Adel Najar

Ultra-low reflection porous silicon nanowires for solar cell applications

High density vertically aligned Porous Silicon NanoWires (PSiNWs) were fabricated on silicon substrate using metal assisted chemical etching process. A linear dependency of nanowire length to the etching time was obtained and the change in the growth rate of PSiNWs by increasing etching durations was shown. A typical 2D bright-field TEM image used for volume reconstruction of the sample shows the pores size varying from 10 to 50 nm. Furthermore, reflectivity measurements show that the 35% reflectivity of the starting silicon wafer drops to 0.1%, recorded for more than 10 μm long PSiNWs. Models based on cone shape of nanowires located in a circular and rectangular bases were used to calculate the reflectance employing the Transfert Matrix Formalism (TMF) of the PSiNWs layer. Using TMF, the Bruggeman model was used to calculate the refractive index of PSiNWs layer. The calculated reflectance using circular cone shape fits better the measured reflectance for PSiNWs. The remarkable decrease in optical reflectivity indicates that PSiNWs is a good antireflective layer and have a great potential to be utilized in radial or coaxial p-n heterojunction solar cells that could provide orthogonal photon absorption and enhanced carrier collection.

Effect of hydrofluoric acid concentration on the evolution of photoluminescence characteristics in porous silicon nanowires prepared by Ag-assisted electroless etching method

We report on the structural and optical properties of porous silicon nanowires (PSiNWs) fabricated using silver (Ag) ions assisted electroless etching method. Silicon nanocrystallites with sizes <5 nm embedded in amorphous silica have been observed from PSiNW samples etched using the optimum hydrofluoric acid (HF) concentration. The strongest photoluminescence (PL) signal has been measured from samples etched with 4.8 M of HF, beyond which a significant decreasing in PL emission intensity has been observed. A qualitative model is proposed for the formation of PSiNWs in the presence of Ag catalyst. This model affirms our observations in PL enhancement for samples etched using HF < 4.8 M and the eventual PL reduction for samples etched beyond 4.8 M of HF concentration. The enhancement in PL signals has been associated to the formation of PSiNWs and the quantum confinement effect in the Si nanocrystallites. Compared to PSiNWs without Si-Ox, the HF treated samples exhibited significant blue PL peak shift of 1...

Efficient coupler between silicon photonic and metal-insulator-silicon-metal plasmonic waveguides

We report the experimental realization of a compact, efficient coupler between silicon waveguides and vertical metal-insulator-silicon-metal (MISM) plasmonic waveguides. Devices were fabricated using complementary metal-oxide-silicon technology processes, with copper layers that support low-loss plasmonic modes in the MISM structures at a wavelength of 1550 nm. By implementing a short (0.5 μm) optimized metal-insulator-silicon-insulator structure inserted between the photonic and plasmonic waveguide sections, we demonstrate experimental coupling loss of 2.5 dB, despite the high optical confinement of the MISM mode and mismatch with the silicon waveguide mode.

On the phenomenon of large photoluminescence red shift in GaN nanoparticles

We report on the observation of broad photoluminescence wavelength tunability from n-type gallium nitride nanoparticles (GaN NPs) fabricated using the ultraviolet metal-assisted electroless etching method. Transmission and scanning electron microscopy measurements performed on the nanoparticles revealed large size dispersion ranging from 10 to 100 nm. Nanoparticles with broad tunable emission wavelength from 362 to 440 nm have been achieved by exciting the samples using the excitation power-dependent method. We attribute this large wavelength tunability to the localized potential fluctuations present within the GaN matrix and to vacancy-related surface states. Our results show that GaN NPs fabricated using this technique are promising for tunable-color-temperature white light-emitting diode applications.

MNOS stack for reliable, low optical loss, Cu based CMOS plasmonic devices

We study the electro optical properties of a Metal-Nitride-Oxide-Silicon (MNOS) stack for a use in CMOS compatible plasmonic active devices. We show that the insertion of an ultrathin stoichiometric Si(3)N(4) layer in a MOS stack lead to an increase in the electrical reliability of a copper gate MNOS capacitance from 50 to 95% thanks to a diffusion barrier effect, while preserving the low optical losses brought by the use of copper as the plasmon supporting metal. An experimental investigation is undertaken at a wafer scale using some CMOS standard processes of the LETI foundry. Optical transmission measurments conducted in a MNOS channel waveguide configuration coupled to standard silicon photonics circuitry confirms the very low optical losses (0.39 dB.μm(-1)), in good agreement with predictions using ellipsometric optical constants of Cu.

