Macroporous Silicon Research Articles

Fabrication and photovoltaic property of ordered macroporous silicon

Wafer-scale highly ordered macroporous silicon (macro-PSi) has been fabricated by the combination of lithography and metal-catalyzed Si etching in hydrofluoric acid. The periodicity and dimensions of the pores can be controlled by the lithography and the etching conditions. Significantly, this catalytic etching method is capable of creating uniform pores on Si substrates of any type of conductivity. Photovoltaic application results showed that the as-prepared macro-PSi samples are photoactive, and became more effective in solar energy conversion via surface modification with metal particles. The present fabrication method of macroporous Si structures could open potential applications in solar cells, photonic crystals, sensors, and batteries.

Composite Si/PS membrane pressure sensors with micro and macro-porous silicon

Porous Silicon (PS) is a versatile material with many unique features making it viable in the field of Microelectromechanical Systems (MEMS). In this paper, we discuss the optimization of formation parameters of micro and macro PS with different porosity and thickness for use in pressure sensors. The optimized material is used in the fabrication of composite Si/PS membranes in piezo-resistive pressure sensors and tested. Pressure sensors with composite membranes have higher sensitivity than those with single crystalline silicon membrane with the sensitivity increasing as the porosity increases. For the same porosity and thickness of the PS layer, Si/micro PS membranes exhibit higher sensitivity than Si/macro PS ones. The offset voltage in these sensors is found to be high and can be due to the stress induced in the membrane during PS formation. Offset voltage and stress values are found to be higher in composite membranes with micro PS as compared to macro PS.

Macroporous Silicon Oxycarbide Fibers with Luffa-like Superhydrophobic Shells

High-surface-area silicon oxycarbide macroporous fibers were fabricated through in situ cross-linking of a preceramic precursor without a prepatterned template. The unique luffa-like shell combined with intrinsic silicon-containing groups accounts for the resultant superhydrophobic property. Meanwhile, the oil-uptake capacity of the corresponding fiber mat is significantly improved by the capsulated nanoparticles.

A periodic array of silicon pillars fabricated by photoelectrochemical etching

A periodic array of silicon pillars was photoelectrochemically fabricated using the two-step etching process with a n-type Si (1 0 0) substrate. Two key factors, backside illumination and anodic bias, were required to obtain a high-aspect ratio macropore array of silicon. It was found that the initial pore could be separated into two different pores when the applied anodic bias was greater than a certain critical value. The pore size of the macroporous silicon with a high porosity was increased by anisotropic etching in an alkaline solution. Due to destruction of the pore sidewalls, KOH etching allowed for the fabrication of silicon pillars on a large-scale wafer with an improved uniformity. The anisotropic etching behavior of KOH solution led to necking of the silicon pillars when the etching time exceeded 60 s.

Conformal P‐N junctions in macroporous silicon for photovoltaic energy conversion

AbstractWe report on the fabrication of a macroporous silicon diode that successfully operates in a photovoltaic mode of energy conversion. Typical device structures are fabricated on 4″, Cz grown, <100>, p‐type, 5 – 30 Ω‐cm, Si substrates and make use of random or pre‐patterned macroporous silicon films up to 100 μm thick with pores ∼1 μm in diameter and a center‐to‐center pore spacing of 2.5 μm. Phosphorous dopants are introduced throughout the porous silicon region with a newly adapted technique based on proximity rapid thermal diffusion. This development effort has enhanced the uniformity of dopant diffusion into the pores and subsequently improved device performance. The resulting p‐n diode within the porous silicon is of a three‐dimensional nature and exhibits an extremely high internal surface area up to ∼6690 cm2/cm3. To our knowledge, this is the first use of such a technique to fabricate conformal, three‐dimensional p‐n junctions for photovoltaic applications. The large internal surface area of this diode makes it applicable to next generation photovoltaics. Moreover, the porous silicon within this device is not simply an antireflection coating; rather it is a light trapping medium that has the ability to become a large surface area diode and can serve as a host matrix for photoactive compounds. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Innovative lift‐off solar cell made of monocrystalline‐silicon thin film by annealing of ordered macropores

