Formation Of Porous Silicon Layers Research Articles

Localized protection of high aspect ratio trenches based on the anisotropy manipulation of plasma-polymerized fluoropolymer coating

Microelectronics, microfluidics and energy storage are among the application fields that could take advantage of the localized protection inside deep cavities or textured substrates. However, standard masking techniques used in the industry, such as photolithography, can be limited when applied to wafers with high aspect ratio topography. We have developed a maskless technique based on the anisotropy manipulation of the deposition and etching of a plasma-polymerized fluoropolymer layer, fully protecting the horizontal surfaces of deep trenches while keeping their sidewalls free of any protective layer. We were able to mask trenches with an aspect ratio up to ∼10, and depth of ∼100 μm. As a proof of concept for this localized protection technique, we demonstrate the formation of porous silicon layers (up to 8 μm in thickness) performed by anodization on these walls, while the horizontal surfaces were fully protected.

Formation mechanisms and photoluminescence properties of self-organized porous Si/SiO2 structures formed by electrochemical etching of n-type of Si

An ordered array of silicon dioxide pyramids were obtained in the course of silicon substrates etching in viscous solution of dimethylformamide. Systematic study was made to establish the influence of etching current and solution composition on the size, density and the shape of SiO2 pyramids. In general, the formation of the structure involves following steps: (i) the porous silicon layer formation at the interface with monocrystalline silicon, (ii) a sol-gel synthesis of the porous SiO2 area on top of the por-Si layer, (iii) formation of cracks in the por-Si layer and (iv) dissolution of the por-Si layer and as consequence the growth of the Si/SiO2 pyramids. The photoluminescence properties of the samples (λex = 405 nm) are determined by morphology of pyramids array.

Preparation of Hydrophobic Porous Silicon by Metal-Assisted Etching with Pd-Catalyst

The process of the porous silicon layer formation by metal-assisted etching of single-crystal Si with the resistivity of ρ = 0.01 Ω cm, coated with thin Pd-film up to 100 nm thick, in HF/H2O2/H2O solution is studied. The porous silicon is studied by scanning electron microscopy and gravimetric analysis. The dependence of the silicon dissolution rate on the HF concentration, in the presence and in the absence of the Pd-film, is investigated. The anodic current facilitating the Si dissolution in the HF solutions of different concentration is studied. By using sessile drop method, the porous silicon formed by the Pd-assisted anodic etching was shown to be superhydrophobic. The porous Si wetting angle reached 172°. The free surface energy for the porous layers and the water surface tension at the porous Si are calculated. The modified Si-surfaces may found their possible use in robotics.

Features of Pore Nucleation in p-Si during Its Electrochemical Etching

Features of the formation of porous silicon layers during anodic etching of p-Si were considered. A fundamental difference between the mechanisms of the formation of nanostructured layers on p-and n-Si was shown.

Modification of Surface Properties of Silicon Wafers by Laser-Assisted Electrochemical Etching

In this paper, the structural properties of porous silicon layer PSL were reported. Photo-assisted (laser) electrochemical etching PECE technique used to fabrication PSL from n-type wafer silicon as a function of etching time. Optical microscopy OM image is confirmed that the surface topography of porous silicon layer formation was a mud-like structure. The porosity and thickness have been determined gravimetrically are varied from 61% to 82% and 7.2 µm to 9.4µm respectively. The XRD patterns show that one diffraction peak for all PSL through anodization duration and it is assigned to the (400) plane and data confirmed the porous silicon PS was nanocrystalline.

Modification of Surface Properties of Silicon Wafers by Laser-Assisted Electrochemical Etching

In this paper, the structural properties of porous silicon layer PSL were reported. Photo-assisted (laser) electrochemical etching PECE technique used to fabrication PSL from n-type wafer silicon as a function of etching time. Optical microscopy OM image is confirmed that the surface topography of porous silicon layer formation was a mud-like structure. The porosity and thickness have been determined gravimetrically are varied from 61% to 82% and 7.2 µm to 9.4µm respectively. The XRD patterns show that one diffraction peak for all PSL through anodization duration and it is assigned to the (400) plane and data confirmed the porous silicon PS was nanocrystalline.

Morphology and FT IR spectra of porous silicon

Abstract The morphology and chemical bods of p-type and n-type porous Si was compared. The surface of n-type sample is smooth, homogenous without any features. The surface of p-type sample reveals micrometer-sized islands. FTIR investigation reveals various distribution of SiOxHy complexes in both p-and n-type samples. From the conditions leading to porous silicon layer formation (the presence of holes) we suggest both SiOxHy and SiFxHy complexes in the layer.

