Thin Porous Silicon Layers Research Articles

Orientation Effect of the Wafer on the Structural Properties of P+ Type Porous Silicon Layers

The structural properties of (111) oriented p+ type Porous Silicon (PS) samples are investigated using various X-ray diffraction techniques and compared to (001) p+ type PS layer structure. High resolution X-ray diffractometry was used to record rocking curves and reciprocal space maps, giving indications about the crystalline quality of the PS samples as well as about the pore orientation. X-ray diffraction and reflectivity performed on thin PS layers allow to estimate the layer thickness, porosity and roughness of the PS/substrate interface.

Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N +/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N +/P, 78.5 cm 2) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current ( I sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I– V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.

A Porous Silicon Optical Biosensor: Detection of Reversible Binding of IgG to a Protein A-Modified Surface

The reversibility, specificity, stability, and scaling of signal response to analyte mass were quantified for a porous silicon-based optical interferometric biosensor. The sensor system studied consisted of a thin layer (5μm) of porous silicon modified with Protein A. The system was probed with various fragments of an aqueous Human IgG analyte. The sensor operates by measurement of the Fabry−Perot fringes in the white light reflection spectrum from the porous silicon layer. Molecular binding is detected as a shift in wavelength of these fringes. IgG was added to and removed from the protein A-modified surface by changing solution pH in a flow cell, and the system was found to be reversible through several on−off cycles. The molecule used to link protein A to the porous Si surface incorporated bovine serum albumin (BSA). This approach was found to completely eliminate signal due to nonspecific binding, tested by exposure of the sensor to the F(ab‘)2 fragment of IgG (which does not bind to protein A). The l...

X-ray reflectivity investigation of thin p-type porous silicon layers

X-ray reflectivity (XRR) was used to investigate the p −- and p +-type porous silicon (PS) layers. For p −-type samples, a linear dependence of the thickness versus the anodization time was revealed in the 10–250 nm thickness range for even very short etching times and the layer structure was homogeneous with a transition layer at the PS/substrate interface. For p +-type materials, a surface film was unexpectedly observed for layers of low porosity. The structure and origin of this surface film is discussed in this article.

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.

Microwave plasma chemical vapor deposition of diamond films with low residual stress on large area porous silicon substrates

Diamond films were deposited on 3-inch diameter p-type 〈100〉 oriented porous silicon substrates using a microwave plasma disc reactor. Thin porous silicon layers were obtained on silicon wafers by anodization in an H2O/HF/C2H5OH solution. Process parameters were varied to obtain the best quality uniform porous silicon films. Diamond films were deposited on these substrates with and without dry seeding techniques utilizing 4-nm diamond particles. Power and pressure were varied in the range of 2800–3300 W and 50–60 Torr, respectively, while the methane concentration was kept constant at 1% by volume in hydrogen. Diamond was successfully deposited on anodized silicon substrates without dry seeding. However, the films were discontinuous in certain regions even after 20 h of deposition and the deposition rate was low. Dry seeding of the porous silicon surface yielded a high deposition rate and uniform films. Porous silicon was found to reduce the intrinsic stress in the deposited diamond film considerably. Furthermore, film adhesion was also improved with porous silicon.

Macroporous p-Type Silicon Fabry−Perot Layers. Fabrication, Characterization, and Applications in Biosensing

We present in this paper that porous silicon can be used as a large surface area matrix as well as the transducer of biomolecular interactions. We report the fabrication of heavily doped p-type porous silicon with pore diameters that can be tuned, depending on the etching condition, from approximately 5 to 1200 nm. The structure and porosity of the matrixes were characterized by scanning force microscopy (SFM) and scanning electron microscopy (SEM), Brunnauer−Emmett−Teller nitrogen adsorption isotherms, and reflectance interference spectroscopy. The thin porous silicon layers are transparent to the visible region of the reflectance spectra due to their high porosity (80−90%) and are smooth enough to produce Fabry−Perot fringe patterns upon white light illumination. Porous silicon matrixes were modified by ozone oxidation, functionalized in the presence of (2-pyridyldithiopropionamidobutyl)dimethylmethoxysilane, reduced to unmask the sulfhydryl functionalities, and coupled to biotin through a disulfide-bon...

