Experimental study of macropore formation in p-type silicon in a fluoride solution and the transition between macropore formation and electropolishing

Anodic dissolution of p-Si is studied in diluted fluoride solution (HF 0.05 M + NH₄F 0.05 M, pH 3), with special focus on the physico-chemical parameters which govern the morphology of pore formation (crystallographic orientation, applied potential, and etching time). The effect of potential has been investigated in the transition region between macropore formation and electropolishing. Upon increasing the anodization potential, the pore cross-section changes from circular to square shape, and the bottom of the pores changes from a rounded to a V-shaped profile. Prolonged etching of the contour of (1 1 0) p-Si disks in the regime of porous silicon formation allows for a comparison of the etching characteristics of the orientations. SEM observation indicates indeed different morphologies as a function of the crystal orientation, and the formation of fractal-like structures is obtained for some orientations. In the same geometry and at a potential just above the onset of the electropolishing regime, prolonged anodization allows for a direct measurement of the Si thickness removed as a function of the crystallographic orientation. We clearly observe the etching anisotropy, with etch depth τ(1 1 1) < τ(1 1 0) < τ(1 0 0). This sequence, similar to that observed for current density in more concentrated HF, differs from that observed for the chemical etching of Si in an alkaline solution.     

The effect of etching temperature on the photoluminescence emitted from, and the morphology of, p-type porous silicon

The temperature at which silicon is electrochemically etched has been found to influence the structure and photoluminescence properties of porous silicon. Decreasing the temperature increased both the current efficiency of the dissolution process and the porosity of the resulting porous layer. Furthermore, a blue-shift was observed in the photoluminescence indicating that the decreased temperature allowed smaller nanocrystals to be formed. An analysis of temperature dependence of the pore initiation and propagation models currently available in the literature failed to yield a satisfactory explanation for the decrease in the average size of the nanocrystals indicated by the results presented in the present paper. Therefore it was proposed that at lower temperature smaller nanocrystals are stabilized due to a combination of their reduced solubility and the increased viscosity of the diffusion layer that leads to a higher localized concentration of silicon ions, thereby allowing smaller nanocrystals to be in equilibrium with their surroundings. The fact that previous authors did not observe blue-shifting highlights the importance of the composition of the etching solution in controlling the quality of the porous silicon produced.     

Metal-assisted etching of p-type silicon under anodic polarization in HF solution with and without H2O2

Pore formation under anodic polarization of a lightly doped p-type Si wafer previously loaded with Pt, Pd and Ag nanoparticles was investigated in HF solution with and without H₂O₂. In HF solution without H₂O₂, a microporous layer was formed in p-Si loaded with Pt or Pd. However, Ag metal nanoparticles yielded pores due to their intrusion in the Si wafer. The addition of H₂O₂ to the etching solution leads to different pore morphologies depending on the metals. Particles of Ag were found at the bottom of most pores. In the presence of Pt nanoparticles, cone-shaped macropores were produced, and the pore depth and diameter increased with increasing H₂O₂ content. Current density influenced the pore morphology. For a sample loaded with Pt or Ag, an increase in applied current density widened the pore diameter. The mechanism of the metal-assisted pore formation was discussed by considering a competitive process between the formation of a microporous layer under polarization and metal-assisted chemical oxidation of the microporous layer by a dissolved oxidizing agent.     

Controlled morphology and optical properties of n-type porous silicon: effect of magnetic field and electrode-assisted LEF

Fabrication of photoluminescent n-type porous silicon (nPS), using electrode-assisted lateral electric field accompanied with a perpendicular magnetic field, is reported. The results have been compared with the porous structures fabricated by means of conventional anodization and electrode-assisted lateral electric field without magnetic field. The lateral electric field (LEF) applied across the silicon substrate leads to the formation of structural gradient in terms of density, dimension, and depth of the etched pores. Apart from the pore shape tunability, the simultaneous application of LEF and magnetic field (MF) contributes to a reduction of the dimension of the pores and promotes relatively more defined pore tips as well as a decreased side-branching in the pore walls of the macroporous structure. Additionally, when using magnetic field-assisted etching, within a certain range of LEF, an enhancement of the photoluminescence (PL) response was obtained.     

Effect of potential steps on porous silicon formation

Porous silicon microstructures were fabricated by applying potential steps through which both anodic and cathodic potentials were periodically applied to silicon wafers. The electrochemical behaviors of porous silicon layers were examined by performing polarization measurements, followed by analyzing the open-circuit potential (E_ocp) and the reaction rate in terms of corrosion current density (j_corr). The surface morphologies and surface products of porous silicon were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It was found that the values of E_ocp and j_corr varied more significantly and irregularly during different polarization stages when the potentials were continuously applied to the wafer surface, while virtually unchanged after 2 min of periodic potential application. In addition, slower reaction rates were observed with applying potential steps, as indicated by smaller values of j_corr. The enhancement on refreshment of silicon surfaces by periodic potential polarization significantly accelerated the growth of porous silicon. The microstructures became more uniformed and better defined due to the improved passivating nature of wafer surfaces.     