EFFECT OF ETCHING TIME ON OPTICAL AND MORPHOLOGICAL FEATURES OF N-TYPE POROUS SILICON PREPARED BY PHOTO-ELECTROCHEMICAL METHOD

Achieving efficient visible photoluminescence from porous-silicon (PSi) is demanding for optoelectronic and solar cells applications. Improving the absorption and emission features of PSi is challenging. Photo-electro-chemical etching assisted formation of PSi layers on n-type (111) silicon (Si) wafers is reported. Samples are prepared at constant current density (~30 mA/cm2) under varying etching times of 10, 15, 20, 25, and 30 min. The influence of etching time duration on the growth morphology and spectral properties are inspected. Room temperature photoluminescence (PL) measurement is performed to determine the optical properties of as-synthesized samples. Sample morphologies are imaged via Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The thickness and porosity of the prepared samples are estimated using the gravimetric method. The emission and absorption data is further used to determine the samples band gap and electronic structure properties. Results and analyzed, interpreted with different mechanisms and compared.

Simulation and optimization of the performance of multicrystalline silicon solar cell using porous silicon antireflection coating layer

Different porous silicon PS formation processes were suggested from the viewpoint of photovoltaic. This work is to analyze on the results of using a porous silicon layer as an antireflection coating to improve the performance of multicrystalline silicon solar cells both in the dark and under illumination. A mathematical method is presented to extract the physical parameters like the diode saturation current, the series resistance, the ideality factor and the shunt resistance. These parameters are extracted to simulate the current–voltage and the power–voltage of multicrystalline silicon solar cell single and double exponential models, before and after PS formation, by using the Newton Raphson method. A good agreement between theoretical model and experimental measurement of electrical characteristics is obtained after and before the porous silicon layer formation which show clearly how the application of a thin chemical vapor etching (CVE)-based PS layer can improve the characteristics of solar cells.

Investigation of Antireflective Porous Silicon Coating for Solar Cells

Solar cell is device that directly converts the energy of solar radiation to electrical energy. So it is important for solar cell to reduce the surface reflection of light in order to improve the efficiency of the device. Texturing and antireflection coating have been used to reduce the reflection of light. Texturing technology has reduced the 10% of incident light. However, there are a few disadvantages of random pyramid texturing that the results are not always reproducible in an industrial environment. And AR coating (MgF2, ZnS) is difficult to apply the standard industrial process because high vacuum is needed and the expense is very heavy. This paper investigates the formation of a thin film of porous silicon on the surface of crystalline silicon substrate without other AR coating layers. The formation of the porous silicon layer was measured with SEM (scanning electron microscopy). The formation of porous silicon layers on the textured silicon wafer resulted in lower than 5% of reflectance in the wavelength region from 400 to 1000 nm.

Passivation of the surface of rear contact solar cells by porous silicon

In this paper we analyse the passivation of the front surface of p-Si interdigitated rear contacts solar cell (IBC) by a thin porous silicon (PS) layer. Effectively, an efficiency improvement of 87% in relative was observed after porous silicon layer formation on the front surface of the IBC cell. The origin of surface passivation by the PS layer was studied by Laser Beam Induced Current (LBIC) method. The front surface of rear contacts cell with thin porous silicon layer was scanned by a modulated red laser beam in presence of a permanent light with different wavelengths and intensities. It was shown that without permanent illumination, the photocurrent of the cell with PS layer is very low, even lower than for a cell with unpassivated surface. However with short permanent wavelength illumination a strong increase of photocurrent was observed (8–10 times!). The light-dependent porous silicon passivation phenomenon is explained by a significant negative charge accumulation at the PS/p-Si interface traps under illumination. This leads to the formation of a hi-low (p +/p) junction at the front surface of the cell and to the reduction of the front surface recombination rate, like in Front Surface Field Solar Cell.

Application of MEMS technology to micro fuel cells

The advantage which we can get it when we apply the micro fabrication technology to the preparation processes of small fuel cells is investigated. The deep reactive ion etching for the gas channel preparation and the formation of porous silicon layers for MEA supports by electrochemical process are the key technologies to improve the MEMS-based micro fuel cells. The MEMS technology can control the fine and precise flows of the fuel, air, and water in the electrodes, which may improve the current and power densities in a small fuel cell.

