Publications (72)43.48 Total impact
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Article: Exciton recombination dynamics in a-plane (Al,Ga)N/GaN quantum wells probed by picosecond photo and cathodoluminescence
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ABSTRACT: We present a combined low-temperature time-resolved cathodoluminescence and photoluminescence study of exciton recombination mechanisms in a 3.8 nm thick a -plane (Al,Ga)N/GaN quantum well (QW). We observe the luminescence from QW excitons and from excitons localized on basal stacking faults (BSFs) crossing the QW plane, forming quantum wires (QWRs) at the intersection. We show that the dynamics of QW excitons is dominated by their capture on QWRs, with characteristic decay times ranging from 50 to 350 ps, depending on whether the local density of BSFs is large or small. We therefore relate the multiexponential behavior generally observed by time-resolved photoluminescence in non-polar (Al,Ga)/GaN QW to the spatial dependence of QW exciton dynamics on the local BSF density. QWR exciton decay time is independent of the local density in BSFs and its temperature evolution exhibits a zero-dimensional behavior below 60 K. We propose that QWR exciton localization along the wire axis is induced by well-width fluctuation, reproducing in a one-dimensional system the localization processes usually observed in QWs.Journal of Applied Physics 03/2010; · 2.17 Impact Factor -
Article: Exciton localization on basal stacking faults in a-plane epitaxial lateral overgrown GaN grown by hydride vapor phase epitaxy
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ABSTRACT: We present a detailed study of the luminescence at 3.42 eV usually observed in a -plane epitaxial lateral overgrowth (ELO) GaN grown by hydride vapor phase epitaxy on r -plane sapphire. This band is related to radiative recombination of excitons in a commonly encountered extended defect of a -plane GaN: I<sub>1</sub> basal stacking fault. Cathodoluminescence measurements show that these stacking faults are essentially located in the windows and the N-face wings of the ELO-GaN and that they can appear isolated as well as organized into bundles. Time-integrated and time-resolved photoluminescence, supported by a qualitative model, evidence not only the efficient trapping of free excitons (FXs) by basal plane stacking faults but also some localization inside I<sub>1</sub> stacking faults themselves. Measurements at room temperature show that FXs recombine efficiently with rather long luminescence decay times (360 ps), comparable to those encountered in high-quality GaN epilayers. We discuss the possible role of I<sub>1</sub> stacking faults in the overall recombination mechanism of excitons.Journal of Applied Physics 03/2009; · 2.17 Impact Factor -
Article: Time-resolved spectroscopy on GaN nanocolumns grown by plasma assisted molecular beam epitaxy on Si substrates
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ABSTRACT: A detailed study of excitons in unstrained GaN nanocolumns grown by plasma assisted molecular beam epitaxy on silicon substrates is presented. The time-integrated and time-resolved photoluminescence spectra do not depend significantly on the (111) or (001) Si surface used. However, an unusually high relative intensity of the two-electron satellite peak of the dominant donor-bound exciton line is systematically observed. We correlate this observation with the nanocolumn morphology determined by scanning electron microscopy, and therefore propose an interpretation based on the alteration of wave functions of excitonic complexes and of donor states by the proximity of the semiconductor surface. This explanation is supported by a model that qualitatively accounts for both relative intensities and time decays of the photoluminescence lines.Journal of Applied Physics 02/2009; · 2.17 Impact Factor -
Conference Proceeding: Radiative lifetimes of negatively charged excitons in CdTe quantum wells
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ABSTRACT: Summary form only given. Using picosecond photoluminescence (PL), we have investigated the temperature dependence of the radiative lifetime of the negatively charged exciton (X<sup>-</sup>) in CdTe quantum wells (QWs). The X<sup>$</sup>lifetime is found to increase from 70 ps at 2 K up to about 200 ps at 30 K. The increase is linear for temperatures bigger than 5 K and reveals that X<sup>$</sup>cannot be treated as a purely localised species. A linear increase is indeed predicted theoretically for delocalised X<sup>-</sup>. In addition, the observed temperature behaviour demonstrates that unlike the donor bound exciton to which X<sup>-</sup> has been previously compared, X<sup>-</sup> radiative recombination can take place with a specific strongly k dependent selection rule. The behaviour of X<sup>-</sup> lifetime at the lower temperatures indicates the presence of a certain degree of localisation.Quantum Electronics Conference, 2000. Conference Digest. 2000 International; 10/2000 -
