Symposium : T
|Dilute Nitride I : Chair to be confirmed|
|14:00||Surface studies of nitrides and Bi-covered III-V substrates|
Authors : P. Laukkanen, M.P.J. Punkkinen, H. Levämäki, K. Kokko, J. Lång, M. Tuominen, V. Tuominen, J. Dahl, M. Kuzmin, J. Puustinen, V.-M. Korpijärvi, V. Polojärvi, A. Aho, A. Tukiainen, M. Guina
Affiliations : University of Turku, Finland; Tampere University of Technology, Finland;
Resume : Engineering of well-defined interfaces on semiconductors is relevant to improving various device materials and also our understanding of the interface properties. The task is not simple because semiconductor surfaces are easily reconstructed and because they strongly react with many adsorbates and impurities. The surface engineering/science might provide a solution to the problem. In this talk, the results concerning (i) behavior of bismuth on III-V surfaces, (ii) effects of the insulator-cap layers on dilute nitride quantum wells, and (iii) initial stages of InN growth on Si(111) are presented. We also show surface-science data which elucidate the N-bonding sites in GaAsN materials.
|14:30||Tuning of the optical properties of In-rich InxGa1-xN (x=0.8-0.4) alloys by light-ion irradiation at low energy|
Authors : M. De Luca1, G. Pettinari1 , A. Polimeni1, M. Capizzi1, G. Ciatto2, L. Amidani3, F. Boscherini3, F. Filippone4, A. Amore Bonapasta4, A. Knübel5, V. Lebedev5, V. Cimalla5, and O. Ambacher5
Affiliations : 1 Dipartimento di Fisica, Sapienza Università di Roma, Piazzale A. Moro 2, 00185 Roma, Italy 2 Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif sur Yvette Cedex, France 3 Department of Physics and CNISM, University of Bologna, V. le C. Berti Pichat 6/2, 40127 Bologna, Italy 4 CNR-Istituto di Struttura della Materia (ISM), Via Salaria Km 29.5, CP 10, I-00016 Monterotondo Stazione, Italy 5 Fraunhofer Institute for Applied Solid State Physics, Tullastr. 72, 79108 Freiburg, Germany
Resume : The effects of H and He irradiation in In-rich InxGa1-xN (x=0.8-0.4) are investigated by optical and structural techniques. Photoluminescence, PL, and PL excitation, PLE, measurements show a remarkable blue-shift of the emission and absorption edge energies, respectively. The extent of such blue-shift increases with H dose (a 0.42 eV maximum is found for x=0.8) and gradually decreases with decreasing In molar fraction (zero for x=0.4). Annealing experiments show the existence of two kinds of complexes responsible for the blue-shift: one dissociates at 300 °C, the other, responsible of a third of the total PL/PLE blue-shift, persists after 36 hours at 350 °C. He-ion irradiation indicates that the latter complex is due to damage-induced defects, whilst the former complex is related to the high chemical reactivity of H in the lattice. Both defects give rise to donor levels and a Moss-Burstein shift, which disappear as soon as the Fermi stabilization energy crosses the conduction band edge (x=0.40). Moreover, strictly H-related complexes could induce a genuine band gap increase of InxGa1-xN. The microscopic origin of these effects is addressed by X-ray absorption measurements. Experiments at the In edge indicate an increasing disorder in the second nearest neighbour shell of the absorbing atom upon H irradiation exclusively. Polarization measurements at the N threshold suggest a preferential orientation of the H-related defects along the wurtzite c-axis.
