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ABSTRACT: New experimental data of trap centres in Pb1−xSnxTe are presented. The chemical trend of the trap activation energy is determined for 0 ≦ x ≦ 0.11. The optical ionization energy of the defect level is obtained from the threshold wavelength of impurity photoconductivity and the results are compared with the thermal activation energy of the trap states. The cross sections of the defects for electron and hole capture, which were not measured in PbTe so far, are obtained from an analysis of photoconductivity transients. Our new data indicate a strong localization of the observed trap states in real space and bonding by the short range central-cell defect potential.Es wird über neue experimentelle Daten von Haftstellen in Pb1−xSnxTe berichtet. Der chemische Trend der Aktivierungsenergie der Haftstellen wird für einen Mischungsbereich 0 ≦ x ≦ 0,11 bestimmt. Die optische Ionisierungsenergie des Defektniveaus wird aus der Einsatzenergie der Störstellenphotoleitung bestimmt und mit der thermischen Aktivierungsenergie der Haftstellen verglichen. Eine Analyse des Abklingverhaltens der Photoleitung ergibt die Einfangsquerschnitte dieser Defekte für Elektronen und Löcher, die bisher noch nicht bekannt waren. Die neuen Daten deuten auf geringe räumliche Ausdehnung der Haftstellenzustände im Ortsraum und eine Bindung durch das kurzreichweitige zentrale Zellenpotential hin.
physica status solidi (b) 02/2006; 123(1):319 - 324. · 1.32 Impact Factor
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ABSTRACT: We present a growth technique which combines wet-chemical growth and molecular beam epitaxy (MBE) to create complex semiconductor nanostructures with nanocrystals as active optical material. The obtained results show that wet-chemically prepared semiconductor nanocrystals can be incorporated in an epitaxally grown crystalline cap layer. As an exemplary system we chose CdSe nanorods and CdSe(ZnS) core-shell nanocrystals in ZnSe and discuss the two limits of thin (d approximately 2R) and thick (d>2R) ZnSe cap layers of thickness d for CdSe nanorods and nanodots of radii R between 2 and 4 nm. In contrast to the strain-induced CdSe/ZnSe Stranski-Krastanow growth of a quantum dot layer in a semiconductor heterostructure, the technique proposed here does not rely on strain and thus results in additional degrees of freedom for choosing composition, concentration, shape, and size of the nanocrystals. Transmission electron microscopy and X-ray diffractometry show that the ZnSe cap layer is of high crystalline quality and provides all parameters for a consecutive growth of Bragg structures, waveguides, or diode structures for electrical injection.
Nano Letters 03/2005; 5(3):483-90. · 13.20 Impact Factor
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ABSTRACT: Due to their large oscillator strengths, ZnSe microcavities with (Zn, Cd)Se quantum wells are particularly suited for investigation of the photon–exciton coupling behaviour in semiconductors. We have observed a strong coupling between the excitonic and photonic modes in a ZnSe microcavity with four (Zn, Cd)Se quantum wells and distributed Bragg mirrors of ZnS and YF3. A very large Rabi splitting meV was observed in temperature dependent photoluminescence investigations.
Journal of Physics Condensed Matter 08/2004; 16(35):S3689. · 2.55 Impact Factor
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ABSTRACT: Planar semiconductor microcavities have found increasing attention since they allow to enhance and control the interaction between light and excitons. When the coupling between photon and exciton is strong enough, polaritons are formed which are observed in a pronounced Rabi-splitting in the cavity spectra. The strong exciton-light coupling regime, necessesary for polariton-based applications depends on the oscillator strength and the exciton binding energy. The wide-gap II–VI (CdTe,ZnSe and CdSe) as well as the group III-nitrides (GaN,InN and AlN) semiconductors excellently fulfil these conditions. We observed a large Rabi-splitting (about 44 meV) with ZnSe-based semiconductor microcavities containing four strained (Zn,Cd)Se quantum wells and ZnS/YF3 distributed Bragg-reflectors. Measurements of the reflectivity and of the photoluminescence revealed clear evidence of the strong coupling regime. We also report on first experiments to optimise the structural and optical properties of (Al,Ga)N/GaN quantum wells. The results obtained so far show that these structures may be used for the investigation of polariton Rabi-splitting at room temperature. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 07/2004; 1(S2):S202 - S209.