AbstractTo answer the challenge of obtaining thinner silicon solar cells, we developed a technique based on the transfer of a thin layer of high‐quality monocrystalline silicon to a foreign low‐cost substrate. The transfer is enabled by the reorganisation of macroporous silicon at high temperature: driven by the minimisation of their surface energy, pores close and, in specific situations, merge to form a large single void, a few micrometers under the wafer surface. As demonstrated in this paper, the micron‐thick film above it may be detached and used to process a solar cell. The difficulty of this approach lies in appropriately ordering the macropores, while its originality is to enable processing of a high‐quality material directly from porous silicon. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Functionalisation of silicon surfaces using tetrazine functionalities

AbstractThe functionalisation of semiconductor (particularly silicon) surfaces have been an active area of research in the past two decades. The surface modification of silicon has opened up new potential applications of this material. In the present work the macroporous silicon surfaces have been effectively functionalised using tetrazine derivatives. The work involved: i) preparation of cyclopentadienyl‐modified silicon surfaces; (ii) modification of the surface by cycloaddition of tetrazines with dimethyl pyrazolyl functionality; (iii) characteristion of functionalised silicon samples by electron microscopy, FT‐IR, Raman, and photoluminescence spectroscopy. The obtained composite materials might have potential application as sensors and components for molecular electronics. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Two‐ and three‐dimensional composite photonic crystals of macroporous silicon and lead sulfide semiconductor nanostructures

AbstractWe describe a new method for direct deposition and growth of nanocyrstalline lead sulfide (PbS) thin films into two‐ and three‐dimensional arrays of macroporous silicon and the formation of composite silicon photonic crystals (PhCs). Infrared transmission measurements exhibit a substantial red‐shift of the photonic stop bands from the composite PhCs relative to the PhC substrates. Photoluminescence spectroscopy reveals the effect of slow‐light modes on the emission spectra from PbS thin films inside a two‐dimensional PhC. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon Photonic Crystals Doped with Colloidally Synthesized Lead Salt Semiconductors Nanocrystals

The fabrication of two-dimensional and three-dimensional silicon photonic crystals doped with lead salt nanocrystals is reported. The silicon based photonic crystals of macro-porous silicon are fabricated by electro-chemical etching via masked silicon wafers with the periodicity along the third dimension is achieved by modulating the anodization current and voltage. The chemical solution deposition technique has been utilized to deposit thin layers of lead salts (PbS and PbSe) nanocrystals into the pores. Infrared transmission measurements revealed a considerable red-shift of the photonic band gap in a good agreement with numerical calculations.

Highly ordered hexagonally arranged sub‐200 nm diameter vertical cylindrical pores on p‐type Si using non‐lithographic pre‐patterning of the Si substrate

AbstractAnodically etched two‐dimensional (2‐D) arrays of highly ordered sub‐200 nm in diameter vertical cylindrical pores were fabricated on p‐type Si wafers, with a resistivity of 6–8 Ω cm, by non‐lithographic pre‐patterning of the silicon substrate through a self‐assembled porous anodic alumina (PAA) thin film, directly grown on the Si wafer. The PAA film was grown by electrochemical oxidation of a thin Al film in an oxalic acid aqueous solution electrolyte. Through the PAA pores, concave etch pits were formed on Si by chemical etching, that were then used as pore initiation sites for electrochemical macroporous silicon formation. The so formed vertical cylindrical pore arrays showed perfect hexagonal arrangement on the Si surface. A pore diameter down to 180 nm and a pore height up to ∼1 μm were achieved for the first time on p‐type Si. The developed technology is particularly interesting for photonic crystals and sensors applications. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of the Micro- and Nanostructured Silicon for Biosensing and Medical Applications

Emerging applications of porous silicon (PS) lies in its ability to incorporate other materials, such as organic groups, organic and inorganic nanoparticles to form (bio)hybrid systems where each individual constituent may be optimized for a particular function. This paper presents our recent experimental results on the fabrication and applications in biosensing of the porous silicon (PS) based microstructures. We have demonstrated that different morphologies of PS, either as-prepared or coated with gold nanoparticles have an important role in biomolecule detection, due to its large internal surface combined with specific electro-optical properties, being in the same time support for immobilization of complementary biomolecules as well as transducer for biochemical interactions. Therefore, we have investigated the photoluminescence properties of nanoporous Si prepared on different Si micropatterned surfaces comparatively with PS/flat Si in order to develop a new simple and versatile process for biosensor transducer fabrication. Meso- and macro-PS have been investigated for protein immobilization and detection using microarray technique or for DNA biomolecule detection by impedance spectroscopy. Finally, we have demonstrated that macroporous silicon constitutes an appropriate substrate for very sensitive SERS biosensors. RAMAN signal of 11-mercaptoundecanoic acid was investigated on Au/macroporous silicon. Various characterisation techniques have been used, optical and scanning electron microscopy (SEM) to investigate samples morphology, X-ray diffraction for nanoparticle structure, Raman and PL spectroscopy, and laser fluorescence detection for chemical and optical properties analysis and impedance spectroscopy for investigation organic molecule attachment on the Au/PS structures.