|14:45||In-plane band gap profiling by laser writing of hydrogenated Ga(AsN)|
Authors : N. Balakrishnan1, G. Pettinari1, O. Makarovsky1, A. Patanè1, M. W. Fay2, A. Polimeni3, M. Capizzi3, F. Martelli4 and S. Rubini4
Affiliations : 1School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK; 2Nottingham Nanotechnology and Nanoscience Centre, University of Nottingham, Nottingham NG7 2RD, UK; 3Dipartimento di Fisica, Sapienza Università di Roma, Piazzale A. Moro 2, 00185 Roma, Italy;
Resume : Hydrogen atoms are highly mobile and reactive elements that passivate both deep and shallow crystal defects and impurities in semiconductors . Here we use a focused laser beam for in-plane band gap profiling of a hydrogen containing GaAs/Ga(AsN) quantum well (QW). Laser excitation provides a means of breaking the H-N bonds within the crystal lattice through photon-assisted processes thus modifying the band structure at the nanoscale . The influence of laser-induced heating and photon-assisted phenomena on the band gap profiling is investigated by micro-photoluminescence and Secondary Electron imaging, thus revealing the mechanisms and the optimal conditions for in-plane laser writing. We pattern band gap profiles of different shapes (triangular and step-like) in the plane of the QW with submicron spatial resolution and high energy accuracy (~1 meV). Thermal annealing studies show that the light emitting areas created by laser are stable up to temperatures T ~ 200 C and that they can be erased at T > 250 C. The proposed in-plane band-gap profiling offers flexibility in the control of the electronic properties of dilute nitrides, thus paving the way for a fast and flexible fabrication approach to nanotechnologies.  C.G. Van de Walle and J. Neugebauer. Nature, 423, 626 (2003).  N. Balakrishnan et al. APL, 99, 021105 (2011).
|15:00||Electron irradiation promoted enhancement of photoluminescence from 1-eV GaInNAs-on-GaAs epilayers|
Authors : E.-M. Pavelescu , J. Puustinen , N. Baltateanu , I. Spanulescu , M. Guina 
Affiliations :  National Institute for Research and Development in Microtechnologies, Erou Iancu Nicolae 126A, 077190 Bucharest , Romania;  Optoelectronics Research Centre, Tampere University of Technology, P.O.Box 692, 33101 Tampere, Finland;  Physics Faculty, Hyperion University, Calea Calarasilor 169, 030615, Bucharest, Romania
Resume : Besides a growing interest of researchers towards the fabrication of GaInNAs/GaAs quantum-wells for 1.3-μm GaAs-based laser diodes, thicker GaInNAs films lattice matched to GaAs substrates are also very attractive for a number of applications, e.g., photovoltaic cells. An addition of a 1-eV band gap GaInNAs cell to a multijunction InGaP-GaAs structure can improve the internal quantum efficiency to record values beyond 50 %. Unfortunately, this alloy is metastable and contains a large number of point defects. The density of defects can be reduced by annealing but annealing induces a large undesired blue shift (BS) in band gap of GaInNAs. This BS is facilitated by the presence of defects. Electron irradiation is a straightforward way to generate point defects in semiconductors. In this work, we have investigated, for the first time, the influence of 7-MeV electron irradiation (1x1014 cm-2 dose) on photoluminescence (PL) from 1-eV GaInNAs epilayers lattice matched to GaAs. The PL has seen to noticeably enhance (35 %) directly upon the irradiation. Upon a thermal treatment (900 oC for 1 min) the PL of the irradiated sample further increased and became much narrower compared to the non-irradiated sample, with no additional BS of PL. This irradiation-promoted enhancement in PL upon annealing was accompanied by very small changes in the alloy macroscopic composition or structure with negligible additional changes in the amount of In-N when compared to the non-irradiated sample.
|15:15||Quasi-Temperature Stable Luminescence at 1.3 μm from Flash Lamp Annealed GaAs|
Authors : Kun Gao1, S. Prucnal1, W. Anwand2, W. Skorupa1, M. Helm1, Shengqiang Zhou1
Affiliations : 1. Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), P.O. Box 510119, 01314 Dresden, Germany 2. Institute of Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf, P.O. Box 51 01 19, 01314 Dresden, Germany
Resume : GaAs is being widely used in optical communication devices in virtue of its outstanding luminescent and electronic performances. Many approaches have been applied to GaAs based materials, to realize the luminescence in fiber-optic transmission windows. In this contribution, we present a novel method to achieve the 1.3 μm light emission by defect-induced luminescent centers. N, Bi and Mn were doped into GaAs wafers by ion-implantation and then their incorporation into GaAs lattice was carried out by flash lamp annealing (FLA). The optical and structural properties of the samples were investigated by micro-Raman spectroscopy, temperature-dependent photoluminescence and positron annihilation spectroscopy. For the intrinsic and the N or Bi incorporated GaAs, a strong luminescence peak occurs at 1.3 μm. On the other hand, Mn-doping had suppressed this luminescence. Results have shown that for the 1.3 μm emission the donor and acceptor pairs are responsible. Furthermore, it is noticeable that the 1.3 μm light emission exhibited outstanding thermal stability (e.g., 20nm red-shift and 58% intensity decline as temperature rose from 20 K to room temperature). Our investigation suggests that GaAs treated by flash lamp annealing (i.e. a chip-relevant technology) presents a promising prospect on applications of light emitters and detectors for optical communication devices.
