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The detection of Raman signals from diluted molecules or biomaterials in complex media is still a challenge. Besides the widely studied Raman enhancement by nanoparticle plasmons, interference mechanisms provide an interesting option. A novel approach for amplification platforms based on supported thin alumina membranes was designed and fabricated to optimize the interference processes. The dielectric layer is the extremely thin alumina membrane itself and, its metallic aluminum support, the reflecting medium. A CVD (chemical vapor deposition) single-layer graphene is transferred on the membrane to serve as substrate to deposit the analyte. Experimental results and simulations of the interference processes were employed to determine the relevant parameters of the structure to optimize the Raman enhancement factor (E.F.). Highly homogeneous E.F. over the platform surface are obtained, typically 370 ± (5%), for membranes with ~100 nm pore depth, ~18 nm pore diameter and the complete elimination of the Al2O3 bottom barrier layer. The combined surface enhanced Raman scattering (SERS) and interference amplification is also demonstrated by depositing ultra-small silver nanoparticles. This new approach to amplify the Raman signal of analytes is easily obtained, low-cost and robust with useful enhancement factors (~400) and allows only interference or combined enhancement mechanisms, depending on the analyte requirements.

Europium-doped La2O3 nanocrystalline powders with sizes ranging from 4 nm to 300 nm have been obtained by the modified Pechini, hydrothermal with conventional furnace, hydrothermal with microwave furnace, and precipitation with ultrasonic bath methods. X-ray diffraction techniques were used to study the evolution of the prepared gels towards the desired crystalline phase. We determined the size and the morphology of the nanoparticles by electronic microscopy. Finally, we studied and analyzed the luminescence properties of the trivalent europium in the hexagonal La2O3 nanocrystals by photoluminescence and cathodoluminescence.

[EN] Silicon is the material of choice for visible light photodetection and solar cell fabrication. However, due to the intrinsic band gap properties of silicon, most infrared photons are energetically useless. Here, we show the first example of a photodiode developed on a micrometre scale sphere made of polycrystalline silicon whose photocurrent shows the Mie modes of a classical spherical resonator. The long dwell time of resonating photons enhances the photocurrent response, extending it into the infrared region well beyond the absorption edge of bulk silicon. It opens the door for developing solar cells and photodetectors that may harvest infrared light more efficiently than silicon photovoltaic devices that are so far developed.

Silicon is the material of choice for visible light photodetection and solar cell fabrication. However, due to the intrinsic band gap properties of silicon, most infrared photons are energetically useless. Here, we show the first example of a photodiode developed on a micrometre scale sphere made of polycrystalline silicon whose photocurrent shows the Mie modes of a classical spherical resonator. The long dwell time of resonating photons enhances the photocurrent response, extending it into the infrared region well beyond the absorption edge of bulk silicon. It opens the door for developing solar cells and photodetectors that may harvest infrared light more efficiently than silicon photovoltaic devices that are so far developed.

10.1186/1556-276X-7-228, We report on an exhaustive and systematic study about the photoluminescent properties of nanoporous anodic alumina membranes fabricated by the one-step anodization process under hard conditions in oxalic and malonic acids. This optical property is analysed as a function of several parameters (i.e. hard anodization voltage, pore diameter, membrane thickness, annealing temperature and acid electrolyte). This analysis makes it possible to tune the photoluminescent behaviour at will simply by modifying the structural characteristics of these membranes. This structural tuning ability is of special interest in such fields as optoelectronics, in which an accurate design of the basic nanostructures (e.g. microcavities, resonators, filters, supports, etc.) yields the control over their optical properties and, thus, upon the performance of the nanodevices derived from them (biosensors, interferometers, selective filters, etc.)

We revisited two different strategies to fabricate 1D photonic crystals of nonlinear optical dielectric materials based on ultrafast laser ablation of the surface of an RbTiOPO4 crystal, and selective etching of ferroelectric domains of the surface of a periodically poled LiNbO4 crystal. We evaluated their behaviour as Bragg diffraction gratings. We also presented the recent advances we developed in a new procedure of fabrication of 2D and 3D photonic crystals of KTiOPO4 (KTP) grown on the surface of a KTP substrate by liquid phase epitaxial means within the pores of a silicon macroporous template. Optical, structural, morphological, and compositional characterization for the photonic crystals produced through this technique are presented., Peer Reviewed, Postprint (published version)

