Nanofabrication with Pulsed Lasers

An overview of pulsed laser-assisted methods for nanofabrication, which are currently developed in our Institute (LP3), is presented. The methods compass a variety of possibilities for material nanostructuring offered by laser–matter interactions and imply either the nanostructuring of the laser-illuminated surface itself, as in cases of direct laser ablation or laser plasma-assisted treatment of semiconductors to form light-absorbing and light-emitting nano-architectures, as well as periodic nanoarrays, or laser-assisted production of nanoclusters and their controlled growth in gaseous or liquid medium to form nanostructured films or colloidal nanoparticles. Nanomaterials synthesized by laser-assisted methods have a variety of unique properties, not reproducible by any other route, and are of importance for photovoltaics, optoelectronics, biological sensing, imaging and therapeutics

Angle-dependent ray tracing simulations of reflections on pyramidal textures for silicon solar cells

International audiencePyramidal textures are commonly used to reduce reflections from silicon solar cells and improve light absorption by light trapping. They are generally modelled or characterized under normal incidence. In this work, a monolayer 3D ray tracing program taking into account the polarisation of light have been developed, validated and used to compute the directional-hemispherical reflectance versus the azimuth and incidence angles for both regular upright pyramids and inverted ones, with (111) facets. Results are given for a wavelength of 0.7 µm. They show that this reflectance is not minimal at normal incidence but for an incidence angle near 20 • and that upright pyramids can have a lower hemispherical reflectance than inverted ones for incidence angles in the middle range. The bihemispherical reflectance is 19.6% for regular upright pyramids and 20.7% for inverted ones. The effect of the pyramids aspect ratio on the hemispherical reflectance at normal incidence is also studied. This reflectance decreases with the aspect ratio of both textures. Above an aspect ratio of 0.51, inverted pyramids have a lower hemispherical reflectance. But their bihemispherical reflectance is lower only for aspect ratios below 0.23

40 GSPS All Optical ADC with ENOB of 6.6 using EO Polymer Optical Deflector and Spatial Quantizer

International audience40GSPS analog-to-digital converters (ADC) arerequired for realization of digital receivers up to 20GHz. An alloptical ADC is presented using an optical deflector for spatialsampling. The optimized design of optical deflector is based onoptical beam propagation and finite element modelingelectromagnetic modeling. An effective number of bits (ENOB) of6.6 ADC is achieved under a ±20V RF voltage swing and byminimalizing angular resolution with a leaky waveguide of 2cmlong. Design and implementation of a polymer waveguide arepresented, meeting the leaky waveguide coefficient of 0.1Np/cmand low optical attenuation with quality optical waveguidesurface flatness better than 20nm. A broadband (2 - 40GHz)transition from coplanar waveguide (CPW) to coupled microstrip(CMS) transmission lines is presented that overcomes the RFattenuation to achieve under 0.06Np/cm variation by using a preemphasisfilter with a flat group delay of less than 70ps

Antireflective sol-gel TiO2 thin films for single crystal silicon and textured polycrystal silicon

International audienceIn this paper, antireflective TiO2 thin films have been prepared on single crystal silicon, and textured polycrystal silicon by sol-gel route using the dip-coating technique. The thickness and the refractive index of the films have been optimised to obtain low reflexion in the visible region, by controlling both the concentration of the titanium isopropoxide (Ti(iOPr)4), and the annealing temperature. We showed that the use of a TiO2 single layer with a thickness of 64.5 nm, heat-treated at 450 or 300 C, reduces the reflection on single crystal silicon at a level lower than 3% over the broadband spectral ranges 670-830 nm and 790-1010 nm, respectively. In order to broaden the spectral minimum reflectance as much as possible, we have proposed to texture polycrystal silicon wafers, and to coat these wafers by a TiO2 single layer with a thickness of 73.4 nm. In this case, the reflectance has been reduced from 27 to 13% in the spectral range 460-1000 nm

Electro-optic and converse-piezoelectric properties of epitaxial GaN grown on silicon by metal-organic chemical vapor deposition

