M. Chamarro

Hole–Nuclear Spin Interaction in Quantum Dots

We have measured the carrier spin dynamics in p-doped InAs/GaAs quantum dots by pump-probe and time-resolved photoluminescence experiments. We obtained experimental evidence of the hyperfine interaction between hole and nuclear spins. In the absence of an external magnetic field, our calculations based on dipole-dipole coupling between the hole and the quantum dot nuclei lead to a hole-spin dephasing time for an ensemble of dots of 14 ns, in close agreement with experiments.

Formation of GaAs nanocrystals by laser ablation

The pulsed laser ablation method has been used to form GaAs nanocrystals. A quadrupled frequency Nd:yttrium–aluminum–garnet laser beam is focused onto a GaAs single crystal target, and a nitrogen flowing gas is sent at the neighborhood of the target in order to transfer in an ethanol bath, the nanoparticles grown in gas phase. The composition of the particles is close to stoichiometry and transmission electron microscopy analyses highlight zinc-blende GaAs nanocrystals with a rather well defined size: 5–8 nm diameter. The low temperature photoluminescence and photoluminescence excitation spectra show quantum confinement of about 870 meV via an emission band in the visible range (500–560 nm) for the GaAs nanocrystals produced by laser ablation without any postannealing treatment.

Photoluminescence polarization of semiconductor nanocrystals

Abstract We review the polarization properties of photoluminescence (PL) in nanocrystals (NCs) from both theoretical and experimental points of view. We show that, under linearly polarized excitation, NCs emit partly polarized light owing to their uniaxial structure or their anisotropic shape. In elongated NCs, the anisotropy may have two origins, the electronic confinement or the effect of depolarizing field created by the light-induced charges on the interfaces. Results of polarization studies in porous silicon are presented. They are explained by the shape of the Si NCs. Experiments in CdSe NCs reveal the fine structure of the excitonic levels and show evidence of the enhancement of the electron-hole exchange energy with decreasing NC size. Spin orientation in wurtzite-type NCs is achieved by optical pumping with circularly polarized light. The effect of a magnetic field on the degree of circular polarization and the mechanisms of spin relaxation are discussed. Results in large-size NCs are presented.

Influence of growth conditions on the structural properties of CdSxSe1−x (x = 0.4 and x = 1) nanocrystals

The structural properties of Cds x Se 1-x nanocrystals elaborated by two different methods have been studied by means of high resolution transmission electron microscopy and optical absorption. In the first method, CdS 0.4 Se 0.6 nanocrystals are grown in glass matrix and show a size distribution characterized by a standard deviation σ nearly proportional to the nanocrystals mean diameter d, the value σ/d being about 0.25. They are crystallized in the stable structure of wurtzite-type from the beginning of growth. Their morphology depends on the size: it changes from sphere to hexagonal prism as the size increases. In the second method CdS nanocrystals are grown in solution and show narrower size distribution; the ratio σ/d is about 0.15. The structure depends on the nanocrystal size: the blende-type structure is dominant at small diameter (d = 3 nm) and the wurtzite-type structure is more important at larger diameter (d > 5 nm). Their morphology also changes from quasi-sphere at small size (< 4 nm) to polyhedron-like at larger size.

Spin decoherence and relaxation processes in zero-dimensional semiconductor nanostructures

In recent years, interest in spin physics has been renewed due to its potential application in spintronics and quantum information. In these frameworks, the main required property is the presence of long spin memory. We present a short review of recent results concerning spin decoherence and relaxation processes in zero-dimensional (0D) nanostructures, especially in quantum dots.

Effects of strain on the optoelectronic properties of annealed InGaAs/GaAs self-assembled quantum dots

The effect of the lattice-mismatch strain and of the charge carrier confinement profile, on the optical properties of thermally annealed self-assembled In x Ga1−x As/GaAs quantum dots (QDs), is theoretically analyzed by using a recently developed 40-band k.p model. First, to evaluate the composition and size of QDs as a function of thermal annealing conditions, we model the In/Ga interdiffusion by a Fickian diffusion. Second, we investigate the decrease of the strain effects on the carrier confinement potentials with annealing by solving the Schrodinger equation separately for electrons and holes. It is clearly found that the strain strongly modifies the QD potential profile, leading to a different electron and hole energy distribution. Finally, we carry on a comparison between theoretical calculations and photoluminescence (PL) experimental results performed in thermal annealed samples. A good agreement is obtained for the energy blueshift and the linewidth narrowing of the PL spectra measured on annealed QD ensemble. These results prove the relevance of the present approach to describe the optoelectronic properties of the nanostructures through the post-growth thermal annealing treatment.

Electron and hole spin cooling efficiency in InAs quantum dots: The role of nuclear field

The spin dynamics of a resident carrier, hole or electron, in singly charged InAs/GaAs quantum dots has been measured by pump-probe experiments. The relative strength of the hole to the electron hyperfine couplings with nuclei is obtained by studying the magnetic-field dependence of the resident-carrier spin polarization. We find, in good agreement with recent theoretical studies, that the hole hyperfine coupling is ten times smaller than the electron one.

Size-dependent electron-hole exchange interaction in CdSe quantum dots

SummaryWe have investigated the fine structure of luminecence of CdSe nanocrystals observed with size-selective excitation. We show that the luminescence line closest to the laser arises from the recombination of the optically forbidden A-exciton. Radiative recombination occurs via a phonon-assisted virtual transition to theB-exciton state. The electron-hole exchange energy obtained from the experimental results strongly increases with decreasing nanocrystal size as expected from the increasing overlap of the electron and hole wave functions. The size dependence is in good agreement with the theoretical predictions.

Optical pumping in CdS1-xSex nanocrystals

We report on the magneto-optical properties of large CdS0.4Se0.6 nanocrystals in a glass matrix. Optical pumping in the two high-energy bands of the photoluminescence spectrum is studied with resonant excitation and in the presence of a magnetic field. We also investigate magnetic field-induced polarization of luminescence for non-polarized excitation. We show that the higher energy band is most likely to be due to excitonic recombination. The Lande g-factor is found to be anisotropic. Its maximum value is estimated from the study of magnetic field-induced circular polarization. The second band is attributed to donor-hole recombination. We show that electrons on the donor level are partly optically oriented by circularly polarized light and that the transition may be excited either from the A or from the B valance band. The maximum degree of circular polarization is calculated in the case of optical pumping with resonant excitation of the A exciton for an assembly of wurtzite nanocrystals with randomly oriented c axes. It is found to be equal to 5/7 instead of 1 in the bulk material. In the case of magnetic field-induced polarization, the effective Lande factors for the different luminescence bands are calculated and compared with the experimental results.

Band parameters of InGaAs/GaAs quantum dots: electronic properties study

We have made a systematic investigation of the band diagram calculation of strained and unstrained InxGa1 − xAs alloys in order to extract accurate and adapted parameters which are useful to the electronic properties of InxGa1 − xAs/GaAs quantum dots. As an application, the 40-band k.p model is used to describe the band offsets as well as the band parameters in the strained InxGa1 − xAs/GaAs system. The κ valence band parameter as well as g* Lande factor depending of the indium concentration were estimated. These results are analyzed and compared with experiment.