Conference Proceeding: Carrier-carrier contribution to intersubband scattering in widequantum wells
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ABSTRACT: Summary form only given. The importance of LO phonon emission for intersubband scattering (ISBS) processes is now well established. In very good agreement with theoretical expectations, such a process gives scattering times of 1 ps or less. The situation is different in wide quantum wells, when the energy separation ΔE<sub>12</sub> between the first two subbands is smaller than hω<sub>LO</sub>. It has often been proposed that ISBS should then be mediated mainly by acoustical phonons, and be quite slow. Such wells would then be promising for laser applications. However, the demonstration of such a laser failed to be obtained. An explanation was in fact proposed in 1989 by Goodnick and Lugli who computed carrier-carrier induced intersubband scattering. It is clear from their calculations that such ISBS could be as short as a few ps, 100 times faster than acoustic phonons. A number of results have nevertheless been published since then, with scattering times ranging from 1 ps to 1 ns. We have carefully studied ISBS by using femtosecond time-resolved luminescence with resonant excitation. The advantage of luminescence in addition to the very good time resolution is the ability to probe the whole distribution of electrons. This of course needs some assumptions on the behavior of holes, but we can show that their contribution to the observed dynamics is minimalQuantum Electronics and Laser Science Conference, 1999. QELS '99. Technical Digest. Summaries of Papers Presented at the; 06/1999 -
Article: Native extended defects in Zn1-yCdySe/InxGa1-xAs heterostructures
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ABSTRACT: Lattice-matched Zn <sub>1-y</sub> Cd <sub>y</sub> Se/In <sub>x</sub> Ga <sub>1-x</sub> As heterojunctions can be fabricated by molecular beam epitaxy on GaAs(001) 2×4 surfaces in a wide range of compositions provided that a suitable strain relaxation profile is achieved within the ternary III–V buffer layer. We focus here on the structural properties of the resulting II–VI/III–V heterostructures and discuss the distribution of native defects, including misfit and threading dislocations, stacking faults, and surface corrugations. © 1998 American Vacuum Society.Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 08/1998; · 1.34 Impact Factor -
Article: Interface composition and stacking fault density in II-VI/III-V heterostructures
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ABSTRACT: The Zn/Se flux ratio employed during the early stages of molecular beam epitaxy of pseudomorphic ZnSe/GaAs(001) as well as lattice-matched ZnSe/ In <sub>0.04</sub> Ga <sub>0.96</sub> As (001) heterostructures controls the density of the native stacking faults, which have been associated with the early degradation of blue-green lasers. In particular, the density of Shockley stacking fault pairs decreases by three to four orders of magnitude and that of Frank stacking faults by one order of magnitude in going from Zn-rich to Se-rich interfaces. © 1997 American Institute of Physics.Applied Physics Letters 02/1997; · 3.84 Impact Factor -
Article: Transmission electron microscopy and cathodoluminescence of tensile‐strained GaxIn1-xP/InP heterostructures. I. Spatial variations of the tensile stress relaxation
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ABSTRACT: We have investigated the optical and structural properties of tensile‐strained Ga x In 1-x P/InP heterojunctions by cathodoluminescence (CL) in the scanning electron microscope and by transmission electron microscopy (TEM). The lattice mismatch of the samples is ranging from 0.4% (x=5.5%) to 0.84% (x=11.8%). We show, in agreement with previous studies, that the relaxation of tensile‐strained epilayers occurs by the emission of partial and perfect dislocations. The numerous twins and stacking faults which are found in the epilayers act as efficient recombination centers for electron‐hole pairs and appear as dark line defects (DLDs) in CL images. ‘‘Ladderlike’’ configurations of these defects are found both by TEM and CL in samples with a lattice mismatch larger than 0.5%. We also demonstrate that DLDs are contaminated by impurities. Areas with networks of perfect dislocations are found between the DLDs. The analysis of the dislocation types allows us to suggest that the growth of low‐mismatched samples is two dimensional, and that it is three dimensional in highly mismatched samples. Finally, the spatial variations of the strain relaxation throughout the samples are studied by 77‐K CL spectroscopic measurements and it is shown that these variations can be correlated with the various types of structural defects. © 1996 American Institute of Physics.Journal of Applied Physics 08/1996; · 2.17 Impact Factor -
Article: Transmission electron microscopy and cathodoluminescence of tensile‐strained GaxIn1-xP/InP heterostructures. II. On the origin of luminescence heterogeneities in tensile stress relaxed GaxIn1-xP/InP heterostructures
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ABSTRACT: We have determined the origin of the spatial luminescence fluctuations observed between the dark line defects present in tensile strained Ga x In 1-x P/InP/n<sup>+</sup>‐InP heterostructures (Part I [F. Cléton et al. J. Appl. Phys. 80, 827 (1996)]). For that purpose, we have undertaken semi‐quantitative and spectroscopic cathodoluminescence (CL) measurements on various specimens in areas exhibiting CL contrasts which could be as large as 80%. The analysis of the variation of the CL polychromatic signal with electron beam energy allowed us to get information on the diffusion‐recombination (DR) parameters of the areas under study. From the correlation between the local relaxation level of these areas and their DR parameters, we can conclude that the variation of the misfit dislocations density at the Ga x In 1-x P/InP interface is at the origin of the luminescence heterogeneities. We also demonstrate that recycling, by the Ga x In 1-x P epilayer, of the photons originating from the heavily doped InP substrate, enhances the CL contrast between areas exhibiting different relaxation levels. © 1996 American Institute of Physics.Journal of Applied Physics 08/1996; · 2.17 Impact Factor -
Article: Background Si‐doping effects on Zn diffusion‐induced disordering in GaAs/AlGaAs multiple‐quantum‐well structures
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ABSTRACT: Identical GaAs/Al 0.2 Ga 0.8 As multiple‐quantum‐well (MQW) structures uniformly doped with Si at various concentrations ranging from 1×10<sup>17</sup> to 1×10<sup>19</sup> cm<sup>-3</sup> are grown by molecular‐beam epitaxy to study the effects of the background Si‐doping level on the Zn diffusion‐induced disordering process. After Zn diffusions at 575 °C for 4 and 16 h, the structures are investigated by secondary‐ion‐mass spectrometry, and by transmission electron microscopy on cleaved wedges of the sample. The results show that the totally and partially disordered regions are always behind the Zn diffusion front. A dependence of the effective Zn diffusivity and of the disordering rate of the structures on the background Si‐doping level is observed. The effective Zn diffusivity and the disordering rate are significantly reduced with increasing background Si concentration. Before Zn diffusion, photoluminescence spectra of the Si‐doped MQW structures exhibit an increase in intensity of the Si donor–column‐III vacancy complex emission band with increasing Si‐doping level. This indicates that the concentration of column‐III vacancies in the MQW structures increases as the background Si concentration increases. After Zn diffusion, an important decrease in intensity of the column‐III vacancy related emission band is observed on the photoluminescence spectra taken in the Zn‐diffused regions. The systematical analysis of the photoluminescence spectra of the Zn‐diffused MQW structures as a function of diffusion time and as a function of etching depth below the sample surface makes it possible to describe the physical processes occurring during Zn diffusion. A model based on the ‘‘kick‐out’’ mechanism of Zn diffusion is proposed to explain the effect of the backgro- und Si‐doping level on the effective Zn diffusivity. The model shows that the effective Zn diffusivity is controlled by the concentration of column‐III interstitials behind the Zn diffusion front and by the donor concentration in the sample. During the incorporation of Zn into the crystal lattice, column‐III interstitials are generated. The supersaturation of these interstitials