Effective antireflection properties of porous silicon nanowires for photovoltaic applications

Porous silicon nanowires (PSiNWs) have been prepared by metal-assisted chemical etching method on the n-Si substrate. The presence of nano-pores with pore size ranging between 10-50nm in SiNWs was confirmed by electron tomography (ET) in the transmission electron microscope (TEM). The PSiNWs give strong photoluminescence peak at red wavelength. Ultra-low reflectance of <;5% span over wavelength 250 nm to 1050 nm has been measured. The finite-difference time-domain (FDTD) method has been employed to model the optical reflectance for both Si wafer and PSiNWs. Our calculation results are in agreement with the measured reflectance from nanowires length of 6 μm and 60% porosity. The low reflectance is attributed to the effective graded index of PSiNWs and enhancement of multiple optical scattering from the pores and nanowires. PSiNW structures with low surface reflectance can potentially serve as an antireflection layer for Sibased photovoltaic devices.

Effect of structure and composition on optical properties of Er-Sc silicates prepared from multi-nanolayer films.

Polycrystalline Er-Sc silicates (Er(x)Sc(2-x)SiO₅ and Er(x)Sc(2-x)Si₂O₇) were fabricated using multilayer nanostructured films of Er₂O₃/SiO₂/Sc₂O₃ deposited on SiO₂/Si substrates by RF- sputtering and thermal annealing at high temperature. RBS, TEM, GIXD, and PL results show the presence of Er(x)Sc(2-x)SiO₅ with an emission peak at 1528 nm for annealing from 900 to 1100 °C, and Er(x)Sc(2-x)Si₂O₇ with an emission peak at 1537 nm for higher annealing temperature. The PL intensity of the Er(x)Sc(2-x)Si₂O₇ phase is five times stronger than that of the Er(x)Sc(2-x)SiO₅ phase at 1250 °C. From PLE and PL spectra of Er(x)Sc(2-x)Si₂O₇ thin film, we schematically illustrate the Er³⁺ Stark energy levels of ⁴I(13/2) to ⁴I(15/2) manifolds due to the crystal field strength effect of Sc³⁺. Temperature-dependent PL of the Er(x)Sc(2-x)Si₂O₇ phase exhibits a variation of the full-width at half-maximum (FWHM) from 1.1 to 2.3 nm. The narrow FWHM is due to the small ionic radii of Sc³⁺, which enhance the crystal field strength affecting the optical properties of Er³⁺ ions located at the well-defined lattice sites of Sc silicate. A large excitation cross-section (σ(ex)) is equal to 3.0x10⁻²⁰ cm² at λ(ex) = 1527.6 nm.

Structural characterization and luminescence properties of ErxSc2-xSi2O7 prepared by RF sputtering

Polycrystalline Er_xSc_2-xSi₂O₇ compounds were fabricated using RF-sputtering by alternating Er₂O₃, Sc₂O₃ layers separated by SiO₂ layer. This new compounds presents excitation cross section at 980nm around 1.39×10^-21cm² with lifetime of 38 μs.

Scandium effect on the luminescence of Er-Sc silicates prepared from multi-nanolayer films.

Polycrystalline Er-Sc silicates (ErxSc2-xSi2O7 and ErxSc2-xSiO5) were fabricated using multilayer nanostructured films of Er2O3/SiO2/Sc2O3 deposited on SiO2/Si substrates by RF sputtering and thermal annealing at high temperature. The films were characterized by synchrotron radiation grazing incidence X-ray diffraction, cross-sectional transmission electron microscopy, energy-dispersive X-ray spectroscopy, and micro-photoluminescence measurements. The Er-Sc silicate phase ErxSc2-xSi2O7 is the dominant film, and Er and Sc are homogeneously distributed after thermal treatment because of the excess of oxygen from SiO2 interlayers. The Er concentration of 6.7 × 1021 atoms/cm3 was achieved due to the presence of Sc that dilutes the Er concentration and generates concentration quenching. During silicate formation, the erbium diffusion coefficient in the silicate phase is estimated to be 1 × 10-15 cm2/s at 1,250°C. The dominant ErxSc2 - xSi2O7 layer shows a room-temperature photoluminescence peak at 1,537 nm with the full width at half maximum (FWHM) of 1.6 nm. The peak emission shift compared to that of the Y-Er silicate (where Y and Er have almost the same ionic radii) and the narrow FWHM are due to the small ionic radii of Sc3+ which enhance the crystal field strength affecting the optical properties of Er3+ ions located at the well-defined lattice sites of the Sc silicate. The Er-Sc silicate with narrow FWHM opens a promising way to prepare photonic crystal light-emitting devices.

Optical transition between Stark levels in (ErSc)2O3 epitaxitial films

Toward quantum information applications, we control the interactions between Er ions by alloying epitaxial Er2O3 with scandium and suppress energy transfer and inhomogeneous broadening of Stark levels in the intra-4f band of Er ions.