Porous Silicon Multilayer Structures Research Articles

Enhanced photoluminescence of active ions in rugate type multilayer structures

Multilayer rugate-like structures based on porous silicon with incorporated active ions are theoretically investigated. Multilayer structures consist on different sinusoidal refractive index profiles designed to show the greatest amplification of electric field at a wavelength of 1 µm. We show that it is possible to increase the localization of the electric field by optimizing the design parameters, and thereby directly enhances the photoluminescence of the active ions. The luminescent properties of active ions incorporated in the porous silicon structures are calculated from a model based on the Förster Resonance Energy Transfer (FRET) theory. This model considers the effect of the electric field due to the structures and the energy transfer processes between donors and acceptors and allows studying the luminescent dynamics in this type of systems. It is shown that a greater intensity in the emission of ions is obtained from the structures with the highest amplification of the electric field. Furthermore, it is observed that the emission of active ions depends on the porous silicon multilayer structure and on the concentration of ions. Likewise, greater luminescence is observed as the acceptor density increases. In the same way, donor and acceptor lifetimes also depend on the acceptor density, although it does not depend on the porous silicon structure. The structures proposed here are prospective for Si-based optoelectronic applications for the near-infrared spectral region.

Temperature distribution inside a porous silicon photonic mirror

Photonic devices require precise and high-cost procedures to evaluate their performance which is related to their temperature increase. The fundamental understanding of thermal phenomena, ergo measurement of temperature, inside radiation controlling devices is of great relevance to study their performance. In this paper, we carry out a comprehensive campaign of experiments to study the temperature profile inside a porous silicon multilayer 1D photonic structure by using a thermographic camera. In particular, we have analyzed broad-range reflective devices and found that the electromagnetic radiation does not travel beyond the photonic structure showing a clear maximum inside of it. We have compared this result with a pure silicon wafer under the interaction with the same radiation. To compare these samples, we used a normalization procedure that can be implemented on many microstructured devices to simplify their performance evaluation.

Theoretical and experimental study of Fermi Bragg reflector as application for thin silicon solar cells

The conception of efficient Bragg reflectors (BRs) based on dielectric layers is needed to improve the light trapping in thin film epitaxial silicon solar cells. Porous silicon multilayer structures (PSMS) used as BR show its potential to enhance optical confinement of low energy photons. In the present work, a novel design of PSMS will be studied in order to extend the reflectivity to long wavelength by adjusting BR thicknesses according to the Fermi profile law. Such new design named Fermi Bragg reflector (FBR) includes different configurations. Several FBR parameters are used in the simulation study, which is carried out by transfer matrix method. The optimal structures were realized using electrochemical etching dissolution of P+ type silicon substrate in hydrofluoric acid solution (HF) at two temperature conditions: room temperature (20 °C) and −20 °C. The measured optical response of FBR shows an enlargement in the width of high reflection range in comparing to the conventional BR. More than 80% of the photons were reflected in the spectral range 636–1155 nm for the FBR performed at ambient temperature and in the range 750–1353 nm to that carried out at −20 °C. This result proves that the FBR formed on crystalline silicon could enhance the light trapping in thin film silicon solar cells.

Reflectivity of 1D photonic crystals: A comparison of computational schemes with experimental results

We report the reflectivity of one-dimensional finite and semi-infinite photonic crystals, computed through the coupling to Bloch modes (BM) and through a transfer matrix method (TMM), and their comparison to the experimental spectral line shapes of porous silicon (PS) multilayer structures. Both methods reproduce a forbidden photonic bandgap (PBG), but slowly-converging oscillations are observed in the TMM as the number of layers increases to infinity, while a smooth converged behavior is presented with BM. The experimental reflectivity spectra is in good agreement with the TMM results for multilayer structures with a small number of periods. However, for structures with large amount of periods, the measured spectral line shapes exhibit better agreement with the smooth behavior predicted by BM.

