelength infrared detectors. In particular, the quantum efficiency, nearest-neighbor crosstalk, and modulation transfer function are evaluated for several device architectures.

Numerical simulation of InAs nBn infrared detectors with n-type barrier layers

Abstract: This paper presents one-dimensional numerical simulations and analytical modeling of ideal (only diffusion current and only Auger-1 and radiative recombination) InAs nBn detectors having n-type barrier layers, with donor concentrations ranging from 1.8×1015 to 2.5×1016 cm-3. We examine quantitatively the three space charge regions in the nBn detector with an n-type barrier layer (BL), and determine criteria for combinations of bias voltage and BL donor concentration that allow operation of the nBn with no depletion region in the narrow-gap absorber layer (AL) or contact layer (CL). We determine the quantitative characteristics of the valence band barrier that is present for an n-type BL. From solution of Poisson’s equation in the uniformly doped BL, we derive analytical expressions for the valence band barrier heights versus bias voltage for holes in both the AL and the CL. These expressions show that the VB barrier height varies linearly with the BL donor concentration and as the square of the BL width. Using these expressions, we constructed a phenomenological equation for the dark current density versus bias voltage which agrees reasonably well with the shape of the J(V) curves from numerical simulations. Our simulations suggest that the nBn detector should be able to be operated at or near zero-bias voltage.

Theoretical and experimental study of dynamics of photoexcited carriers in GaN

Abstract: We present a theoretical and experimental study of the sub-picosecond dynamics of photo-excited carriers in GaN. In the theoretical model, interaction with an external ultrafast laser pulse is treated coherently and to account for the scattering mechanisms and dephasing processes, a generalized Monte-Carlo simulation is used. The scattering mechanisms included are carrier interactions with polar optical phonons and acoustic phonons, and carrier-carrier Coulomb interactions. We study the effect of different scattering mechanisms on the carrier densities. In the case that the excitation energy satisfies the threshold for polar optical scattering, phonon contribution is the dominant process in relaxing the system, otherwise, carrier-carrier mechanism is dominant. Furthermore, we present the temperature and pulse power dependent normalized luminescence intensity. The results are presented over a range of temperatures, electric field, and excitation energy of the laser pulse. For comparison, we also report the experimental time-resolved photoluminescence studies on GaN samples. There is a good agreement between the simulation and experiment in normalized luminescence intensity results. Therefore, we show that we can explain the dynamics of the photo-excited carriers in GaN by including only carrier-carrier and carrier-phonon interactions and a relatively simple two-band electronic structure model.

Auger recombination in bulk InGaN and quantum wells: a numerical simulation study

Abstract: The theory of pure-collision and phonon-assisted Auger recombination mechanisms in bulk InGaN alloys is reviewed. The model is based on a Green function formalism and uses realistic electronic structures obtained by nonlocal empirical pseudopotential calculations and phonon spectral density functions determined from first-principles lattice dynamical cal- culations. The effect of phonons is formally included to the infinite order of perturbation theory by means of the spectral density function which contains summation over all possible phonon momenta. Auger transitions in quantum wells may significantly differ from their bulk counterpart since momentum conservations is lifted along the confining direction. A preliminary analysis indicate that direct Auger transitions in confined structures exhibit an enhancement with respect to the bulk case. The analysis is based on a full-zone description of the electronic structure in which confined and unbound states are represented as a superposition of bulk states of the underlying lattice, thus allowing a fair comparison between Auger coefficients in bulk and quantum wells.

Numerical simulation of quantum efficiency and surface recombination in HgCdTe IR photon-trapping structures

Abstract: We have investigated the quantum effiency in HgCdTe photovoltaic pixel arrays employing a photon-trapping structure realized with a periodic array of pillars intended to provide broadband operation. We have found that the quantum efficiency depends heavily on the passivation of the pillar surface. Pillars passivated with anodicoxide have a large fixed positive charge on the pillar surface. We use our three-dimensional numerical simulation model to study the effect of surface charge and surface recombination velocity on the exterior of the pillars. We then evaluate the quantum efficiency of this structure subject to different surface conditions. We have found that by themselves, the surface charge and surface recombination are detrimental to the quantum efficiency but the quantum efficiency is recovered when both phenomena are present. We will discuss the effects of these phenomena and the trade offs that exist between the two.

Full-band electronic structure calculation of semiconductor nanostructures: a reduced-order approach

Abstract: We propose an efficient reduced-order technique for electronic structure calculations of semiconductor nanostructures, suited for inclusion in full-band quantum transport simulators. The model is based on the linear combination of bulk bands obtained by the empirical pseudopotential method, combined with the use of problem-matched basis functions numerically generated from the singular value decomposition. The efficiency and accuracy of the proposed approach are demonstrated in the case of the dispersion relation of hole subbands in an unstrained GaN layer.