Free-standing Porous Silicon Films Research Articles

Free-Standing Porous Silicon Film Produced by a Pulsed Anodic Etching of n+-Silicon Substrate in an HF: HCl: C2H5 OH: H2O2:H2O Electrolyte: Characterization and Adsorption of Colchicine

Porous silicon (PSi) has been established as a promising candidate for a variety of applications in many fields of science and technology. Currently, free-standing PSi is receiving an increasing attention for biomedical applications, such as drug delivery carriers and bio-sensing. In this study, fabrication of free-standing PSi film with attractive structural and optical properties for a prospective biomedical uses, is reported. The film is produced in the dark by a pulsed anodic etching of n+- type silicon substrate in an unconventional (HF: HCl: C2H5 OH: H2O2:H2O) electrolyte. The macro-porous nature of the produced film is revealed by scanning electron microscopy (SEM) imaging. X-ray diffraction (XRD) and Raman scattering investigations show both amorphous and nano-crystalline phases coexist in the produced film. On ageing it for two months, Fourier transformation infrared (FTIR) spectroscopy results reveal that the produced film exhibits the characteristic of efficiently oxidized PSi with a good stability. In addition to the strong S-band, the room temperature photoluminescence (PL) spectrum of the film exhibits a weaker F-band, which is attributed to the recombination in SiO2 allocated at surface defects. Moreover, the film adsorption of colchicine drug via immersion process has been investigated by FTIR and UV-spectrophotometric measurements. The results demonstrate that the colchicine is specifically and efficiently adsorbed, with adsorption capacity of around 33%. The results in this study suggest that our fabricated free-standing PSi film can be effectively utilized to prepare PSi nanoparticles for forthcoming drug delivery applications.

Silicon quantum dots for energetic material applications

In its history as an energetic material, porous silicon has demonstrated flame speeds in excess of 3 km s−1, tunable combustion behavior, and high energy output, which in theory makes it a very attractive energetic system. In practice, its application within the field is limited by porous silicon's typical substrate-adhered form and caustic chemical processing requirements that limit how and when porous silicon is made. In this work, we have relieved porous silicon of these constraints by creating reactive silicon quantum dots from free-standing porous silicon films. The resulting material is composed of crystalline silicon nanoparticles with diameters as small as 2 nm that retain the chemical properties of the original films including the SiH2 termination layer. The fabricated silicon particles were characterized using FTIR Spectroscopy, TEM, and EDS for determining the size and the chemical composition. For testing as an energetic material fuel, porous silicon was mixed with an oft used oxidizer, sodium perchlorate. During open-channel combustion tests, silicon quantum dots mixed with sodium perchlorate demonstrated flame speeds over 2.5 km s−1, while bomb calorimetry tests showed an average heat of combustion of 7.4 kJ g−1. These results demonstrate the ability to retain the porous silicon material properties that allow for highly energetic material reactions to occur, despite the additional processing steps to create silicon quantum dots. This opens the door for the use of porous silicon in the bulk of the energetic material application space, much of which was previously limited due to the substrate-attached nature of typical porous silicon.

다공성실리콘내 Fe3O4나노입자의 압력침착과 채움밀도 모니터링 방법

In this paper, we propose a new method to infiltrate Fe3O4-nanoparticles into a porous silicon film and a monitoring tech- nique to detect packing density of nanoparticles within the film. Recently, research to use porous silicon as a drug carrier or a new functional sensor material by infiltrating Fe3O4-nanoparticles has been extensively performed. However, it is still necessary to enhance the packing density and to develop a monitoring technique to detect the packing density in real time. In this light, we forcibly injected a nanoparticle solution into a rugate-structured free-standing porous silicon (FPS) film by applying a pressure difference between the two sides of the film. We found that the packing density by the pressure-infil- tration method proposed in this paper is enhanced, relative to that by the previous diffusion method. Moreover, a continuous shift in wavelength of the rugate reflectance peak measured from the film surface was observed while the nanoparticle solu- tion was being injected. By exploiting this phenomenon, we could qualitatively monitor the packing density of Fe3O4-nano- particles within the FPS film with the injection volume of the nanoparticle solution.