The deposition of specific materials into ordered pores arranged in a regular lattice allows the fabrication of ordered fiber arrays. Polymer micro- and nanofibers using macroporous silicon and nanoporous alumina templates were fabricated employing a vacuum infiltration method for Poly(methyl methacrylate) (PMMA) structures or a melt-assisted template wetting into the pores for Poly(3-hexylthiophene) (P3HT) structures. We have studied how the aspect ratio of the micro- and nanopores and the kind of polymer influence the structures fabrication., Peer Reviewed, Postprint (published version)

A study was conducted to demonstrate the infiltration of polystyrene (PS) spheres inside the pores of silicon membrane. The study conducted significant investigations of the particle arrangement for D values between 1 and 3. The study employed charge-stabilized colloidal particles to conduct investigations. It was demonstrated that sulphate-stabilized PS spheres were infiltrated into the silicon membrane. A combination of PS spheres, with different diameters and high-quality porous silicon membranes were used to cover the values of D and conduct investigations. The silicon membrane was manufactured by electrochemical etching procedure, which allows very long pores with vertical walls. It was also observed that the pore diameter decreases to a value around 50% or less at the bottom from a normal value at the surface.

Microtubes and microfibres composed of poly(3-hexylthiophene) (P3HT) were fabricated by melt-assisted templates using ordered macroporous silicon. We have studied the influence of the pore depth and the template type on the microstructure fabrication, where the templates were membranes or structures opened only at one end. Current– voltage (I–V) measurements demostrated that the resistivity of these P3HT microstructures was in the same order as that of homogeneous films, which allows them to be used in electronic devices., Peer Reviewed, Postprint (published version)

This work focuses on the fabrication of three-dimensional (3D) microstructures by electrochemical etching of n-type silicon. We exploit the possibility of modulating the pore diameter along the growing direction to produce quasi-3D structures which exhibit periodicity also in third dimension. The fabricated structures are further processed to convert them into truly 3D networks consisting of interconnected silicon veins embedded in air. In particular, we show how a variety of complex 3D networks can be obtained starting from a simple initial pore arrangement, e.g. square or hexagonal. These 3D structures can be easily fabricated with very high precision and uniformity on a large scale. They constitute ideal hosts for sensing applications as well as perfect molds for casting 3D structures of unusual materials., Peer Reviewed, Postprint (published version)

We report the experimental characterization of two-dimensional (2D) macroporous silicon photonic crystals using angular-dependent reflectance spectroscopy in the mid-IR region. We have investigated different sample structures and we have shown that an adequate post-processing of the measured data is crucial in order to recognize the photonic bands and to achieve a good agreement of the measured data with the theoretical predictions for the studied structures., Peer Reviewed, Postprint (published version)

Macroporous silicon membranes, prepared by photo-electrochemical etching, were subjected to pore widening performed by multiple oxidation/oxide-stripping cycles. We study the dependence of pore cross section and diameter on the oxidation parameters. The anisotropy of the oxidation process allows us to correct the faceted pore shape after the etching and to obtain pores with circular cross section. The shown post-treatment of macroporous silicon extends the possibilities of the electrochemical etching process to produce structures with a desired geometry., Peer Reviewed, Postprint (published version)

In this work, we study the changes in the optical properties of 300-nm-thick hydrogenated amorphous silicon carbide layers after an annealing process. Both intrinsic and phosphorus-doped amorphous silicon carbide layers (a-SiCx:H) were deposited on silicon wafers by plasma enhanced chemical vapour deposition (PECVD) at 400 °C and annealed in a quartz furnace at 800 °C. The presence of randomly oriented silicon nanocrystals was confirmed by X-ray diffraction (XRD) measurements after the partial recrystallization process only in the doped layers. The presence or the absence of the nanocrystals clearly changes the Fourier transform infrared (FTIR) spectra. From the fitting of the experimental curves with the model of Lorentz oscillators, the refractive index and the extinction coefficient of the different layers were obtained., Peer Reviewed, Postprint (published version)

This article describes the benefits of including cross-layer information in the scheduling mechanism of a UMTS downlink channel. In particular, the information obtained from the fast power control algorithm is used to properly schedule transmissions. A prioritization function that exploits the short-term channel variations is proposed. This strategy is shown to be a feasible approach to improve system performance in terms of capacity and delay. This enhancement is obtained as a benefit of intrinsic multi-user diversity. The proposal is applicable within the current UMTS radio resource management framework., Peer Reviewed, Postprint (published version)

