Showing papers by "Le Si Dang published in 2003"

Direct comparison of recombination dynamics in cubic and hexagonal GaN/AlN quantum dots

Abstract: We report on time-integrated and -resolved photoluminescence (PL) data on self-assembled GaN quantum dots (QD's) embedded in AlN, in both cubic [zinc-blende (ZB)] and hexagonal [wurtzite (Wz)] crystallographic phases. The comparison of the optical properties of ZB and Wz nitride QD's allows us to distinguish pure dimensionality effects from the influence of the large polarization-induced electric fields present in the Wz nanostructures. Specifically, the PL energy of the ZB QD's is always higher than the bulk cubic GaN band-gap energy, in contrast to the Wz QD's where a 7-MV/cm polarization field gives rise to below-gap PL emission for sufficiently large QD's. As a further consequence of the internal field, the low-temperature PL decay times of the Wz QD's are significantly longer than the ZB ones, and increase strongly with the QD height in contrast to the ZB ones, which exhibit only a small size dependence. For both types of QD's, the PL intensity is found to be weakly dependent on temperature, underscoring the strong zero-dimensional exciton localization in the GaN/AlN system. In spite of the strong localization, however, we show that the nonradiative channels cannot be neglected and have a significant contribution in the PL decay time for both ZB and Wz QD's.

Surfactant effect of In for AlGaN growth by plasma-assisted molecular beam epitaxy

Abstract: In this article, the surfactant capability of In for AlGaN growth by plasma-assisted molecular beam epitaxy has been assessed. We have determined the range of substrate temperatures and In fluxes to form a self-regulated 1×1 In adlayer on AlxGa1−xN(0001). The presence of this In film favors two-dimensional growth of AlGaN under stoichiometric conditions. The formation of metal droplets on the surface is inhibited. In incorporation, if any, is lower than 0.01%. The structural quality of the layers is verified by high-resolution x-ray diffraction, both in symmetric and asymmetric reflections.

Molecular-beam epitaxial growth and characterization of quaternary III–nitride compounds

Abstract: We report on the controlled growth and characterization of quaternary AlGaInN compounds by plasma-assisted molecular beam epitaxy. Two-dimensional growth is achieved with a monolayer of In segregating at the growth front. In incorporation is hindered by increasing growth temperature and Al mole fraction, which is explained by the lower binding energy of InN compared to GaN and AlN. The mosaicity of the layers is determined by the substrate quality, whereas the alloy disorder increases with the Al content, independent of the In mole fraction. Room temperature photoluminescence is dominated by a narrow band-edge emission, whose Stokes shift and activation energy increase with the In content. This behavior is interpreted in terms of carrier localization in self-formed alloy inhomogeneities. An In-related band bowing parameter of 2.5 eV has been estimated.

Polariton parametric amplifier pump dynamics in the coherent regime.

Abstract: We studied the pump coherent dynamics in a II-VI microcavity parametric amplifier, using angle-resolved four-wave mixing. The polariton parametric amplification is found to result in a strong quenching and saturation of the pump coherence lifetime above the threshold. For the polariton scattering processes that remain below the amplification threshold, we find an angle-dependent collision broadening associated with the efficiency of the polariton scattering towards the excitonic reservoir.

In incorporation during the growth of quaternary III-nitride compounds by plasma-assisted molecular beam epitaxy

Abstract: Epitaxial growth of quaternary AlGaInN compounds by plasma-assisted molecular beam epitaxy has been demonstrated. Two-dimensional growth is achieved under In excess, with an In film segregating at the growth front. The maximum In incorporation is significantly affected by the substrate temperature and the Al mole fraction of the alloy. This behavior has been attributed to the enhancement of In segregation due to the high binding energy of AlN compared to InN and GaN.

Interfacial strain in 3C‐SiC/Si(100) pseudo‐substrates for cubic nitride epitaxy

Abstract: After a short review of the present stage of knowledge of the Si[100] and β-SiC surface reconstructions and Si carbonisation mechanism, we discuss the possible origin for the change in sign of the interfacial strain recently observed in this strongly mismatched cubic hetero-epitaxial system as a function of the initial carbonisation temperature. Both the interfacial and anisotropic character of this strain are confirmed by micro-Raman analysis of cleaved CVD-grown samples, which reveal also how extended defects relieve the residual strain after the growth of a few micrometers of 3C-SiC. Such pseudo-substrates have been used as a cubic template for the MBE growth of cubic nitrides and Raman scattering results on β-AlN materials and cubic GaN/AlN stacked quantum boxes are presented. Finally, the potential and limitations of such 3C-SiC pseudo-substrates for nitride growth are discussed.

Single quantum dot spectroscopy of CdSe/ZnSe grown on vicinal GaAs substrates

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.

Collision broadening in II-VI semiconductor microcavities

Abstract: We present a comprehensive study of the collision broadening in II-VI semiconductor microcavities. The polariton nonlinear coherent dynamics was investigated using angle-resolved four-wave mixing, which allows us to study the polariton dephasing on a large scale within reciprocal space. The low-energy polariton dephasing time is found to present a threshold dependence with excitation angle as well as excitation density, associated with the possibility, for polaritons with large enough in-plane wave vectors to scatter towards the high density of states excitonic reservoir. The validity of the polariton-polariton scattering calculation based on the effective exciton-exciton interaction hamiltonian is also discussed.

