Vibrational study on styrene functionalized porous silicon: A method for determining the relative yield of different grafting routes

A new semi-quantitative method providing the relative efficiency of three different organic functionalization reactions onto porous silicon has been set up, based on infrared absorption data. Compared to previously reported techniques, it enables a direct titration of the grafted molecules. We demonstrated that grafting of Si-styrenyl moieties by ethylaluminium dichloride mediated hydrosilylation of phenylacetylene leads to higher yields than organometallic addition onto either hydrogenated or brominated silicon.     

Irreversible quenching of luminescence in porous silicon

The influence of applied voltage on photoluminescence (PL) in porous silicon was studied. A strong PL band around 680 nm was observed when excited by a 300 nm ultraviolet light with no voltage applied, but upon increasing the bias voltage, a strong and progressive decrease of the PL intensity was observed leading finally to a complete quenching of the emitted light at 1.80 V. The peak position of the emission appears to be stable. This effect is completely irreversible, and the spectra depend on the increased voltage to the sample and corresponding temperature increase. Nonradiative recombination resulting from the thermal oxidation was suggested to be responsible for the quenching.     

Photoluminescent properties of silicon carbide and porous silicon carbide after annealing

Photoluminescent (PL) p-type 6H porous silicon carbides (PSCs), which showed a strong blue-green photoluminescence band centered at approximately 490 nm, were annealed in Ar and vacuum conditions. The morphological, optical, and chemical states after annealing are reported on electrochemically etched SiC semiconductors.The thermal treatments in the Ar and vacuum environments showed different trends in the PL spectra of the PSC. In particular, in the case of annealing in a vacuum, the PL spectra showed both a weak red PL peak near 630 nm and a relatively intense PL peak at around 430 nm in the violet region. SEM images showed that the etched surface had spherical nanostructures, mesostructures, and islands. With increasing annealing temperature it changes all spherical nanostructures. The average pore size observed at the surface of the PSC before annealing was of the order of approximately 10 nm.In order to investigate the surface of a series of samples in detail, both the detection of a particular chemical species and the electronic environments at the surface are examined using X-ray photoelectron spectroscopy (XPS). The chemical states from each XPS spectrum depend differently before and after annealing the surface at various temperatures. From these results, the PL spectra could be attributed not only to the quantum size effects but also to the oxide state.     

Self-affinity study of nanostructured porous silicon-crystalline silicon interfaces

Self-affinity was used to analyze the roughness at the porous silicon (PS)-crystalline Si (cSi) interfaces fabricated under different conditions. Using the variable bandwidth method, the self-affinity behavior was, qualitatively and quantitatively, analyzed from the cross-section micrographs of the PS samples obtained by field emission scanning electron microscope. The results show that correlation length is related with the average pore width. Roughness exponent is found to be correlated with the interface roughness. In addition, similar experimental roughness exponents were obtained for several interfaces grown by different methods, indicating the intrinsic fractal nature of the PS-cSi interfaces. The results were confirmed through the self-affinity analysis done on the atomic force microscopy profiles.     

Ellipsometric study of refractive index anisotropy in porous silicon

Porous Si layers of different thicknesses were prepared by anodising p⁺-type Si substrates with a resistivity of 0.01 Ω cm. The porosity of the samples ranged from 23% to 62%. The refractive index values for the ordinary and extraordinary rays were determined by multiple angle of incidence ellipsometry, from which an optical anisotropy parameter varying from 13% to 20% was obtained. The porous Si layers were modelled as uniaxially anisotropic films on an isotropic substrate, with an optical axis perpendicular to the sample surface. The morphological anisotropy which is typical for the p⁺-type porous Si with a predominating cylindrical geometry is responsible for these optical properties. All the porous Si layers studied were found to be optically negative.     

