Patrice Miska

Exciton and core-level electron confinement effects in transparent ZnO thin films

The excitonic light emission of ZnO films have been investigated by means of photoluminescence measurements in ultraviolet-visible region. Exciton confinement effects have been observed in thin ZnO coatings with thickness below 20 nm. This is enhanced by a rise of the intensity and a blue shift of the photoluminescence peak after extraction of the adsorbed species upon annealing in air. It is found experimentally that the free exciton energy (determined by the photoluminescence peak) is inversely proportional to the square of the thickness while core-level binding energy is inversely proportional to the thickness. These findings correlate very well with the theory of kinetic and potential confinements.

Experimental and theoretical investigation of carrier confinement in InAs quantum dashes grown on InP(001)

Carrier confinement in InAs quantum dashes (QDas) grown on InP(001) is investigated both experimentally and theoretically. The aim of these studies is to reconstruct the electronic structure of the QDas. QDas with low size dispersion are achieved by improving growth conditions. Optical transitions between ground and excited states are studied by continuous-wave-photoluminescence and photoluminescence-excitation experiments at low temperature. We also report on infrared spectroscopy of conduction-band intersubband transitions. A simplified theoretical model is developed, yielding results consistent with the experimental data. Combining experimental and theoretical results, we propose an interpretation of the optical transitions occurring in these QDas, and we give a first theoretical absorption spectrum of these structures.

Transmittance enhancement and optical band gap widening of Cu2O thin films after air annealing

Cu2O thin films have been grown on glass substrates at room temperature by reactive magnetron sputtering. As-deposited films exhibit high electrical resistivity and low optical transmittance. To improve the film properties, post annealing treatments in air at various temperatures have been performed. Low temperature annealing (<300 °C) avoids the film oxidation into CuO and the films remain single-phased. In this temperature range, the annealing in air enhances the transmittance in the visible region due to the decrease of the defect scattering. Moreover, the optical band gap of Cu2O thin films is enlarged from 2.38 to 2.51 eV with increasing annealing temperature. The increase of optical band gap accompanying the reduction of Urbach energy indicates that the widening of optical band gap may result from the partial elimination of defect band tail after thermal annealing in air. Combining experimental results with recent reported calculations, the peak at about 1.7 eV in photoluminescence spectra is assign...

Photoluminescence of Nd-doped SnO2 thin films

Structural, optical, and electrical properties of Nd-doped SnOx thin films are reported. The atomic structure was characterized by x-ray diffraction and infrared absorption spectrometry. Investigation of the photoluminescence properties revealed Nd-related bands at 920 and 1100 nm for samples annealed at 700 °C, which present the tetragonal structure of the SnO2 rutile phase. Nd3+ ions can be indirectly excited and no concentration quenching was observed up to 3 at. %. It is concluded that Nd3+ ions are efficient optically active dopants in addition to be responsible of the observed electric conductivity improvement. These materials are then interesting for solar cell applications.

Plasmonic engineering of spontaneous emission from silicon nanocrystals

Silicon nanocrystals offer huge advantages compared to other semi-conductor quantum dots as they are made from an abundant, non-toxic material and are compatible with silicon devices. Besides, among a wealth of extraordinary properties ranging from catalysis to nanomedicine, metal nanoparticles are known to increase the radiative emission rate of semiconductor quantum dots. Here, we use gold nanoparticles to accelerate the emission of silicon nanocrystals. The resulting integrated hybrid emitter is 5-fold brighter than bare silicon nanocrystals. We also propose an in-depth analysis highlighting the role of the different physical parameters in the photoluminescence enhancement phenomenon. This result has important implications for the practical use of silicon nanocrystals in optoelectronic devices, for instance for the design of efficient down-shifting devices that could be integrated within future silicon solar cells.

Vertical electronic coupling between InAs∕InP quantum-dot layers emitting in the near-infrared range

Stacked InAs quantum dots (QDs) grown on InP(113)B are analyzed both experimentally and theoretically in order to study the influence of the electronic vertical coupling between the QD layers. Improved growth conditions enable us to control the optimum QD height of the samples, thus yielding an emission wavelength of our nanostructures at about 1.55μm at room temperature. Three samples containing three QD layers with different vertical spacing are studied. The QD electronic structure is studied by continuous-wave photoluminescence and time-resolved photoluminescence experiments at low temperature. A simplified theoretical model is developed, yielding results consistent with experimental data. This analysis evidences the electronic coupling between the QD layers.

Experimental and theoretical studies of electronic energy levels in InAs quantum dots grown on (001) and (113)B InP substrates

An experimental and theoretical comparative study of InAs quantum dots grown on (001) and (113)B InP substrates is performed. The difference between the optical transitions in the dots on the two substrates is attributed to strain effects. The influence of the first InP capping layer is also studied.

Carrier relaxation dynamics in InAs∕InP quantum dots

The electronic properties of InAs∕InP(113)B double-cap quantum dots (QDs) emitting around 1.55μm are investigated. The carrier dynamics in QDs is studied by nonresonant time-resolved photoluminescence experiments. This analysis reveals the electronic structure of the QDs and the transient filling of the confined levels. Under low excitation densities, the spontaneous exciton lifetime is estimated and compared to time-resolved experiments. Under high excitation density, a direct Auger recombination effect is identified. The temperature analysis enables us to distinguish Auger and phonon-assisted relaxation processes.

Influence of the annealing temperature on the photoluminescence of Er-doped SiO thin films

Er-doped amorphous silicon suboxide thin films were prepared by the coevaporation method. The Er concentration was varied from 0.4to6at.% and the samples were annealed at different temperatures up to 900°C. The samples exhibit a broad photoluminescence band in the visible range. Both energy and intensity of this band were dependent on the annealing temperature. For as-deposited films and samples annealed below 500°C, this band was assigned to defects in the oxide films. For higher annealing temperatures, this photoluminescence band shifted to higher wavelengths and was correlated to the appearance of amorphous silicon clusters. Two narrow bands in the near-infrared range at 0.98 and 1.54μm were also observed for the annealed samples. The intensity of these Er-related luminescence was maximal for an annealing temperature equal to around 700°C. The effective absorption cross section of Er was dependent on the annealing temperature and was equal to 6.6×10−16cm2 for the sample annealed at 700°C. The strong Er...

Luminescence efficiency at 1.5μm of Er-doped thick SiO layers and Er-doped SiO∕SiO2 multilayers

The luminescence from Er-doped thin films is studied in two different systems. The first one is a SiO single layer. The second one is a SiO∕SiO2 multilayer allowing us to obtain size-controlled silicon nanocrystals. In both systems, the annealing-temperature dependence of the luminescence is investigated. It is shown that the optimal annealing temperatures are equal to 700 and 1050°C for the single layer and the multilayer, respectively. Moreover the luminescence efficiency at 1.5μm is one order of magnitude higher in the single Er-doped SiO layer. These results are discussed in relation to the formation of silicon nanoparticles with annealing treatments.