Handbook of Porous Silicon

Porous silicon has been fabricated by both “top-down” techniques from solid silicon and “bottom-up” routes from silicon atoms and silicon-based molecules. Over the last 50 years, electrochemical etching has been the most investigated approach for chip-based applications and has been utilized to create highly directional mesoporosity and macroporosity. Chemical conversion of porous or solid silica is now receiving increasing attention for applications that require inexpensive mesoporous silicon in powder form. Very few techniques are currently available for creating wholly microporous silicon with pore size below 2 nm. This review summarizes, from a chronological perspective, how more than 30 fabrication routes have now been developed to create different types of porous silicon.

The key aspects of porous silicon manufactured by anodization are reviewed, with the following subjects being addressed: anodization of different wafer types, wafer cell design, post-anodization handling requirements (rinsing/drying/storage), parameters affecting layer uniformity, the use of nonaqueous electrolytes and electrolyte additives (surfactants, oxidizers, and other types), methods for tuning porosity, process control and natural variability, different electrode materials, and the requirements for maintaining health and safety.

The different classes of electrochemical etching of silicon are briefly compared and contrasted, and then the literature on galvanic etching is comprehensively reviewed. Thick uniform mesoporous films with surface areas as high as 910 m2/g have been achieved with optimized galvanic etching (Fig. 1).

Spontaneous electroless etching of silicon surfaces with hydrofluoric acid and chemical oxidant-based solutions is often referred to as “stain etching” due to the color change it imparts. The field is comprehensively reviewed with regard to the etching mechanisms and the range of chemical oxidants explored to date.

Essential aspects in the fabrication of porous silicon and nanostructures by metal-assisted particle etching are presented. Basic processes using 1-step or 2-step method are described as well as mechanism of metal-assisted chemical etching. Influence of various parameters such as nature of metal, temperature, etching solution composition, intrinsic properties of silicon substrate on the morphology of porous silicon, or nanostructures is discussed. Applications of silicon nanostructures obtained by metal-assisted etching are briefly introduced, showing the promising potential of this etching method whose main properties are simplicity, low cost, easy process control, reproducibility, and reliability for fabrication of silicon nanostructures including silicon nanowires.

The literature on the photoetching technique of preparing photoluminescent mesoporous silicon films using both hydrofluoric acid-based and alkali electrolytes is reviewed. The benefits of using an incoherent light source and specific oxidizing agents are highlighted. The technique is particularly useful for creating thin porous regions in n-type Si wafers, SOI wafers, micromachined wafers, or those that contain electronic circuitry.

The formation of porous silicon (PS) via a HNO₃/HF vapor-etching (VE)-based technique is described. VE of silicon was found to lead to the growth of PS layers (PSLs) and/or a thick white layer identified as the (NH₄)₂SiF₆ compound. Which phase dominates depends on the HNO₃/HF volume ratio. High-resolution SEM observations show that p- and p+-type PSLs are composed of dot-like crystalline Si particles embedded in an amorphous phase, having sizes not exceeding 5 nm. Almost pure (NH₄)₂SiF₆ may be obtained in a wide range of thicknesses for a HNO₃/HF volume ratio exceeding 1/4. The (NH₄)₂SiF₆ is highly soluble in water, enabling fabrication of grooves in Si wafers. Control over the growth of the (NH₄)₂SiF₆ is demonstrated to realize groove patterns, which could be used in a variety of applications.

The direct creation of highly porous silicon by chemical conversion of highly porous silica is receiving increasing attention for application areas that require relatively inexpensive material. Silica can be reduced to silicon at moderate temperatures with magnesium vapor which is the most popular route. The range of different silica feedstocks utilized to date is surveyed, as is the morphology of the porous silicon structures achieved. Key technical challenges are then highlighted and alternative Reduction chemistriesreduction chemistries mentioned.

In this chapter a mechanical method of porous silicon formation is described. By applying high-energy ball milling to polycrystalline silicon powder or single crystalline silicon wafers, highly dispersed and nanocrystalline silicon powders are produced. Pressing and sintering then lead to a porous matrix. The macroporous structures made in this way can then be permeated by meso- and micropores. The sinters have isotropic character of the pore distribution and morphology; this method is not limited by the wafer dimensions, and it is possible to make large-scale porous bodies, which is an advantage in comparison to lithographic methods.

The electrochemical formation of macropores in porous silicon is briefly reviewed. Various morphologies are obtained as a function of the substrate type and etching conditions. On n-Si, macropores are generally growing along preferential crystallographic directions. On p-Si, in aqueous conditions far from electropolishing, the growth direction is rather determined by the current lines in the space-charge region. A summary of macropore characteristics is given as a function of the preparation conditions. Various models have been developed in order to account for the morphologies and characteristic sizes. These joint experimental and theoretical works have provided a good understanding of macropore growth, opening the way to many applications, and the most significant ones are mentioned. An impressive level of control has eventually been achieved for the fabrication of regular macropore arrays of high aspect ratio, including the incorporation of intentional defects or pore-wall shaping.

Many physical properties of mesoporous silicon are intimately related to details of the structural organization of its silicon skeleton. This information can hardly be accessed directly but can be deduced by studying the complementary pore space surrounding this skeleton. By profiting from the potentials of nuclear magnetic resonance cryoporometry to probe fine details of the pore structure in mesoporous silicon, the results of systematic studies of the correlations between the fabrication conditions by electrochemical etching and the resulting mesostructure are presented. A deeper insight into advanced fabrication methods aimed at structure improvement and microstructure design is provided.

Virtually all porous silicon structures studied to date via electrochemical etching are mesoporous or macroporous, rather than microporous. Very high surface areahigh surface area (540–840 m2/g) porous silicon structures with high micropore content have now been demonstrated by wafer anodization in very concentrated hydrofluoric acidconcentrated hydrofluoric acid, metal-assisted stain etching, galvanic etchinggalvanic etching, and silica reductionsilica reduction. The smallest electrochemically generated pores to date are probably “supermicropores” lying in the range of 1–2 nm diameter since they do not exhibit molecular sieving over the 0.4–0.85 nm molecule size range. Highly microporous silicon could be advantageous for a number of specific applications such as hydrogen storage, gettering, explosives, and gas sensing, but the low chemical stability of the material could pose challenges with its industrial exploitation.