Vapor sensing using a bio-inspired porous silicon photonic crystal

Naturally occurring photonic crystals present on the wing scales of Papilio butterflies are known to exhibit vivid iridescent coloration as well as various optical properties, including vapor sensing properties. This work reports the fabrication of a periodic array of concave multilayers in porous silicon mimicking the Papilio blumei nanostructure and its application as an optical vapor sensor. We compared the variations in the reflectance spectrum due to ethanol vapor adsorption and condensation inside the bio-inspired concave porous silicon multilayer structure to the variations of a standard flat porous silicon Bragg mirror. This results in an enhanced response time in the case of the bio-inspired concave structure.

Light Absorption Enhancement of Silicon-Based Photovoltaic Devices with Multiple Bandgap Structures of Porous Silicon.

Porous-silicon (PS) multi-layered structures with three stacked PS layers of different porosity were prepared on silicon (Si) substrates by successively tuning the electrochemical-etching parameters in an anodization process. The three PS layers have different optical bandgap energy and construct a triple-layered PS (TLPS) structure with multiple bandgap energy. Photovoltaic devices were fabricated by depositing aluminum electrodes of Schottky contacts on the surfaces of the developed TLPS structures. The TLPS-based devices exhibit broadband photoresponses within the spectrum of the solar irradiation and get high photocurrent for the incident light of a tungsten lamp. The improved spectral responses of devices are owing to the multi-bandgap structures of TLPS, which are designed with a layered configuration analog to a tandem cell for absorbing a wider energy range of the incidental sun light. The large photocurrent is mainly ascribed to an enhanced light-absorption ability as a result of applying nanoporous-Si thin films as the surface layers to absorb the short-wavelength light and to improve the Schottky contacts of devices. Experimental results reveal that the multi-bandgap PS structures produced from electrochemical-etching of Si wafers are potentially promising for development of highly efficient Si-based solar cells.

Optical characterization of one-dimensional porous silicon photonic crystals with effective refractive index gradient in depth

In this paper, we report the results of the investigation about the optical properties of one-dimensional photonic crystals based on porous silicon multilayer structures having unit cells with optical properties that vary as a function of the device thickness. These devices were obtained by anodizing crystalline silicon in electrolyte aqueous solution of HF at different concentrations. The effective refractive index and extinction coefficients, of porous silicon layers of unit cells had been obtained by fitting the experimental reflectance spectra using the transfer matrix method. The effective–refractive indexes, for devices obtained in electrolyte solution of high HF concentration, were shown to have a significant gradient in the depth of the layers, whereas it was low (negligible) for devices obtained in electrolyte solution with low HF concentration, but the mechanical stability of these latter devices is poor. It was found that this fragility depends on the layer number and upon their physical thicknesses ratio.

Tunneling times of acoustic phonon packets through a distributed Bragg reflector.

The longwave phenomenological model is used to make simple and precise calculations of various physical quantities such as the vibrational energy density, the vibrational energy, the relative mechanical displacement, and the one-dimensional stress tensor of a porous silicon distributed Bragg reflector. From general principles such as invariance under time reversal, invariance under space reflection, and conservation of energy density flux, the equivalence of the tunneling times for both transmission and reflection is demonstrated. Here, we study the tunneling times of acoustic phonon packets through a distributed Bragg reflector in porous silicon multilayer structures, and we report the possibility that a phenomenon called Hartman effect appears in these structures.

Tunable optical filters with wide wavelength range based on porous multilayers.

A novel concept for micromechanical tunable optical filter (TOF) with porous-silicon-based photonic crystals which provide wavelength tuning of ca. ±20% around a working wavelength at frequencies up to kilohertz is presented. The combination of fast mechanical tilting and pore-filling of the porous silicon multilayer structure increases the tunable range to more than 200 nm or provides fine adjustment of working wavelength of the TOF. Experimental and optical simulation data for the visible and near-infrared wavelength range supporting the approach are shown. TOF are used in spectroscopic applications, e.g., for process analysis.

Rationally designed porous silicon as platform for optical biosensors

Optical porous silicon multilayer structures are able to work as sensitive chemical sensors or biosensors based in their optical response. An algorithm to simulate the optical response of these multilayers was developed, considering the optical properties of the individual layers. The algorithm allows designing and customizing the porous silicon structures according to a given application. The results obtained by the simulation were experimentally verified; for this purpose different photonic structures were prepared, such as Bragg reflectors and microcavities. Some of these structures have been derivatized by the introduction of aminosilane groups on the porous silicon surface. The algorithm also permits to simulate the effects produced by a non uniform derivatization of the multilayer.