GaN Quantum Dots: Physics and Applications

Abstract: Recent works by our group on hexagonal and cubic GaN/AlN quantum dots grown by molecular beam epitaxy are reviewed. It is shown that the growth of GaN on AlN can occur either in a layerby- layer mode to form quantum wells or in the Stranski-Krastanow mode to form self-assembled quantum dots. High resolution transmission electron microscopy reveals that quantum dots are truncated pyramids (typically 3 nm high and 15 nm wide), nucleating on top of a wetting layer. The existence of internal electric elds of 7 MV/cm in hexagonal quantum dots is evidenced by observations of various physical e ects related to the quantum con ned Stark e ect, e.g. energy redshift of the interband transition, decrease of its oscillator strength, or enhancement of the exciton interaction with LO phonons. Prospects for UV and near-IR applications, using interband and intersubband transitions of GaN/AlN quantum dots, respectively, will be discussed also in this presentation.

Non-linear polariton dynamics in II–VI microcavities

Abstract: Angle-resolved optical spectroscopy is employed to investigate polariton dynamics under conditions of high excitations in CdTe-based microcavities. Time-resolved photoluminescence measurements under non-resonant excitation clearly show the suppression of the relaxation bottleneck on the lower polariton branch, followed by the onset of stimulated emission from the ground polariton states at k|| = 0. To probe the coherent dynamics along the lower polariton branch, we use for the first time angle-resolved four wave mixing measurements. A sharp decrease of the dephasing time T2 is observed when pumping at the magic angle at high powers, as a result of stimulated parametric scattering.

Optical properties of hexagonal and cubic GaN self‐assembled quantum dots

Abstract: Optical characteristics of hexagonal GaN (h-GaN) and cubic GaN (c-GaN) self-assembled quantum dots (QDs) were investigated by means of photoluminescence (PL), PL excitation, cathodoluminescence (CL), and time-resolved PL experiments. We observed a Stokes-like shift between PL excitation absorption edge and PL emission from the h-GaN QDs as well as Al(Ga)N base layer. With increasing temperature, the PL intensity of Al(Ga)N base layer or GaN quantum wells was dramatically decreased while that of GaN QDs was not changed much. We also observed wavelength-resolved CL images reflecting strong carrier localization in the QD confinement. From the time-resolved PL experiments, we found that the measured lifetime of the h-GaN QDs emission increased with decreasing emission energy (i.e., increasing QD size), while that of the c-GaN QDs kept almost constant. Therefore, we conclude that the h-GaN QD emissions are strongly influenced by built-in interal electric field as well as carrier localization in the QDs.

Visible red light emission from Eu‐doped GaN quantum dots grown by plasma‐assisted MBE

Abstract: We demonstrate the growth of GaN quantum dots doped with Eu by plasma-assisted MBE. Intense emission from the intra-atomic 4f transition of Eu3+ was observed at 622 nm. GaN quantum dots (QDs) doped with 1% Eu showed the maximum emission. The full-width at half maximum (FWHM) of the CL spectrum of Eu-doped GaN QDs was 2.1 nm at room temperature. Eu doping increased the size of QDs by 4 times and decreased the density of QDs by one order of magnitude. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of growth conditions on optical properties of CdSe/ZnSe single quantum dots

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 line shapes 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.

Intersubband Absorptions in Doped and Undoped GaN/AlN Quantum Wells at Telecommunication Wavelengths Grown on Sapphire and 6H-SiC Substrates

Abstract: In this paper we present experimental and theoretical study of intersubband transitions at telecommunication wavelengths in GaN/AIN hexagonal-phase quantum wells grown by molecular beam epitaxy on sapphire substrates. Crossed structural and photoluminescence experiments show that strong in-plane carrier localization occurs due to thickness fluctuations at GaN/AIN interfaces. Fourier transform infrared spectroscopy and photo-induced absorption spectroscopy performed on doped and undoped samples reveal a systematic blue-shift of the e1-e2 transitions with doping due to many body interactions. A good agreement is achieved between experiments and self-consistent Schrodinger-Poisson calculations.

Structural and Optical Properties of GaN Quantum Dots

Abstract: Growth conditions, structural and optical properties of GaN quantum dots (QDs) grown by plasma-assisted molecular beam epitaxy will be examined. It will be shown that, depending on the Ga/N ratio value and on growth temperature, the growth mode of GaN deposited on AlN can be either of the Stranski-Krastanow or of the Frank-Van der Merwe type. It will be shown that vertical correlation results in a red shift and in a narrowing of the photoluminescence spectra. Growth of Eu-doped GaN quantum dots embedded in AlN will be described. Intense photoluminescence associated with Eu has been measured, with no GaN band-edge emission, as an evidence that carrier recombination mostly occurs through rare earth ion excitation. Persistent photoluminescence of Eu-doped GaN quantum dots as a function of temperature has been put in evidence, as a further confirmation of the recombination of confined carriers through Eu ion excitation.

II–VI semiconductor microcavity angle‐resolved coherent dynamics

Abstract: We used angle-resolved time-integrated four-wave mixing to study the pump dynamics in a II–VI semiconductor microcavity polariton parametric amplifier. The low-energy polariton dephasing time decreases at large angles when the excitonic reservoir becomes accessible to polariton–polariton scattering. Additionally, in the parametric amplification regime, with phase matching for the polariton–polariton scattering at the magic angle, we observe a strong reduction of the FWM decay time. Numerical analysis of the polariton coherent dynamics in this regime allows to interpret the observed features in terms of time dependent pump depletion.

Angle-dependent polariton collisional broadening in semiconductor microcavities

Abstract: The polariton nonlinear dynamics in a II–VI semiconductor microcavity has been studied using angle-resolved four-wave mixing. The low-energy polariton dephasing time is found to decrease at large angles, when polariton can escape towards the excitonic reservoir, in good agreement with a calculation of the angle-dependent collisional broadening. Moreover, we find that the parametric amplification of k =0 polaritons is associated to a strong quenching of the coherence at the magic angle.