Porous Silicon Layers Research Articles

Structural Features of Porous Silicon Formed on Heavily Doped Plates of Single-Crystal Silicon with Electron Conductivity

Using scanning electron microscopy, the structures of the surface and internal regions of porous silicon obtained by anodizing heavily doped plates of single-crystal silicon with electron conductivity in a hydrofluoric acid solution at different current densities were studied. It is found that the porous silicon surface has dark gray and light gray pores, which differ in size and surface distribution density. Dark gray pores possess larger sizes, and their density is about 5–10 times less than that of light gray pores. Based on the cross-section imagery, it is shown that light gray pores correspond to underdeveloped channels of small depth, while dark gray pores are the entrance points of deep bottle-shaped channels passing from the surface into the depth of the silicon wafer. The equivalent diameters of light gray pores on the surface of porous silicon are 12–15 nm and are practically independent of the anodic current density. At the same time, the equivalent diameters of dark gray pores and average distances between their centers increase linearly from 15 to 35 nm on the surface and from 35 to 120 nm in the volume of porous silicon when the current density is increased from 30 to 90 mA/cm2. The average thickness of silicon skeleton elements is about 3 nm on the surface and increases to 5–6 nm in the volume. By setting the density of the anode current, it is possible to obtain layers of porous silicon with different structural parameters. The obtained research results have practical significance for the formation of composite materials based on porous silicon, which can be used as a porous matrix for the deposition of metals and semiconductors.

Improved the Sensitivity and Limit of Detection of Surface Alloying SERS Sensors by Controlling Mixing Ratio of Trimetallic (Ag-Au-Pd) Nanoparticles

In this study, three different types of hybrid structures SERS sensors made from different mixing ratio of trimetallic (Ag-Au-Pd) nanoparticles of [Au1: Ag1: Pd1] , [Au1: Ag2: Pd1] , [Au1: Ag2: Pd2] in surface alloying forms deposited on macro porous silicon (macroPsi) layer were created and extensively tested. By using a simple and fast immersion process, trimetallic (Ag-Au-Pd) nanoparticles were created utilizing ion reduction of numerous metallic salts on a macro porous silicon (macroPsi) layer. The macroPsi layers were created using a laser assisted etching (LAE) technique for 15 minutes with a constant density of approximately (28mA/c.m2). At a constant concentration (10-3 M) of HAu.Cl4, Ag.NO3 and Pd.Cl2 to synthesize Au-NPs, Ag-NPs and Pd-NPs, immersion processes with various mixing ratios were performed. The hybrid structures of SERS sensors were examined using XRD, FE-SEM, and Raman microscopes. The sensors were tested at different concentrations from 10-6 to 10-12 of MB target molecules. The SERS trimetallic sensors demonstrated a considerable reliance on hot spot zones among trimetallic nanoparticles. Higher enhancement factor with lower detection limit of Raman signal was obtained from [Au1: Ag2: Pd2] hybrid structures SERS sensor of about 1.5×1010 and 10-14 respectively, due to extra ordinary specific surface area and high surface density forming trimetallic nanoparticles . The variations of mixing ratio of trimetallic (Ag-Au-Pd) provide an effective pathway to develop the sensors performance towards detection of lower concentrations.

FT-IR detection of DMMP on TiO2-Modified porous silicon substrates

In this study, we present a new chemical sensor based on a functionalized porous silicon and sensitive to dimethyl methyl phosphonate (DMMP), a simulant of sarin gas, known as one of the most toxic warfare agents. The porous silicon was prepared by anodising a boron-doped p-Si (100) wafer. A metal oxide TiO2 layer was then deposited by thermal evaporation of Ti and oxidized under atmospheric conditions to form a TiO2 film which was used as a reactive coating. The characterization of the PSi and TiO2/PSi films was investigated using SEM, EDX and XPS spectroscopies. The results indicated that the TiO2 was well incorporated on the surface and slightly inside the porous silicon layer. FT-IR studies were then carried out before and after exposure to DMMP and the detection performance was compared with PSi. The results obtained showed that our surfaces have the ability to detect DMMP down to 0.5 ppm compared to uncoated PSi layer. We have also shown that a TFA pre-treatment of our TiO2 modified PSi, prior the exposure to DMMP, is crucial for increasing its sensitivity and that our sensor can be reused several times with a simple hexane cleaning step, a key criterion for routine applications.