behind the Zn diffusion front is responsible for the enhancement of Al–Ga interdiffusion. Since column‐III interstitials and column‐III vacancies can mutually annihilate, the concentration of column‐III interstitial and column‐III vacancy in the Zn‐diffused region is reduced with increasing Si‐doping level, leading to a retardation of Zn diffusion into the MQW structure. On the other hand, a decrease of the effective Zn diffusivity caused by an increase in donor concentration in the samples is also demonstrated. Our results give evidence for the Fermi‐level effect and the interactions between different point defects during Zn diffusion‐induced disordering of GaAs/AlGaAs multilayered structures. © 1996 American Institute of Physics.Journal of Applied Physics 05/1996; · 2.17 Impact Factor -
Article: Comparison of the lateral carrier transport between a GaAs single quantum well and the AlGaAs barrier during cathodoluminescence excitation
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ABSTRACT: In a recent paper we measured the lateral hole diffusion in a GaAs/AlGaAs single quantum well (SQW) by a novel method. At helium temperature, we estimate a lateral hole diffusion length in the QW of 1.5 μm. However, the assumption that diffusion takes place mainly in the SQW needs to be checked, as the measured diffusion length is the result of two competing processes: (i) hole diffusion in the SQW plane itself and (ii) hole diffusion in the barrier followed by recombination in the SQW. We present here a comparison between the lateral hole distribution in the SQW and in the AlGaAs barrier. First, we estimate the hole diffusion length in the barrier fitting experimental cathodoluminescence linescans on simulated ones. Second, using the measured diffusion lengths in the QW plane and in the bulk barrier and modeling the carrier transport, we deduce the lateral hole distribution in both layers. It is found that even for very large barriers (1.2 μm), the hole diffusion in the barrier contributes less than 0.1% of the total lateral hole diffusion. The lateral transport is mainly carried by holes in the QW (2D diffusion) due to their confinement in the well.Journal of Applied Physics 08/1994; · 2.17 Impact Factor -
Article: Effects of post‐diffusion annealing on Zn‐diffused GaAs:Si
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ABSTRACT: After Zn diffusion into Si‐doped GaAs (n≊1.5×10<sup>18</sup> cm<sup>-3</sup>), the Zn‐diffused samples are annealed under different conditions: (i) in vacuum, (ii) in arsenic vapor, and (iii) with a Si 3 N 4 mask capping the sample surface. The Zn concentration profiles obtained by secondary‐ion‐mass spectroscopy and the photoluminescence (PL) spectra taken at different depths below the sample surface are studied in detail. After annealing in vacuum, the steep (p<sup>+</sup>‐n) Zn diffusion front advances into the bulk. We observe that the intensity ratio between the Si donor‐gallium vacancy complex (Si Ga ‐V Ga ) related emission band and the band‐to‐band (e‐h) transition is enhanced in the region ahead of the Zn diffusion front. In contrast, Zn atoms diffuse deeper into the bulk of the samples annealed in arsenic vapor with or without capping layer. These samples show the kink‐and‐tail (p<sup>+</sup>‐p‐n) Zn concentration profiles with a decrease in the intensity ratio around the tail region. The analysis of the PL data suggest a supersaturation of gallium vacancies ahead of the diffusion front of the sample annealed in vacuum and an undersaturation of this defect around the tail region of the samples annealed in As vapor. Our results underline the important role of the nonequilibrium of the defect concentration during the postdiffusion annealing, which permits explanation of the anomalous double profile of Zn by the interstitial‐substitutional mechanism.Journal of Applied Physics 12/1993; · 2.17 Impact Factor -
Article: Cathodoluminescence and photoluminescence studies of dislocations in GaAs/AlGaAs quantum wells
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ABSTRACT: The optical properties of low pressure metal organic vapor deposition grown GaAs/Al x Ga 1-x As (x=0.5) single quantum well structures (SQW) with grown‐in dislocations (GD) were studied by low temperature cathodoluminescence (CL) and photoluminescence (PL). High luminescence efficiency around the GD was observed and attributed to impurity decoration. CL spectra show a region surrounding the GD that consists of Si impurities and native defects in the SQW and barrier layers. The diameter of this region was found to be in the order of 1 μm using spectrally resolved CL micrographs.Journal of Applied Physics 09/1993; · 2.17 Impact Factor -
