The Experts below are selected from a list of 6312 Experts worldwide ranked by ideXlab platform
Matthew R. Phillips - One of the best experts on this subject based on the ideXlab platform.
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Distribution of visible luminescence centers in hydrogen-doped ZnO
Journal of Materials Research, 2011Co-Authors: Laurent L. C. Lem, Cuong Ton-that, Matthew R. PhillipsAbstract:ZnO crystals have been investigated by scanning cathodoluminescence microscopy and spectroscopy at 80 K following hydrogen incorporation by plasma exposure. The intensity of the ZnO near-band-edge (NBE) emission is greatly enhanced while the defect-related green emission is quenched following plasma treatment. These effects are attributed to the passivation of zinc vacancies by hydrogen. The green and yellow intensities and their intensity ratios to the NBE vary with Excitation depth for both undoped and H-doped ZnO crystals. The intensities of the green and yellow emissions exhibit sublinear dependencies on electron beam Excitation Density while the NBE intensity increases linearly with the Excitation Density. These saturation effects with increasing Excitation Density must be taken into account when assessing defects in ZnO by luminescence characterization.
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Cathodoluminescence efficiency dependence on Excitation Density in n-type gallium nitride.
Microscopy and microanalysis : the official journal of Microscopy Society of America Microbeam Analysis Society Microscopical Society of Canada, 2003Co-Authors: Matthew R. Phillips, Sergei O. Kucheyev, Hagen Telg, Olaf Gelhausen, Milos TothAbstract:Cathodoluminescence ~CL! spectra from silicon doped and undoped wurtzite n-type GaN have been measured in a SEM under a wide range of electron beam Excitation conditions, which include accelerating voltage, beam current, magnification, beam diameter, and specimen temperature. The CL intensity dependence on Excitation Density was analyzed using a power-law model ~ICL @ J m ! for each of the observed CL bands in this material. The yellow luminescence band present in both silicon and undoped GaN exhibits a close to cube root ~m 5 0.33! dependence on electron beam Excitation at both 77 K and 300 K. However, the blue ~at 300 K! and donor-acceptor pair ~at 77 K! emission peaks observed in undoped GaN follow power laws with exponents of m 5 1 and m 5 0.5, respectively. As expected from its excitonic character, the near band edge emission intensity depends linearly ~m 5 1! in silicon doped GaN and superlinearly ~m 5 1.2! in undoped GaN on the electron beam current. Results show that the intensities of the CL bands are highly dependent not only on the defect concentration but also on the electron-hole pair Density and injection rate. Furthermore, the size of the focussed electron beam was found to have a considerable effect on the relative intensities of the CL emission peaks. Hence SEM parameters such as the objective lens aperture size, astigmatism, and the condenser lens setting must also be considered when assessing CL data based on intensity measurements from this material.
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Effects of Excitation Density on cathodoluminescence from GaN
Applied Physics Letters, 2001Co-Authors: Sergei O. Kucheyev, Milos Toth, Matthew R. Phillips, James Williams, Chennupati JagadishAbstract:Wurtzite GaN epilayers are studied by cathodoluminescence ~CL! spectroscopy. Results show that the intensities of donor‐acceptor pair ~DAP! and yellow luminescence ~YL! peaks sublinearly depend on Excitation Density, presumably, due to saturation effects. The intensity of near-gap emission, however, exhibits a superlinear dependence on electron-beam Excitation. In contrast to photoluminescence measurements, CL studies of GaN are usually performed in a regime with a strongly nonlinear dependence of luminescence intensities on Excitation due to a large difference in carrier generation rates for these two techniques. As a result, the ratios of near-gap to YL and DAP emission intensities strongly depend on electron-beam current. Moreover, electron-beam spot size ~i.e., beam focusing! dramatically affects CL intensity. An understanding of such saturation effects is necessary for a correct interpretation of CL spectra from GaN. © 2001 American Institute of Physics. @DOI: 10.1063/1.1408273#
Hideyo Okushi - One of the best experts on this subject based on the ideXlab platform.
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Excitation Density dependence of luminescence spectrum of electron-hole plasma in diamond
Diamond and Related Materials, 2009Co-Authors: Yoshihiro Sakamoto, Kazuro Murayama, Yasushiro Nishioka, Hideyo OkushiAbstract:Abstract Time-resolved band edge luminescence spectrum in IIa diamond has been measured with the 5th harmonics of a pulsed YAG laser (5.82 eV) and an ICCD image intensifier of 5 ns gate width at 290 K. The time-resolved luminescence spectrum is decomposed into three components of free exciton (FE), excitonic complex (EC) and electron-hole plasma (EHP). The decay times of the FE and EC luminescence are 45 and 27 ns, respectively and that of the EHP luminescence has been seen to be shorter than the gate width, 5 ns. The low energy onset of the EHP luminescence spectrum has been observed to decrease with increasing Excitation Density and attains the onset of the electron-hole drop luminescence spectrum at the Excitation Density of 0.6 J/cm 2 , at which the electron-hole pair Density is 1.2 × 10 20 cm − 3 . Furthermore, the Excitation Density dependences of the FE, EC and EHP luminescence intensities are explained with the percolation theory.