The porosity (void fraction per unit volume) of silicon has been varied from less than 1 % to as high as 97 % using electrochemical etching of solid silicon with supercritical drying or silica aerogel reduction with supercritical drying. Fifteen techniques are identified for quantifying porosity in specific physical forms, and a conversion table is provided between porosity, pore volume (void content per unit weight), and bulk density in air. Finally ten applications are given which exploit medium to high (25–95 %) porosity, or the ability to vary porosity significantly within a given structure.

Investigations of the structure and morphology of ultrathin PS films are reviewed, of relevance to the technological control of miniaturized PS-based devices. Several characterization tools with high reliability and precision are available; however, many of them are destructive or could affect the ultrathin PS structure. Grazing incidence X-ray reflectivity (XRR) is a powerful tool to probe the structural and morphological characteristics of ultrathin PS films. Homogeneity, thickness, surface and interface roughness, porosity, and density of ultrathin PS films were accurately determined using XRR. Nonetheless, prior to XRR measurements, ultrathin PS films should be submitted to complementary nondestructive morphological and optical examinations (thickness, roughness, oxidation, etc.).

Electrochemical etching of silicon can generate porous silicon where porosity is modulated with depth. The overall fabrication technique, experimental tips for improving uniformity, typical porosity profiles, and methods of patterning and stabilization are reviewed. Due to its ease of fabrication, such multilayers have been extensively fabricated and applied in different fields, such as photonics, phononics, sensing etc.

Porosified silicon membranes of defined thicknesses were first studied in the 1990s and have now been realized by electrochemical anodization, micromachining techniques, and the annealing of ultrathin deposited films. The three fabrication routes produce very different morphologies and levels of porosity. A variety of applications have been explored for both macroporous and mesoporous membranes and these are also surveyed. Wholly microporous membranes in silicon, where all pores have diameters less than 2 nm, have not been achieved to date.

Metal-assisted chemical etching (MACE) of silicon is receiving much interest as a controllable method of generating silicon nanostructures of varied forms, including porous silicon. The various morphologies, etch chemistry variables (e.g., metal catalysts, substrate type, electrolyte temperature), and potential applications of the resulting nanostructures are reviewed.

A variety of PSi-polymer composites have been developed, from the perspectives of different polymers, different composite morphologies, and different targeted applications.The field is comprehensively reviewed, focusing on the different design and synthesis strategies, together with a brief discussion of the emerging fields of application.

Data and literature are collated that emphasize the high tunability of porous silicon properties, either via manipulation of its structural parameters, via the chemistry of the large internal surface area, or via impregnation of other materials. An overview of quantitative data on more than 30 properties is tabulated and compared to those of nonporous silicon. Where available, the range of values reported to date is given. The properties showing the widest tunability to date include the visible photoluminescence (optical bandgap), mechanical stiffness, thermal conductivity, optical refractive index, electrical resistivity, biodegradability kinetics, optical reflectivity, and surface wettability.

Mesoporous silicon has tunable thermal properties that can be radically different from those of nonporous silicon. Published data on the major thermal constants and the techniques of measurement are reviewed and values compared to those of other materials. The combination of very low thermal conductivity and heat capacity of highly porous silicon has led to a number of specific applications that exploit these thermal properties.

Introducing nanoscale porosity into silicon dramatically lowers its stiffness and hardness in a tunable manner over a wide range. Available data is collated on Young’s modulus and Vickers hardness as a function of porosity and layer morphology. There is little quantitative data on fracture toughness and strength, but theoretical work predicts that optimized nanocomposites could be very mechanically durable. The exceptional plasticity recorded for individual silicon nanowires is yet to be demonstrated in mesoporous silicon. A number of application areas are highlighted that rely heavily on the mechanical properties of porous silicon.

In many applications exploiting mesoporous silicon (PSi), their performance may be controlled by the rate of diffusive propagation of the confined molecules. The pulsed field gradient technique of nuclear magnetic resonance provides the most direct access to molecular diffusion. The different factors determining the diffusivities in PSi are reviewed. In particular, diffusivities in liquid state are shown to be most strongly affected by mesoscale disorder. Atomistic disorder is shown to control surface diffusion in applications in which PSi is brought into contact with gas phases at low vapor pressures. Correlations between the compositions of phases coexisting within the pore space, namely, liquid and gaseous, and liquid and solid ones, respectively, are briefly discussed.

The optical properties of porous silicon layer described as a mixture of air, silicon, and silicon dioxide are determined by the thickness, porosity, refractive index, and the shape and size of pores. Refractive index of porous silicon is reviewed in this chapter. Full theoretical solutions can be provided by different effective medium approximation methods such as Maxwell–Garnett’s, Looyenga’s, or Bruggeman’s. In the case of semiempirical approaches, the refractive indices are measured using spectroscopic ellipsometry, and then, the model parameters, such as the layer thickness and calculated effective dielectric function, are adjusted to fit the spectra. Various methods based on optical transmission and reflection measurements are used to calculate the refractive index data for both fresh and oxidized porous silicon using the envelope, Maxwell–Garnett, and Goodman method. The coherent transfer matrix technique and Heavens theory were used to determine the complex refractive index of porous silicon for the reflectance of an absorbing layer on an absorbing substrate. An ab initio quantum mechanical study of the effects of oxidation process in porous silicon using an interconnected supercell structure and its complex refractive index was compared with experimental data obtained from spectroscopic ellipsometry. The spectroscopic ellipsometry evaluation of a porous silicon multilayer was reported for the preparation of porous silicon multilayer stacks. The effects of oxidation on the Bragg reflector parameters and the variations in the refractive index and thickness after oxidation were reported.