|15:30||Role of light emission and sub-bandgap defects in the dielectric response of semiconductor junction under charge carrier injection|
Authors : Kanika Bansal, Shouvik Datta
Affiliations : Division of Physics, Indian Institute of Science Education and Research, Pune 411021, Maharashtra, India
Resume : To understand the dielectric behavior of the material during light emission and specifically during lasing, one requires a general understanding of electronic processes when the optically active junction is under significant charge carrier injection. With this intent, we studied dielectric response of red electroluminescent devices under charge carrier injection (forward bias). We used impedance spectroscopy and voltage modulated electroluminescence spectroscopy at frequencies lower than 100 kHz (K. Bansal and S. Datta, J. Appl. Phys. 110, 114509, 2011). Results show that the reactance of the device acquires inductive like behavior with increasing bias, identified as negative capacitance. Magnitudes of negative capacitance and voltage modulated electroluminescence increase when modulation frequency is lowered and their onsets shift to lower biases. Similar dependence of both electrical and optical properties on frequency indicates the participation of sub-bandgap defects in the radiative recombination dynamics. We explain the origin of frequency dependent negative capacitance as a competition between fast (radiative recombination) and slow (charge carrier trapping and de-trapping from defect levels) processes governing the charge carrier dynamics in these red electroluminescent devices. We developed voltage modulated electroluminescence spectroscopy to qualitatively characterize such active defects under forward bias. However, quantitative characterization of defects is not trivial in this case due to the presence of a large number of free charge carriers. Conventional defect characterization techniques (DLTS, TPC etc.) used for reverse bias Schottky junctions fail in this case. These techniques are based on depletion approximation which breaks down under significant charge carrier injection. Therefore, one needs to develop new techniques or generalize the existing ones to quantitatively characterize and analyze such devices under heavy charge carrier injection. However, the observed low frequency response can cause the efficiency compromise at high frequency applications such as direct modulation of laser diodes and light emitting diodes in optical communication. Hence the characterizations we have done can be useful during device design for optimum performance. We further studied non- electroluminescent devices such as Silicon diodes under forward bias to get generalized understanding. In this case, frequency dependence of negative capacitance shows exactly opposite trend than that of red electroluminescent devices. We anticipate that the slow and fast processes which compete and give rise to frequency dependent negative capacitance couple differently in Silicon which is intrinsically inefficient for light emission. To comprehend the charge carrier dynamics of a forward bias junction in general, with plenty of injected charge carriers and defects playing role, one may need to go beyond the electrostatic description of the junction based on Poisson’s equation. Further work is being pursued in this direction. The similar studies would be useful to carry out on laser diodes near lasing threshold for better insight of the lasing process.