Fabrication of patterned porous silicon using high-energy ion irradiation

P-type silicon has been patterned using high-energy protons beam prior to electrochemical etching in hydrofluoric acid. The ion beam selectively damages the silicon lattice, resulting in an increase in the local resistivity of the irradiated regions. It is found that the photoluminescence intensity of the irradiated regions increases with proton irradiation into a 0.02 Ω.cm resistivity p-type silicon. By immersing the etched sample into potassium hydroxide, the porous silicon is removed to reveal the underlying three-dimensional structure of the patterned area.     

Morphological and nanostructural features of porous silicon prepared by electrochemical etching

Porous layers were produced on a p-type (100) Si wafer by electrochemical anodic etching. The morphological, nanostructural and optical features of the porous Si were investigated as functions of the etching conditions. As the wafer resistivity was increased from 0.005 to 15 Ω·cm, the etched region exhibited ‘sponge’, ‘mountain’ and ‘column’-type morphologies. Among them, the sponge-type structured sample showed the largest surface area per unit volume. Silicon nanocrystallites, 2.0 to 5.3 nm in size, were confirmed in the porous layers. The photoluminescence peaks varied in the wavelength range of 615 to 722 nm. These changes in the maximum peak position were related to the size distribution of the Si crystallites in the porous silicon. The doping levels of the wafers significantly affect the size distribution of the Si crystallites as well as the light-emitting behavior of the etched Si, which contains nanoscale Si crystallites.     

Finite element modeling of reactive liquid silicon infiltration

The LSI process, i.e. the infiltration of molten silicon into porous structures, is one of the most economical techniques for the production of C/C-SiC and C/SiC ceramics. However, despite decades of development, the infiltration behavior affected by phenomena at the infiltration front has not been understood sufficiently. In the present work, a numerical model, based on the finite element method, was developed to simulate the infiltration process. The 3D model includes the penetration of silicon into the porous preform as well as the exothermal reactions at the infiltration front caused by the growth of SiC layers. For model validation, a special measuring furnace was used, enabling in situ optical inspection and weight measurement during liquid silicon infiltration into C/C-preforms in a controlled atmosphere. For the first time, a numerical model could be established which provides a tool to simulate the infiltration kinetics as well as the thermal processes during the LSI process in three dimensions. The model enables the optimization of melt infiltration processes with complex components within reasonable computer times.     

In-situ atomic force microscopy of silicon(100) in aqueous potassium hydroxide

Growth dynamics of pyramid-shaped features that emerge during etching of silicon(100) surfaces in 2 M aqueous potassium hydroxide solutions have been investigated using in-situ atomic force microscopy. Micropyramids were found to grow continuously from a scale-shaped structure that is present on the surface during etching. In addition, two characteristic removal mechanisms of fully developed pyramids could be identified. It is suggested that these etching mechanisms are unique to pyramids and not comparable to the etching properties of single crystal surfaces.     

The mechanism of galvanic/metal-assisted etching of silicon

Metal-assisted etching is initiated by hole injection from an oxidant catalyzed by a metal nanoparticle or film on a Si surface. It is shown that the electronic structure of the metal/Si interface, i.e., band bending, is not conducive to diffusion of the injected hole away from the metal in the case of Ag or away from the metal/Si interface in the cases of Au, Pd, and Pt. Since holes do not diffuse away from the metals, the electric field resulting from charging of the metal after hole injection must instead be the cause of metal-assisted etching.     

Diffusion in porous silicon: effects on the reactivity of alkenes and electrochemistry of alkylated porous silicon