Porosity depth profiling of thin porous silicon layers by use of variable-angle spectroscopic ellipsometry: a porosity graded-layer model

Variable-angle spectroscopic ellipsometry was used to determine nondestructively the porosity depth profile and thickness of thin porous silicon layers produced by anodization of p(+)-doped silicon wafers. A porosity graded-layer model is presented and used in the analysis of the material. In the porosity graded-layer model an inhomogeneous layer is built up by several thin sublayers with the porosity changing slightly from one sublayer to the next. Results from the ellipsometry analysis and from transmission electron microscopy reveal inhomogeneous layers whose porosity and thereby optical properties change with their depth in the layers.

Negative-Resistance Effects in Light-Emitting Porous Silicon Diodes

Definite negative-resistance (NR) effects in electroluminescence(EL)-emissive porous silicon (PS) diodes have been investigated in relation to the EL characteristics. The PS diode is composed of a semitransparent thin Au film, a relatively thin PS layer, a n+- or p-type Si substrate and an ohmic back contact. When the forward bias voltage is applied to the diode, a reversible NR behavior is observed in the current–voltage (I–V) characteristics at room temperature. At a low temperature of 10 K, the peak-to-valley current ratio in the n+-type substrate case extends to about 104. It is shown that the EL emission is switched on at the onset of the NR effect. For the p-type substrate diode, the NR effects can be activated by photoillumination. The mechanism of the observed NR effects is also discussed from the viewpoint of electron injection into the silicon nanocrystallites in PS. The NR effects can be regarded as one of the field-induced functions of PS as a quantum-sized crystalline system.

X-ray Reflectivity Study of Porous Silicon Formation

AbstractX-ray reflectometry is used to study the first stages of formation of thin n-type porous silicon layers. Results on classical n−-type porous silicon prepared under illumination are first reported. Then, the effect of the illumination during the formation is observed by comparing n+/− - type samples prepared in darkness or under illumination. X-ray specular reflectivity measurements allow to observe an increase of the surface porosity even for the short formation times and a macroporous layer under the nanoporous layer is also identified for illuminated samples. The presence of a crater at the top of the layer is observed by profilometer measurements, especially in the case of illuminated samples. Specular and diffuse x-ray scattering results show important effects of light during the porous silicon formation.

Vapor sensitivity of thin porous silicon layers

Spectroscopic ellipsometry has been used to study optical changes in thin porous silicon layers due to exposure to vapors from different liquids. The changes in the ellipsometric parameters Ψ and Δ were caused by changes in the total refractive index of the layers. The detection limit threshold for acetone vapors by the layers was 12 ppm. Using a 4-layer optical model, it was determined that capillary condensation was involved in the vapor sensitivity of the layers. Based on these findings, it was concluded that thin porous silicon layers can be used as an optical sensor material for gas sensing purposes.

Blocking effect of charge transfer at the porous silicon/silicon interface

The photoluminescence (PL) of a thin porous silicon (por-Si) layer and of bulk c-Si is investigated in situ after the formation in 0.2 M NH4F (pH 3.2) and in water, ethanol, and their vapors. The excitation light of the used pulsed N2 laser is absorbed in the por-Si surface layer. This allows the contactless study of changes of the diffusion of excess carriers from the por-Si layer into the c-Si by using the PL of c-Si as a probe. The radiative recombination of c-Si decreases after exchanging the electrolyte by the other media while the PL intensity of the por-Si increases strongly. The strongest increase of the PL of por-Si is observed in ethanol atmosphere. The results are discussed on the basis of blocking of the out diffusion of excess carriers from the por-Si layer into the bulk c-Si due to electrical charging of interface states.

Photosensitivity of silicon–porous silicon heterostructures

The photosensitivity of silicon–porous silicon heterostructures has been studied. The measured photocurrent intensity as a function of photon energy showed oscillations in both polarized and unpolarized light. Assuming that photocurrent oscillations were due to light interference in thin porous silicon layers, the refractive index was estimated as 1.6–1.8. At normal light incidence there was no effect of the direction of light polarization, but at oblique light incidence the photocurrent when the light electrical vector coincided with the incidence plane exceeded that at perpendicular polarization, due to a difference in reflectivity.

Preparation of thin porous silicon layers by stain etching

A method of preparation of thin (1000 Å) porous silicon layers by chemical etching of c-Si in HF:HNO 3:H 2O=1:3:5 solution is reported wherein a thin Al film is deposited by evaporation on the silicon surface prior to etching. The method is characterized by no “incubation” period to the onset of the stain etching. The reaction between Al and HNO 3 produces the required holes for the very fast start of the chemical etching of Si. Using Al patterns deposited through a mask or prepared by photolithography it is possible to achieve selective formation of porous silicon. The porous silicon layers exhibit visible photoluminescence and when prepared on p-type c-Si and a shallow p–n c-Si homojunction the resulting heterostructures also display electroluminescence and photovoltaic properties. © 1997 Elsevier Science S.A.