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Free-exciton luminescence spectrum broadening due to excitonic complex in diamond
Diamond and Related Materials, 2006Co-Authors: Kazuro Murayama, Yoshihiro Sakamoto, T. Fujisaki, Satoshi Yamasaki, Hideyo OkushiAbstract:Abstract The Excitation Density evolution of a band-edge luminescence spectrum of diamond in the photon energy region 5.14 to 5.40 eV was investigated at 190, 240 and 290 K with the fifth harmonic (5.82 eV) of a pulsed YAG laser in the Excitation Density range 2.2 to 272 mJ/cm 2 . A broadening of a free-exciton luminescence (FE) spectrum with the peak energy ( E peak ) of 5.28 eV due to the growth of the low-energy tail was observed with increasing Excitation Density. From a detailed spectrum analysis, it is found that the broadening originates in excitonic complex (EC) luminescence ( E peak = 5.26 eV) in addition to electron-hole plasma (EHP) luminescence ( E peak = 5.23 eV). The EC luminescence intensity was observed to be proportional to the 1.5 power of the FE luminescence intensity, which is the same manner as in the case of the EC luminescence in crystalline silicon. The Excitation Density dependence of the EHP luminescence intensity is discussed with percolation theory.
Matteo Vannini - One of the best experts on this subject based on the ideXlab platform.
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InGaN/GaN multiple quantum well for superfast scintillation application: Photoluminescence measurements of the picosecond rise time and Excitation Density effect
Journal of Luminescence, 2019Co-Authors: Guido Toci, Martin Nikl, Leonida A. Gizzi, P. Koester, F. Baffigi, Lorenzo Fulgentini, Luca Labate, Alice Hospodková, Vitezslav Jary, Matteo VanniniAbstract:Abstract We report the study of the fast rise time and decay time in the ps time scale of the excitonic luminescence of a multiple quantum well (MQW) heterostructure of InGaN/GaN, including the Excitation Density effect. These structures were proposed as ultrafast scintillators for soft X-ray detectors and particle beam diagnostics. Measurements were carried out with a Hadland Imacon 500 streak camera following Excitation of the sample by laser pulses of few tens of µJ at 266 nm and 400 nm, with pulse duration less than 200 fs. The rise time of the detected MWQ luminescence was less than 10 ps, with a possible contribution from the detection system and signal collection geometry. The calibration and the signal processing techniques employed to fully exploit the time resolution of the detection system are also described.
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The role played on the Yb:LuAG laser performance by high doping levels and high ion Excitation Density
CLEO: 2014, 2014Co-Authors: Angela Pirri, Vladimir Babin, Guido Toci, Martin Nikl, Matteo VanniniAbstract:We present the laser performance achieved by 15 at.% Yb:LuAG crystal. Experimental evidences of a non-linear loss mechanism which occurs at high ion Excitation Density is observed and characterized.
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effects of the Excitation Density on the laser output of two differently doped yb yag ceramics
Optics Express, 2010Co-Authors: Angela Pirri, Guido Toci, Daniele Alderighi, Matteo VanniniAbstract:We report the behavior of two Yb3+ doped ceramics (i.e. 10% at. and 20% at.) under quasi-continuous wave laser pumping. Two different behaviors are found depending on the Density of Yb3+ in the excited level. Experimental results show that at low population inversion Density, the maximum output power and the efficiency are almost independent on the doping concentration. In particular, an output power as high as 8.9 W with a corresponding slope efficiency of 52% with respect to the injected pump power was reached with the 20% at. sample. Conversely, at high population inversion densities, the 20% doped sample shows a sudden decrease of the laser output for increasing pump power, due to the onset of a nonlinear loss mechanism. Finally, we report a comparison of the experimental results with numerical simulations for the evaluation of the inversion Density and of the temperature distribution.
Ryoichi Akimoto - One of the best experts on this subject based on the ideXlab platform.
