Henry Mathieu

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.

Fractional‐dimensional calculation of exciton binding energies in semiconductor quantum wells and quantum‐well wires

We propose a fractional‐dimensional approach of excitonic characteristics in semiconductorquantum wells and quantum‐well wires with cylindrical or rectangular cross sections. This type of approach has proved to provide accurate and convenient methods for extracting excitonic binding energies, either from optical spectroscopy experiments, or from simple envelope function calculations. In this paper, we first try and extend the simple description previously developed for single quantum wells and superlattices. Next, we show how the accuracy of the model is dramatically improved by invoking microscopic considerations, in order to describe the anisotropy of the relative motion of confined electron‐hole pairs. This original approach allows a rather simple and quick determination of eigenenergies of confined excitons, whatever the quantum numbers of the conduction and valence subbands, and whatever the shape of the confining medium. The results of our calculations compare favorably to those of available variational theories and to experimental findings.

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.

Analytical model for the refractive index in quantum wells derived from the complex dielectric constant of Wannier excitons in noninteger dimensions.

Absorption spectra of low-dimensional structures such as quantum wells or wires have been strikingly well reproduced by expressions based on solutions of the Schrodinger equation for the Coulomb potential in noninteger dimensions, which require much less computational effort than more elaborate calculations. The compact and analytical complex dielectric constant of Wannier excitons in d dimensions is given, and included in a simple model of the refractive index in quantum well structures in the vicinity of the absorption threshold.

Excitons in semiconductor quantum wells: A straightforward analytical calculation

A new and very simple method is presented for calculating exciton binding energies in quantum confined semiconductor structures. The aim of the model calculation, which is developed in the framework of the fractional‐dimensional space, is not to compete with the very advanced ones already proposed but, on the contrary, to avoid tedious and expensive calculations to obtain, with a good accuracy, the exciton binding energy in most of the confined structures. Furthermore, in the cases where the 1s and 2s transition energies can be experimentally measured, the method permits one to obtain the exciton binding energy without any hypothesis nor calculation.

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.

A single equation describes excitonic absorption spectra in all quantum-sized semiconductors

The relative motion of the electron-hole pairs which constitute Wannier-Mott excitons in semiconductor quantum wells, superlattices, and quantum wires can never be considered strictly 1-D, 2-D, or 3-D. We propose an exact generalization of the well-known calculations of Elliott in the 3-dimensional case, and of Shinada and Sugano for 2-dimensional media-we calculate the absorption spectrum by bound and unbound excitonic states, by using a metric space with a noninteger dimension /spl alpha/>1. Whatever the dimensionality, i.e., for any quantum-sized structure, the whole optical density spectrum is obtained from a single compact equation, in excellent agreement with experimental data and with the most accurate available theories. We present examples of calculated spectra for quantum wells under applied perpendicular electric fields, and for quantum wires. >