Investigation of nanostructured porous silicon by Raman spectroscopy and atomic force microscopy

A set of porous silicon layers was produced by stain etching using a solution of HF:HNO 3 in a composition ratio of 500:1 with etching time varying from 1 to 10 min. The residual stress and the correlation length of the layers were obtained through the analysis of the micro-Raman spectra using a phonon confinement model including a term to account for the amorphous phase. The correlation length, which corresponds to the crystallite size, is found to decrease as the etching time increases, and agrees with the values obtained by atomic force microscopy analysis. The residual compressive stress tends to increase with etching time, as expected by the smaller crystallite size. However, an oscillatory behavior is detected for etching times higher than 4 min. This result correlates well with the photoluminescence measurements and can be understood by the porous silicon layer formation observed with the atomic force microscopy.

Formation of Porous Silicon Layers on Insulating Substrate for Microbridge – Type Sensor Applications

ABSTRACTFormation processes of porous silicon on insulating substrate were studied. It was demonstrated that both electrochemical and chemical formation methods allow to transform heavily doped p-type polycrystalline silicon into homogeneous porous silicon. Porous silicon was successfully used as sacrificial layer in the fabrication process of microbridge structures.

Open-Circuit Study of Stain Etching Processes Leading to the Formation of Porous Silicon Layers

Layers of monocrystalline silicon were used as a working electrode in a three-electrode electrochemical cell with platinum as reference electrode in order to carry out stain etching processes leading to the formation of porous silicon layers. The potential at which the corrosion process takes place was measured as a function of the concentration of the oxidizing agent, and the reaction order with respect to the nitrate concentration was found to be close to one. From the experimental results a mechanism is formed for the stain etching process and a model proposed for growth of the porous layer. © 2003 The Electrochemical Society. All rights reserved.

Micro- and nanopatterning of sensor chips by means of macroporous silicon

In this contribution, the authors present a novel technique to prepare new three-dimensional porous silicon layers. The base material is n-type Si with a thickness of 60μm. The preparation of the new microstructures is an one-step process. Hence, it should be possible to adopt this technique in up-to-date sensor fabrication processes to enhance the properties of the sensors. As Jeske et al. [Surf. Interface Anal. 22 (1994) 363.] showed, p-type silicon can also be utilised as substrate material for the formation of porous silicon layers to be filled up with sensing components. In upcoming experiments it is tried to tailor this process also to p-type silicon substrates. First deposition experiments with Ni-P were carried out to characterize the deposits on the porous silicon samples.

Humidity sensors using porous silicon layer with mesa structure

A capacitance-type humidity sensor in which a porous silicon layeris used as a humidity-sensing material was developed. This sensor wasfabricated monolithically to be compatible with the typical IC processtechnology except for the formation of porous silicon layer. As the sensor ismade as a mesa structure, the correct measurement of capacitance is expectedbecause it can remove the effect of the parasitic capacitance from the bottomlayer and other junctions. To do this, the sensor was fabricated using processsteps such as the localized formation of porous silicon, oxidation of theporous silicon layer, and etching of the oxidized porous silicon layer. Fromcompleted sensors, capacitance response was measured at a relative humidity of25-95% at room temperature. As a result, the measured capacitance showed anincrease over 300% at the low frequency of 120 Hz, and showed littledependence on temperature between 10 and 40 °C.

Long-range gettering of microdefects in silicon single crystals during the formation of porous silicon layers on their surface and ion irradiation

Experimental data on the dissolution of microdefects in the near-surface regions of silicon single crystals during the electrochemical formation of porous silicon layers followed by argonion irradiation are presented. A decrease in the microdefect concentration is detected near the interface with porous silicon and near the opposite surface of the samples.

Formation of shallow n+-p junction in silicon by spin-on technique

A simple mixture based on tetraethoxysilane was used to form shallow and highly phosphorus-doped n+ layers by the spin-on technique. Simultaneous doping with Yb was also used to improve the characteristics of the structures. The n+-p junctions obtained were investigated using optical methods, electron microscopy and Auger spectroscopy. Additionally, recombination in surfaces prepared under different conditions was monitored by a transient microwave absorption technique. It was found that the depth of the junction is about 0.4 µm and the phosphorus concentration at the surface reaches 2 · 1020 cm-3. Adding Yb to the spinning solution and the highly doped surface coating by porous silicon layer formation both reduce the surface recombination. The lowest obtained value of the surface recombination velocity was close to 170 cm/s.

Depth Gradients in Porous Silicon: How to Measure Them and How to Avoid Them

AbstractFor the formation of single layers and layer systems the depth homogeneity of porous silicon (PS) plays a key role. We have measured qualitatively and quantitatively structural gradients in p-PS layers with increasing layer thicknesses by fitting reflectance spectra in the infrared and by measuring thickness oscillations in the reflectance during the formation of PS layers. These results allow us to give recipes for the formation of PS layers with a homogenous optical thickness in depth.