Study on surface chemistry and particle size of porous silicon prepared by electrochemical etching

In the present study, surface chemistry of porous silicon was tailored by thermal oxidation and thermal hydrosilylation method. Porous silicon nanoparticles were prepared by applying ultrasound to freestanding porous silicon films. The results showed a mesoporous silicon layer with an average pore diameter of 28.26nm, porosity of 70.7% and thickness of 4660nm. Porous silicon surface was modified and compared with native surface by Fourier transform infrared spectroscopy. Ultrasonic fracture of porous silicon films for 8h resulted in nanoparticles with average particle size of 149nm. Zeta potential measurements of nanoparticles at varying pH reveal presence of different surface chemistry of particles.

Fabrication and Characterization of Free-Standing DBR Porous Silicon Film

Distributed Bragg reflector porous silicon of different characteristics were formed to determine their optical constants in the visible wavelength range using a periodic square wave current between low and high current densities. The surface and cross-sectional SEM images of distributed Bragg reflector porous silicon were obtained using a cold field emission scanning electron microscope. The surface image of distributed Bragg reflector porous silicon indicates that the distributions of pores are even. The cross-sectional image illustrates that the multilayer of distributed Bragg reflector porous silicon exhibits a depth of few microns and applying of square current density during the etching process results two distinct refractive indices in the contrast. Distributed Bragg reflector porous silicon exhibited a porosity depth profile that related directly to the current-time profile used in etch. Its free-standing film was obtained by applying an electro-polishing current.

Biofunctionalization of free-standing porous silicon films for self-assembly of photonic devices

Here we test chemical strategies for the fabrication of free-standing porous silicon thin films that are covalently modified on one of their faces with biorecognition elements to guide the vertical self-assembly of optical structures. Chemical modification viahydrosilylation of alkenes followed by coupling of biomolecules is carried out after the porous silicon film is lifted off from the silicon wafer to avoid damage to the organic monolayer during the lift-off step. Micron-sized biotinylated particles produced from asymmetrically modified porous silicon films are deposited via biorecognition in the correct orientation onto a substrate modified with avidin to form optical resonant microcavities.

Investigation of hydrogen storage behavior of silicon nanoparticles

Porous Silicon (PS) freestanding film is a derivative of single crystal Si wafer. PS films obtained on electrochemical etching of p-type silicon (Si) wafer were used to synthesize Si nanoparticles by ultrasonication. 12 μm thick and 29% porous freestanding PS films were sonicated for 4 h in 120 W ultrasonication bath at 42 kHz. HRTEM image shows Si nanoparticles in the range of 8–20 nm in size. In this paper we present results of hydrogen absorption experiments conducted on Si nanoparticles. Standard Seivert’s type apparatus was used to carry out hydrogen absorption pressure composition isotherm measurements in the pressure range of 1–10 bar and in the temperature range of 29 °C–150 °C. Theoretically SiHx system has 3.44, 6.66 and 9.67 wt% of hydrogen for x = 1, 2, 3 respectively. Experimental results show maximum hydrogen uptake of 2.25 wt% at the temperature of 120 °C and at 9.76 bar pressure. Hydrogenation of Si nanoparticles exhibits frequency downshifts from 510.7 to 507.3 cm−1 in Raman spectra. Raman peaks were de-convoluted in two bands to study effect of hydrogenation on FWHM, crystallanity and elastic strain of the nanoparticles. Bonding between Si, O and H atoms were investigated using Fourier transform infrared spectroscopy(FTIR) spectroscopy. UV–Vis spectra and Tauc plots were used to discuss the relation between hydrogenation and optical band gap of the Si nanoparticles. Optical band gap was found to increase from 1.6 to 2.25 eV on subjecting Si nanoparticles to hydrogenation.