Alkenes are known to react with hydrogen-terminated silicon surfaces to produce robust organic monolayers that are attached to the surface via covalent SiC bonds. In this report we investigate the dependence of the rate of alkylation of porous silicon samples on the reaction time using photochemical initiation. The kinetics of the photochemical alkylation of hydrogen-terminated porous silicon by undec-1-ene in toluene were observed to be pseudo first order, however the apparent rate constant decreased as the concentration of undec-1-ene increased. This behaviour is opposite to what would be expected if the rate-limiting process was an elementary chemical reaction step involving the alkene. Instead, it suggests that transport of the alkene to reactive sites and in the correct orientation is the rate-limiting step. Comparison of the rates of alkylation of porous silicon by undec-1-ene and dimethoxytrityl (DMT)-undecenol is consistent with such an interpretation as the bulky DMT headgroup gives a lower rate of alkylation. The diffusion of some simple redox-active probe molecules in porous silicon was investigated using a scanning electrochemical microscope (SECM). The probe molecules are converted at diffusion-controlled rate at an inlaid disk ultramicroelectrode (UME) consisting of the cross-section of a microwire sealed in glass. If the microelectrode is placed a short distance above the porous silicon, the microelectrode current depends on kinetics of the electrochemical reactions at the porous silicon and the mass transport properties within the open thin layer cell formed by the microelectrode and the alkylated porous silicon. In order to differentiate the effects of finite heterogeneous kinetics at silicon from diffusion limitations, current-distance curves were fitted over a wide range of applied potentials (on the Si) and it was observed that the diffusion coefficient in the porous layer was strongly anisotropic. The measured diffusion rates are comparable to those in bulk water along the pores, but with negligible diffusion between pores. This indicates that few pore-pore interconnections exist in the porous silicon.     

Structure,morphology, and photoluminescence of porous Si nanowires: effect of different chemical treatments

The structure and light-emitting properties of Si nanowires (SiNWs) fabricated by a single-step metal-assisted chemical etching (MACE) process on highly boron-doped Si were investigated after different chemical treatments. The Si nanowires that result from the etching of a highly doped p-type Si wafer by MACE are fully porous, and as a result, they show intense photoluminescence (PL) at room temperature, the characteristics of which depend on the surface passivation of the Si nanocrystals composing the nanowires. SiNWs with a hydrogen-terminated nanostructured surface resulting from a chemical treatment with a hydrofluoric acid (HF) solution show red PL, the maximum of which is blueshifted when the samples are further chemically oxidized in a piranha solution. This blueshift of PL is attributed to localized states at the Si/SiO₂ interface at the shell of Si nanocrystals composing the porous SiNWs, which induce an important pinning of the electronic bandgap of the Si material and are involved in the recombination mechanism. After a sequence of HF/piranha/HF treatment, the SiNWs are almost fully dissolved in the chemical solution, which is indicative of their fully porous structure, verified also by transmission electron microscopy investigations. It was also found that a continuous porous Si layer is formed underneath the SiNWs during the MACE process, the thickness of which increases with the increase of etching time. This supports the idea that porous Si formation precedes nanowire formation. The origin of this effect is the increased etching rate at sites with high dopant concentration in the highly doped Si material.     

Potentiostatic formation of porous silicon in dilute HF: Evidence that nanocrystal size is not restricted by quantum confinement

The role that applied potential has in controlling the properties of porous silicon formed on highly conductive p-type silicon in diluted HF has been investigated by studying the photoluminescence characteristics along the current-voltage curve and using high resolution transmission electron microscopy (HRTEM) evidence to support the conclusions drawn. A dramatic decrease in the average nanocrystal size was found to take place after the etching current density switched from an exponential dependence on the applied potential to a linear relationship. Importantly this event occurred prior to reaching the U_ps potential (usually consider the onset of electropolishing). This rapid decrease in particle sizes has been explained in terms of the partial formation of an oxide film. The presence of oxygen terminated porous silicon allows a trapped exciton states model to be invoked, which removes the quantum confinement restrictions on the minimum particle size. Support for the presence of a partial oxide prior to U_ps comes from both FTIR measurements and previous literature related to the location of the flat-band potential.     

A comparative electrochemical study of iron deposition onto n- and p-type porous silicon prepared from lightly doped substrates

Electrodeposition of iron from acidic sulfate solutions onto porous silicon (PS) prepared from n- and p-type (1 0 0) substrates is studied by electrochemical measurements. Results from current-potential curves show that deposition of iron on p-type PS can only be achieved under illumination and cathodic polarization, whereas the deposition is found to proceed on n-type even in the dark. The measurements of the cathodic current efficiency indicate that the fraction of current used for iron deposition decreases with the applied potential due to hydrogen evolution reaction which is a competing reaction to metal deposition. Scanning electron microscopy shows that very fine iron crystallites with an average size of 40-70 nm are formed under double potential step conditions. The energy band diagrams of silicon-solution interfaces determined by electrochemical impedance measurements reveal that the iron deposition mechanism on both substrates is electron transfer from the conduction band.     

