Use Of Porous Silicon Research Articles

Porous silicon as a substrate for hydroxyapatite growth

Porous silicon has been shown to be an excellent candidate biomaterial, following studies establishing its biostability and non-toxicity. These favorable properties, coupled with the ease of its topographical manipulation, make it a promising material for the growth of hydroxyapatite, which is used as an artificial bone material. In this paper, the use of porous silicon as a substrate for hydroxyapatite growth induced by two methods is reported: a simple soaking process in a simulated body fluid and a laser–liquid–solid interaction process which allows interaction between a scanning laser beam and the substrate immersed in the simulated body fluid. The grown layers are investigated by light microscopy, electron microprobe analysis and X-ray diffraction. It is found that the layer topography obtained after applying the two processes is different.

Molecular Identification by Time-Resolved Interferometry in a Porous Silicon Film

Porous silicon is becoming increasingly important for the sensitive detection of liquid and gaseous analytes due to its high surface area and tunable porosity and functionality. These authors report the first use of porous silicon for the analysis of mixtures of chemical compounds. Using time-resolved reflectivity measurements to analyze ethanol and acetone mixtures, they found that the components could be separated temporally in the porous Si film in a process that is analogous to gas chromatography, the porous Si film playing the dual role of separation column and detector.

Applications of Microstructured Porous Silicon as a Biocatalytic Surface

The use of porous silicon (PS) as an efficient surface enlarging carrier matrix for immobilised enzymes is demonstrated. An optimal porous matrix in p-type silicon (10–20 Ωcm) with respect to enzymatic substrate turn-over was obtained when anodising the sample at 100 mA/cm2 for 5 min. An increase in glucose turn-over of ≈︂220 times as compared to an enzyme activated polished surface was recorded. The highest increase in turn-over was found to be 350 times for an n-epilayer on n+ substrate. The application of glucose monitoring is demonstrated showing a linear range to 15 mM glucose with a satisfactory storage and operational stability. The use of ascorbate oxidase activated porous silicon for the elimination of ascorbate (an electrochemical interferent) in glutamate monitoring is reported. An upper elimination level of 1 mM ascorbate was found. PS is also reported as an efficient protein cleavage surface when activated with proteases. Cleavage times normally ranging between 6 and 24 h were found to be 60 s in the microreactor. Myoglobin cleaved on a trypsin microreactor is shown with the corresponding mass spectra having a sequence coverage of 79%.

Permeated Porous Silicon Suspended Membrane as Sub-ppm Benzene Sensor for Air Quality Monitoring

To realise advanced microsensors for a reliable monitoring of very low concentrations of pollutant species such as NOx, SO2, CO, O3 and aromatic hydrocarbons, the use of porous silicon (PS) permeated with semiconducting oxides has been explored. To reduce the power consumption and to make feasible the device to operate in a fast pulsed temperature mode, a novel sensor architecture has been designed. The main feature of the device is represented by a permeated suspended macroporous Si membrane, few tens of microns thick. In this paper the porous silicon formation through a suspended silicon membrane and the morphological characterization of the PS layer are reported. Moreover, the performance of a C6H6 gas sensor based on the suspended macroporous Si membrane (≅30 μm thick), permeated with the chemical precursor of Sn oxide is presented. The results have demonstrated the feasibility to realize a macroporous silicon suspended membrane with high specific surface area, efficient electrical insulation and negligible warpage. Furthermore, the permeation of the oxidized macroporous silicon membrane with SnO2 has been proved to be a valuable approach to fabricate gas sensors suitable to detect aromatic hydrocarbons in a sub-ppm range.

Use of porous silicon antireflection coating in multicrystalline silicon solar cell processing

The latest results on the use of porous silicon (PS) as an antireflection coating (ARC) in simplified processing for multicrystalline silicon solar cells are presented. The optimization of a PS selective emitter formation results in a 14.1% efficiency multicrystalline (5/spl times/5 cm/sup 2/) Si cell with evaporated contacts processed without texturization, surface passivation, or additional ARC deposition. Specific attention is given to the implementation of a PS ARC into an industrially compatible screen-printed solar cell process. Both the chemical and electrochemical PS ARC formation method are used in different solar cell processes, as well as on different multicrystalline silicon materials. Efficiencies between 12.1 and 13.2% are achieved on large-area (up to 164 cm/sup 2/) commercial Si solar cells.

Porous silicon in solar cell structures: a review of achievements and modern directions of further use

Porous silicon, which is being obtained by electrochemical etching of silicon wafers in electrolytes on the base of hydrofluoric acid, recently attracted the attention of specialists in photovoltaics even more due to a number of its unique properties. However, at present, acceptable results are obtained for the use of porous silicon as antireflecting coating for silicon solar cells only. In the present paper, previous experience of the use of por-Si in the silicon solar cells has been reviewed. On the base of examination of the porous silicon properties, a number of new directions of improvement of photoconversion efficiency of structures with optimized layers of porous silicon are proposed. The results of numerical calculations carried confirm perspectiveness of use of porous silicon for efficiency improvement for different types of silicon solar cells. These can be increased of their internal quantum efficiency, expansions of operating spectral range toward ultra-violet and infrared spectrum range, decrease of losses of photogenerated power due to the influence of bulk and surface recombination.