The effect of etching time of porous silicon on solar cell performance

Porous silicon (PS) layers based on crystalline silicon (c-Si) n-type wafers with (1 0 0) orientation were prepared using electrochemical etching process at different etching times. The optimal etching time for fabricating the PS layers is 20 min. Nanopores were produced on the PS layer with an average diameter of 5.7 nm. These increased the porosity to 91%. The reduction in the average crystallite size was confirmed by an increase in the broadening of the FWHM as estimated from XRD measurements. The photoluminescence (PL) peaks intensities increased with increasing porosity and showed a greater blue shift in luminescence. Stronger Raman spectral intensity was observed, which shifted and broadened to a lower wave numbers of 514.5 cm⁻¹ as a function of etching time. The lowest effective reflectance of the PS layers was obtained at 20 min etching time. The PS exhibited excellent light-trapping at wavelengths ranging from 400 to 1000 nm. The fabrication of the solar cells based on the PS anti-reflection coating (ARC) layers achieved its highest efficiency at 15.50% at 20 min etching time. The I-V characteristics were studied under 100 mW/cm² illumination conditions.     

Low-porosity porous silicon nanostructures on monocrystalline silicon solar cells

In this work we present a study of low-porosity porous silicon (PS) nanostructures stain etched on monocrystalline silicon solar cells. The PS layers reduce the reflectance, improve the diffusion of dopants by rapid thermal processes, and increase the homogeneity of the sheet resistance. Some samples were subjected to chemical oxidation in HNO₃ to reduce the porosity of the surface layer. After the diffusion process, deposition of a SiN_x antireflection layer, and screen printing of the samples, an efficiency of 15.5% is obtained for low-porosity PS solar cells, compared with an efficiency of 10.0% for standard PS cells and 14.9% for the reference Cz cells.     

Comparative study of the oxidation of thin porous silicon layers studied by reflectometry, spectroscopic ellipsometry and secondary ion mass spectroscopy

We present a systematic study on ultrathin porous silicon (PS) layers (40–120 nm) of different porosities, formed by electrochemical etching and followed by thermal oxidation treatment (300°C and 600°C) and by electrochemical oxidation. The oxidised and non-oxidised PS layers have been analysed by spectroscopic reflectometry (SR), spectroscopic ellipsometry (SE) and secondary ion mass spectroscopy (SIMS). The SR and SE spectra were fitted by a multiparameter fit program and the composition and the thickness of the PS layers were evaluated by different optical models. PS layers, formed electrochemically in the outermost layer of a p/n⁺ monocrystalline silicon junction were successfully evaluated using a gradient porosity optical model. The non-oxidised PS, formed in p-type silicon, can be well described by a simple optical model (one-layer of two-components, silicon and voids). The spectra of the oxidised PS layers can be fitted better using an optical model with three interdependent components (crystalline-silicon, silicon-dioxide, voids). The SIMS results give a strong support for the optical model used for SR and SE.     

Simulation and fabrication study of porous silicon photonic crystal

The present work reports design and fabrication of porous silicon based one-dimensional (1D) photonic crystal. Distributed Bragg reflector (DBR) is a 1D photonic crystal composed of multilayer stack of high and low refractive index layers. Design of porous silicon DBR is a complex one and requires appropriate control in optical parameters of its constituent layers. In order to design DBR, two porous silicon single layer samples were fabricated using current density of 10 and 50 mA/cm². Optical characterization of single layer samples showed series of interference fringes. Reflective interferometric Fourier transform spectroscopy (RIFTS) method was employed to determine optical constants of porous silicon single layers. DBR simulation was carried out based on transfer matrix method. DBR was then fabricated using optical parameters obtained from RIFTS method. Reflection bandwidth of prepared DBR was found to be 216 nm, which is comparable to the simulated value of 203 nm.     