Article: Lateral transport in GaAs/AlGaAs quantum wells
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ABSTRACT: The lateral transport of excess carriers in metal organic chemical vapor deposition grown GaAs/Al x Ga 1-x As(x=0.5) 20 Å thick single quantum well structures is studied by cathodoluminescence (CL) between 5 K≤T≤170 K. Impurity and native defect related transitions are found to take place around grown‐in dislocations where well thickness variations are also found. The CL intensity of free excitons and/or transitions involving impurities and native defects is measured as a function of distance from the dislocation to the point of excitation. Using a simple diffusion model, we are able to determine the hole diffusion length, L=1.5 μm, in slightly n‐doped SQW. This represents a novel method for the direct determination of the diffusion length in sufficiently defect‐free material.Applied Physics Letters 07/1993; · 3.84 Impact Factor -
Article: Self‐interstitial mechanism for Zn diffusion‐induced disordering of GaAs/AlxGa1-xAs (x=0.1-1) multiple‐quantum‐well structures
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ABSTRACT: GaAs/Al x Ga 1-x As multiple‐quantum‐well (MQW) structures with identical well thicknesses but with different Al contents x in the barrier (x≊0.1, 0.2, 0.45, and 1) were grown by molecular‐beam epitaxy to study the impurity‐induced disordering mechanism. The disordering of the structures is observed directly by transmission electron microscopy on cleaved wedges of the sample, by the secondary electron imaging mode of scanning electron microscopy, and by secondary‐ion‐mass spectroscopy after Zn diffusions at 575 °C during different times (1, 4, 9, and 16 h). The results show that the totally and partially disordered regions are always behind the Zn diffusion front. The partially disordered extent depends on x. As x increases, the disordering rate increases due to the increase in Zn diffusivity. The effect of high Zn concentration is investigated by photoluminescence and by Raman scattering measurements. The systematical analysis of the photoluminescence spectra of the MQW structures diffused for different times and of the photoluminescence spectra taken on different depths below the sample surface makes it possible to describe the physical processes occurring during Zn diffusion. The column‐III vacancies are created at the sample surface. They diffuse into the bulk of the sample where they are filled by other defects. Using the x‐ray‐diffraction technique, an expansion of the lattice constant in the region behind the Zn diffusion front was observed. This is due to a supersaturation of column‐III interstitials. During the incorporation of Zn into the crystal lattice, column‐III interstitials are generated. These interstitials could be responsible for the enhancement of the Al‐Ga interdiffusion. The important role of the electric field at the p‐n junction formed by Zn diffusion is discussed. The negatively charged columnȁ- 0;III vacancies and the positively charged column‐III interstitials are confined, respectively, on the n and p sides of the p‐n junction. The results give evidence for the self‐interstitial mechanism of Zn diffusion‐induced disordering in GaAs/AlGaAs MQW structures.Journal of Applied Physics 05/1993; · 2.17 Impact Factor -
Article: Role of point defects in the silicon diffusion in GaAs and Al0.3Ga0.7As and in the related superlattice disordering
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ABSTRACT: The mechanism of silicon diffusion in GaAs, Al 0.3 Ga 0.7 As, and the silicon diffusion‐induced layer disordering of multiquantum wells have been studied by photoluminescence, secondary‐ion‐mass spectroscopy, and transmission electron microscopy across a corner of a wedge‐shaped sample. The diffusion source was a grown in highly Si‐doped layer. The main photoluminescence properties of point defects in GaAs and Al 0.3 Ga 0.7 As are reviewed to interpret the experimental data. The depth profile of the photoluminescence allows the spatial correlation between the luminescence spectra and the Si concentration profile obtained from secondary‐ion‐mass‐spectroscopy measurements. On the basis of the photoluminescence results, the physical processes occurring during the Si diffusion are discussed. Frenkel defects (pairs of element‐III vacancies and interstitials) are generated in the highly Si‐doped region. The element‐III interstitials rapidly diffuse towards the surface where they react with the element‐III vacancies generated at the surface when annealing is performed in an external As pressure. This induces a supersaturation of element‐III vacancies in the Si‐doped region which drives the Si diffusion. Annealing in vacuum reduces the oversaturation of element‐III vacancies and, hence, reduces the Si diffusion. A domination of the Si donor–element‐III vacancy complex emission band was found in the spectra taken in the Si‐diffused region. This gives evidence for the vacancy‐assisted mechanism in the Si diffusion and in the impurity‐induced disordering.Journal of Applied Physics 04/1992; · 2.17 Impact Factor -
