Light-emitting porous silicon

Although porous silicon has been known for more than 35 years, only in 1990 was it recognized that porous silicon shows an increased bandgap and efficient room-temperature photoluminescence in the visible. This paper will give an overview of porous silicon research, with special emphasis on the formation mechanism of microporous silicon in terms of a depletion of holes in the porous region due to quantum confinement and the understanding of the origin of the visible luminescence. The status of research on electroluminescent and other devices based on porous silicon will be discussed, as well as results for other luminescent forms of nanocrystalline silicon.     

The potential of porous silicon gas sensors

Recent developments in porous silicon gas sensors have been reviewed. Monitored species detection levels, and the mechanisms of sensing for different sensor designs are also discussed. Porous silicon surface modification methods have been employed for detecting different gas molecules; H₂O, ethanol, methanol, isopropanol, CO_x, NO_x, NH₃, O₂, H₂, HCl, SO₂, H₂S and PH₃.     

Blue luminescence from porous layers produced by metal-assisted chemical etching on low-doped silicon

Photoluminescent porous layers were formed on highly resistive p-type silicon by a metal-assisted chemical etching method using K₂Cr₂O₇ as an oxidizing agent. A thin layer of Ag is deposited on the (1 0 0) Si surface prior to immersion in a solution of HF and K₂Cr₂O₇. The morphology of the porous silicon (PS) layer formed by this method as a function of etching time was investigated by scanning electron microscopy (SEM). It shows that the surface is formed by macropores filled with microporous silicon. The porous layers were characterized by backscattering spectrometry (BS) as a function of etching time in random and channelling mode. Channelling spectra show that the porous layer remains crystalline after etching. On the other hand, random and channelling spectra show that the deposited silver diffuses into the pore. Luminescence from metal-assisted chemically etched layers was measured. It was found that the PL intensity increases with increasing etching time. This behaviour is attributed to increase of the density of the silicon nanostructure. Finally, the PL spectra show two peaks of emission at 450 and 600 nm.     

The modification of the properties of n-type conductivity porous silicon by argon ion irradiation

The results of the research into the influence of argon ion irradiation at 3 keV on the composition and structure of porous silicon are presented. At a certain angle of incidence of the particles relative to the surface of the monocrystallites, an undulating λ∼60 nm nanorelief is formed, while the crystallite sizes and structure remain unchanged. The IR-spectroscopy data show that SiH groups are mainly localized in a thin 120 nm near-surface layer. During exposure of the samples to the air in the dark, monohydride groups are removed from the surface within a month. Dihydride groups, located in deeper layers, are oxidized considerably more slowly than the monohydride ones. The experimental data show that the 0.1 μm-thick surface layer serves as a diffusion barrier preventing oxygen from penetrating deep into the porous silicon.     

The dependence of hydrofluoric acid concentration during anodization on photoluminescence of porous silicon

The mixture of hydrofluoric (HF) acid and ethanol is used as an electrolyte during anodization of silicon. We investigated the effect of the ratio of HF acid to ethanol on photoluminescence. It is concluded that porous silicon anodized with the electrolyte containing 35 or 40% HF acid provides strong photoluminescence. The fact implies the existence of a chemical reaction including ethanol during anodization other than electrochemical reaction.     

Visible luminescence in photo-electrochemically etched p-type porous silicon: Effect of illumination wavelength

The effect of low power density of ~ 5 μW/cm² monochromatic light of different wavelengths on the visible photoluminescence (PL) properties of photo-electrochemically formed p-type porous silicon (PS) has been investigated. Two-peak PL “red” and “green” is resolved in PS samples etched under blue-green wavelength illumination; 480, 533 and 580 nm. It is found that the weight of “green” PL has maxima for the sample illuminated with 533 nm wavelength. Whereas, PL spectra of PS prepared under the influence of red illumination or in dark does not exhibit “green” PL band, but shows considerable enhancement in the “red” PL peak intensity. Fourier transform infrared (FTIR) spectroscopic analysis reveals the relationship between the structures of chemical bonding in PS and the observed PL behavior. In particular, the PL efficiency is highly affected by the alteration of the relative content of hydride, oxide and hydroxyl species. Moreover, relative content of hydroxyl group with respect to oxide bonding is seen to have strong relationship to the blue PL. Although, the estimated energy gap value of PS samples shows a considerable enlargement with respect to that of bulk c-Si, the FTIR, low temperature PL and Raman measurements and analysis have inconsistency with quantum confinement of PS.     

Estimation of oxide related electron trap energy of porous silicon nanostructures

Estimation of electron trap energy (E_t), with respect to bulk Si valence band, of oxidized porous silicon (PS) nanostructures is reported. Photoluminescence (PL) spectra of oxidized PS prepared with different formation parameters have been investigated and the room temperature PL characteristics have been successfully explained on the basis of oxide related trap assisted transitions. PL peak energy for the oxidized samples with low porosity exhibited a blue shift with increasing formation current density (J). For the high porosity samples double peaks appeared in the PL spectra. One of these peaks remained constant at ∼730 nm while the other was blue shifted with increase in J. Evolution of PS nanostructure was correlated to the formation parameters using a simple growth mechanism. PS nanostructure was modelled as an array of regular hexagonal pores and the average value of E_t was estimated to be 1.67 eV.     