An application of Raman spectroscopy on the measurement of residual stress in porous silicon

The porous silicon film, at micron level, and the bulk silicon substrate is a basic structure in MEMS components. The residual stress, due to the lattice mismatch between the film and the substrate, exists on the interface and may cause cracking and damaging on the component. Micro-Raman spectroscopy is an optical measurement method that was rapidly applied into the fields of chemistry, physics, material science and mechanics. In this paper, the method is introduced and applied to the study of the stress problems in porous silicon as follows. (1) In the electrochemical etching technique for porous silicon preparation, the distribution of the tensile residual stress along the transitional region between etched and un-etched area is experimentally studied and the result reveals the stress is continuous across the region. In the etched region it reaches GPa level, and in the transition region the gradient of the stress is high. (2) In chemical etching preparation of porous silicon, the residual stress rises up seriously in the cracked area, up to 0.92 GPa. With the porosity increasing, the tensile stress on the porous silicon film increases accordingly. The appearance of the porous silicon film surface is also given by metalloscope and atomic force microscope. The structure of the micro-pores is expected to have a close relation with the distribution of the residual stress.     

High sensitivity of palladium on porous silicon MSM photodetector

In this work, the nanocrystalline porous silicon (PS) is prepared through the simple electrochemical etching of n-type Si (1 0 0) under the illumination of a 100 W incandescent white light. SEM, AFM, Raman and PL have been used to characterize the morphological and optical properties of the PS. SEM shows uniformed circular pores with estimated sizes, which range between 100 and 500 nm. AFM shows an increase in its surface roughness (about 6 times compared to c-Si). Raman spectra of the PS show a stronger peak with FWHM=4.3 cm⁻¹ and slight blueshift of 0.5 cm⁻¹ compared to Si. The room temperature photoluminescence (PL) peak corresponding to red emission is observed at 639.5 nm, which is due to the nano-scaled size of silicon through the quantum confinement effect. The size of the Si nanostructures is estimated to be around 7.8 nm from a quantized state effective mass theory. Thermally untreated palladium (Pd) finger contact was deposited on the PS to form MSM photodetector. Pd/PS MSM photodetector shows lower dark (two orders of magnitude) and higher photocurrent compared to a conventional Si device. Interestingly, Pd/PS MSM photodetector exhibits 158 times higher gain compared to the conventional Si device at 2.5 V.     

Efficient luminescence from porous silicon

Photoluminescence measurements are carried out on porous silicon layers. We show the enhancement and stabilization of the luminescence when depositing a silicon nitride layer on top of porous layers.We also demonstrate that direct- and remote-plasma nitridation are good ways to reduce the ageing effect of porous silicon layers due to a passivation of dangling bonds.     

The effect of anti-reflection coating of porous silicon on solar cells efficiency

Anti-reflection coatings of solar cells have been fabricated using different techniques. The techniques used include SiO₂ thermal oxidation, ZnO/TiO₂ sputtering deposition and porous silicon prepared by electrochemical etching. Surface morphology and structural properties of solar cells were investigated by using scanning electron microscopy and atomic forces microscopy. Optical reflectance was obtained by using optical reflectometer. I-V characterizations were studied under 80 mW/cm² illumination conditions. Porous silicon was found to be an excellent anti-reflection coating against incident light when it is compared with another anti-reflection coating and exhibited good light-trapping of a wide wavelength spectrum which produced high efficiency solar cells.     

Photoluminescence and electroluminescence from porous silicon

The properties and origins of the red, blue and infrared photoluminescence bands of porous silicon are reviewed and discussed in the light of the models that have been proposed to explain the experimental and theoretical results. The red band is due to quantum confinement possibly supplemented by surface states; the blue band is linked to the presence of silicon dioxide; the infrared band is correlated with dangling bonds and bandgap luminescence in large crystallites. The fabrication and characterization of light-emitting devices made of porous silicon are reported and discussed with respect to critical issues such as the device stability, efficiency, modulation speed, emission wavelength, and compatibility with microelectronic processing.     