Electrochemically etched porous silicon can exhibit pronounced optical anisotropy even though bulk silicon is basically optically isotropic. The origin of this effect, the most significant parameters, and potential applications in sensing and micro-optic devices are reviewed.

The visual color of a material is often not important for many applications but can be crucial for those that involve consumer acceptance and branded products. Solid silicon is gray, but porous silicon can have varied colors depending on its physical form and pore contents. Silicon chip-based layers can exhibit vivid colors, tunable across the visible spectrum through their lowered refractive index and optical interference with the underlying bulk silicon. Highly columnar morphologies, referred to as “black silicon,” include highly porous forms. Even white silicon is possible via photonic crystals. Polydisperse mesoporous silicon microparticle powders are typically dark brown through light tan, depending on bandgap widening, particle size, and the level of oxidation, which is useful for matching skin tone in cosmetic products, but disadvantageous with various foodstuffs, beverages, and oral care products. The color of such powders can be better tuned chemically by the impregnation of specific food nutrients that themselves have vivid colors. Some such natural pigments can themselves benefit with improved fading resistance as a result of UV protection via oxidized porous silicon impregnation.

The future development of porous silicon (PS)-based optoelectronic devices depends on a proper understanding of electrical transport properties of the PS material. Electrical transport in PS is influenced not only by each step of processing and fabrication methods but also by the properties of the initial base substrate. This chapter endeavors to chronologically document how the knowledge base on the nature of carrier transport in PS and the factors governing the electrical properties has evolved over the past years. The topics covered include the proposed electrical transport models including those based on effective medium theories, studies on contacts, studies on physical factors influencing electrical transport, anisotropy in electrical transport, and attempts to classify the PS material.

After a brief introduction to diamagnetism, the magnetic properties of silicon are briefly outlined. The magnetic behavior of silicon consists of a diamagnetic and a paramagnetic term, whereas the diamagnetism predominates. Furthermore, various types of porous silicon like as-etched and oxidized porous silicon are discussed, and the dependence of the diamagnetism on the surface treatment and thus on the paramagnetic defects is outlined. Nanostructuring of silicon results in a modification of the magnetic behavior with reduced diamagnetic contribution, and a further posttreatment of the samples leads to a smaller diamagnetic susceptibility.

This chapter reviews first the intrinsic magnetic behavior of empty porous silicon; next the filling of the pores with ferromagnetic metals (Ni, Co, NiCo, Fe) by electrodeposition; and finally the adjustability of the magnetic properties of the nanocomposite due to the modification of the porous silicon, as well as due to a variation of the geometric characteristics of the metal deposits. The magnetic characteristics of these hybrid materials strongly depend on the pore structure of the template as well as the geometry and the spatial distribution of the embedded metal structures. Besides the shape and size of the nanostructures, a key factor is the magnetic interaction between metal structures which determines the properties of the system. The ferromagnetic behavior of the nanocomposites, which is also present at room temperature, opens the opportunity for new applications.

In this chapter the paramagnetic properties of nanostructured silicon are outlined and furthermore the magnetic properties of a composite material consisting of porous silicon with infiltrated superparamagnetic iron oxide nanoparticles are discussed. The magnetic behavior of the system depends on the nanoparticle size as well as on the magnetic coupling between them. Both influence the so-called blocking temperature, the transition between superparamagnetic behavior and blocked state. A particle size-related assessment shows that the blocking temperature increases with increasing particle size if the distances between the particles are equal. The blocking temperature can be decreased by weakening the magnetic interaction between the particles. Due to the good biocompatibility of both porous silicon and iron oxide nanoparticles, the composite system is of interest for biomedical applications in the fields of therapy and diagnosis.

The photoluminescence of mesoporous silicon and silicon nanocrystals has received enormous study over the last 25 years. The spectroscopic nature and efficiency of various emission bands from the near-infrared to the ultraviolet are briefly reviewed, as are mechanistic studies on individual nanocrystals. Improvements in surface passivation and size control of silicon nanocrystals have led to impressive photoluminescence quantum efficiencies in the visible range.

The performance of porous silicon visible light-emitting diodes is reviewed and compared to those of silicon nanocrystals prepared by other fabrication routes. Efficiencies up to 1 % have been achieved with porous silicon but almost up to 10 % with silicon nanocrystal-polymer composites. Challenges with simultaneously achieving high efficiency, output power, stability, and modulation speed are highlighted. Further improvements could be realized from narrowing the skeleton size distribution and improving the electronic transport and surface passivation of porous silicon.

The characteristics of electronics states in porous silicon (PS) from measurements of thermoluminescenceThermoluminescence (TL) are presented. The observed shape of the TL peaks at low temperatures (4–250 K) is explained by quasi-continuous spectrum of the electron traps with activation energy in range of 0.03–0.4 eV. The high-energy peaks observed at 100–300 °C are associated with radiation-induced defects E′ (≡ Si •) and nonbridging oxygen hole centers (≡ Si − O •) that generated in insulating SiO_x layer which covers the PS surface. Currently, the TL of PS is not exploited as a radiation dosimeter, due to the low activation energies of the traps and strong fading. Nevertheless, the observations of high temperature peaks of TL in oxidized PS, its biocompatibility and other properties, suggest a potential use of this material for in vivo dosimetry. An additional application could be the use of PS as a template for more established scintillation materials.

Realization of a silicon-based laser, integrated with optical and electronic components on a single chip, remains one of the main challenges in optoelectronic technology. Achievement of optical amplification in silicon nanocrystals is a complex problem depending on many factors; however, it is theoretically possible. The chapter discusses the potential of luminescent porous silicon for lasing and lists observations of positive optical gain in various types of silicon nanocrystals. Several different approaches lead to observation of small positive optical gain in porous silicon-based samples; however, a real injection silicon-based laser has not yet been demonstrated.

