Porous silicon optical microcavity for chemical sensing application using tris-(8-hydroxyquinoline) aluminum (Alq

The present paper reports the study of a porous silicon based microcavity for a potential chemical sensing application using tris-(8-hydroxyquinoline) aluminum (Alq3) molecules. Porous silicon based planar microcavity was first designed and fabricated using the electrochemical etching technique. Photoluminescence emission of a single porous silicon layer after immersion in an Alq3 solution was first carried out in order to verify that the Alq3 molecules were bound to the porous surface. A wide green band centered at 519 nm, typical of a nano-structured Alq3 film, was observed. Reflectivity measurements of the porous silicon microcavity were then performed for different aluminum concentrations of the Alq3 solution. The microcavity device showed a good sensibility for the Alq3 molecules and an important shift of the microcavity photonic resonance was observed. This device might be considered for a potential aluminiun sensing application

Morphological, structural and ellipsometric investigations of Cr doped TiO2 thin films prepared by sol-gel and spin coating

International audiencePure and chromium doped titanium dioxide (TiO2) thin films at different atomic percentages (0.5%, 1.3% and 2.9%) have been elaborated on ITO/Glass substrates by sol-gel and spin-coating methods using titanium (IV) isopropoxide as a precursor. The surface morphology of films was investigated by scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM), the structure was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and high resolution transmission microscopy (HRTEM). SEM and HRTEM show homogenous and polycrystalline films. XRD patterns indicate a phase transition from anatase to anatase-rutile leading to expand the absorption band of TiO2 molecules around 520 cm(-1) in FTIR spectra. The optical constants such as the refractive index (n), the extinction coefficient (K) and the band gap (E-g) as well as the film thickness are determined using spectroscopic ellipsometry technique and Fourouhi-Blommer dispersion model. Results show three major changes; (i) the thickness of pure TiO2 layer is 54 nm, which linearly decreases when the layer is doped with chromium and reaches 33 nm for a doping concentration of 2.9%, (ii) the band gap energy (E-g) is also linearly reduced from 3.24 eV to 2.80 eV when the Cr-doping agent increases, and, (iii) a phase transition from anatase to anatase-rutile is observed causing an increase in values of n(lambda) for wavelength greater than 350 nm. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved

Photoluminescence study of nitrogen effects on confined states in GaAs

Modulation doped heterostructures GaAs/GaAs$_{1-x}$Nx/GaAs/Al0.3Ga0.7As:δSi with GaAs$_{1-x}$Nx Quantum Wells (QW) with different nitrogen contents x have been grown by molecular beam epitaxy and investigated by photoluminescence (PL) spectroscopy. We have found that at low temperature the photoluminescence spectra are essentially formed by two structures observed at 1.51 eV and 1.47 eV attributed to excitonic transitions in GaAs layer, and in GaAs$_{1-x}$Nx QW, respectively. The Band Anticrossing Model (BAC) has been adopted in order to confirm the nature of the transitions in GaAs$_{1-x}$Nx QW's. The band structure of $\delta$-doped GaAs/GaAs$_{1-x}$Nx/GaAs/Al0.3Ga0.7As:δSi has been studied theoretically by using the finite differences method to self-consistently and simultaneously solve Schrödinger and Poisson equations written within the Hartree approximation. We find in this way good agreement between our measured and our calculated values for the transition energies in our GaAs$_{1-x}$Nx QW's

Effect of rapid oxidation on optical and electrical properties of silicon nanowires obtained by chemical etching

In the present work, we report the investigation of passivated silicon nanowires (SiNWs) having an average radius of 3.7 μm, obtained by chemical etching of p-type silicon (p-Si). The surface passivation of the SiNWs was performed through a rapid oxidation conducted under a controlled atmosphere at different temperatures and durations. The morphology of the SiNWs was examined using a scanning electron microscope (SEM) that revealed a wave-like structure of dense and vertically aligned one-dimensional silicon nanostructures. On the other hand, optical and electrical characterizations of the SiNWs were studied using a UV-Vis-NIR spectrometer, the Fourier transform infrared spectroscopy (FTIR) and I-V measurements. The reflectance of SiNWs has been dropped to approximately 2% in comparison to that of bare p-Si. This low reflectance slightly increased after carrying out the rapid thermal annealing. The observed behavior was attributed to the formation of a SiO2 layer, as confirmed by FTIR measurements. Finally, the electrical measurements have shown that the rapid oxidation, at certain conditions, contributes to the improvement of the electrical responses of the SiNWs, which can be of great interest for photovoltaic applications