Micro-Patterning of Semiconductors by Metal-Assisted Chemical Etching through Self-Assembled Colloidal Spheres

Macroporous semiconductors such as Si and GaAs with ordered pore intervals were fabricated by the site-selective chemical etching of a substrate using patterned Pd-Pt thin film catalyst through self-assembled colloidal spheres as a mask. The obtained macroporous silicon using noble metal catalyst was conical because the pore wall of macropores was chemically dissolved presumably due to the diffusion of positive holes injected at the metal catalyst. In contrast, relatively straight pores with uniform diameter were grown when the concentration of HF was high. With increasing concentration of HF, the etching rate at the pore bottoms in a vertical direction to the Si surface increased, while the etching rate in a lateral direction was suppressed markedly to yield high aspect ratio of pores up to 10. The change in features of macroporous silicon was thought to be involved in the difference in diffusion behavior of injected positive holes at the silicon/metal interface.

Humidity Sensing of Ordered Macroporous Silicon With ${\rm HfO} _{2}$ Thin-Film Surface Coating

Porous silicon (PS), as a gas/chemical sensing material, has been widely investigated. In this paper, the humidity sensing characteristics of n-type macroporous silicon with ordered structure and metal oxide thin-film coating is studied. The ordered PS has uniform pore size, pore shape and distribution. All pores are aligned vertically and open to the environment. The PS heterostructure (PS/Si substrate) and self-supporting membrane are fabricated and their resistance responses are measured under room temperature. A resistance variation of 23.5% and 28.3% for each structure are obtained, respectively. Surface modification for sensing enhancement is also investigated. The resistance and capacitance responses of ordered PS heterostructure with HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin-film coating are characterized. The HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> modified PS show high sensing variation and near-linear response to a wide range of relative humidity (RH). It is also demonstrated that PS with HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin-film coating is able to sense the RH change faster than a commercial humidity sensor. To the best of our knowledge, this is the first time that humidity sensing with ordered porous silicon and HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin-film is reported. Possible sensing mechanisms and future work are also discussed.

Macroporous silicon based simple and efficient trapping platform for electrical detection of Salmonella typhimurium pathogens

A thermally oxidized macroporous silicon substrate with simple electrode structure without interdigitated electrode configuration has been reported for the detection of Salmonella typhimurium pathogens by electrical impedance measurement using antibody–antigen binding method. Macroporous silicon which has been fabricated by anodizing silicon in HF and DMF solution is a regular network of pores of 1–2 μm diameters. This has been thermally oxidized to yield the surface hydrophilic for antibody immobilization as well as provide suitable electrical insulation of the metal contacts from the underlying conducting silicon substrate. The macroporous silicon surface has been immobilized by Salmonella specific antibody and has been tested with different concentration of S. typhimurium pathogens in phosphate buffer solution (PBS). It has been found that such macroporous silicon substrates is capable of detecting down to 10 3 CFU/ml in pure culture using a 3 mm × 1 mm electrode structure with a wide spacing of 1 mm. The selectivity of the macroporous silicon substrates with reference to S. typhimurium has been tested to be satisfactory by carrying out controlled experiments with Escherichia coli O157:H7.