DETERMINATION OF THE COMPLEX REFRACTIVE INDEX OF POROUS SILICON LAYERS ON CRYSTALLINE SILICON SUBSTRATES

In this work, we show an algorithm to calculate the complex refractive index of porous silicon (PS) on its crystalline silicon (c-Si) substrate in UV-NIR range by means of the reflectance spectra only. The algorithm is based on the analytical relations established by Heavens to obtain both complex refractive index and thickness of an absorbing thin film on an absorbing substrate. Based on this model, some simplification is introduced at different wavelengths. We start with the NIR range (1000–2500 nm), where the c-Si substrate has a low extinction coefficient. Then, we continue with the near infrared to the optical range (300–1000 nm), where PS has a strong extinction coefficient and dispersion. The calculated n and k values are in agreement with those reported in the literature obtained from separated measurements of a free standing PS film. We consider that the algorithm can be applied to any thin film on a substrate with similar optical properties.

Tunable NIR filter based on a free-standing porous silicon film containing nematic liquid crystal

We present the design of a thermally tunable interference filter for the NIR range. The filter is based on a free-standing porous silicon multilayer filled with the nematic liquid crystal mixture E7. The porous silicon stack is constituted by a quarter-wave microcavity sandwiched between two distributed Bragg reflectors. Reversible thermal tuning of the microcavity resonance is obtained in the range of 13 nm. Thermal dependence of the effective refractive index of the liquid crystal in the sample is characterized using variable angle spectroscopic ellipsometry. The non-axial liquid crystal director configuration is found inside silicon mesopores. The distribution of the liquid crystal director within pores is simulated using the Frank's free energy approach. Assuming the molecular configuration of the liquid crystal to be escaped radial, we have determined the value of the surface anchoring strength in the mesoporous silicon to be less than 10−6 J m−2. By fitting the transmittance spectra, using an effective medium approximation and a transfer matrix method, the liquid crystal is found to fill 84% of the void inside the porous sample. The good agreement between the experimental data and the theoretical curves justifies the Frank's approach and the hypothesis of escape radial configuration of nematics confined in silicon mesopores.

Label-free porous silicon membrane waveguide for DNA sensing

We report a label-free porous silicon membrane waveguide biosensor based on a 1μm thick freestanding porous silicon film with 100nm diameter pores. The sensor operates in the Kretschmann configuration. A formvar polymer film provides robust adhesion of the porous silicon membrane to a rutile prism and enables confinement of guided modes in the porous silicon membrane. Attenuated total reflectance measurements are performed, along with theoretical calculations, to fully characterize the waveguide. The sensitivity of the sensor is investigated through DNA hybridization in the porous silicon membrane. A detection limit of 42nM was demonstrated for 24-base pair DNA oligonucleotides.

A low‐power sensor for volatile organic compounds based on porous silicon photonic crystals

AbstractA miniaturized, low‐power chemical sensor for volatile organic compounds using a porous Si photonic crystal is demonstrated. The device uses simple electronic circuitry a light‐emitting diode, a phototransistor, and thin film of a 1‐dimensional photonic crystal of porous Si, drawing 0.5 W of power. Integration of the freestanding porous silicon film with the conventional circuitry is discussed as well as the current detection limits for ethanol vapor in air. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strong anisotropy of lateral electrical transport in (110) porous silicon films

We study the lateral electrical transport in free-standing porous silicon films prepared by anodic etching of heavily boron-doped (110) Si wafers. It is shown that the lateral dark conductivity and photoconductivity are significantly higher along the [10] in-plane crystallographic direction than that along the [001] one. The electrical transport anisotropy decreases under light illumination and with increasing temperature. The experimental results are explained by using an effective-medium approximation and taking into account potential barriers between anisotropic Si nanocrystals assembling the films. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

SHORT COMMUNICATION: Thin-film free-standing monocrystalline si solar cells with heterojunction emitter

We propose a novel approach to thin-film silicon solar cells, namely the freestanding monocrystalline silicon layer transfer process with heterojunction emitter (FMS-HJ). High crystallographic quality mono-Si films were deposited on freestanding porous silicon (PS) films by chemical vapor deposition (CVD). These free-standing mono-Si (FMS) films were processed into solar cells by creating a-a-Si/c-Si heterojunction. In our preliminary experiments a thin-film FMS-HJ solar cell with 9.6% efficiency was realized in a 20-μμm-thin active layer. Copyright © 2005 John Wiley & Sons, Ltd.

