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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.
05/2013;
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ABSTRACT: Two alternative approximations of the electronic structure of CdTe and HgTe are proposed, both suited to the needs of accuracy
and numerical efficiency of full-band carrier transport simulation: a local empirical pseudopotential (EPM) parametrization
including relativistic corrections, and an original full-Brillouin-zone (FBZ)
\user2 k\user2 p{{\user2 {k}}\cdot {\user2 {p}}} model using two expansion points (Γ and W). The EPM and
\user2 k\user2 p{{\user2 {k}}\cdot {\user2 {p}}} band structures closely match the available experimental and abinitio information, complemented with the results of new density functional theory (DFT)-local density approximation (LDA) calculations,
for the conduction and valence bands relevant in transport phenomena. The EPM description of the binary compounds, featuring
transferable Te pseudopotentials, is the basis for a computation of the electronic structure of the ternary alloy Hg1−x
Cd
x
Te in the framework of disorder-corrected virtual crystal approximation. The composition dependence of energy gaps, effective
masses, and high-frequency dielectric constants are discussed and compared with available experimental data, and the novel
FBZ approach is applied to the case of x=0.7.
Journal of Electronic Materials 04/2012; 38(8):1717-1725. · 1.47 Impact Factor
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ABSTRACT: Band-to-band Auger recombination mechanisms in HgCdTe are investigated as functions of temperature in the small modulation
limit by using realistic electronic structures obtained by empirical pseudopotential calculations and their corresponding
wavevector-dependent dielectric functions. The calculated Auger lifetimes are compared with the semiempirical Beattie-Landsberg-Blakemore
(BLB) model, which has been extensively employed to reproduce experimental data. The Auger-1 lifetime can be fitted well to
the BLB model with a constant overlap integral |F
1
F
2|=0.16, near the lower limit of the range reported in the literature. The role of the Auger-7 process in p-type HgCdTe is also investigated and the ratio γ between the intrinsic Auger-7 and Auger-1 lifetimes is found to be about 10.
KeywordsAuger recombination–HgCdTe–avalanche photodiodes–Monte Carlo transport simulation
Journal of Electronic Materials 04/2012; 40(8):1663-1667. · 1.47 Impact Factor
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ABSTRACT: This work presents a numerical simulation study of HgCdTe-based avalanche photodetectors (APDs). The two-dimensional model
used is based on a full-band Monte Carlo approach in which the electronic structure is computed using a nonlocal empirical
pseudopotential model with spin–orbit corrections. The carrier–phonon scattering rates have been computed from first principles
using a rigid pseudo-ion model. The most attractive feature of these devices is the potential for single-carrier ionization
when electrons are used as the primary injection carrier. For this reason, this work focuses on two front-illuminated (electron-injection)
device structures: a planar diffused PIN structure and a planar diffused PN photodiode with guard rings. To predict the performance
of these APDs, the electron multiplication gain has been studied as a function of the position where photogenerated carriers
are injected and as a function of the curvature of the p-type diffusion region. We find that, in the diffused PIN structure, the limited lateral spatial extent of the high-electric-field
region leads to a reduction of the multiplication gain from the center of the device to the periphery. Furthermore, the higher
the curvature, the more abruptly the gain decreases. For the simple PN structure, we find that the presence of the guard rings
removes the high electric field from the surface and induces a more gradual roll-off of the gain from the center of the device
to the periphery.
KeywordsHgCdTe–avalanche photodetectors–eAPD–numerical simulation–Monte Carlo–multiplication gain
Journal of Electronic Materials 04/2012; 40(8):1651-1656. · 1.47 Impact Factor
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ABSTRACT: The low-field electron mobility in bulk ZnO has been the object of extensive experimental studies, mainly through measurement
of the temperature-dependent Hall effect. In this work, we reassess the experimental results through direct simulations of
Hall measurements, performed with a Monte Carlo transport model and taking into account all the major scattering mechanisms.