Sensing characteristics of the organic vapors according to the reflectance spectrum in the porous silicon multilayer structure

We fabricated a porous silicon multilayer and investigated its reflectance spectra before, during, and after exposure to various organic vapors. During exposure of the porous silicon multilayer to isopropanol, ethanol, methanol, and acetone vapors, the reflectance peak shifted toward longer wavelengths by about 5, 12, 26, and 39 nm, respectively. The shift of the reflectance peak arises from refractive index changes induced by capillary condensation of the organic vapor in the pores of the porous silicon multilayer. In addition, we observed that the shift value of the reflectance peak increased with increasing organic solvent concentration in the organic solvent-water mixture. After removing the organic vapor, the reflectance spectrum returned completely to its original state.

Hypersonic acoustic mirrors and microcavities in porous silicon

Periodic solid state structures exhibit transmission stop bands for waves of certain frequencies. We report the realization and direct measurement of acoustic band gaps in porous silicon multilayer structures which exhibit ∼50 dB stop bands for longitudinal acoustic waves in the gigahertz range. Furthermore, realization of an acoustic microcavity structure in porous silicon is demonstrated.

Elaboration of one-dimensional photonic structure on silicon by electrochemical etching

We present a study on the formation of One-Dimensional Photonic Band Gap structure with electrochemical etching of p+-type Si (100) with resistivity 0.01 Ω.cm in HF/ CH3COOH/H2O solution. The process can be precisely controlled by varying the experimental parameter (current density, etching time, number of porous layer). The elaborated structures consisting of porous layers with periodically modulated current densities provide an opportunity to create multilayer structures: Distribution Bragg Reflector’s (DBR), Optical Micro-Cavities (OMC).To obtain a periodic porous silicon multilayer structure we switched the current density between low (7–14 mA/cm2) and high (50–100 mA/cm2) values. Reflection spectra measured from DBR and OMC are acquired by spectroscopy Cary 500. Morphological analysis of porous silicon surface was carried out by scanning electron microscope. The reflectivity for DBR shows an increase reflectivity from 55% to 95%, when the periodic layers number increases from 20 to 40. It is clearly shown that the changing of the experimental parameter induces a shift of the reflectivity of OMC from 2030 nm to 1265 nm. Moreover, it is noted that the reflectivity increases from 65% for the 1st OMC to 70% for the second one.

Preparation and analysis of porous silicon multilayers for spectral encoding applications

AbstractThis paper compares two methods used to encode information into the optical spectrum of a porous Si film using a modulated current‐time waveform. In the first approach, a waveform consisting of a sequence of current steps is used to produce a porous silicon multilayer structure. Each layer in the multilayer acts as a Fabry‐Perot interference film, and the superposition of their contributions results in a complicated reflectivity spectrum. The spectrum can be deconvoluted by means of a Fast Fourier Transformation (FFT), which yields the optical thickness of each of the individual layers and their combinations.The second type of waveform consists of several superimposed cosine waves of differing frequencies. The resulting reflectivity spectrum consists of a series of peaks, each of whose position in the spectrum (frequency at which a peak is at maximum amplitude) corresponds to the frequency of a cosine wave component in the current‐time waveform. The two methods of encoding information are compared in terms of repeatability, tunability, and information capacity. The superimposed cosinusoidal waves are found to provide greater information density and a simpler means of decoding. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical reorientation of liquid crystal molecules inside cylindrical pores for photonic device applications

We present the simulated distribution of the local director of a nematic liquid crystal inside cylindrical macropores under the influence of an electric field. The Frank free energy approach is used to describe the nematic behavior. The influence of both molecular anchoring strength and pore radius is investigated. The results of this analysis are applied for simulation of an electrically tunable microcavity based on porous silicon infiltrated with a liquid crystal. The Bruggeman approximation is used while calculating the effective refractive index of each layer in the porous silicon multilayer structure. The reflectivity spectrum of the latter is simulated using the transfer matrix approach. The electrical tuning range of a microcavity designed for near-infrared waves is found to vary from 10.5 up to 23 nm for weak and strong surface anchoring conditions, respectively.