Identification of luminous region in porous silicon layers

Photoluminescence spectra were measured experimentally and compared with the results of the optical model calculations in oder to identify the region of photoluminescence in a porous silicon layer. It is found that the upper surface region in the porous silicon layer contributes most of the luminescence, by analyzing the interference patterns of the photoluminescence spectra and those of reflectances. It is confirmed by comparing the results of a detailed computer simulation with the experiments, and by analyzing the photoluminescence characteristics of a free-standing porous silicon film. Analyses of the photoluminescence spectra measured using several different excitation wavelengths also support our conclusion.

Tunable Porous Silicon Mirrors for Optoelectronic Applications

ABSTRACTTunable porous silicon mirrors were fabricated as building blocks for optical interconnects and as a first step towards efficient routing of information on a small size scale. The basic structures for the tunable mirrors are porous silicon microcavities infiltrated with liquid crystals. The optical properties of the mirrors are influenced by thermal or electric field modulation. When the device is heated or a voltage is applied, the orientation of the liquid crystals changes, causing a change in the effective refractive index of the liquid crystals within the mirror. The position of the reflectance resonance of the porous silicon microcavity is particularly sensitive to such changes in the refractive index. A reversible 10nm shift of the reflectance resonance of the mirror, leading to a 30% change in the amplitude of reflectance, has been observed due to thermal effects. Not only do these results show potential for future devices, but they also confirm that the liquid crystals are able to rotate in the constricted geometry of the porous silicon microcavities. For voltage driven devices, careful attention needs to be given to the configuration of the electrical contacts. The effectiveness of various device geometries has been investigated. Using standard lithographic techniques, aluminum contacts with minimum feature sizes of 10 microns were patterned directly on the porous silicon surface. A study on the use of free-standing porous silicon films was also performed.

Phase matching of second-harmonic generation in birefringent porous silicon

Optical second-harmonic generation is investigated in free-standing (110) porous silicon films having strong inplane birefringence. Based on the measured values of the refractive indices, the conditions of phase matching are calculated and verified in the experiments on angular and polarization dependences of second-harmonic generation. Filling pores with a transparent dielectric liquid allows phase matching to be achieved and the second-harmonic signal to be increased by two orders of magnitude. The results of these experiments suggest an opportunity to form new-type nonlinear-optical media consisting of birefringent porous silicon as a phase-matching matrix for the materials embedded in the pores.

On the relationship between the optical transmission and photoluminescence characteristics of porous silicon

The characteristics of free-standing porous silicon (por-Si) films were studied using their optical transmission, photoluminescence (PL), and photoluminescence excitation spectra. The transmission spectra exhibit no features within the emission bands or near PL excitation thresholds, which is evidence that nonluminescent por-Si fragments play a dominating role in the process of light absorption. This fact indicates that the optical transmission spectra cannot be used as a source of information on the bandgap energy of charge carriers in luminescent silicon nanocrystallites. The required energy spectrum parameters can be roughly evaluated using the PL excitation spectrum.

Thermally based light modulation through free-standing porous silicon film

A large laser induced increase of light absorption has been observed in free-standing porous silicon films, in the wavelength range 500–700 nm. Thermal origin of the observed modulation is suggested by two hitherto unreported observations: the spectral dependence and the comparison between the time decay of modulation with photoluminescence. A simple thermal model, taking into account the porosity of the porous silicon film, provides a good fit over the probed spectrum.