The deformation potentials required to compute acoustic and optical phonon scattering are derived from first-principles computations,
and an original theory for charged-dislocation-line scattering is proposed and implemented. Monte Carlo results for the electron
mobility and the Hall factor are compared with analytical expressions derived with the relaxation-time approximation, which
is found to be adequate at low temperatures where inelastic scattering effects due to optical phonons are negligible.
Journal of Electronic Materials 04/2012; 38(8):1677-1683. · 1.47 Impact Factor
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ABSTRACT: 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 p{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.
Optical and Quantum Electronics 04/2012; 40(14):1135-1141. · 0.82 Impact Factor
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ABSTRACT: A full-band Monte Carlo model has been developed for understanding the carrier multiplication process in HgCdTe infrared avalanche
photodiodes. The proposed model is based on a realistic electronic structure obtained by pseudopotential calculations and
a phonon dispersion relation determined by abinitio techniques. The calculated carrier–phonon scattering rates are consistent with the electronic structure and the phonon dispersion
relation, thus removing adjustable parameters such as deformation potential coefficients. The computation of the impact ionization
transition rate is based on the calculated electronic structure and the corresponding wavevector-dependent dielectric function.
The Monte Carlo model is applied to investigate key performance figures of long-wavelength infrared (LWIR) and mid-wavelength
infrared (MWIR) HgCdTe avalanche photodetectors such as carrier multiplication and noise properties. Good agreement is achieved
between simulations and experimental results. The multiplication process in LWIR (λ
c=9.0μm at 80K) and MWIR (λ
c=5.1μm at 80K) devices is found to be initiated only by electrons, as expected from excess noise measurements. This single-carrier
multiplication behavior can be traced back to the details of the computed valence-band structure and phonon scattering rates.
KeywordsAvalanche photodiodes (APDs)-HgCdTe-Monte Carlo simulation
Journal of Electronic Materials 04/2012; 39(7):912-917. · 1.47 Impact Factor
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ABSTRACT: 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.
Applied Physics Letters 12/2010; 97(23):231118-231118-3. · 3.84 Impact Factor
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ABSTRACT: High field electron and hole transport in wurtzite phase GaN is studied using an ensemble Monte Carlo method. The model includes the details of the full band structure derived from nonlocal empirical pseudopotential calculations. The nonpolar carrier-phonon interaction is treated within the framework of the rigid pseudoion approximation using ab initio techniques to determine the phonon dispersion relation. The calculated carrier-phonon scattering rates are consistent with the electronic structure and the phonon dispersion relation thus removing adjustable parameters such as deformation potential coefficients. The impact ionization transition rate is computed based on the calculated electronic structure and the corresponding wave-vector dependent dielectric function. The complex band structure of wurtzite GaN requires the inclusion of band-to-band tunneling effects that are critical at high electric fields. The electric-field-induced interband transitions are investigated by the direct solution of the time dependent multiband Schrödinger equation. The multiband description of the transport predicts a considerable increase in the impact ionization coefficients compared to the case in which tunneling is not considered. In the second part of this work it will be shown that the proposed numerical model correctly predicts the carrier multiplication gain and breakdown voltage of a variety of GaN avalanche photodetectors that have been recently fabricated by several research groups.
Journal of Applied Physics 10/2009; · 2.17 Impact Factor
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ABSTRACT: The coming to age of GaN-based ultraviolet avalanche photodiodes (APDs) has made them increasingly preferred over PIN photodetectors in several areas spanning from communication to defense systems, and from commercial to scientific applications. In this work, which is the second article of a two-part series, we study the physics and performance of GaN APDs using the full-band Monte Carlo (FBMC) model described in Part I. The proposed FBMC model is based on a realistic electronic structure obtained by pseudopotential calculations and a phonon dispersion relation determined by ab initio techniques. We determine the key performance figures such as the carrier multiplication gain and the breakdown voltage for several GaN APD structures that have been fabricated by a number of experimental groups. The calculated electron and hole multiplication gains as a function of the applied bias, as well as the breakdown voltage, are found to be in good agreement with the experimental data available. Based on the FBMC results we also propose an efficient recurrence equation model, which provides a first-order estimate of the multiplication gain without resorting to the full fledge microscopic approach.