Porous silicon field emitters for display applications

A fabrication technology for porous silicon (PS) field emitters for display applications is described. Silicon blunt emitters prepared on p-type silicon wafers of 30 Ω cm resistivity were covered with a thin layer of porous silicon using an electrochemical anodization in ethanoic hydrofluoric solution. A PS layer with similar characteristics was also prepared on flat silicon surfaces. The PS layer morphology was investigated by scanning electron microscopy (SEM). The composition of the PS layers after the stabilization treatments was analyzed by secondary ion mass spectrometry (SIMS). Similar values of the vacuum field emission current were obtained for both cases of PS on silicon blunt tips and on flat surfaces. The oxidized porous silicon layers did not show vacuum field emission properties at all. Good field emission results were obtained on samples that were immediately processed to prevent the PS oxidation or on samples with a stabilized porous surface.

Controllability And Homogeneity Of Optical Properties Of Thin Porous Silicon Films

AbstractReflection spectroscopy was applied to evaluate the optical homogeneity and the refractive index of thin porous silicon (PS) layer. Variation in PS layer optical thickness was evaluated by measuring and mapping the reflectance over the surface area. For the circular anodization area of 17 mm diameter, the measured variation was less than 5 % on an area of 11 mm diameter. The anodization electrode position was found to have little influence on the homogeneity. A method to derive refractive index from a reflection spectrum is studied. The wavelength dependence of PS index measured by the method showed fair coincidence with the calculation based on effective medium approximation. In the two layer PS formation, optical thickness was found to change whether high porosity or low porosity layer is formed first.

X-ray diffraction investigation of porous silicon superlattices

High-resolution X-ray diffraction has been used for the structural characterization of thin single porous silicon (PS) layers and superlattices. Thin PS layers, in the range of 100–1000 nm, exhibit several thickness interfringes, giving evidence for the lateral homogeneity of the layer thickness. For PS superlattices, several satellite reflections are visible in the diffraction pattern. The analysis of these experimental results using simulations allows us to deduce structural information of this new kind of silicon heterostructure.

X-Ray Diffraction and Reflectivity Studies of Thin Porous Silicon Layers

AbstractHigh resolution X-ray diffraction and reflectivity have been used for the structural characterization of thin porous silicon layers of p and p+ doping type. Thin porous silicon layers studied either by diffraction or reflectivity, in the range of 10–1000 nm, exhibit several thickness fringes, corresponding to a lateral homogeneity of the layer thickness. The comparison between the experimental results with simulations enables one to deduce structural information relative to the porosity, thickness, lattice parameter as well as interface thickness. For p+ type samples a double fringe system was observed, showing the existence of a surface film probably at the porous silicon layer top surface.

Nonlinear Electrical Functions of Porous Silicon Light-Emitting Diodes

AbstractSome nonlinear electrical characteristics in electroluminescent porous silicon (PS) diodes with a relatively thin PS layer (0.5–5 μm thick) are described. The experimental PS diodes were composed of a semitransparent Au film, a PS layer, p- or n-type Si substrate, and an ohmic back contact. The PS layers were prepared by anodizing Si wafers in an ethanoic HF solution. In some cases, the PS layers were treated by rapid thermal oxidization (RTO) process. When the bias voltage is applied, the PS diodes show the electrical behavior like the metal-insulator-semiconductor (MIS) diodes. The negative-resistance characteristics and memory effect are also observed. These results indicate that the quantum-structured nature of the PS layer appears not only in the optical properties but also in the electrical properties.

Elaboration and Light Emission Properties of Low Doped p-Type Porous Silicon Microcavities

AbstractWe account for the elaboration of Bragg reflectors and microcavities based on efficiently luminescent porous silicon. A characterisation of very thin porous silicon layers obtained with current densities of formation varying from 1.5 mA to 300 mA is presented. The resulting refractive index variation (typically from 1.37 to 1.86 at 700 nm) enables the elaboration of high quality Bragg reflectors and Fabry-Perot filters from the yellow to the near infrared. Although low doped p-type porous silicon develops rougher interfaces than highly doped p-type porous silicon, its better luminescence efficiency has enabled us to elaborate microcavities with a strong emission in a narrow band.