Hydrogen in porous silicon — A review

This review is devoted to summarising the hydrogen-assisted properties and applications of porous silicon (PS). The role of hydrogen as an intermediate product in silicon porosification technology is accentuated. The regularities of hydrogen bonding in PS and its applications for hydrogen storage are listed. The models of hydrogen influence on luminescence and electrical properties of PS are analysed. The corresponding applications of PS for H₂ gas sensors and pH metres are illustrated. Hydrogen-assisted explosion and grafting of PS are discussed. Such a review can be useful for the tailoring of PS properties.     

Crystallite-size-dependent characteristics of porous silicon

Porous silicon prepared with anodic currents of 5 to 30 mA/cm² are characterized for structural and electronic properties of surface using photoluminescence, grazing angle X-ray diffraction, photoconductivity, thermally stimulated exo electron emission and work function measurements. The observed results indicate that with increasing porosity the crystallite size decreases and the amount of silicon hydride and oxide-type species increases, exhibiting a tendency similar to that of hydrogenated amorphous silicon and hydrogenated microcrystalline silicon. Free-standing powder of porous silicon, characterized by bright photoluminescence at 730 nm, showed crystallites of nanometre dimensions under the transmission electron microscope.     

Optical and structural analysis of porous silicon coated with GZO films using rf magnetron sputtering

In the production of porous silicon (PS) to optoelectronic application one of the most significant constrains is the surface defects passivation. In the present work we investigate, gallium-doped zinc oxide (GZO) thin films deposited by rf magnetron sputtering at room temperature on PS obtained with different etching times. The X-ray diffraction (XRD), Fourier transform infrared (FTIR) and atomic force microscopy (AFM) analysis have been carried out to understand the effect of GZO films coating on PS. Further, the XRD analysis suggests the formation of a good crystalline quality of the GZO films on PS. From AFM investigation we observe that the surface roughness increases after GZO film coating. The photoluminescence (PL) measurements on PS and GZO films deposited PS shows three emission peaks at around 1.9 eV (red-band), 2.78 eV (blue-band) and 3.2 eV (UV-band). PL enhancement in the blue and ultraviolet (UV) region has been achieved after GZO films deposition, which might be originated from a contribution of the near-band-edge recombination from GZO.     

Raman scattering and photoluminescence study of porous silicon formed on n-type silicon

We report Raman scattering and photoluminescence studies on porous silicon film formed on n-type silicon. The Raman spectra over the sample surface exhibit considerable variation whereas the photoluminescence spectra are practically identical. Our results indicate that, well inside the film surface, it consists of spherical nanocrystals of typical diameter ≈ 100Å, while on the edge these nanocrystals are ? 300Å. We further observe that there is no correlation between the photoluminescence peak position and the nanocrystal diameter. This suggests that the origin of the photoluminescence is due to radiative recombination between defect states in the bulk as well as on the surface of the nanocrystal.     

Determination of Vickers microhardness on porous silicon surfaces

The Vickers microhardness values of two different sets of porous silicon layers were determined at applied load of 98 mN. The sets consisted of Boron-doped substrates anodized at diverse current densities for two different amounts of hydrofluoric acid (HF) in the etching solution. We found that the microhardness of the samples with lower content of HF at the anodization process showed higher values, whereas the Vickers parameter diminishes consistently for higher current densities. A possible explanation of this behavior is proposed.     

制备温度对多孔硅光学特性的影响(英文)

本文不同的温度下制备多孔硅.通过荧光光谱、光吸收谱、X射线光电子谱研究了多孔硅的光和结构特性.研究结果表明存在着一个制备临界温度343 K,当制备温度从临界温度之下提高到临界温度之上时,多孔硅的荧光和光吸收从红移转向蓝移,同时硅2p电子结合能也从减小转向增大.     

Improvement of the durability of porous silicon through functionalisation for biomedical applications

The durability of porous silicon (PS) in solutions was improved by grafting a molecule, 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl-1,3,5,7,2,4,6,8-tetraoxatetrasilocane (TE), with four terminal vinyl groups. With a native PS sample as control, we compared the long-term durability of three modified PS samples: TE-, undec-10-enoic acid (UA)-, and TE/UA(TE first and UA followed)-grafted PS, in a weak organic base of dimethyl sulfoxide, an aqueous mineral solution of CuBr₂, and phosphate buffered saline respectively. Results indicate that TE-grafting is a straightforward and impactful approach to protect PS from oxidation and degradation. Further we used the TE-grafted PS to fabricate a prototype protein microarray by post-grafting UA and subsequently converting UA to nitrilotriacetic acid/Ni²⁺ for binding histidine-tagged proteins.     