This paper presents a novel technique for silicon dioxide (SiO2) microneedle fabrication. Microneedles are hollow microcapillaries with tip diameters in the range of micrometers. They can be used in the fabrication of microsyringes. These structures can be of high interest in medical and biological applications, such as DNA injection, antibody manipulation and drug delivery, and cell manipulation. Fabrication process is based on electrochemical etching of n-type silicon in hydrofluoric acid (HF) solutions. Basic process flow and etching conditions that ensure a stable pore growth are described. These conditions also determine the geometry of the resulting microneedle structure. Microneedle arrays of different dimensions can be fabricated in a single run on the same wafer. In this work, microneedle arrays with pore diameters ranging from 2 to 5 µm, pore lengths from 30 to 140 µm and wall thicknesses in the range of 70–110 nm are reported., Peer Reviewed, Postprint (published version)

This paper reports on the fabrication of two-dimensional macropore arrays by electrochemical etching of silicon. We describe some critical effects accompanying the etching process that can generate imperfections in the etched pore pattern. We also give the solutions to improve the pore uniformity and to produce high-aspect-ratio macropores with good structural characteristics. The obtained macroporous structures can be further processed to produce high-aspect-ratio pillars, instead of pores. By modulating the applied etching current we have also induced periodicity in the third dimension., Peer Reviewed, Postprint (published version)

We investigate the absolute photonic band gap (PBG) formation in two-dimensional (2-D) photonic crystals designed using symmetry reduction approach. The lattice symmetry, shape and orientation of dielectric scatterers affect the photonic gap parameters. We use symmetry reduction, achieved either by including additional rods into the lattice unit cell or by reorienting noncircular scatterers to engineer the photonic band gaps in 2-D square and triangular structures. The case of air rods drilled into silicon background is considered. We show that for square structures symmetry reduction can be an effective way to enlarge the absolute PBG, but for triangular lattices any modification of the crystal structure considerably reduces the absolute PBG width. We also discuss the practical technological feasibility of the proposed structures., Peer Reviewed, Postprint (published version)

We investigate different aspects of the absolute photonic band gap (PBG) formation in two-dimensional photonic structures consisting of rods covered with a thin dielectric film. Specifically, triangular and honeycomb lattices in both complementary arrangements, i.e., air rods drilled in silicon matrix and silicon rods in air, are studied. We consider that the rods are formed of a dielectric core (silicon or air) surrounded by a cladding layer of silicon dioxide (SiO2), silicon nitride (Si3N4), or germanium (Ge). Such photonic lattices present absolute photonic band gaps, and we study the evolution of these gaps as functions of the cladding material and thickness. Our results show that in the case of air rods in dielectric media the existence of dielectric cladding reduces the absolute gap width and may cause complete closure of the gap if thick layers are considered. For the case of dielectric rods in air, however, the existence of a cladding layer can be advantageous and larger absolute PBG’s can be achieved., Peer Reviewed, Postprint (published version)

We investigate different aspects of the absolute photonic band gap (PBG) formation for two-dimensional photonic crystals, consisting of air rods drilled into silicon. Specifically, square lattices of square and circular rods are considered. The lattice symmetry, shape and orientation of the rods affect the photonic gap parameters. A symmetry reduction by addition of a smaller different shaped rod into the center of the lattice unit cell can produce significantly larger band gap. Combining the symmetry reduction and rotation of the noncircular rods yields the greatest improvements in the size of absolute band gap. We discuss the maximization of the absolute PBG width as a function of lattice parameters and the practical fabrication feasibility of these optimized structures., Peer Reviewed, Postprint (published version)

In this work we investigate the effects of typical thermal treatments on c-Si surface passivation by intrinsic a-SiCx:H films. First, a forming gas anneal (FGA), (400 °C, 30 min in H2/N2) is applied resulting in an improvement in surface passivation. The best result is a surface recombination velocity lower than 23 cm s-1. A second thermal step (730 °C, 30 s) is studied to try and simulate the firing step for screen-printed contacts. This annealing has no effect in surface passivation indicating that a-SiCx:H films are promising candidates for c-Si surface passivation in the photovoltaic industry. In order to analyze the improvement of surface passivation after FGA, a physical model of surface recombination at the a-SiCx:H/c-Si interface is developed. This improvement is directly connected to a reduction in fundamental recombination velocities of electrons and holes at the interface. A possible explanation could be the reduction in the interface state density due to hydrogen saturation of dangling bonds., Peer Reviewed, Postprint (published version)