The chemical reactions of porous Si, involving formation of Si-O, Si-C, Si-N, or Si-metal surface bonds, is reviewed. The reactivity of as-formed porous Si is dominated by the chemistries of silicon-hydrogen (Si-H) and silicon-silicon (Si-Si) bonds, which are strong reducing agents. Depending on the oxidant, various surface species can be generated in oxidation-reduction reactions of porous Si: in particular metal nanoparticles, silicon oxides, or silicon-carbon species. The oxidation chemistry of porous Si, involving air, water, chemical oxidants, or electrochemical oxidation is discussed. The aqueous stability of these various silicon oxides is quite dependent on the means by which a particular oxide is formed. Si-C bond forming reactions including hydrosilylation, hydrocarbonization, carbonization, and reductive electrochemical grafting, and the chemical method used to confirm Si-C bond formation are presented. Because much interest in the chemistry of porous Si is focused on the generation of functional nanostructures to graft molecules such as drugs, proteins, targeting agents, or biological receptor molecules to porous Si surfaces, the review emphasizes the covalent chemistry of Si-O and Si-C surface species for the attachment of functional species (particularly biomolecules) to porous Si.

The biocompatibility of porous siliconPorous silicon is critical to its potential biomedical uses, both in vivo within the human body for therapy and diagnostics, and in vitro for biosensing and biofiltration. Published data from cell culture and in vivo studies are reviewed, and a number of emerging applications for bioactive or biodegradable silicon are discussed.

In the biomaterials field there is an increasing interest in medically biodegradable materials. The medical biodegradability of mesoporous silicon is now established both in vitro and in vivo. The review highlights the techniques used to date to characterize this phenomenon, the degradation kinetics, and the various factors that can influence the kinetics of dissolution into orthosilicic acid.

Mesoporous silicon is a complex nanostructure whose optoelectronic properties and morphology have received intense study over the last 25 years. Its properties often depend on both its skeleton size distribution and the chemical nature of its high internal surface area. This review collates some of the lessons learned with regard characterization, highlighting potential issues that need to be considered and artifacts that can arise. These have in the past both complicated data interpretation and even caused problems in reproducing published data.

The use of microscopy in the structural characterization of mesoporous silicon has been widespread with the most popular techniques being several electron microscopies, Raman, and atomic force microscopy. The general field is reviewed including other techniques such as luminescence and multiphoton microscopy to study the optoelectronic properties, and acoustic microscopy to study the mechanical properties.

X-ray diffraction (XRD) is a useful, complementary tool in the structural characterization of porous silicon (pSi), providing information not readily available from direct visualization techniques such as electron microscopies. This review outlines key considerations in the use of diffraction techniques for analyses of this material in both thin film form and freestanding porous Si nano or microparticles. Examples of the range of content in the analysis of pSi are provided, including formation mechanisms, layer thickness, extent of pSi oxidation, and degree of crystallinity. Such properties influence practical properties of pSi such as its biodegradability. We also focus on selected key properties where XRD has been particularly informative: (a) strain, (b) the structural analysis of pSi multilayers, and (c) an analysis of pSi loaded with small molecules of fundamental or therapeutic interest.

Pore volume and surface area of porous silicon are key parameters to consider when developing applications that rely on the capacity to carry a payload, such as drug delivery, or that are dependent on the degree of “reactivity,” such as sensing or energetics. The ability to define and tune surface areas and pore size distributions is a necessity for clinical use of the material. Herein, the historical assessment of these physical parameters by gas adsorption is reviewed, the methodology behind the measurements is described, the limitations are highlighted, and data related to its use in determining the effects associated with different anodization parameters and post-anodization processing is presented.

Nuclear magnetic resonance (NMR) cryoporometry is an experimental technique of structural characterization of mesoporous materials. In this contribution, different aspects of its application to study details of the pore structure in mesoporous silicon are presented. In particular, the information obtained with help of NMR cryoporometry is compared to that assessed using more conventional gas sorption techniques. The potentials of NMR cryoporometry to reveal fine details of the pore structure in intentionally multilayered mesoporous silicon are demonstrated.

Thermal analysisThermal analysis and calorimetry Calorimetryoffer versatile possibilities to study different physical and chemical changes in materials. Calorimetry gives accurate and valuable information on phase transitionsPhase transitions of nanostructured materials and is therefore very useful in studies of mesoporous materials. The use of calorimetry in porous silicon research focusing on drug delivery applications and pore morphologyPore morphology determination is reviewed.

Characterization methods for magnetic materials, especially nanostructured ones such as mesoporous silicon, are reviewed. Besides magnetometers, which are one of the most important instruments to investigate magnetic properties, magnetic force microscopy and magneto-optical microscopy are briefly outlined. Magnetometers measure in an integrative way over the entire sample, whereas magnetic force microscopy and magneto-optical methods probe the magnetic properties of a local region or an individual nanostructure. With magnetometers in general, field- and temperature-dependent measurements can be performed; magneto-optical microscopy can be used to get knowledge about the domain structure, and with magnetic force microscopy, the magnetization reversal of, e.g., a nanowire, can be studied.

We provide a literature survey of a number of classical techniques used to quantify the chemical composition of porous silicon, highlighting their general merits and potential limitations with the material. Much of the early literature was focused on photoluminescent material, but increasingly there are studies on nanocomposites where chemical composition analysis is required to assess the degree and uniformity of impregnation or surface attachment.

The surface of electrochemically etched porous silicon is passivated with hydrogen just after preparation. The surface is gradually oxidized under ambient atmosphere, and the rate depends upon the ambient condition. The chemical and physical changes affect the properties of porous silicon-based devices. Proper understanding of the surface is important, and infrared (IR) spectroscopy is an effective and easy tool for monitoring and/or characterizing the surface state. Silicon is almost transparent to IR light, and hence the convenient transmission measurement is applicable to films and membranes of porous silicon. The measurement technique is first described, and then assignments of absorption bands in the spectra are given for the hydrogen-terminated and oxidized surface. The prevention of oxidation and the functionalization of porous silicon surface are important for many practical uses, where IR measurements can be used to monitor the surface. In addition, methods other than the transmission mode are briefly introduced.