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I. C. Robin,
R. André,
Le Si Dang,
H. Mariette,
S. Tatarenko,
J. M. Gérard,
K. Kheng,
F. Tinjod,
M. Bartels, K. Lischka,
D. Schikora
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ABSTRACT: We investigated the transition between two different exciton recombination regime in quantum dots depending on temperature. When temperature is raised above a threshold value that we determine experimentally non-radiative recombination channels, characterized by their activation energy, dominate over radiative recombination. The analyses of time resolved photoluminescence spectroscopy versus temperature shows that the maximum temperature for dominant radiative recombination scales linearly with the activation energy of the non-radiative channels over a large range of values (10–60 meV) measured for various kind of II–VI-based quantum dots: CdTe/ZnTe, CdTe/ZnMgTe, CdSe/ZnSe. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 02/2004; 241(3):542 - 545. · 1.32 Impact Factor
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ABSTRACT: We investigated the optical properties of CdSe quantum dots (QDs) grown by molecular-beam epitaxy on GaAs substrates using micro-photoluminescence spectroscopy. Comparison was made between the QDs grown on a substrate with a vicinal tilt of 2° in the [111] direction and those on an on-axis substrate. We have studied the evolution of lineshapes of QD photoluminescence spectra under the improved condition of spatial resolution. It was found that the use of a substrate with the vicinal surface leads to the suppression of excitonic PL emitted from a wetting layer. The PL studies revealed that the thermo-stability up to 150 K was obtained in the sample on the on-axis substrate, whereas rapid temperature-induced quenching starting from 6 K was observed in that grown on the vicinal substrate. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 02/2004; 1(4):791 - 794.
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ABSTRACT: Successful p-type doping of cubic GaN by carbon grown under Ga-rich conditions is reported with maximum hole concentration of 6 × 1018 cm−3 and hole mobility of 19 cm2/Vs at room temperature, re-spectively. Cubic GaN:C was grown by rf-plasma assisted molecular beam epitaxy (MBE) on a semiinsulating GaAs (001) substrate (3 inches wafer). C-doping of the c-GaN was achieved by e-beam evaporation of a graphite rode with an C-flux of 1 × 1012 cm−2 s−1. Optical microscopy, Hall-effect measurements and room temperature photoluminescence were used for the investigation of the morphological, electrical and optical properties of cubic GaN:C. Under Ga-rich growth conditions most part of the carbon atoms were incorporated substitutially on N-site giving p-type conductivity. Our results verify that effective p-type doping of c-GaN can be achieved under Ga-rich growth conditions.
physica status solidi (c) 11/2003;
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ABSTRACT: Molecular beam epitaxy has successfully been used to grow crystalline layers of group III-nitrides (GaN, AlN and InN) with cubic (zinc-blende) structure on GaAs substrates. In this article, we discuss these efforts that, despite inherent difficulties due to the metastability of the c-III nitrides, led to substantial improvements of the structural, electrical and optical quality of these wide gap semiconductors. We review experimental work concerned with the epitaxy of c-GaN and the control of the growth process in-situ, the important issue of p- and n-type doping of c-GaN and investigations of the structural and optical properties of c-InGaN and c-AlGaN.
physica status solidi (c) 08/2003;
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ABSTRACT: The energetics and thermodynamic properties of cubic (c-)InxGa1−xN alloys are investigated by combining first-principles total energy calculations, a concentration-dependent cluster-based model, and Monte Carlo simulations. The search for the ground-state energies leads to the conclusion that biaxial strain suppresses phase separation, and acts as a driving force for chemical ordering in c-InxGa1−xN alloys. Ordered superlattice structures, with composition x ≅ 0.5 and stable up to T = 1000 K, arises as the relevant thermodynamic property of the strained alloy. We suggest that the In-rich phases recently observed by us in c-GaN/InxGa1−xN/GaN double heterostructures are ordered domains formed in the alloy layers due to biaxial strain. © 2003 American Institute of Physics.
Applied Physics Letters 06/2003; 82(24):4274-4276. · 3.84 Impact Factor
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ABSTRACT: We investigated the optical properties of two monolayers of CdSe sandwiched by ZnSe layers grown by molecular-beam epitaxy on GaAs substrates with a vicinal tilt of 2° in the [111] direction. By varying the spatial resolution from 10 μm down to 500 nm, sharp photoluminescence lines due to the recombination of excitons confined into quantum dots could be observed at low temperature. The dot density could be as low as ≈ 109 dots/cm2, which is smaller than previously reported values by at least one order of magnitude. © 2003 American Institute of Physics.