|15:45||An optically detected magnetic resonance study of effects of hydrogenation on non-radiative defects in GaNP and GaNAs alloys|
Authors : D. Dagnelund 1, I. P. Vorona 1,2, G. Nosenko 1,2, X. J. Wang 1,3, C. W. Tu 4, H. Yonezu 5, A. Polimeni 6, M. Capizzi 6, W. M. Chen 1, and I. A Buyanova 1
Affiliations : 1 Department of Physics, Chemistry and Biology Linköping University, S-581 83 Linköping, Sweden 2 Institute of Semiconductor Physics of National Academy of Sciences of Ukraine, Kiev 03028, Ukraine 3 National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 200083 Shanghai, China 4 Department of Electrical and Computer Engineering, University of California, La Jolla, California, USA 5 Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Toyohashi, Aichi, 441-8580, Japan 6 INFM and Dipartimento di Fisica, Universita` di Roma “La Sapienza”, Piazzale A. Moro 2, I-00185 Roma, Italy
Resume : For full exploration of dilute nitrides in device applications, a better understanding and control of defects which dominate carrier recombination processes and severely degrade performance of devices is required. Post-growth hydrogenation is a common method that has been employed for decades to passivate defects in semiconductors. In this work, effect of post-growth hydrogenation on defects and their role in carrier recombination in molecular beam epitaxial GaNP and GaNAs alloys are examined by photoluminescence and optically detected magnetic resonance techniques. It is found that in GaNP, H incorporation surprisingly leads to effective activation of several defects. Among them, two defect complexes are identified to contain a Ga interstitial atom in their cores and may also involve a H atom as a partner. The observed activation critically depends on the presence of N in the alloy, as it does not occur in GaP with a low level of N doping. Carrier recombination via these defects is shown to efficiently compete with the near band-edge radiative recombination, explaining the observed degraded optical quality of the alloys after the H treatment. In sharp contrast, in GaNAs hydrogen is found to efficiently passivate Ga interstitial related defects present in the as-grown material. A possible mechanism responsible for the observed difference in the H behavior in GaNP and GaNAs will be discussed.
|Dilute Nitride II : Chair to be confirmed|
|16:30||Optical transitions between localized and delocalized states in dilute nitrides studied by photoreflectance and micro-photoluminescence|
Authors : Robert Kudrawiec
Affiliations : Institute of Physics, Wroclaw University of Technology, Wybreze Wyspianskiego 27, 50-370 Wroclaw, Poland
Resume : Photoreflectance (PR) is known as a very sensitive and nondestructive absorption-like technique to investigate optical transitions between delocalized states (band-to-band and free exciton transitions) in bulk-like materials and quantum wells. In contrast to PR, photoluminescence (PL) is an emission-like technique, which is very sensitive to optical transitions between localized states. In micro-PL measurements the excitation area is reduced to the diameter of ~1 um and hence sharp PL lines, which are related to individual excitons localized on different potential fluctuations, can be resolved in micro-PL spectra [1, 2]. The combination of the two techniques (PR and micro-PL) allows to perform very careful studies of various properties/effects in dilute nitrides including i) the carrier localization phenomenon, ii) the character of optical transitions, iii) channels of radiative and non-radiative recombination, iv) activation energies of localized excitons, v) the free exciton binding energy, and vi) the temperature dependence of energy gap. In this presentation I am going to present a review our recent application of PR and micro-PL techniques [1-3] to study optical transitions between localized and delocalized states in dilute nitrides (GaNAs, GaInNAs, GaNAsSb, and GaInNAsSb layers and quantum wells).  Appl. Phys. Lett. 94, 011907 (2009).  Appl. Phys. Lett. 98, 131903 (2011).  J. Physics: Cond. Matter, 23 205804 (2011).
|17:00||Distribution of localized nitrogen atoms in GaAsN thin films grown by a Chemical Beam Epitaxy investigated by a photoreflectance spectroscopy|
Authors : Atsuhiko Fukuyama(1), Susumu Yamamoto(1), Akio Suzuki(1), Makoto Inagaki(2), Hidetoshi Suzuki(3), Masafumi Yamaguchi(2), and Tetsuo Ikari(1)
Affiliations : 1)Faculty of Engineering, Univ. of Miyazaki, 1-1 Gakuen Kibanadai-nishi, Miyazaki 889-2192, Japan 2)Toyota Technological Institute 2-12-1 Hisakata, Tempaku, Nagoya, Aichi 468-8511, Japan 3)Interdisciplinary Research Organization, Univ. of Miyazaki
Resume : Dilute nitride semiconductor (In)GaAsN has been expected as an absorbing layer material for ultrahigh-efficiency tandem solar cells. Several growth techniques have been used to fabricate this material. Recently, it was found GaAsN films grown by chemical beam epitaxy (CBE) technique showed high hole mobility and long minority carrier lifetime compared with that by other techniques. This improvement was related to homogeneity of N atom distribution. To evaluate homogeneity of N atoms distribution, we focused attention on the energy level of localized nitrogen state (E_N) and its temperature dependence (-dE_N/dT). This is because E_N is related to the distance between adjacent N atoms, which is modified by the temperature as well as the distribution of N atoms. The values of E_N for CBE grown GaAsN films were calculated from conduction band splits (E+ and E-) measured by a photoreflectance spectroscopy from 100 to 300 K. The estimated values were compared with those obtained by other growth technique such as molecular beam epitaxy (MBE). It was found that -dE_N/dT for CBE grown films decreased with increasing the N composition. In contrast, the -dE_N/dT for MBE grown films showed an opposite tendency. In addition, values of -dE_N/dT for CBE grown samples were smaller than those for MBE grown samples. These results suggested that the homogeneity of N atoms distribution strongly depended on the crystal growth technique and might be improved by CBE.