Porous silicon pillar and bilayer structure as a nucleation center for the formation of aligned vanadium pentoxide nanorods

Porous silicon single layer (PSM), bilayer (PSB) and pillar (PSP) structures have been evaluated as nucleation centers for vanadium pentoxide (V₂O₅) crystals. Deposition of vanadium precursor over different substrates (drop casting technique), followed by annealing treatment under Ar-H₂ (5% H₂) atmosphere, induced crystallization of vanadium oxide. With respect to c-Si/SiO₂ substrate, V₂O₅ nanorods with relatively large aspect ratio were formed over and within PSP structures. On the other hand, pores in PSM and PSB were found to be filled with relatively smaller crystals. Additionally, PSB provided a nucleation substrate capable to align the nanocrystals in a preferential orientation, while V₂O₅ crystals grown on PSP were found to be randomly aligned around the nanoporous pillar microstructure. Nanorods and nanocrystals were identified as V₂O₅ by temperature-controlled XRD measurements and evidence of their crystalline nature was observed via transmission electron microscopy. A careful analysis of electronic microscopy images allows the identification of the facets composing the ends of the crystals and its corresponding surface free energy has been evaluated employing the Wulff theorem. Such high surface area composite structures have potential applications as cathode material in Lithium-ion batteries.     

Purification of silicon powder by the formation of thin porous layer followed by photo-thermal annealing

ABSTRACT: Porous silicon has been prepared using a vapor-etching based technique on a commercial silicon powder. Strong visible emission was observed in all samples. Obtained silicon powder with a thin porous layer at the surface was subjected to a photo-thermal annealing at different temperatures under oxygen atmosphere followed by a chemical treatment. Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) results indicate that silicon purity is im-proved from 99.1 to 99.994 % after annealing at 900 degreesC.     

Anodic electrode reaction of p-type silicon in 1-ethyl-3-methylimidazolium fluorohydrogenate room-temperature ionic liquid

The anodic electrode behavior for a p-type silicon single crystal electrode ((1 0 0), ρ = 0.01-0.02 Ω cm, boron doped) was examined in the 1-ethyl-3-methylimidazolium fluorohydrogenate, EtMeIm(FH)_2.3F, room-temperature ionic liquid (RTIL). The electrochemical behavior was very similar to that in conventional HF aqueous solution. After the anodic electrode reaction, the Si electrode was uniformly covered with a mesoporous Si layer having a pore size of ∼25 nm. The mesoporous layer did not exhibit a photoluminescence spectrum in the visible region due to the lack of Si-H termination. However, after chemical treatment with an ethanolic HF solution, a subset of the porous Si samples showed a very weak photoluminescence.     

Characterization of hybrid cobalt-porous silicon systems: protective effect of the Matrix in the metal oxidation

ABSTRACT: In the present work, the characterization of cobalt-porous silicon (Co-PSi) hybrid systems is performed by a combination of magnetic, spectroscopic, and structural techniques. The Co-PSi structures are composed by a columnar matrix of PSi with Co nanoparticles embedded inside. The oxidation state, crystalline structure, and magnetic behavior are determined by X-Ray Absorption Spectroscopy (XAS) and Alternating Gradient Field Magnetometry (AGFM). Additionally, the Co concentration profile inside the matrix has been studied by Rutherford Backscattering Spectroscopy (RBS). It is concluded that the PSi matrix can be tailored to provide the Co nanoparticles with extra protection against oxidation.     

Light room anodic etching of highly doped n-type 4H SiC in high concentration HF electrolytes: difference between C and Si crystalline faces

In this paper, we study the electrochemical anodization of n-type heavily doped 4 H-SiC wafers in a HF-based electrolyte without any UV light assistance. We present, in particular, the differences observed between the etching of Si and C faces. In the case of the Si face, the resulting material is mesoporous (diameters in the range of 5 to 50 nm) with an increase of the ‘chevron shaped’ pore density with depth. In the case of the C face, a columnar morphology is observed, and the etch rate is twice greater than for the one for the Si face. We''ve also observed the evolution of the potential for a fixed applied current density. Finally, some wafer defects induced by polishing are clearly revealed at the sample surfaces even for very short etching times.     

Mechanical properties of sintered meso-porous silicon: a numerical model

Because of its optical and electrical properties, large surfaces, and compatibility with standard silicon processes, porous silicon is a very interesting material in photovoltaic and microelectromechanical systems technology. In some applications, porous silicon is annealed at high temperature and, consequently, the cylindrical pores that are generated by anodization or stain etching reorganize into randomly distributed closed sphere-like pores. Although the design of devices which involve this material needs an accurate evaluation of its mechanical properties, only few researchers have studied the mechanical properties of porous silicon, and no data are nowadays available on the mechanical properties of sintered porous silicon. In this work we propose a finite element model to estimate the mechanical properties of sintered meso-porous silicon. The model has been employed to study the dependence of the Young’s modulus and the shear modulus (upper and lower bounds) on the porosity for porosities between 0% to 40%. Interpolation functions for the Young’s modulus and shear modulus have been obtained, and the results show good agreement with the data reported for other porous media. A Monte Carlo simulation has also been employed to study the effect of the actual microstructure on the mechanical properties.