Cell cultureCell culture is a powerful in vitro characterization technique to optimize the properties of a biomaterial for in vivo biomedical use by conversely revealing potential sources of cytotoxicity. A comprehensive literature survey of the range of cell types cultured on porous silicon is given, together with a discussion of how surface chemistry, topography, and porosity gradients affect cell behavior.

This chapter summarizes the main theoretical approaches to model the porous silicon electronic band structure, comparing effective mass theory, semiempirical, and first-principles methods. In order to model its complex porous morphology, supercell, nanowire, and nanocrystal approaches are widely used. In particular, calculations of strain, doping, and surface chemistry effects on the band structure are discussed. Finally, the combined use of ab initio and tight-binding approaches to predict the band structure and properties of electronic devices based on porous silicon is put forward.

Besides the well-known effect of photoluminescence, the impinging of photons and other kinds of particles such as electrons, ions, and muons on porous silicon produces important effects. Some of these effects can modify the structure and properties of the material, distorting the interpretation of data based on the use of irradiation. Some of the irradiation effects are useful in different applications such as photodynamic therapy or display applications. This work is a review of the effects of irradiation on porous silicon.

Porosified silicon wafers or powders from electrochemical etching often require a series of additional processing steps, prior to use. These can include patterning, drying, comminution, manipulation of surface chemistry, sintering, impregnation, or coating, in order to achieve the desired substrates, membranes, microparticles, or nanoparticles. Typical process flows are illustrated, giving examples of where the choice and order of processing steps can be important. Examples are also given of processing topics yet to be widely explored with porous silicon, but which could become important for some of the emerging applications.

Methods to transfer a pattern into porous silicon using light are reviewed. These methods can be applied before, during, or after the anodization processes. The advantages and disadvantages of each method are noted and technical performance compared using the aspect ratio of the pattern transferred into the porous silicon as a key metric. Based on this comparison, it is possible to group the various methods in a manner that allows specific applications to use the most appropriate patterning method.

This chapter is a visual guide to the numerous approaches to nanolithography nanofabrication on large area based on supramolecular self-assembly. A short history of this recent scientific and technological field, an outline of the most-cited methods of self-assembly, and tables reporting different nanofabrication methods are reported. Indications on requirements, advantages, and drawbacks of the various approaches are also listed in the table. Thanks to the recently developed metal-assisted catalytic etching (MACE), the colloidal patterns can be easily propagated to silicon and other semiconductors opening a wide field of morphology, nanostructures, and applications.

This chapter describes the capabilities of the direct imprinting of porous substrates (DIPS) technique for patterning and modifying the physical properties of porous silicon films. DIPS can achieve very high-resolution two-dimensional and three-dimensional patterning with feature sizes below 100 nm while eliminating the need for intermediate masking materials and etch recipes that complicate and increase the expense of other patterning techniques. The DIPS process utilizes a reusable master stamp to imprint the desired pattern into porous silicon by directly applying the stamp to the porous substrate with a pressure on the order of 100 MPa. This process is performed in a matter of seconds at room temperature. In addition to enabling the fabrication of patterned porous silicon structures, DIPS also enables morphological control over material properties including porosity, pore size, and refractive index. Examples of designs fabricated by DIPS include grating-coupled wave guides, free-standing particles, and curvilinear structures such as lenses.

Wet-etched mesoporous silicon is normally dried in air, but this limits the range of porosities and surface areas achievable, due to capillary force-induced collapse of the silicon skeleton. The various alternative drying techniques are reviewed with particular attention paid to supercritical drying, a powerful technique applicable to all physical forms of porous silicon.

Homoepitaxy on porous Si aims at producing monocrystalline thin silicon-on-insulator wafers or monocrystalline thin Si solar cells. There are two methods of preparation of the porous Si layer for homoepitaxy: on the one hand, the porous Si is reorganized at elevated temperatures to close the surface as a seed layer for epitaxy, and on the other hand, the reorganization of the porous Si is avoided to keep the open pore structure. For homoepitaxy on the porous Si layers, most research has been reported on the usage of atmospheric pressure chemical vapor deposition (APCVD). The quality of epitaxially grown Si layers, using different deposition techniques and various types of porous silicon, was assessed by etch pit density, minority carrier lifetime, Hall mobility, microscopy, and device performance. It can be concluded that APCVD is in combination with a closed porous Si surface layer as a seed layer, the most promising approach to produce high-quality epitaxial layers.

The literature on epitaxial growth of different materials on porous silicon substrate has been surveyed. This field was stimulated by the theoretical prediction in 1986 of stress field and hence defect reduction in lattice-mismatched film grown on a mesoporous substrate. Data now exists not only on Ge, SiC, and diamond films but also on a range of both III–V and II–VI semiconductors, as well as other crystalline materials. Recently there has been most interest in GaN growth on porous silicon for optoelectronic applications.

Thermal oxidation is a common technique to manipulate macropore shape and realize a variety of novel structures in silicon. The stress and deformation induced by such oxidation, kinetics of oxide growth, and its anisotropy are reviewed. Uniform arrays of both silicon and silica microstructures such as needles and tubules have been realized via thermal oxidation.

High surface area mesoporous silicon is prone to sintering, which can be an issue for some applications and be exploited for others. The mechanisms of silicon sintering, conditions that promote it, methods of characterizing, effects on properties, and potential uses are reviewed.

Conductive polymer nanostructures synthesized using porous silicon (PSi) templates are described, with an emphasis on PSi template advantages, pore-filling phenomenon, mechanism of polymerization, and selective removal of PSi to release the polymeric structures. The interaction of pyrrole monomers, as a case study, on the entire surface of PSi under both galvanostatic and potentiostatic deposition modes is presented with discussion on the processing issues associated with the electrochemical deposition process inside the pores. Additionally, various materials infiltrated into PSi templates are briefly described. Examples of free-standing conductive polymer structures formed by selective dissolution of PSi are provided.

Loading porous silicon by bringing it into contact with a molten substance facilitates complete pore filling through capillary action, or melt intrusion. If the intrusion is carried out at elevated temperature, the molten active can return to a solid form at room temperature, remaining in the pores. Reviewed herein are the methodologies used for loading freestanding porous silicon flakes and powders.