|17:15||Impact of strain on the microstructure of N δ-doped (In,Ga)As quantum wells|
Authors : R.Gargallo-Caballero1, E. Luna1, F. Ishikawa2 and A. Trampert1
Affiliations : 1 Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany 2 Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Resume : Nitrogen δ-doped (In,Ga)As quantum well (QW) structures emerge as an attractive solution to overcome the difficulties found in the growth of dilute nitrides, in particular those related to the structural deterioration of the alloy with increasing N content. The δ-doping approach involves a spatial constraint of the elements and the introduction of a locally high tensile strained layer during the growth. In this work, we focus on the determination, using transmission electron microscopy techniques, of the local element distribution and on the structural analysis of tensile-strained (In,Ga)As/GaAs QWs with a sub-monolayer (ML) N insertion in the center of the QW. The investigated samples were grown by plasma-assisted molecular beam epitaxy. We find that the sub-ML insertion results in a several MLs-thick (In,Ga)(As,N) layer with lateral composition fluctuations. We also find an inhomogeneous In incorporation across the QW, with a minimum In content ([In]min), exactly at the position of the N-insertion, where N content is maximum ([N]max). Regardless of the position along the QW, [N]max corresponds to [In]min so that an (In,Ga)(As,N) layer of this composition has a lattice parameter close to the one of GaAs. Thus, N and In atoms tend to combine to minimize the epitaxial strain at the position of the N insertion, suggesting an intricate strain-induced element incorporation process. The influence of strain will be discussed in dependence on the amount of sub-ML insertion.
|17:30||Micro-photoluminescence investigation and simulation of thermal quenching of individual exciton lines in GaInNAs layers|
Authors : M. Latkowska1,M. Baranowski1, R. Kudrawiec 1, G. Sęk1, J. Misiewicz1, J. Ibáñez2, M. Henini3, and M. Hopkinson4
Affiliations : 1Institute of Physics, Wrocław University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wrocław, Poland; 2Institut Jaume Almera, Consell Superior d’Investigacions Científiques, 08028 Barcelona, Spain; 3School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom; 4Department of Electronic and Electrical Engineering, University of Sheffield, S3 3JD Sheffield, United Kingdom
Resume : The quality of GaInNAs alloys deteriorates significantly after nitrogen incorporation. This leads to a strong carrier localization at low temperatures. It is accepted that broad PL band, which is attributed to recombination of localized excitons, is composed of sharp lines related to individual excitons. Unfortunately, the exact nature of this lines is still unknown and controversial. In this work the authors applied the temperature-dependent micro-photoluminescence to study the thermal quenching of individual exciton lines from GaInNAs layers grown by MBE. It was observed that the localization energy varies from 0 to 150meV, whereas the activation energy for each individual line is the same within the experimental uncertainty and equals ~6meV. This observation means that the main source of sharp lines are excitons localized on deep donor- (acceptor-) like states. At low temperatures these states are able to attend in radiative recombination because of coulomb attraction between electrons and holes. We propose that the dissociation of excitons is responsible for PL quenching at low temperatures regime. Our simulations clearly show that the individual sharp PL lines observed at low temperatures appear for this material due to exciton hopping between localization centers. Taking into account saturation effect and exciton dissociation phenomenon we show that observed changes in power and temperature dependent micro-PL spectra can be excellently reproduced by our model.