The stability under gate bias stress of unpassivated thin film transistors was studied by measuring the transfer and output characteristics at different temperatures. The active layer of these devices consisted of in nanocrystalline silicon deposited at 125°C by Hot-Wire Chemical Vapour Deposition. The dependence of the subthreshold activation energy on gate bias for different gate bias stresses is quite different from the one reported for hydrogenated amorphous silicon. This behaviour has been related to trapped charge in the active layer of the thin film transistor., Peer Reviewed, Postprint (published version)

Hydrogenated microcrystalline silicon films obtained at low temperature (150–280°C) by hot wire chemical vapour deposition at two different process pressures were measured by Raman spectroscopy, X-ray diffraction (XRD) spectroscopy and photothermal deflection spectroscopy (PDS). A crystalline fraction >90% with a subgap optical absortion 10 cm-1 at 0.8 eV were obtained in films deposited at growth rates >0.8 nm/s. These films were incorporated in n-channel thin film transistors and their electrical properties were measured. The saturation mobility was 0.72 ± 0.05 cm2/V s and the threshold voltage around 0.2 eV. The dependence of their conductance activation energies on gate voltages were related to the properties of the material., Peer Reviewed, Postprint (published version)

Silicon–carbon emitters were deposited by plasma enhanced chemical vapour deposition (PECVD) on a conventional silicon transistor base. The resulting transistors were annealed after the emitter had been deposited which led to the dopant diffusing from the amorphous layer into the crystalline base. This paper analyses the effect of the annealing temperature on the properties of the amorphous layer and on the electrical characteristics of the transistors. First, after isochronal annealing between 740°C and 880°C, the physical properties of PECVD a-Si0.8C0.2:H(n) layers were measured. Then, the base current of the transistors was analysed as a function of the annealing temperature to gain deeper insight into its physical origin., Peer Reviewed, Postprint (published version)

Polysilicon thin film transistors (TFT) are of great interest in the field of large area microelectronics, especially because of their application as active elements in flat panel displays. Different deposition techniques are in tough competition with the objective to obtain device-quality polysilicon thin films at low temperature. In this paper we present the preliminary results obtained with the fabrication of TFT deposited by hot-wire chemical vapor deposition (HWCVD). Some results concerned with the structural characterization of the material and electrical performance of the device are presented., Peer Reviewed, Postprint (published version)

We fabricated and characterized annealed n-type amorphous Si1-xCx on p-type crystalline silicon heterojunction diodes with three different base doping concentrations: NA~1016,¿1018, and 1020¿cm-3. The conduction mechanisms were determined by analyzing the temperature dependence of the current–voltage characteristics. The results show that the diodes with low doping concentrations (1016¿cm-3) are ideal, because the phosphorous slightly diffuses into the crystalline silicon, whereas diodes with higher doping concentrations (1018–1020¿cm-3) are dominated by multitunneling capture emission. The increase in the base acceptor doping concentration also causes excess current over the ideal diode current at low forward bias and an increase in the leakage reverse current., Peer Reviewed, Postprint (published version)

The fabrication and characterization of in situ-doped amorphous Si/sub 0.8/C/sub 0.2/ emitter transistors are presented. Emitter Gummel numbers exceeding 10/sup 14/ s/cm/sup 4/ are reported for the first time in this type of structure. The high values obtained for G/sub E/ are believed to be due to the valance band discontinuity between the Si/sub 0.8/C/sub 0.2/ layer and the crystalline part of the emitter, which effectively blocks the minority carrier injection from the base into the noncrystalline part of the emitter., Peer Reviewed, Postprint (published version)

This paper addresses the problem of the space charge region Shockley-Read-Hall (SRH) recombination currents in heterojunctions with one noncrystalline side. A formulation which generalizes previous works is discussed. The approach is based on the drift-diffusion model with a thermionic-field emission boundary condition. The main physical parameters which determine the relative contribution of each zone of the space charge region (SCR) to the total recombination current are identified. The general analysis is applied for the first time to amorphous/crystalline heterojunctions and design criteria are established to minimize the total recombination current., Peer Reviewed, Postprint (published version)