Applied Physics Letters 04/2003; 82(14):2227-2229. · 3.84 Impact Factor
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O.C. Noriega,
J.R. Leite,
E.A. Meneses,
J.A.N.T. Soares,
S.C.P. Rodrigues,
L.M.R. Scolfaro,
G.M. Sipahi,
U. Köhler,
D.J. As,
S. Potthast,
A. Khartchenko, K. Lischka
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ABSTRACT: The optical properties of cubic AlGaN/GaN multiple quantum well (MQW) structures are investigated in the temperature range 1.7–300 K by photoluminescence (PL) and modulated photoreflectance (PR) techniques. The PL and PR spectra of the MQWs display features, which can be clearly ascribed to strong radiative recombinations taking place at the GaN QW regions. The measured carrier confinement energies in the QWs are in good agreement with the results of calculations carried out using a k · p eight-band Kane model assuming ideal sharp barrier/well interfaces.
physica status solidi (c) 12/2002;
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ABSTRACT: We have performed annealing experiments with c-InGaN/GaN double heterostructures in order to obtain information on the thermal stability and the formation process of In-rich clusters in the InGaN layers. While the as grown samples showed a dominating luminescence at about 2.3 eV, the annealed samples showed a new luminescence peak at 2.8–3.0 eV which may be due to a band gap emission of a regenerated layer with an In-content of about x = 0.20. These results are corroborated by micro Raman spectroscopy. Our annealing experiments show that at elevated temperatures In-atoms can diffuse in c-InGaN layers while In-rich aggregates are stable at growth temperature.
physica status solidi (c) 12/2002;
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ABSTRACT: In this work, we have investigated the optical properties of two samples of CdSe quantum dots by using submicro-photoluminescence spectroscopy. The effect of vicinal-surface GaAs substrates on their properties has been also assessed. The thinner sample, grown on a substrate with vicinal surface, includes only dots with a diameter of less than 10 nm (type A islands). Islands of an average diameter of about 16 nm (type B islands) that are related to a phase transition via a Stranski-Krastanow growth process are also distributed in the thicker sample grown on an oriented substrate. We have studied the evolution of lineshapes of PL spectra for these two samples by improving spatial resolution that was achieved using nanoapertures or mesa structures. It was found that the use of a substrate with the vicinal surface leads to the suppression of excitonic PL emitted from a wetting layer. Comment: 2pages, 2 figures, Proceedings of International Conference On Superlattices Nano-Structures And Nano-Devices, July, Toulouse, France, to appear in the special issue of Physica E
11/2002;
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ABSTRACT: The plasmon-longitudinal optical (LO) phonon collective excitations were studied by Raman scattering in cubic GaN doped with Si and in intrinsically doped cubic AlxGa1−xN alloys. The specific asymmetry of the corresponding Raman lines, associated with the dispersion of the coupled excitations was clearly detected in GaN. The spatial coherence of the relevant coupled oscillations was determined from the comparison of the experimental Raman spectra with the calculated ones. A broad line, which was previously attributed to the overdamped plasmon-LO phonon oscillations, was also observed in the GaN and AlxGa1−xN samples in the range between the transversal optical and LO phonon frequencies. © 2002 American Institute of Physics.
Journal of Applied Physics 04/2002; 91(9):6197-6199. · 2.17 Impact Factor
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A. Tabata,
L. K. Teles,
L. M. R. Scolfaro,
J. R. Leite,
A. Kharchenko,
T. Frey,
D. J. As,
D. Schikora, K. Lischka,
J. Furthmuller,
F. Bechstedt
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ABSTRACT: Phase separation suppression due to external biaxial strain is observed in In <sub>x</sub> Ga <sub>1-x</sub> N alloy layers by Raman scattering spectroscopy. The effect is taking place in thin epitaxial layers pseudomorphically grown by molecular-beam epitaxy on unstrained GaN(001) buffers. Ab initio calculations carried out for the alloy free energy predict and Raman measurements confirm that biaxial strain suppress the formation of phase-separated In-rich quantum dots in the In <sub>x</sub> Ga <sub>1-x</sub> N layers. Since quantum dots are effective radiative recombination centers in InGaN, we conclude that strain quenches an important channel of light emission in optoelectronic devices based on pseudobinary group-III nitride semiconductors. © 2002 American Institute of Physics.