|17:45||Theoretical study of broad-band gain in 1.3 μm GaInNAs quantum well optical amplifiers|
Authors : Xiao Sun, Judy Rorison
Affiliations : Dept. of Electrical & Electronic Engineering, University of Bristol; Dept. of Electrical & Electronic Engineering, University of Bristol;
Resume : It has been observed experimentally that the band edge photoluminescence of GaInNAs Quantum well (QW) materials is broadened resulting from band-tailing, localised states or conduction band edge fluctuations. N compositional fluctuations will cause conduction band edge fluctuations which localise the electrons into the resulting quantum dots. A laser model was previously shown to investigate the electron dynamics in the QW and conduction band fluctuations states using a rate equation approach. We further developed a more detailed laser model to study the gain characteristics. A broadened gain was observed (> 100 nm) if we add gain from both QW and fluctuations lasing centres. Moreover, a systematic model on electron dynamics and gain properties in the quantum well optical amplifiers is under investigation. We believe these conduction band fluctuations could lead to a more broadened gain in the GaInNAs QW optical amplifiers for optical communications. Detailed results will be presented in my talk.
|18:00||Experimental study and simulations of carrier dynamics in GaInNAs/GaAs quantum well|
Authors : M. Baranowski1, R. Kudrawiec1, M. Latkowska1, M. Syperek1 and J. Misiewicz1, J. Gupta2
Affiliations : 1 Institute of Physics, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wroclaw, Poland 2 Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Canada K1A 0R6
Resume : The incorporation of nitrogen atoms into Ga(In)As deteriorates its optical properties and characteristic features appear in the photoluminescence (PL) spectra at low temperatures such as S-shape and PL asymmetry. This phenomena are explained by the localization of excitons on a potential fluctuations induced by the nitrogen incorporation. Presence of localizing centers has also impact on a carriers dynamic in this material system. The aim of this report is to show that model of hopping excitons  is able to explain the PL dynamic in GaInNAs/GaAs QW and influence of the localizing states on it. In the following we present the results of the time resolved photoluminescence studies of GaInNAs/GaAs QW at different temperatures and corresponding PL dynamics simulations. We show that the model of hopping excitons explains all of the characteristic effects observed in the experiment - the decay times dispersion and the impact of temperature on the decay times. This model enables us to take in to account, temperature and nonradiative recombination impact on the PL spectra and dynamics. Taking a few parameters from time integrated PL measurements at low temperature (i.e. FWHM, Stoke-shift) we are able to reconstruct all of the results of temperature dependent time resolved PL measurements. Using this model we can investigate theoretically impact of localizing centers density and distribution on PL dynamics.  M. Baranowski, et al., J. Physics: Cond. Matter, 23 205804 (2011).
|18:15||Modelling of the effects of conduction band fluctuations caused by nitrogen clustering in GaInNAs materials|
Authors : kevin sun
Affiliations : university of south ampton
Resume : For future optical metro and access networks it is important to develop cost-effective, reliable components with good performance at long telecommunication wavelength window. GaInNAs/GaAs Quantum Well (QW) devices were initially proposed due to the decrease of bandgap, reduced temperature sensitivity and better lattice match to substrate GaAs for 1.3 and 1.55 µm emissions. Unlike other general III–V alloy semiconductors, the large electronegativity of N in GaInNAs and its small covalent radius cause a strong negative bowing parameter and the injection of N to InGaAs dramatically decreases the bandgap thus increases the electron confinement, as modelled successfully by the band anti-crossing (BAC) model. In the original BAC model, a single N state was assumed to be randomly located within the QW leading a splitting of the conduction band (CB) of InGaAs thus a reduction of the fundamental bandgap occurs. Recent studies have investigated N pair and N cluster states noticing they all offer turning of the conduction band position and hence the bandgap and effective mass of GaInAs with spatial resolution. This GaInNAs/GaAs material system allows novel optical and electrical components to be considered especially for multi-wavelength amplification due to its broad band gain properties.
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