Correlation of atomic force microscopy and photoluminescence analysis of GaAs nanocrystallites elaborated by electrochemical etching of n

GaAs nanocrystallites are elaborated by electrochemical etching of n+ type GaAs substrates. Photoluminescence (PL) and atomic force microscope (AFM) images analysis are used to study the porous layers obtained with different etching times. Two kind of quantum confinement due to the formation of two nanostructures of different sizes are observed from room temperature photoluminescence (RTPL) spectra. The surface topography and the density of grains of the porous GaAs (π-GaAs) samples are investigated using the AFM technique. AFM images showed that the structure of films was nanocrystalline with a grain size close to 7 nm, this was confirmed by photoluminescence spectroscopy (PL) and the effective mass theory

Design and optimization of an OR gate all optical circuit based on silicon photonic crystals

In this paper we investigate and optimize the design of an OR gate all optical circuit based on silicon photonic crystals. The OR gate is formed by two ring resonators placed between three waveguides, obtained by removing specific rods from the photonic-crystal structure. To optimize the design parameters, the fill factor (r/a), corresponding to the ratio between the rod radius “r”’ and the inter-rod lattice “a’’, was varied using the plane wave expansion method. The Q-factor has been determined to achieve the optimal performance of the ring resonators. The optical properties and the normalized transmission spectra for the proposed gate based on the photonic-crystal ring resonators have been calculated by the finite difference time domain (FDTD) technique. We have noted that the two rings must be symmetric to the central waveguide to obtain the OR gate function in the output

Ellipsometric investigation of porous silicon layers for the design of a DBR

Porous silicon layers (PSL) were fabricated by electrochemical etching and investigated by spectroscopic ellipsometry (SE) in the energy range 0.6−5 eV. Within the effective medium approximation (EMA) and through an optical model consisting of a mixture of void and crystalline silicon (cSi), we were able to determine the porosity (void concentration) and the thicknesses of the PSL. The PSL were divided into several sublayers in order to obtain the best agreement between measured and simulated spectra. Once the etching parameters have been controlled and by choosing the appropriate conditions, it was possible to design a distributed Bragg reflector (DBR) with a high reflectivity band centered at 800 nm. This DBR consists on stacks of alternate PSL having two different refractive indices

Correlation between physical properties and growth mechanism of In

Indium sulfide (In2S3) thin films were grown on ITO-coated glass substrate using the electrodeposition method. The effect of the deposition time on the structural, morphological, optical and electrical properties of the as-grown In2S3 thin films was studied. XRD spectra of the obtained films reveal the polycrystalline nature of (β-In2S3) with a tetragonal crystal structure along the (109) plane, and exhibit a sharp transition to the (0012) plane when the deposition time is extended beyond 20 min. Using atomic force microscope (AFM), the surface morphology shows a remarkable change in the grain size, thickness, and surface roughness when varying the deposition time. UV-VIS spectrophotometer show that the optical band gap values of In2S3 decrease from about 2.82 to 1.93 eV by extending the electrodeposition duration from 5 to 20 min. All films were found to have an n-type character with a lower electrical resistivity of about 1.8×10-3 Ω cm for films deposited at 20 min

Synthesis and characterization of ZnO/Cu₂O core–shell nanowires grown by two-step electrodeposition method

International audienceZnO/Cu2O core/shell nanowires have been grown by two-step electrodeposition method on ITO-coated glass substrates. The sample's morphology was explored by means of scanning electron microscopy (SEM). SEM images confirm the homogeneity of the nanowires and the presence of Cu2O shell on ZnO core. X-ray diffraction and Raman scattering measurements were used to investigate the purity and the crystallinity of the samples. Optical transmission measurements reveal an additional contribution at about 1.7 eV attributed to the type-II interfacial transition witch confirms the advantage of using the ZnO/Cu2O structure in photovoltaic application