We report the measurement of the (r13, r33) Pockels electro-optic coefficients in a GaN thin film grown on a Si(111) substrate. The converse piezoelectric (d33) and electro-absorptive coefficients are simultaneously determined. Single crystalline GaN epitaxial layers were grown with a AlGaN buffer layer by metal organic chemical vapor deposition, and their structural and optical properties were systematically investigated. The electro-optic, converse piezoelectric, and electro-absorptive coefficients of the GaN layer are determined using an original method. A semi-transparent gold electrode is deposited on the top of the GaN layer, and an alternating voltage is applied between top and bottom electrodes. The coefficients are simultaneously and analytically determined from the measurement of the electric-field-induced variation ΔR(θ) in the reflectivity of the Au/GaN/buffer/Si stack, versus incident angle and light polarization. The method also enables to determine the GaN layer polarity. The results obtained for a Ga-face [0001] GaN layer when using a modulation frequency of 230 Hz are for the electro-optic coefficients r13 = +1.00 ± 0.02 pm/V, r33 = +1.60 ± 0.05 pm/V at 633 nm, and for the converse piezoelectric coefficient d33 = +4.59 ± 0.03 pm/V. The value measured for the electro-absorptive variation at 633 nm is Δko/ΔE = +0.77 ± 0.05 pm/V

Electrooptic and converse-piezoelectric properties of epitaxial GaN/Si structures for optoelectronic applications

International audienceIn order to take advantage in all-optic optoelectronic devices, we have investigated the optical and piezoelectric properties of GaN films deposited on (111) silicon. Films are epitaxially grown by MOCVD, thanks to a buffer made of (Al, Ga) N intermediate layers [1]. Structural properties of GaN are analyzed using TEM and the influence of threading dislocations density is discussed. Optical properties are investigated using a prism coupling [2]. Electrooptic measurements are performed using an original technique [3]. A semi-transparent gold electrode is deposited on top of GaN layer and an alternating voltage is applied between top and bottom electrodes. The electro-optic, converse piezoelectric, and electro-absorptive coefficients are simultaneously determined from the measurement of the electric field induced variation ΔR(θ) in the reflectivity of the Au/GaN/buffer/Si stack versus incident angle. The method also enables to determine the GaN layer polarity. The results obtained for a Ga-face [0001] GaN layer when using a modulation frequency of 230 Hz are for the electro-optic coefficients r13 = +1 pm/V, r33 = +1.60 pm/V at 633 nm, and for the transverse piezoelectric coefficient d33 = +4.59 pm/V. The value measured for the electro-absorptive variation is Δko/ΔE = +0.77 pm/V. The electro-optic coefficients for GaN /Si and the electro-absorptive coefficient are measured for the first time. The converse piezoelectric value agrees with values previously reported

Antireflective sol–gel TiO2 thin films for single crystal silicon and textured polycrystal silicon

In this paper, antireflective TiO2 thin films have been prepared on single crystal silicon, and textured polycrystal silicon by sol–gel route using the dip-coating technique. The thickness and the refractive index of the films have been optimised to obtain low reflexion in the visible region, by controlling both the concentration of the titanium isopropoxide (Ti(iOPr)4), and the annealing temperature. We showed that the use of a TiO2 single layer with a thickness of 64.5 nm, heat-treated at 450 or 300 °C, reduces the reflection on single crystal silicon at a level lower than 3% over the broadband spectral ranges 670–830 nm and 790–1010 nm, respectively. In order to broaden the spectral minimum reflectance as much as possible, we have proposed to texture polycrystal silicon wafers, and to coat these wafers by a TiO2 single layer with a thickness of 73.4 nm. In this case, the reflectance has been reduced from 27 to 13% in the spectral range 460–1000 n

Structuration de surface de Silicium par gravure chimique assistée par des métaux nobles pour la texturisation de cellules solaires : Simulation de la modulation de la courbure de bande à l’interface Metal/Electrolyte/Silicium

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Modélisation 2D de la courbure de bandes de l'interface Pt-Si-Electrolyte, vers une meilleure compréhension des mécanismes de gravure chimique

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