Macroporous silicon templated from silicon nanocrystallite and functionalized Si [sbnd]H reactive group for grafting organic monolayer

This paper reports the development of a new fabrication process for highly porous and highly functional macroporous silicon (m-PSi). This new fabrication process involves two steps of electrochemical etching and one step of sonication detachment, and it uses silicon nanocrystallites as a template to form a honeycomb-like macroporous structure. The surface fabricated by this process has been characterized in comparison with the m-PSi surface fabricated by a one-step etching process. Scanning electron microscopy (SEM) images show that both m-PSi surfaces have nearly similar pore diameters (1–2 μm), but their porous microstructures are very different: the surface fabricated by two-step etching exhibits a smooth and shallow pore structure, while the other surface exhibits a rough and deep pore structure. Fourier transform infrared spectroscopy (FTIR) analyses reveal that the former is functionalized with a reactive Si H group, while the latter is functionalized with a stable Si O Si group. To evaluate the Si H reactive group, an allyl polyethylene glycol (PEG) is employed to modify the surface through hydrosilylation. SEM, FTIR, X-ray photoelectron spectroscopy, and water contact angle measurements are used to characterize the PEG-grafted m-PSi surface. PEG-grafted m-PSi substrates may have wide applications in biosensors, chemosensors, and biochips.

Highly Proton‐Conducting Self‐Humidifying Microchannels Generated by Copolymer Brushes on a Scaffold

Filling in the gaps: Macroporous silicon membranes modified with sulfonated polymer brushes have been synthesized by pore-filling surface polymerization (see picture) to give proton-conducting channels with tailor-made, finely tuned physicochemical characteristics. These membranes display high conductivity values (ca. 10(-2) S cm(-1)) regardless of the humidity, thus surpassing the performance of nafion.

Macroporous silicon hydrogen diffusion layers for micro-fuel cells

In this study, we have demonstrated the possibility of using macroporous silicon as a hydrogen diffusion layer in integrated micro fuel cells. Macroporous silicon was used to increase an exchange surface between hydrogen and active layers of micro fuel cells. Moreover, electrochemical etching is an interesting alternative to costly Deep Reactive Ion Etching techniques. A maximum density power of 250 mW/cm2 was demonstrated using conventional proton exchange membrane micro fuel cell. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Porous silicon for the development of capacitive microstructures

Macroporous silicon is developed in p-type substrates in order to extend the active surface area of the electrodes in a silicon integrated capacitive element. Some laboratory prototypes with a 3D architecture and Si/SiO 2/PolySi layers have been developed. The contribution of the surface enlargement to the final capacitance is analyzed by comparing it with a 2D planar reference capacitive device. By this method, a capacitance gain of 2400% has been obtained at low frequencies with a capacitance density of 180 nF/cm 2. Based on the physical microstructure, an equivalent electrical circuit of the porous capacitive element is proposed and its electrical behavior discussed.

Photo-electrochemical etching of macro-pores in silicon with grooves as etch seeds

Role of linear nucleation centres (surface grooves) in formation of macro-pores has been studied for n-type Si. It has been found that linear nucleation centres, contrary to the case of point nucleation pits, initiate formation of semi-regular pore pattern: along the grooves the pores are arranged randomly as a result of self organization process, across the grooves they follow the prescribed period a. The average distance between the pores along a groove line depends on a, and decreases as a increases. At the optimal value of etching current density and seed period a, the distinct pores can merge, thus forming a periodic structure of one-dimensional photonic crystal. Optical properties of grooved structures with merged pores have been investigated by means of Fourier infrared reflection micro-spectroscopy in the spectral range from 1.45 μm to 16.5 μm. Reflectance spectra reveal presence of photonic band gaps which positions in the middle IR range are in a good agreement with calculations by transfer matrix method. The fabrication approach developed is promising for application in silicon-based photonics.

Effect of noble metal catalyst species on the morphology of macroporous silicon formed by metal-assisted chemical etching

Macroporous silicon with ordered pore intervals was fabricated by the site-selective chemical etching of a Si substrate using patterned noble-metal thin films as a catalyst. The morphology of the etched silicon surface and the etching rate was affected by the shape of deposits and metal catalyst species such as Pt–Pd, Au, and Pt. The etching rate increased in the following order: Au < Pt ≤ Pt–Pd. The pores of macroporous silicon prepared by using Pt–Pd catalyst were conical in shape because of the chemical dissolution of the surface of the macropores. On the other hand, by using Au catalyst, relatively straight pores with uniform diameter were formed in the direction of pore depth. The morphology of macroporous silicon was assumed to be affected by the difference in the shape of metal catalysts and the diffusion behaviour of injected positive holes at the silicon/metal interface.