Permeated porous silicon for hydrocarbon sensor fabrication

To realise advanced solid-state microsensors for a reliable monitoring of very low concentrations of pollutant species such as NO x , SO 2, CO, O 3 and aromatic hydrocarbons, the use of porous silicon (PS) layers permeated with mixed semiconducting oxides (Sn–V–O) has been proposed and explored. A low concentration C 6H 6 sensor made of a permeated macroporous silicon layer on a massive Si substrate has exhibited an excellent performance. To reduce the power consumption down to the level reported for micromachined gas sensor (0.1 W), and to make the device feasible for operation in a fast-pulsed temperature mode, a novel sensor architecture has been designed. The main feature of the device is represented by a permeated suspended macroporous Si membrane (few tens of microns thick) on top of which a heater resistor and a temperature sensor are integrated. In this paper the PS formation and processing are presented. Results on the morphological characterization of permeated macroporous silicon membrane are also illustrated. Finally, the electrical response of the device exposed to gas mixtures of different compositions is shown.

Porous silicon : a new material for microsensors and microactuators

Since the use of porous silicon for microsensors and microactuators is in the euly stage of study, only several application devices, such as light-emitting diodes and chemical sensors have so far been demonstrated. In this paper we present an overview of the present status of porous silicon sensors and actuators research with special emphasis on the applications of chemical sensors and optical devices. The capacitive type porous silicon humidity sensors had a nonlinear capacitance-humidity characteristic and a good sensitivity at higher humidity above . The porous silicon device showed a sharp increase in current when exposed to an ethanol vapor. The diode fabricated on porous silicon diaphragm exhibited an optical switching characteristic, opening up its utility as an optical sensor or switch. The photoluminescence (PL) spectrum, taken from porous silicon under 365 nm excitation, had a broad emission, peaked at -610 nm. The electroluminescence(EL) from ITO/PSi/In LED had a broader spectrum with a blue shifted peak at around 535nm than that of the PL.

Solvent detection using porous silicon optical waveguides

Since the first report on the use of porous silicon as an optical waveguide medium in 1995, significant development has been made towards the understanding and applicability of such material. Here, the introduction of solvents (acetone, methanol, and propan-2-ol) into the pores is shown to dramatically reduce the loss of the waveguides, in a reversible manner. Both the magnitude and duration of this effect are sensitive to the solvent introduced. In some waveguides, for example, the measured loss (at 0.633 μm) falls by as much as 34 dB cm −1 on the introduction of acetone. Theoretical estimates of the effect of solvents on the interfacial scattering loss confirm this as the origin of the observed reductions. These results, combined with the fact that a substantial portion of the guided-mode field interacts with the solvent, indicate an enhanced sensitivity for sensor applications may be achievable.

Morphology of porous silicon layers: image of active sites from reductive deposition of copper onto the surface

We report a scanning electron microscopy (SEM) investigation of the morphology of porous silicon layers and comparisons between our observations and a recent theory for its formation and morphology. Also examined was the use of porous silicon as a reducing agent in the preparation of Cu dispersed on the surface of the porous silicon. The morphology of the resulting copper deposits on the surface of the porous silicon layers in turn was used to infer the morphology of the hydrogenated, chemically active sites of the porous silicon. Conditions necessary for the reduction to take place, and the effect of the deposition upon the optical luminescence of the porous silicon are noted.

Porous silicon layer permeated with Sn–V mixed oxides for hydrocarbon sensor fabrication

To realise advanced microsensors for a reliable monitoring of very low concentrations of pollutant species such as NO x , SO 2, CO, O 3 and aromatic hydrocarbons, the use of porous silicon (PS) layers permeated with sensing compounds has been explored. For a conformal coating of the inner structure of the PS layer, a wet chemical permeation technique rather than a physical deposition technique has been explored. Mixed semiconducting oxides (Sn–V–O) characterized by peculiar catalytic properties towards hydrocarbons have been used as sensing compound. Results on the morphological, structural, chemical, and electrical characterization of permeated PS layer are presented and discussed.

Silicon-based visible light-emitting devices integrated into microelectronic circuits

MICROELECTRONIC device integration has progressed to the point where complete 'systems-on-a-chip' have been realized1–3. Now that optoelectronics is becoming increasingly important for information and communication technologies, there is a need to develop optoelectronic devices that can be integrated with standard microelectronics. Conventional semiconductor technology is largely based on crystalline silicon, which (being an indirect bandgap semiconductor) is an inefficient light-emitting material. This has stimulated significant effort towards developing silicon-based optoelectronic components and, of the several strategies explored so far4,5, the use of porous silicon appears the most promising; porous silicon produces high-efficiency, room-temperature, visible photoluminescence6, and its material and optical properties have been studied in detail7,8. But the extreme reactivity and fragility of porous silicon have hitherto prevented its integration with conventional silicon processing technology. We have recently shown9,10 that the thermal and chemical stability of porous silicon can be greatly enhanced — while retaining desirable light-emitting and charge-transport properties — by partial oxidation. Here we take advantage of these improvements in material properties to demonstrate the successful integration of silicon-based visible light-emitting devices into a standard bipolar microelectronic circuit.