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Type-I interband transition in undoped ZnSe/BeTe type-II quantum wells under high Excitation Density
Semiconductor Science and Technology, 2009Co-Authors: Hirofumi Mino, Kenichi Oto, Ryoichi AkimotoAbstract:A spatially direct photoluminescence (PL) spectrum associated with type-I interband transition of a ZnSe layer in undoped ZnSe/BeTe type-II quantum structures was investigated by varying the photo-Excitation Density. For a sample with a narrower ZnSe layer both PL intensity and linewidth of the trion show a superlinear increase with increasing Excitation Density in comparison to that with a wider ZnSe layer. The results are explained by the effective increase of the electron concentration and an enhanced dephasing rate of the trion resulting from the electron–trion scattering. The effective increase of the electron concentration in the ZnSe layer is considered to be originating from both the greater transfer rate of the hole into the BeTe layer due to the narrower ZnSe layer and the efficient spatially indirect transition PL of the complex states through the interface. With decreasing Excitation Density, no indication of any shift in peak energy Density was observed indicating that the studied structures are of excellent quality.
Adnen Melliti - One of the best experts on this subject based on the ideXlab platform.
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Influence of the Excitation Density and temperature on the optical properties of type I InAs/GaAsSb quantum dots
Journal of Luminescence, 2020Co-Authors: Afef Ben Mansour, Adnen Melliti, Rihab Sellami, Mohamed Souhail Kehili, A. Salhi, Radhouane ChtourouAbstract:Abstract In this study, the optical properties of InAs quantum dots (QDs) were characterized using photoluminescence (PL) measurements. The QDs, capped with GaAs and GaAs1−xSbx (x = 6%) strain-reducing layer (SRL), were grown by Molecular Beam Epitaxy. Temperature-dependent photoluminescence (TDPL) of both ground state (GS) and first excited state (ES) was carried out through the analysis of the PL peak position as well as the integrated PL intensity. The temperature dependence of the integrated PL intensity shows the carrier trapping in the potential barrier at the interface between the capping layer and QDs in both samples at low temperatures for the GS and ES. The Excitation Density dependent photoluminescence (EDPL) showed a redshift of the GS and ES PL peak energies with increasing Excitation Density. We attribute this variation to the bandgap renormalization (BGR) effect. The potential barrier reduction for the GaAsSb-capped QDs increases carrier injection efficiency inside the QDs, giving rise to a larger BGR effect compared to the QDs capped with GaAs. With increasing temperature, BGR redshift varies considerably for the GaAs- capped QDs but less for the Q with GaAsSb SRL. This effect was explained using the population rate of carriers inside the QDs while taking into account the nonradiative recombination process for the two samples. Furthermore, the variation of the integrated intensity with the Excitation power Density grows superlinearly for the two samples for a temperature range from 10 K to 220 K. This behavior was explained by the random capture of carriers in the dots. The sample with GaAsSb SRL having a smaller potential barrier has reduced superlinear dependence at low temperatures. Losses mechanism has a significant impact on increasing the superlinear dependence at high temperatures. The ES showed a stronger superlinearity compared to the GS. This study helps to understand the optical mechanisms in some devices, such as QD lasers.
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The effect of the Excitation and of the temperature on the photoluminescence circular polarization of AlInAs/AlGaAs quantum dots
Applied Surface Science, 2009Co-Authors: N. Sellami, Adnen Melliti, Abdeljalil Sahli, M. A. Maaref, Christophe Testelin, R. KuszelewiezAbstract:Abstract In this paper, we present a study of photoluminescence (PL) from AlInAs/AlGaAs quantum dots (QDs) structures grown by molecular beam epitaxy. Specifically, we describe the effects of the temperature and of the Excitation Density on the photoluminescence circular polarization. We have found that the circular polarization degree depends on temperature. On the other hand, the study of the Excitation Density dependent circular polarization PL degree shows that the last increases in the case of the sample of weak dot Density. However, in the case of large dot Density, it is almost constant in the Excitation Density range from 0.116 W cm−2 to 9 W cm−2.
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Optical anisotropy and photoluminescence Excitation Density dependence for auto-organized Al0.28In0.72As/Al0.28Ga0.72As quantum dots
Physica E: Low-dimensional Systems and Nanostructures, 2005Co-Authors: Adnen Melliti, Abdeljalil Sahli, Walid Ouerghi, Karima Kerkani, M. A. Maaref, Aristide Lemaître, Paul VoisinAbstract:Abstract In this paper, we present a study of photoluminescence (PL) from Al0.28In0.72As/Al0.28Ga0.72As quantum dot (QD) structures grown by molecular beam epitaxy. We have found that the surface-emitted PL is linearly polarized with a polarization degree of 0.2. The study of the Excitation-Density-dependent PL spectra reveals an unusual behaviour of the full-width at half-maximum (FWHM), which increases, then decreases with increasing Excitation Density. On the other hand, the decay time decreases from 630 to 430 ps when the Excitation Density is varied from 140 to 5600 W cm−2.