Porous silicon is a promising template for the preparation of metal nanostructures by electrochemical deposition. Because porous silicon is a semiconductive porous electrode, electrochemical deposition of metals occurs not only at the bottom of pores but also on the pore wall and pore openings. Thus, the control of electrochemical deposition within porous silicon has been a challenging issue. Electrochemical deposition on porous silicon is influenced by illumination conditions. Metal deposition on porous silicon is possible by displacement deposition. Many studies have reported on electrochemical deposition of metal for practical applications. In this chapter, electrodeposition under polarization is firstly reviewed. Secondly, displacement deposition on porous silicon is explained. Finally, the microscopic structure formation by electrodeposition on porous silicon is summarized.

There is now both experimental and theoretical data relating to conductivity changes in porous silicon and other silicon nanostructures arising from the adsorption of specific molecules. The phenomenon is reviewed with emphasis on the potential mechanisms involved and its exploitation with regard sensing applications.

This chapter reports on the variety of functional coatings for porous silicon structures, and reviews the methods developed for their respective deposition, the spectroscopic and analytical techniques used for their characterization, and the functionalities imparted to porous silicon related with their specific domains of application.

Electroencapsulation is an application of electrospraying in which liquid is atomized into micro- or even nanosized droplets using electrostatic forces alone. The method allows better controllability of the capsulation process than, e.g., the most commonly used spray drying. Due to the applied electrostatic forces, it also enables production of complex capsule structures, like solid shell covered liquid core particles. In this review, we will focus on electroencapsulation processes used to improve the handling, processing, and administration of porous silicon-based drug delivery systems.

Photoluminescent porous silicon nanoparticles have many potential medical uses if their properties can be optimized. The techniques for both fabricating such particles and their surface chemistry manipulation are reviewed, including recent approaches whereby such nanoparticles are embedded in other biomaterial matrices.

Porous silicon has enormous potential for a variety of applications as a high surface area variant of single crystalline silicon. Its high surface area is its defining feature, which can dominate the properties of the material. The native surface produced after an electrochemical etch is typically a hydride-terminated surface that is only metastable in ambient air; this surface will oxidize over time. Surface chemical functionalization can enable stabilization of the porous silicon with respect to demanding chemical and biologically relevant environments and can enable precise tailoring of properties to endow the material with particular characteristics on demand. This chapter examines the surface chemistry of porous silicon that produces direct silicon-carbon bonds, to enable the covalent binding of just about any organic molecule of interest to the surface.

The production of microparticles from anodized silicon wafers requires a “top-down” approach in that, after anodization, the porous silicon is in the form of a layer that is detached from the silicon wafer as whole intact membranes or large flakes. Such macroscale layers can then be milled into powder using either of four main comminution techniques – hand-milling, rotor-milling, ball-milling, and jet-milling – each described in detail herein. The choice of technique governs the particle size distribution achievable. It is shown that the physical properties and purity of porous silicon powders are significantly influenced by the milling technique and conditions used – an important consideration for practical applications.

Porous silicon (PS) is a promising material for photonic, optoelectronic, and sensor devices. However, achieving stable metallic contacts to porous silicon has been a challenge. Oxidation of the Si-H_x bond on porous silicon surface on exposure to aerial atmosphere is the main reason of the instability. This review highlights the attempts made to modify the PS surface and make stable ohmic and rectifying contacts. Data on different metals, alloys, and conducting polymers utilized to treat the surface of porous silicon prior to the formation of ohmic and rectifying contacts are provided in tabular form. The methods deployed to deposit the contact materials on porous silicon are also summarized. The performance of noble metal treatment of porous silicon surface by electroless deposition is highlighted.

Macroporous silicon layers, with micron diameter pores, can be subjected to many standard processes developed by the microelectronics industry for silicon wafers, but these processes require tuning and optimization. A variety of processing techniques to manipulate macropore morphology, chemical composition, and patterning are reviewed.

All current applications of mesoporous and macroporous silicon under investigation are briefly surveyed, and more than 50 reviews over the period 1985–2014 are collated. Applications are grouped into twelve domains: electronics, optoelectronics, optics, diagnostics, energy conversion, catalysis, filtration, adsorbents, medical, food, cosmetics, and consumer care. Targeted product/function examples are given for each domain, together with the current level of industrial and academic activity. Comparisons are made with the major industrial uses of silica, comparing high-value and low-value product areas. Although porous silicon uses in electronics have the longest history of development, the most active R&D domains currently, as gauged by volume of literature, are energy conversion (lithium batteries), medical (drug delivery), and diagnostics (chip-based biosensing and mass spectrometry).

The increasing expansion of telecommunication applications leads to the integration of complete system on chip associating various processing units mixing passive and active elements. Nevertheless, passive component performances are limited by the underlying lossy silicon wafer. Then, obviously, the use of innovative substrates becomes crucial for monolithic RF systems to reach high performances. So, looking for IC compatible processes, porous silicon seems to be a promising candidate as it can provide localized isolating regions from various silicon substrates. In this chapter, we propose a synthesis of RF device technologies that use porous silicon for reducing losses into the substrate. The state-of-the-art performances of widespread RF devices, that is to say, inductors or coplanar waveguides, are then presented. Other RF devices which use porous silicon as a substrate are also described.

The exceptionally low thermal conductivity of highly porous silicon has led to its use as a thermal insulator within microsystems. A comprehensive review of thermal conductivity literature is provided, together with examples of its use in microsensing and microphotonic systems.

Gettering, the process whereby unwanted impurities are moved to noncritical regions of devices, is achievable with porous silicon, as primarily a result of its very high surface areas. The mechanisms, different techniques, and classification of gettering in silicon, the methods of characterization, and the gettering performance of mesoporous silicon with regard transition metals are reviewed.