Optimization Of Porous Silicon Reflectance For Solar Cell Applications

AbstractThe potential use of porous silicon as an antireflective coating on solar cells has recently been recognized. This study investigates the effect of current density, anodization time, and surface conditions on the reflectance of porous silicon which was fabricated by anodizing (100) float zone single crystal Si wafers. The wafers were coated on one side with Al prior to anodization, and a HFbased solution was used as the electrolyte. Current densities of 5 – 100 mA/cm2 were used to anodize both polished and unpolished wafers over time intervals ranging from 2sec - 30 minutes. Reflectance properties were tested over the 400 - 1100 nm range, and minimum reflectances of 3 – 5% were achieved. The reflectance of the best porous Si sample normalized with respect to the sun's spectrum compares favorably with the reflectance of a double layer ZnS/ MgF2 with prior texturing.

Theoretical Thermal Conductivity of Porous Silicon: Nonlinear Behavior

AbstractThe use of porous silicon (PS) in fabricating optoelectronic devices is in progress. However, the performance of such applications still needs to be improved. In particular, in solar cells heat must be dissipated to avoid a decay in their efficiency and one of the properties of PS that must be evaluated is the effective thermal conductivity. It is well known that the thermal conductivity of silicon is temperature dependent. Thus we cannot use a standard effective medium approach to obtain its effective thermal response. In this work, we extend the averaging volume and surface methods [1] to consider nonlinear effects in the effective transport coefficients. We model PS as composed of c-Si cylindrical columns covered by different overlayers (i.e. a-Si or SiO2) immersed in another medium. In our model the effective thermal conductivity has an explicit dependence on the temperature gradient. We present a parametric analysis of the model, compare it with the c-Si behavior and evaluate the importance of the nonlinear contribution.

Application of Porous Silicon to Bulk Silicon Micromachining

ABSTRACTA fully C-MOS compatible process for bulk silicon micromachining using porous silicon technology and front-side lithography is developed. The process is based on the use of porous silicon as a sacrificial layer for the fabrication of deep cavities into monocrystalline silicon, so as to avoid back side lithography. Cavities as deep as several hundreds of micrometers are produced with very smooth surface and sidewalls. The process is used to produce : a) suspended monocrystalline silicon membranes, b) free standing polysilicon membranes in the form of bridges or cantilevers with lateral dimensions from a few μms to several hundreds of μms. Important applications to silicon integrated devices as sensors, actuators, detectors etc., are foreseen.

Use of Porous Silicon to Minimize Oxidation Induced Stacking Fault Defects in Silicon

ABSTRACTPresent methods for minimizing stacking fault defects, generated during oxidation of silicon, include damaging the back of the wafer or depositing poly-silicon on the back. In either case a highly defective structure is created and mis is capable of gettering either self-interstitials or impurities which promote nucleation of stacking fault defects.A novel method of minimizing these defects is to form a patch of porous silicon on the back of the wafer by electrochemical etching. Annealing under inert gas prior to oxidation may then result in the necessary gettering. Experiments were carried out in which wafers were subjected to this treatment. Subsequent to oxidation, the wafers were etched to remove oxide and reveal defects. The regions of the wafer adjacent to the porous silicon patch were defect-free, whereas remote regions had defects. Deep level transient spectroscopy has been used to examine the gettering capability of porous silicon, and the paper discusses the mechanism by which the porous silicon getters.

Analysis of porous silicon silicon-on-insulator materials

Silicon-on-insulator materials are being used in the development of radiation hard VLSI electronic circuits and one promising route is the use of porous silicon. Porous silicon is produced by electrochemically etching selected regions of silicon wafers to produce volumes of highly reactive porous silicon which can be converted to dielectric oxide or conducting metal or suicide. Results are presented of accelerator-based nuclear reaction measurements of the behaviour of porous silicon under a range of conditions. Oxidation behaviour as a function of temperature and different annealing conditions is reported together with evidence of the pick-up and removal of carbon. Fluorine, which has been reported to improve radiation hardness, has been implanted into porous silicon and is found to be very mobile at 1090 ° C. Preliminary results are also reported on the distribution of chemical vapour deposited tungsten in masked porous silicon structures.

High Voltage Considerations for Silicon-on-Insulator Devices Using Porous Silicon

We describe a Silicon-on-Insulator (SOI) structure for high voltage BICMOS uniquely suited to the use of porous silicon (PS). In this SOI structure, bulk, high speed bipolar devices are readily integrated with CMOS high voltage and logic devices (smart power). To investigate the processing compatibility of PS with this structure, we measured breakdown strength and etch rate of thermally treated PS in 7:1 buffered oxide etch (BOE) and determined that they can approach values typical of thermal silicon oxides/nitrides.