Thèse doctorat : Electronique et automatique : Ecully, Ecole centrale de Lyon : 2007. / 207 références.

Thèse doctorat : Physique et Chimie de la Matière et des Matériaux : Nancy 1 : 2007. / Titre provenant de l'écran-titre.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

In this thesis, the carrier localization effect, the quantum confinement Stark effect (QCSE) and nonlinear optical properties of the as-grown InGaN/GaN multiple-quantum-wells (MQWs) structure and nanopillars with diameters of 100 nm and 160 nm and height of 700 nm have been investigated with linear and nonlinear photoluminescence (PL) techniques, In order to investigate the carrier localization effect and QCSE, “S-shaped” temperature dependent PL peak positions of the samples are quantitatively simulated and analyzed with the localized-state ensemble luminescence model. It is found that after nanotexturing both the carrier localization effect and QCSE become weakened. Moreover, the smaller the pillars the weaker the two effects will be. In addition, the nanotexturing introduced the new radiative recombination pathways of carriers are confirmed on the sidewalls of the nanopillars with cathodoluminescence (CL) spectrum and panchromatic CL image. Two-photon absorption (TPA) induced PL spectra of the three samples are measured to investigate the nonlinear optical properties. A peculiar excitation-power dependence, say I~P1.53, of the PL intensity is revealed. It was proposed that a mixed excitation mechanism, namely two-step successive one-photon absorption occurring in the InGaN well layers and one-step two-photon absorption mainly taking place in the GaN barrier and buffer layers, to interpret the observed phenomenon. Besides, the steady-state energetic distribution of carriers excited via this mixed excitation mechanism is very different from that of carriers via one-step one-photon excitation. In contrast with the case of one-photon PL in the samples, the influence of carrier localization effect becomes weaker in the TPA PL of the two nanopillars. / published_or_final_version / Physics / Master / Master of Philosophy

Not available / text

published_or_final_version / Chemistry / Master / Master of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

The porous layers were formed on n and p-type GaAs (100) oriented wafers, which were doped with Cr and Zn. The resulting holes concentration in bulk was equal n - 1014 cm-3 and p - 1018 cm-3. The porous layers were formed by anodization process in different solutions such as HF:HNO3:H2O (40:1:59), HF:C2H5OH:H2O (1:1:1), (6:1:1), (15:1:1) (concentration of HF was 45%). Duration of anodization process varied from 0.5 to 60 minutes at a current density of (10 – 100) mA/cm2. After the etching, the samples were rinsed with distilled water and dried. The analysis of surface morphology and photoluminescence of porous GaAs were given at this work. The performed observations have shown that the surface morphology strongly depends on the composition of etching solution, specific resistance of GaAs wafers, etching time and current density during the etching procedure. A mat film is observed on the surface of electrochemically-etched GaAs. The colour of the film varied from black to light brown and to greenish. Colour variation is determined by anodization conditions determined by the composition of etching solution, current density during anodization, etching time and specific resistance of wafers. The observed films were solid. Some films were fragmented in a chaotic way or composed by oriented units. In order to assess the luminescence properties of the porous material and their dependence on etching conditions the photoluminescence spectra of porous GaAs were measured at room... [to full text]

<p>The mechanism for defect related green emission from zinc (ZnO:Zn) and sulfur doped ZnO (ZnO:S) are determined through optical characterization of the green and UV emission bands. ZnO:Zn is prepared by heating ZnO in a slightly reducing atmosphere for 1 hour and sulfur doped ZnO is similarly obtained with a small amount of sulfur added. Photoluminescence (PL), photoluminescence excitation spectra (PLE), and quantum efficiency measurements are analyzed to determine the mechanism of the green defect emission. Low temperature PL and PLE measurements are used to assign activation energies to the emission processes and connect them with donor bound excitons in ZnO. It was determined that both ZnO:Zn and ZnO:S have a similar green emission mechanism. This common mechanism involves the formation of donor bound excitons <italic>I_3a</italic> and <italic>I₉</italic>, which were determined to be the mediators between photoexcitation of excitons and the transfer of energy to the defect responsible for green emission. The most efficient excitation processes for both the green and band edge emissions at low temperatures is through direct excitation of the neutral donor bound exciton <italic>I₉</italic> or by ionizing the neutral donor bound exciton <italic>I_3a</italic>. The ionization of <italic>I_3a</italic> eliminates this exciton localization site and simultaneously creates a bound exciton at <italic>I₉</italic>. The <italic>I₉</italic> bound exciton can then either transfer energy to the defect responsible for the green emission or contribute to the free exciton population through a phonon assisted transition. At room temperature a resonant absorption peak associated with <italic>I₉</italic> is still present in the absorption band for ZnO:Zn suggesting partial localization at <italic>I_3a</italic> and <italic>I₉</italic> of free excitons with low kinetic energy (excitations below the band gap) continues to be the intermediate between excitons and the energy transfer to the green emitting defect. </p><p>In ZnO:S, the addition of sulfur creates ZnS domains within the lattice leading to a type II band alignment at the interface of ZnO and ZnS domains. This band alignment at the interface increases the density of free electrons in ZnO, which may then encounter an ionized <italic>I_3a</italic> site converting it to its neutral form. Increasing the density of free electrons, a result of the type II band alignment, increases the chances of returning an ionized <italic>I_3a</italic> to its neutral form and thus increases the green emission. These results can lead to informed optimization of ZnO:S as a potential white light emitting phosphor.</p> / Dissertation