Journal of Applied Physics 10/2009; · 2.17 Impact Factor
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ABSTRACT: 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.
Journal of Applied Physics 11/2007; · 2.17 Impact Factor
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ABSTRACT: Electron and hole transport in wurtzite phase ZnO is studied using an ensemble full band Monte Carlo method. The model includes
an accurate description of the electronic structure obtained with the nonlocal pseudopotential method and numerically calculated
impact ionization transition rates based on a wavevector-dependent dielectric function. Results of transport simulations at
both low and high electric fields are presented. It is found that the low field electron mobility is close to 300cm2V−1s−1 at room temperature, and the peak electron drift velocity is 2.2×107cm/s at a field of 275kV/cm. The determination of the ionization coefficients is affected by some uncertainties due to the
incomplete knowledge of the high energy phonon scattering rates. Nevertheless, the present calculations of the ionization
coefficients provide a reasonably accurate estimate of the impact ionization process.
Journal of Electronic Materials 07/2007; 36(8):857-863. · 1.47 Impact Factor
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ABSTRACT: 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.
Journal of Applied Physics 06/2007; 101(12):123706-123706-8. · 2.17 Impact Factor
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03/2007: pages 69 - 93; , ISBN: 9783527610723
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03/2007: pages 117 - 143; , ISBN: 9783527610723
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ABSTRACT: The article addresses the frequency conversion of low-frequency noise deriving from trap-assisted generation-recombination (GR) noise in RF devices under forced, nonlinear operation through a physics-based noise model. The superposition of the stationary (small-signal) GR spectra originating from noninteracting trap levels with properly distributed energies is shown, in simple yet significant device case studies, to yield a 1/f or 1/f-like behaviour over a prescribed frequency range. The same trap distribution is also exploited for large-signal, cyclostationary noise simulation in forced periodic conditions. In this case, low-frequency 1/f-like noise is shown to be upconverted from the baseband to all noise sidebands. Circuit-level compact modeling strategies for noise-frequency conversion based on the modulation of small-signal low-frequency noise are also investigated and compared to the fundamental approach. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2006.
International Journal of RF and Microwave Computer-Aided Engineering 11/2005; 16(1):4 - 12. · 0.59 Impact Factor
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ABSTRACT: The evaluation of the microwave propagation characteristics of traveling-wave optoelectronic devices by the full-wave finite element method leads to large generalized eigenvalue equations which have to be solved for each frequency point. This paper describes a novel approach, based on the singular value decomposition, which drastically reduces the size of the eigenvalue problem without loss of accuracy. Moreover, by employing higher order elements, conductor losses are evaluated with a smaller number of unknowns, and the discontinuity of the normal field components at material interfaces is correctly modeled. The effectiveness of our numerical technique is demonstrated by discussing applications to electrooptic and electroabsorption modulators.
Microwave Conference, 2001. 31st European; 10/2001
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ABSTRACT: The Schwarz--Christoffel toolbox, a free MATLAB package for the computation of conformal maps, is applied to the quasi-static analysis of coplanar waveguides (CPWs) of arbitrary cross section in order to provide computationally efficient and very accurate estimates of their capacitance, inductance, characteristic impedance, and skin-effect attenuation. A few examples of many-sided polygonal waveguides are discussed, and the trapezoidal CPW, important, for example, for electrooptic modulators, is described in full detail, providing general guidelines for the electrode geometry optimization. The technique is validated through a comparison with the results of a full-wave finite-element method, and excellent agreement is demonstrated both in vacuo and with two-layer dielectric substrates.
09/2001;
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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.
Solid-State Electronics.