Article: Luminescence peculiarities on (AlGa)As single quantum well
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ABSTRACT: This paper reports on single quantum well characterization by means of luminescence excited with a laser (photoluminescence) and an electron beam (cathodoluminescence) at liquid helium temperatures. Small quantum well regions with smaller confinement were observed. They seem to be associated with dislocations originated at the substrate. The observed shift of the X(e‐hh) peak position versus the excitation level can be explained by luminescence generation in different lateral regions of the quantum well.Journal of Applied Physics 03/1992; · 2.17 Impact Factor -
Article: Thermal conversion of n‐type GaAs:Si to p type in excess arsenic vapor
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ABSTRACT: Annealing in excess arsenic vapor at 650 °C introduces thermal conversion of n‐type Si‐doped GaAs samples (n = 1.3 × 10<sup>18</sup> cm<sup>-3</sup>) into p type. The observations are made by current‐voltage and electron‐beam induced current measurements. The donor concentration on the n side near the junction decreases after annealing. We present a comparison between the photoluminescence spectra of samples annealed under different conditions and an analysis of depth profile of the photoluminescence spectra. Our results underline the important role of gallium vacancies and gallium vacancy‐silicon donor complex in the thermal conversion.Journal of Applied Physics 11/1991; · 2.17 Impact Factor -
Article: A model for the Zn diffusion in GaAs by a photoluminescence study
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ABSTRACT: To study the mechanism of zinc diffusion in GaAs, we diffused zinc from a ZnAs 2 source into Si‐doped GaAs samples (n ≊ 1.3 × 10<sup>18</sup> cm<sup>-3</sup>) at different temperatures (from 575 °C up to 700 °C) in sealed evacuated quartz tubes. The samples are characterized by the depth profile of the photoluminescence at different temperatures. The photoluminescence spectra show characteristic emission associated to deep levels of gallium and arsenic vacancies. A detailed analysis of the spectra demonstrates the role played by vacancies in the Zn diffusion process. The spatial correlation between the luminescence spectra and the Zn concentration obtained from secondary ion mass spectroscopy measurements has been demonstrated.Journal of Applied Physics 07/1991; · 2.17 Impact Factor -
Article: Room temperature vertical cavity GaAs/AlGaAs surface emitting injection laser
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ABSTRACT: A vertical cavity GaAs/AlGaAs top surface emitting injection laser (SEIL) has been fabricated using a simple planar technology. The laser has a low series resistance (40 Omega ) and works at room temperature in pulsed operation with a duty cycle up to 35% and a threshold current of 55 mA. The authors obtained monomode operation at 896 nm wavelength, with a linewidth less than 1.5 AA. Both single SEILs and two dimensional SEIL arrays has been fabricated.Electronics Letters 10/1990; · 0.96 Impact Factor
Top co-authors
Top Journals
Institutions
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1988–2010
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École Polytechnique Fédérale de Lausanne
- Laboratoire d'optique
Lausanne, VD, Switzerland
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1997
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University of Minnesota Duluth
Duluth, MN, USA
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1996
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Université des Sciences et Technologies de Lille 1
Lille, Nord-Pas-de-Calais, France
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1992
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University of Leipzig
Leipzig, Saxony, Germany
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