Laser-induced thermal effects on the optical properties of free-standing porous silicon films

A detailed study of the unique optical properties of free-standing oxidized porous silicon films has been performed. Under continuous-wave laser irradiation, a strongly superlinear light emission and a very large laser-induced absorption are observed. The nonlinear emission is characterized by a sharp intensity increase that is in proportion to at least the eighth power of the excitation intensity. This emission has a broad peak (600–1300 nm), slow time constant (⩾10 ms), and very low polarization memory (⩽0.01 near the emission peak). The induced absorption increases linearly with the pump laser intensity and can be as large as several times the linear absorption. The increase in the normalized absorption coefficient is almost independent of the oxidation temperature and emission wavelength, with essentially no polarization dependence. These experimental results are discussed in terms of laser-induced thermal effects. An evaluation of the temperature rise under the laser irradiation is performed both experimentally and theoretically. A remarkably high-temperature rise of ⩾700 °C is estimated for a moderate excitation intensity of 20 W/cm2.

The diffusion redistribution of hydrogen and oxygen in porous silicon films

A porous silicon (PS) layer prepared by electrochemical anodization of a single crystal silicon surface has been known for many years. Crystalline structure of the PS is network of silicon is nanometer-sized regions, surrounded by void space, with very large surface to volume ratios [1]. The PS surfaces are covered by silicon hydrides and silicon oxides. Observation of visible photoluminescence from PS has generated widespread interest because of the potential application of PS in Si-based optoelectronic technology. The luminescence, optical and other properties of PS are determined by incorporation of hydrogen and oxygen on surfaces and the formation of Si-H, Si-H2 and Si-O bonds. Therefore stability of luminescence and optical characteristics of PS depend on processes of concentration redistribution of hydrogen and oxygen atoms under treatments. This, in turn, is limited by oxygen and hydrogen diffusion along inner PS surfaces with simultaneous penetration into silicon. The formation mechanism of PS and the luminescence mechanism have been actively discussed, whereas the composition changes of PS caused by hydrogen and oxygen diffusion, and by appropriate changes in the number of Si-H, Si-H2 and Si-O bonds due to thermal treatments are rarely considered. From FTIR measurements, a reduction of photoluminescence intensity of PS with simultaneous decrease of intensity of Si-H2 peaks as results of annealing at 25–400 ◦C was observed by Tsai et al. [2]. Hydrogenand oxygenrelated bonds changes during PS oxidation for the range 20–500 ◦C at atmospheric pressure has also been reported [3, 4]. The aim of the present work is to determine the diffusion parameters of hydrogen and oxygen atoms in free-standing PS films from analysis of changes intensity of Si-H and Si-O absorption peaks as a function of temperature and duration of annealing in air. The PS films were prepared on n-type Si (100) substrates of resistivity ρ= 0,09A cm by anodic etching in a solution HF : H2O= 1 : 3 at a current density of 15 mA/cm2 under the white light illumination. The PS films were then detached from silicon substrate by electropolishing under former solution with a current density of 0.8–1.0 A/cm2. The free-standing PS films and PS film on Si substrates were characterized by porosity, photoluminescence and FTIR measurements at room temperature. Porosity of films was determined by gravimetry. Freestanding PS films of thickness 10–20μm and porosities 70–80% were obtained. FTIR spectra of free-standing PS films were measured at room temperature by using a Mattson 1000 spectrometer. The photoluminescence spectra were excited by the 325 nm line of He-Cd laser under a power output of 20 mW. This investigation focuses on the analysis of changes of absorption coefficients of Si-H, Si-H2 (doublet at 2100 cm−1) and Si-O (1100 cm−1) peaks as a result of isothermal annealing at temperatures in the range 65–185 ◦C at atmospheric pressure. Isothermal annealings of PS films were carried out at 65, 80, 110, 150 and 185 ◦C. Time-dependence FTIR and photoluminescence spectra were measured after 5-min cycles of isothermal annealings. The distribution of photoluminescence intensity along thickness of PS films (on silicon substrates) has been analyzed by sequential removal of thin layers (about 1μm) from PS film and the photoluminescence intensity measurements. The absorption coefficient (α) is deduced from transmission spectra by solving α in the equation