Jacques Allegre

High internal electric field in a graded-width InGaN/GaN quantum well: Accurate determination by time-resolved photoluminescence spectroscopy

Time-resolvedphotoluminescence (PL), at T=8 K, is used to study a graded-width InGaN/GaN quantum well. Across the sample, the well width continuously varies from ∼5.5 to 2.0 nm corresponding to PL peak energies varying between 2.0 and 2.9 eV and to PL decay rates covering four orders of magnitude. The plot of decay times versus PL energies is very well fitted by a calculation of the electron–hole recombination probability versus well width. The only fitting parameter is the electric field in the well, which we find equal to 2.45±0.25 MV/cm, in excellent agreement with experimental Stokes shifts for this type of samples.

Quantum confinement effects of CdS nanocrystals in a sodium borosilicate glass prepared by the sol‐gel process

Experimental evidences of both weak and strong confinement regimes are reported on CdSnanocrystals embedded in a sodiumborosilicate glass matrix. A method, based on the sol‐gel technique, is used for the preparation of CdS‐activated glass. This route is capable of providing nanocrystals covering a wide range of radii with small size dispersion. Low‐temperature linear‐absorption spectra have been analyzed in terms of excitons and electron‐hole confinements by fitting the results of a numerical calculation to experimental findings. The model used, in the envelope‐function formalism, involves both a Lorentzian broadening of the exciton energy states inside each nanocrystal and a Gaussian size distribution. The improvement of crystal quality and the sharpening of the size distribution by thermal annealing is also studied versus both time and temperature of treatment. It is shown that we can keep a tight control on the crystallinity, average size, and size distribution of the nanocrystals by rather simple adjustments and short treatments.

Effects of GaAlN barriers and of dimensionality on optical recombination processes in InGaN quantum wells and quantum boxes

We compare several InGaN-based low-dimensional systems, by time-resolved photoluminescence (PL), versus temperature (8<T<280 K). We investigate the influence of growing or not an AlGaN barrier on top of the active layer. We address the differences between quantum wells and quantum boxes 5–10 nm in diameter and 2 nm in height. Our results are consistent with carrier localization on potential fluctuations with spatial extension much smaller than the size of the quantum boxes. Growing an AlGaN barrier reduces the carrier mobility between fluctuations, thus maintaining an effective PL dominated by localized carriers up to room temperature.

Absorption properties of CdS nanocrystals in glasses; evidence of both weak and strong confinement regimes

Abstract Experimental evidence of both weak and strong confinement regimes is reported on CdS nanocrystals embedded in glass matrix. Classic methods for nanocrystallite preparation meet a principal difficulty, the polydispersion in size of crystallites due to several difficulties such as coalescence and decomposition of particles with time and high temperatures. In this work, we used a new route to elaborate CdS-doped sodium borosilicate glass from gel formed in an aqueous medium. Our vitreous matrix is fully and swiftly densified at relatively low temperature, providing us with CdS nanocrystallites yielding fine band-edge structure and small size dispersion. Low temperature absorption spectra have been interpreted in terms of excitons and electron-hole confinements, taking account of both a Lorentzian broadening of the energy states inside each nanocrystal and a Gaussian size-distribution. It has been shown that, for very small crystallites, the strong blue-shift of the absorption edge due to the quantum size effects, is partially compensated by a red-shift associated to the size distribution.

Scale Effects on Exciton Localization and Nonradiative Processes in GaN/AlGaN Quantum Wells

Previous experimental studies have allowed us to observe peculiar localization effects of excitons in GaN/AlGaN quantum wells grown by MBE, as well as efficient nonradiative inter-well carrier transfers. In this work, we use the envelope-function approximation to calculate exciton energies and wave functions. We show that the typical spatial extension of the electron and hole can by itself explain the localization and transfer processes, mainly because, due to its large effective mass, the in-plane extension of the hole is smaller than the average distance between two aluminum atoms in the barriers.

Time-Resolved Spectroscopy of MBE-Grown InGaN/GaN Self-Formed Quantum Dots

GaInN/GaN quantum dots have been grown by molecular beam epitaxy on sapphire substrates. By changing the size and composition of the dots, the emission energy can be tuned over the entire visible spectrum. We present time-resolved photoluminescence obtained on such samples with emission energies ranging from 2.4 to 3.0 eV. We observe that the radiative recombination rate of electron-hole pairs varies over several decades, in correlation with the transition energy. When the decay time of the ground-state recombination reaches several microseconds, a much faster (nanoseconds) recombination is observed at higher energy. This is tentatively explained in terms of the partial screening of the internal field by a finite number of electron-hole pairs.

Dynamics of Excitons in GaN–AlGaN MQWs with Varying Depths, Thicknesses and Barrier Widths

Picosecond time-resolved photoluminescence is used to investigate the recombination dynamics of excitons in samples which ail contain four GaN-AlxGa1-xN quantum wells of respective widths of 4, 8, 12 and 16 molecular monolayers, grown by molecular beam epitaxy. The compositions and thicknesses of the barriers have been varied, in order to change the electric fields induced by piezo- and pyro-electric effects. The dependences of experimental decay times with barrier characteristics indicate the presence of efficient inter-well carrier escaping. Calculations of electronic tunneling times versus barrier width show that the present carrier escaping is enhanced by some additional process, such as scattering by impurities or composition fluctuations in the barriers.

Reduction of carrier in-plane mobility in group-III nitride based quantum wells: The role of internal electric fields

Localized and free excitons in GaN/Al0.17Ga0.83N quantum wells are studied by time-resolved photoluminescence (PL) versus temperature. We focus more particularly on 16-monolayer wide quantum wells for which, at T = 8 K, we observe double excitonic features. We assign the latter to two different localization states of excitons, from the temperature dependence of PL energies and of recombination dynamics. We discuss the reasons why double PL lines are observed in terms of the limitation of carrier in-plane mobility by interface roughness. We emphasize, in particular, the strong effect of the longitudinal electric field on the in-plane motion of carriers.

Evidence of confinement of fullerene C60 in microporous VPI-5 zeolite

Andersons method of incorporation of C60 within the cages of a VPI-5 zeolite has been used as well as a chemical vapor deposition procedure. 129Xe nuclear magnetic resonance has confirmed that, starting from an organic solution of fullerene, C60 was actually incorporated into the molecular cages of the inorganic matrix. Thermal analysis methods and X-ray diffraction show that the treatment does not affect the integrity of the crystalline structure of VPI-5. Photoluminescence experiments gave similar results to those already mentioned by other authors, indicating the probable confinement effect of the inorganic matrix on the C60 isolated molecules.

Electronic structure of (1 1 3)-grown GaAs-(GaAl)As single quantum wells under biaxial strain fields

Optical properties of (1 1 3)-grown GaAs-(GaAl)As single quantum wells (SQWs) have been studied by wavelength- and piezomodulated spectroscopy. A supermodulation of light hole states is observed in piezomodulation spectroscopy. The theoretical methods required to compute the envelope functions and the stress dependence of the transition energies are detailed. A close agreement is obtained between the experimental observations and the theoretical predictions.