Fluorescence tuning of confined molecules in porous silicon mirrors

Emission signal from fluorescent molecules (fluorescein-5-maleimide) in a porous silicon mirror is enhanced by tuning the pore size and reflectance spectrum of the porous silicon multilayer structure. This is achieved when the reflectance spectrum of the silicon mirror overlaps the fluorescent excitation and emission wavelengths of the fluorescent molecule, and chemical linkers assure the molecular confinement.

Porous silicon microcavities fabricated using ion irradiation

By employing microcavity (MC) techniques, the broad spectral band and wide emission angle of photoluminescence from porous silicon (PSi) can be narrowed, directed and tuned in PSi microcavities. The microcavity structure is formed with a porous silicon active layer sandwiched between two Bragg reflectors. We have characterised the optical properties of the porous silicon multilayer structures. A number of Bragg reflectors and Fabry–Perot optical microcavities are fabricated with tuning of emission wavelengths over a wide range in heavily doped p-type silicon. The optical properties of these microstructures were investigated using reflectivity and photoluminescence measurements at different temperatures. This paper reports the ability of focussed 2 MeV proton beam irradiation to controllably blue shift the resonant wavelength of porous silicon microcavities in heavily doped p-type wafers. Using this process wafers are patterned on a micrometer lateral scale with microcavities tuned to different resonant wavelengths, giving rise to high-resolution full-colour reflection images over the full visible spectrum.

Surface wave photonic device based on porous silicon multilayers

Porous silicon is widely studied in the field of photonics due to its interesting optical properties. In this work, we present theoretical and first experimental studies of a new kind of porous silicon photonic device based on optical surface wave. A theoretical analysis of the device is presented using plane-wave approximation. The porous silicon multilayered structures are realized using electrochemical etching of p +-type silicon. Morphological and optical characterizations of the realized structures are reported.

Coupling of one-dimensionally confined optical modes in porous silicon: Spectral observation and photonic-quantum-well description

Single and double photonic-quantum-wells (PQWs) have been fabricated, using refractive-index-modulated porous silicon multilayer structures. The one-dimensionally confined optical modes observed are consistent with those calculated using the transfer matrix method. While single PQWs exhibit atom-like discrete spectral features, molecule-like energy level splitting is accessible by combining two single PQWs into a double PQW structure. Similar to an electron in a double-quantum-well structure, the energy separation between the split ‘anti-bonding’ and ‘bonding’ photonic states can be ‘tuned’ by changing the strength of inter-well coupling.

Rayleigh scattering in multilayered structures of porous silicon

The possibility of growing high-quality porous silicon (por-Si) multilayered structures[1,2], which can consist of hundreds of layers with different, controllable porosity [3], revealed variousnew ways of using this material in optoelectronics and photonics. Apart from obvious applications of por-Si multilayered structures as mirrors, optical filters, etc., the most attractive goal is to combine the photonicmanipulation with the por-Si luminescence, which, in principle, would allow the fabrication of Si-basedlasers with adjustable properties [4].The photon transport through the por-Si multilayered structure is usually described using the transfermatrix method [5]. In this approximation it is assumed that the individual por-Si layers can be characterizedby a well defined refractive index, which depends on porosity. The general features of the reflection andtransmission coefficients are reproduced by this method (see, e.g., the recent observation of the photonicBloch oscillations [6]). However, even in the case when the absorption is negligible, not all interferencepeaks predicted by the transfer matrix calculations are seen experimentally, and the observed peaks arenoticeable suppressed [6,7]. To get more insight in this phenomenon it is convenient to study some simplemultilayered structures, in particular those containing a single microcavity, where the resonant transmit-tance peak is well separated and could be analyzed in details. Below we present the experimental data onthe transmittance spectra of several structures of this sort. The observed suppression of the resonant peaksis then interpreted by taking into account the Rayleigh scattering of light on the microscopic structuredisorder of por-Si—the effect, which is not allowed for in the transfer matrix calculations.