In this chapter, silicon electrochemical micromachining (ECM) technology is reviewed with particular emphasis to the fabrication of complex microstructures and microsystems, as well as to their applications in optofluidics, biosensing, photonics, and medical fields. ECM, which is based on the controlled electrochemical dissolution of n-type silicon under backside illumination in acidic (HF-based) electrolytes, enables microstructuring of silicon wafers to be controlled up to the higher aspect ratios (over 100) with sub-micrometer accuracy, thus pushing silicon micromachining well beyond up-to-date both wet and dry microstructuring technologies. Both basic and advanced features of ECM technology are described and discussed by taking the fabrication of a silicon microgripper as case study.

This chapter presents a literature survey of the applications of porous silicon in BioMEMS (biological/biomedical microelectromechanical systems). This material possesses properties particularly suitable for biomedical purposes: biocompatibility, biodegradability, photoluminescence, ability to precisely control the pore size and shape, and possibility to easily modify the surface chemistry. Many applications can, for instance, be found in the fields of sensing and delivery of therapeutics. It is expected that the number of BioMEMS using porous silicon will continue to increase in the future with the development of lab-on-a-chip/microfluidic devices.

A literature survey is made of the various uses of both macroporous and mesoporous silicon in individual microdevices and complex microsystems. The material has been used as a silicon wafer processing tool where it is sacrificial: in a passive role where it provides, for example, thermal or electrical isolation and in an active role where it performs a range of functions. Examples include delivering drugs, sensing, emitting light, storing hydrogen, providing filtration, or having a catalytic role.

Porous silicon based one, two and three dimensional photonic crystals (PCs) have been reviewed. Apropriate selection of the fabrication parameters can be used to form a photonic band gap of tunable width in the required spectral region. Due to the ease of fabrication of 1D PCs, it has been shown to be useful for the demonstration of some physical phenomenon and numerous works have been dedicated to photonic and sensing applications. The fabrication of 2D PCs requires the prestructuring of the silicon surface and the structures with the defects for the incorporation of different materials have opened a wide range of applications. Although the fabrication of 3D PCs, needs the combination of more than two tehniques, the fabrication of perfect mirrors in the infrared region has been shown.

Porous silicon optical waveguides have attracted attention for applications ranging from optoelectronics to chemical and biological sensors. This chapter describes various waveguide geometries that have been achieved in porous silicon, including single and multilayer architectures. Depending on the processing parameters, oxidized porous silicon waveguides can be fabricated with losses below 0.5 dB/cm. While these losses are a challenge for some optoelectronics applications, sensor applications of porous silicon waveguides have flourished. Recent innovations have led to demonstrations of the label-free detection of nucleic acid molecules, proteins, and toxins with sensitivities as low as 10−6/RIU.

This work reviews progress in the fabrication of high-resolution porous silicon gratings. A comparison of the various fabrication methods is presented, including prepatterning of silicon, stamping, laser writing, and photolithographic and photo-assisted dissolution. Various types of sensors incorporating gratings are discussed which use either intensity and/or spatial (angular) measurement. The sensing capabilities, features, and limitations of these types of gratingsGratings are presented.

Work on phononic crystals in porous silicon is first put in context of the more general interest in controlling wave behavior in periodic media in both photonic and phononic crystals. The condition for a stop band for acoustic waves is derived by analogy with the Bragg condition with optical multilayers and then also obtained from the Rytov theory of layered acoustic media. The modeled acoustic bandgap in porous silicon is then defined and examined with the parameters used to model acoustic velocities in the material. A historical overview of papers covering theory and experiments on acoustic bandgaps in porous silicon is then given with an emphasis on their hypersonic nature and phoxonic properties. Finally, applications of acoustic distributed Bragg reflectors (ADBRs) with integrated transducers for high-performance bulk acoustic resonators (BAW) and possible applications for thermoelectric devices are presented.

In this chapter, the state of the art on porous silicon gas sensorsGas sensors, both electrical and optical, is reviewed by paying special emphasis on the advancement of gas sensor architectures that has occurred over the two last decades, as well as on the different functionalization approaches implemented in and chemical species sensed with such architectures. Ten main architectures, five for the electrical domain (capacitor, Schottky-like diode, resistor, FET-like transistor, and junction-like diode) and five for the optical domain (single layer, waveguide, Bragg mirror, resonant cavity, and rugate filter), have been proposed so far for improving gas sensor features. Several functionalization schemes have been integrated in such architectures to improve sensor performance, and more than 50 different chemical species have been sensed using porous silicon gas sensors. The latest trends on multiparametric sensing on single devices as well as on multisensor integration in a single chip, for both optical and electrical domains, are also discussed.

The rapidly developing field of porous silicon-based biosensors that utilize optical transduction is comprehensively reviewed by distinguishing the differing strategies for small- and moderate-size biomolecular analytes and the challenges with analysis of complex biofluids. A number of topics are identified for future research that should lead to one-shot disposable chip-based systems becoming commercially avialable.

The literature on the use of porous silicon as a matrix to capture and assist in mass spectroscopy of molecular species is comprehensively reviewed. The different analytical techniques are first described; then the range of compounds analyzed and the different application areas that utilize such mass spectrometry are covered.

Porous silicon with immobilized recognition biomolecules is an attractive platform for many microfluidic chip-based bioanalytical applications. We review the progress in the field since its earliest developments in the 1990s. An improved assay for early detection of prostate cancer has reached clinical evaluation, but there are also exciting developments in both aptamer-based biosensing and mass spectrometry-based biosensing.

Highly targeted radiotherapy (“brachytherapy”) was the first clinical application of stain-etched silicon microparticles. We briefly review here the manufacture, testing, and clinical use of the32P:Si beta-emitting formulation for unresectable hepatocellular and pancreatic carcinoma. Clinical data from a number of trials suggests that the technology offers patients with inoperable solid tumors an attractive alternative to external beam radiotherapy.

Biodegradable porous siliconBiodegradable porous silicon is under preclinical assessment for a range of drug delivery applications. Studies to date on oral, subcutaneous, intravenous, and intravitreal modes of delivery are reviewed. Both the merits of this nanostructured carrier technology and some existing challenges are briefly discussed.