Effects of buffer layer thickness on properties of ZnO thin films grown on porous silicon by plasma-assisted molecular beam epitaxy

ZnO thin films with different buffer layer thicknesses were grown on Si and porous silicon (PS) by plasma-assisted molecular beam epitaxy (PA-MBE). The effects of PS and buffer layer thickness on the structural and optical properties of ZnO thin films were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL). The ZnO buffer layers, the intensity of the (002) diffraction peak for the ZnO thin films and its full width at half maximum (FWHM) decreased with an increase in the thickness of the ZnO buffer layers, indicating an improvement in the crystal quality of the films. On introducing PS as a substrate, the grain sizes of the ZnO thin films became larger and their residual stress could be relaxed compared with the ZnO thin films grown on Si. The intensity ratio of the ultraviolet (UV) to visible emission peak in the PL spectra of the ZnO thin films increased with an increase in buffer layer thickness. Stronger and narrower UV emission peaks were observed for ZnO thin films grown on PS. Their structural and optical properties were enhanced by increasing the buffer layer thickness. In addition, introduction of PS as a substrate enhanced the structural and optical properties of the ZnO thin films and also suppressed Fabry-Perot interference.     

Dual detection biosensor based on porous silicon substrate

Due to the high surface-to-volume ratio (hundreds of m²/cm³) porous silicon became during the last years a good candidate material as substrate for biosensor application. Moreover, the versatility of surface chemistry allows different functionalization approaches and large number of molecules to be captured on well-defined areas. This paper reports a dual detection method for protein recognition processes developed on different nanostructured porous silicon (PS) substrates, based on using two complementary spectroscopic techniques: fluorescence and electrochemical impedance. The structures were tested for biomolecular recognition – biotin–strepavidin couples – in order to achieve an optimum surface for protein's immobilizations. Comparative analyses of the attachment degree and preservation of the biomolecules activity on the porous silicon surfaces and silicon slides are also described.     

Horseradish peroxidase-modified porous silicon for phenol monitoring

In this study, horseradish peroxidase enzyme (HRP) was covalently immobilized on porous silicon (PSi) surface using multistep strategy. First, acid terminations were generated on hydrogenated PSi surface by thermal hydrosilylation of undecylenic acid. Then, the carboxyl-terminated monolayer was transformed to active ester (succinimidyl ester) using N-hydroxysuccinimide (NHS) in the presence of the coupling agent N-ethyl-N′-(3-dimethylaminopropyl) carbodiimide (EDC). Subsequently, the enzyme was anchored on the surface via an amidation reaction. The structure of the PSi layers was observed by scanning electron microscopy (SEM). Infrared spectroscopy (FTIR) and contact angle measurements confirmed the efficiency of the modification at each step of the functionalization. Cyclic voltammetry was recorded using the HRP-modified PSi as working electrode. The results show that the enzymatic activity of the immobilized HRP is preserved and in the presence of hydrogen peroxide, the enzyme oxidizes phenolic molecules which were subsequently reduced at the modified-PSi electrode.     

Optical sensing of flammable substances using porous silicon microcavities

A porous silicon multilayer, constituted by a Fabry–Pèrot cavity between two distributed Bragg reflectors, is exposed to vapor of several organic species. Different resonant peak shifts in the reflectivity spectra, ascribed to capillary condensation of the vapor in the silicon pores, have been observed. Starting from experimental data, the layer liquid volume fractions condensed in the sensing stack have been numerically estimated. Values ranging between 0.27 (for ethanol) and 0.33 (for iso-propanol) have been found. Time-resolved measurements show that the solvent identification occurs in less then 10 s.     

Carbon nanotubes initiate the explosion of porous silicon

Here we show that a mixture of multi-walled carbon nanotubes (MWCNTs) and ferrocene doped with sodium perchlorate, as an oxidant, can be combusted using a camera flash as an initiator. We optimize the MWCNT to oxidant ratio by monitoring the intensity and spectral characteristics of the light emission. In addition, we construct a novel nanostructured energetic material combining MWCNTs with porous silicon (pSi) impregnated with sodium perchlorate and show that pSi can be exploded using carbon nanotubes as photosensitive initiators.     

Ellipsometric study of the influence of chemical etching on thin porous silicon structures

The effect of chemical etching on Porous Silicon (PS) samples is studied and quantified by using variable angle spectroscopic ellipsometry (VASE). The main aim of this work is to assess the impact of such etching on the physical properties of electrochemically etched, thin PS antireflection coatings (ARC) for solar cell applications. In this study, detailed models of PS layers etched at constant current densities are created using a graded uniaxial Bruggeman Effective Medium Approximation (BEMA). Changes in porosity, thickness, and optical anisotropy of the PS samples due to chemical etching are determined as a function of etching time after PS formation. Three series of PS films, etched at three different current densities, are investigated. It is shown that significant changes in physical properties occur for chemical etching times longer than ~ 60 s. The anodic etching process for fabricating PS ARC structures can be performed in less than 10 s. Therefore, chemical etching does not lead to significant deviations from the intended PS structure and is not seen as a hindrance to accurate control of processes for fabricating thin PS ARCs.