Applied Physics Letters 03/2002; · 3.84 Impact Factor
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ABSTRACT: We present first-principles calculations of the thermodynamic and electronic properties of the zinc-blende ternary InxGa1-xN, InxAl1-xN, BxGa1-xN, and BxAl1-xN alloys. They are based on a generalized quasi-chemical approximation and a pseudopotential-plane-wave method. T-x phase diagrams for the alloys are obtained. We show that due to the large difference in interatomic distances between the binary compounds a significant phase miscibility gap for the alloys is found. In particular for the InxGa1-xN alloy, we show also experimental results obtained from X-ray and resonant Raman scattering measurements, which indicate the presence of an In-rich phase with x[approximate]0.8. For the boron-containing alloy layers we found a very high value for the critical temperature for miscibility, \textasciitilde9000 K, providing an explanation for the difficulties encountered to grow these materials with higher boron content. The influence of a biaxial strain on phase diagrams, energy gaps and gap bowing of these alloys is also discussed.
Physica E: Low-dimensional Systems and Nanostructures. 03/2002; 13:1086--1089.
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ABSTRACT: Cubic phase GaN/AlxGa1-xN Multi Quantum Well structures were grown by rf-plasma assisted molecular beam epitaxy (MBE) on GaAs (001) substrates. X-ray measurements showed a high phase purity of the epilayers and revealed an Aluminum incorporation between 9 % and 49 %, respectively. The QW luminescence was tuned between 3.25 eV and 3.4 eV by means of the variation of QW barrier Aluminum content and QW width. Strong Cathodoluminescence (CL) from the GaN QWs and the underlying cubic AlxGa1-xN bulk material was observed at room temperature. The spatial localization of the QW emission was unambiguously determined by depth-resolved CL measurements. Combined with a model of energy-dependent penetration, diffusion, and recombination, these variations indicate a value of about 20 nm for the minority carrier diffusion length within the AlxGa1-xN confinement layer. The assignment of AlxGa1-xN bulk and GaN luminescence was further supported by employing a simple effective-mass quantum mechanical model.
MRS Proceedings. 12/2001; 743.
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ABSTRACT: This paper describes structural and optical investigations of molecular beam epitaxy grown strained ZnSe/(Zn,Cd)Se quantum wells. Strain state, cadmium content, and quantum well layer thickness have a strong influence on the emission energy, which allows a luminescence tuning between 2.25 and 2.50 eV. Furthermore a ZnSe based microcavity structure completed with ZnS/YF3 distributed Bragg mirrors was processed. These reflectors are an alternative to several epitaxial grown II–VI materials and other reported approaches with respect to oxide layers. A blueshift in the photoluminescence emission energy measured by angle resolved microcavity detuning indicates an influence of the cavity mode on the quantum well mode.
physica status solidi (a) 12/2001; 188(3):983 - 988. · 1.21 Impact Factor
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ABSTRACT: We have measured photoluminescence spectra of molecular-beam-epitaxy-grown cubic GaN/InxGa1−xN/GaN double heterostructures with x between 0.09 and 0.33. We observe a luminescence peak at about 2.3–2.4 eV which is almost independent of the InGaN layer composition. High-resolution x-ray diffraction measurements revealed a pseudomorphic In-rich phase with x = 0.56±0.02 embedded in the InGaN layers. Including strain effects we calculate a gap energy Eg = 2.13 eV of this phase. In cubic InGaN, spontaneous polarization and strain-induced piezoelectric fields are negligible. Therefore, the observed difference between the luminescence energy and the gap of the In-rich phase is assumed to be due to the localization of excitons at quantum-dot-like structures with a size of about 15 nm. © 2001 American Institute of Physics.
Applied Physics Letters 08/2001; 79(9):1243-1245. · 3.84 Impact Factor
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T. Frey,
D. J. As,
M. Bartels,
A. Pawlis, K. Lischka,
A. Tabata,
J. R. L. Fernandez,
M. T. O. Silva,
J. R. Leite,
C. Haug,
R. Brenn
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ABSTRACT: The radio-frequency plasma-assisted molecular beam epitaxy of cubic AlyGa1−yN/GaN heterostructures on GaAs(001) substrates is reported. Rutherford backscattering spectroscopy, high resolution x-ray diffraction, and first-order micro-Raman spectroscopy measurements were used to characterize the structural and vibrational properties of the alloy epilayers. The Al content of the alloy is in the range from 0.07<x<0.20. X-ray diffraction reciprocal space maps demonstrate the good crystal quality of the cubic (Al, Ga)N/GaN films. The measured Raman shift of the TO phonon mode of the AlyGa1−yN alloy is in good agreement with theoretical calculations. © 2001 American Institute of Physics.
Journal of Applied Physics 02/2001; 89(5):2631-2634. · 2.17 Impact Factor