Porous silicon possesses a number of favorable properties that make it relevant to the field of regenerative medicine / tissue engineering (i.e. the idea that the human body can heal itself with the assistance of a temporary scaffold). These properties include tunable scaffold dissolution based on Si nanostructure thickness/porosity and surface chemistry; in vitro and in vivo compatibility; and facile formation of easily processible porous silicon / biocompatible polymer constructs. In this review, selected formulations of porous Si relevant to scaffold evaluation are described, followed by key tissue engineering milestones for porous silicon, and brief summaries of studies of porous silicon-containing materials with relevance to specific therapies.

Porous silicon has been under evaluation as both a photosensitizer and a photothermal agent against cancer. Photodynamic therapy approaches exploit the ability of mesoporous silicon to generate reactive oxygen species under illumination; photothermal therapy exploits its ability to absorb near-infrared light and the low thermal conductivity. Both the in vitro and in vivo data acquired to date are reviewed.

This chapter focuses on cell immunoisolation and bio-filtration applications of porous silicon membranes. After an introduction on immunoisolation for the treatment of diabetes, the different materials used for that function are reviewed and compared. Applications involving porous silicon are then presented in more detail. Other uses of microfabricated porous silicon membranes in hemofiltration and protein sorting are also discussed.

Today over 90 % of all photovoltaic solar cells produced worldwide are composed of the silicon (single crystal, multicrystalline, amorphous, etc.). Despite this, the relatively high cost of silicon solar cells remains the main obstacle for their even wider applications. Physical principles of working of photovoltaic solar cell and action of antireflecting coating have been briefly presented. This chapter reviews investigations carried out over the last 10–15 years concerning the use of porous silicon layers in silicon solar cells. Data on photovoltaic parameters of silicon solar cells with thin porous silicon layer as antireflecting coating and also reflectance data of PS layers have been summarized. Advantages of nanostructured PS use in silicon solar cells, concerning increase of the cell effective surface area, lowering of the reflectance, broadening of the effective band gap of near-surface region of cell, etc., which finally promote improved silicon cell efficiency and simplify the technology, have been presented.

Miniaturization of fuel cells (FC) can offer a possibility in the field of small energy sources. Many silicon-based technologies can be used to perform micro-fuel cells and, in particular, porous silicon. In this chapter, after general consideration on fuel cells, we describe the state of the art of porous silicon integration in micro-fuel cells. In particular, we show how porous silicon has arisen as a promising material to perform many functions necessary to the core fuel cell such as proton exchange membrane, gas diffusion layer and catalyst support or flow fields. The performances of the several final devices reported in the literature are discussed.

Research for advanced Li-ion batteries (LIBs) has been following the direction toward higher energy and power densities. As an anode active material for LIBs, Si has a maximum theoretical capacity far greater than that of the currently commercial graphite anode. However, Si lithiation/de-lithiation is accompanied by large volume expansion/contraction, leading to mechanical instability and hence fast capacity fading. Significant advancement in overcoming this problem has been demonstrated by adopting nanostructured porous Si anode materials, which contain “preset” voids to accommodate volume expansion of the Si particles so that the dimensional variations of the entire electrode layer can be mitigated. This chapter reviews reports of porous Si anode materials synthesized by different methods, including etching, magnesiothermic reaction, templating, and electro-spraying, and the electrochemical performance of the resulting Si anode materials.

The room-temperature energetic propertiesEnergetic properties of porous silicon filled with an oxidant was accidentally discovered in 2002. Since then, this new branch of the porous silicon technology has made some progress, and several possible applications of this technology have been proposed. Of importance in the technology development are aspects like the type of oxidizer to be used, stabilization of the oxidizer in the pores, the effect of surface hydrogen on the long-term stability and energetic properties, the initiation of the reaction via electrical hot wires or laser illumination, the energy released from the reaction, as well as the reaction velocity. In this chapter, the above criteria are discussed, but emphasis is also placed on the porous silicon growth conditions, the effect of porous layer thickness, porosity, and the pore dimensions. The method to deposit the solid-state oxidizer in the pores via solvents is investigated, as well as the gas being generated during the energetic reaction. Some applications of this relatively new technology are proposed and discussed. As an interesting new development, the correlation between the nano-explosions of porous silicon and the appearance and development of the natural phenomenon of ball lightning, caused by small silicon particles covered with hydrogen atoms, is discussed.

Functional foods are often described as those that can have a positive effect on health beyond basic nutrition. Examples include cholesterol-lowering oatmeal, bacteria-loaded yogurt for gut health, and iodine-fortified bread for prevention of thyroid disease. There is growing evidence that orthosilicic acid, the biodegradation product of porous silicon, can have a positive contribution to optimizing bone healthbone health. The relevant nutritional literature on silicic acid and trials related to osteoporosis are collated and discussed. Silica microparticles (and inadvertently nanoparticles) have been used for decades as an approved food additive. Preliminary studies have shown that porous silicon has high chemical stability in many stored foodstuffs and dissolves in intestinal fluid faster than in gastric fluid and that the taste and mouthfeel of oxidized porous silicon microparticles can be acceptable. The potential uses of mesoporous silicon or silica particles in both protecting and raising bioavailability of ingested high-value nutrients are analyzed. Despite its technical potential, inexpensive and very scalable fabrication routes are required if porous silicon is to have significant uptake by the food industry.

Porous silicon particles could be utilized in a variety of consumer care applications if their cost of manufacture becomes low enough. Silica and porous silica are already widely used by the toothpaste and cosmetic industries, and porous silicon offers superior mechanical properties for teeth cleaning and very different optical properties for cosmetics. Partial thermal oxidation has been used to improve shelf life in liquid formulations and to match brown skin tones. Preliminary mouthfeel testing for oral hygiene products has been carried out, but formalized skin-feel testing and optimization is required for numerous dermatological products. Product examples discussed include multifunctional dentifrice abrasives, sunscreens, bronzers, foundation and makeup additive, and anti-aging formulations.