Tuning the cathodoluminescence of porous silicon films

We have obtained intense cathodoluminescence (CL) emission from electron beam modified porous silicon films by excitation with electrons with kinetic energies below 2 keV. Two types of CL emissions were observed, a stable one and a non-stable one. The first type is obtained in well-oxidized samples and is characterized by a spectral peak that is red shifted with respect to the photoluminescence (PL) peak. The physically interesting and technologically promising CL is however the CL that correlates closely with the PL. Tuning of this CL emission was achieved by controlling the average size of the nanostructure thus showing that the origin of this CL emission is associated with the quantum confinement and the surface chemistry effects that are known to exist in the porous silicon system. We also found that the electron bombardment causes microscale morphological modifications of the films, but the nanoscale features appear to be unchanged. The structural changes are manifested by the increase in the density of the nanoparticles which explains the significant enhancement of the PL that follows the electron irradiation.     

Effects of thermal oxidation on the photoluminescence properties of porous silicon

The effects of thermal oxidation on the photoluminescence (PL) properties of powdered porous silicon (PSi) are studied using X-ray photoelectron spectroscopy (XPS). It is found that the PL intensity is steeply quenched after annealing at and recovered at above . The XPS intensity of oxides formed on the PSi surface is also found to strongly depend on the annealing temperature. The comparison between the annealing temperature dependence of PL intensity and that of the oxide XPS intensity suggests that the formation of thin disordered SiO₂ layer accompanies the quenching of the PL intensity, and that the formation of thick high-quality SiO₂ layer results in the PL intensity recovery. These results indicate that the thickness and quality of SiO₂ layer play a crucial role in the PL properties of thermally oxidized PSi.     

Optical properties of erbium-doped porous silicon waveguides

Planar and buried channel porous silicon waveguides (WG) were prepared from p⁺-type silicon substrate by a two-step anodization process. Erbium ions were incorporated into pores of the porous silicon layers by an electrochemical method using ErCl₃-saturated solution. Erbium concentration of around 10²⁰ at/cm³ was determined by energy-dispersive X-ray analysis performed on SEM cross-section. The luminescence properties of erbium ions in the IR range were determined and a luminescence time decay of 420 μs was measured. Optical losses were studied on these WG. The increased losses after doping were discussed.     

Effect of oxygen implantation on ionoluminescence of porous silicon

We report on ionoluminescence investigations of porous Si prepared from the p⁺-type Si, which exhibited, after prolonged ambient air exposure, moderate photon emission with a maximum in the red–orange region. In an attempt to activate a shorter wavelength emission, the samples were implanted with 225 keV O⁺ ions at the dose of 1×10¹⁷ cm⁻². The strong blue band at 2.7 eV, well known in silica, has emerged in the ionoluminescence spectra following the oxygen implantation. The results of the comparative ionoluminescence experiments, performed on both porous Si and two forms of silica, show the important role of SiO₂ defect-related states in ion-induced optical emission from porous Si.     

Annealing and amorphous silicon passivation of porous silicon with blue light emission

The photoluminescence (PL) of the annealed and amorphous silicon passivated porous silicon with blue emission has been investigated. The N-type and P-type porous silicon fabricated by electrochemical etching was annealed in the temperature range of 700-900 °C, and was coated with amorphous silicon formed in a plasma-enhanced chemical vapor deposition (PECVD) process. After annealing, the variation of PL intensity of N-type porous silicon was different from that of P-type porous silicon, depending on their structure. It was also found that during annealing at 900 °C, the coated amorphous silicon crystallized into polycrystalline silicon, which passivated the irradiative centers on the surface of porous silicon so as to increase the intensity of the blue emission.     

Polarization phenomena in the optical properties of porous silicon

We examine the polarization memory effect for porous Si excited by linearly polarized light. The various observations for the red-luminescing, slow band are discussed in the general framework of particle shape asymmetry. We show that because of the intrinsically nonlinear luminescence response, measurement parameters influence the polarization response. The preparation of porous Si with photoassisted etching is found to control the polarization retention parameter ρ. Using linearly polarized light during etching produces in-plane asymmetries. We find a substantial ρ-anisotropy linked to crystal symmetry planes and axes as a consequence of anisotropic etching. The effects are discussed with reference to current models of the light emission mechanism.