Non-thermal atmospheric jet plasma effects on the morphological, electrical and optical properties of nanocrystalline silicon

In this study, a non-thermal atmospheric argon plasma jet (NTAPJ) was designed and used to treat the porous silicon layer, prepared by photoelectrochemical etching (PECE). The porous silicon layers were distinguished before and after the cold plasma treatment utilized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL), and dark and photo current-voltage characteristics. The results confirmed the nanostructure of the formed porous silicon. After the treatment with argon plasma, the intensity of the XRD peak increased, and the porosity decreased from 93% to 62%. Additionally, porous silicon's average roughness (Sa) and root mean square (Sq) were reduced from 60.10 to 12.31nm and 73.70 to 16.53nm, respectively. The current-voltage characteristic indicated the increase in photocurrent and the ideality factor, as well as a decrease in dynamic resistance. The photoluminescence results showed a change in PL peaks, represented by a blue shift, an increase in intensity, and a vanishing of another peak.

Non-thermal atmospheric jet plasma effects on the morphological, electrical and optical properties of nanocrystalline silicon

In this study, a non-thermal atmospheric argon plasma jet (NTAPJ) was designed and used to treat the porous silicon layer, prepared by photoelectrochemical etching (PECE). The porous silicon layers were distinguished before and after the cold plasma treatment utilized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL), and dark and photo current-voltage characteristics. The results confirmed the nanostructure of the formed porous silicon. After the treatment with argon plasma, the intensity of the XRD peak increased, and the porosity decreased from 93% to 62%. Additionally, porous silicon's average roughness (Sa) and root mean square (Sq) were reduced from 60.10 to 12.31nm and 73.70 to 16.53nm, respectively. The current-voltage characteristic indicated the increase in photocurrent and the ideality factor, as well as a decrease in dynamic resistance. The photoluminescence results showed a change in PL peaks, represented by a blue shift, an increase in intensity, and a vanishing of another peak.

Infiltration of Erbium ions (Er3+) in Porous Silicon Layer Synthesized by Electrochemical Method: Structural and Optical Properties Studies

Infiltration of Erbium ions (Er3+) in Porous Silicon Layer Synthesized by Electrochemical Method: Structural and Optical Properties Studies

Radiation sensor based on a 1D-periodic structure infiltrated by (B-co-MP) a conjugated copolymer

In this study, a novel gamma-ray radiation sensor has been developed depending on a 1D photonic crystal (1D-PhC). Based on porous silicon (PSi) layer that has been penetrated by a conjugated copolymer (B-co-MP) which consists of BEHP-PPV and MEH-PPV, with a fractional ratio of 60:40. The suggested method for the development of the dosimeter is based on the shift of photonic band-gap to shorter wavelengths, where exposure to gamma-ray radiation at doses ranging from 0 to 20 kGy alters the refractive index of the (B-co-MP) copolymer. The fitted experimental data, the equation of Bruggeman effective medium, and the transfer matrix method (TMM) are the main axes in the framework of the current theoretical approach. The collected data shows that, within the visible range, the suggested sensor's sensitivity (224 nm/RIU) is high and stable over a 0-20 kGy applied-dose range. Also, we compared these results with previous research.

Dichloro-tin (IV) hexadeca-fluoro-phthalocyanine (F16PcSnCl2) thin film on porous silicon layers by ultrasonic spray pyrolysis, for possible application in optoelectronics devices

Phthalocyanines represent a significant class of organic semiconductors that have garnered attention for their potential applications in conducting polymers and organic electronics. The unique structural characteristics of phthalocyanines, coupled with the intriguing chemical behavior and variations in bandgap associated with different substitution sites, offer exciting prospects for designing novel application devices. In this study, we have successfully fabricated a heterostructure incorporating dichloro tin (IV) hexa deca fluoro phthalocyanine (F16PcSnCl2) on both porous silicon (PS) and crystalline silicon (c-Si). The PS substrate was prepared using metal-assisted chemical etching. To explore the optoelectronic applications, we thoroughly characterized the optical, electrical, and morphological properties of the heterostructure. F16PcSnCl2 exhibits the lowest reflectance within the visible light spectrum, making it highly advantageous for photosensitive applications that necessitate efficient light absorption, diffusion, or scattering. The morphological analysis of the F16PcSnCl2 film reveals the presence of nanosphere-type structures uniformly distributed on both PS and c-Si substrates. The absorbance spectrum exhibits three distinct bands, which serve as typical indicators of the F16PcSnCl2 complex. Several hybrid heterostructures were fabricated for electrical characterization, displaying rectifying ohmic behavior and demonstrating a photocurrent effect in the I-V curves. Notably, when the heterostructures were polarized at 1 V, a pronounced response to pulses of white light was observed in the current–time curves. Overall, the integration of organic and inorganic materials in heterostructures holds great promise for innovative applications in optoelectronics.

Preparation and characterization of silicon nanoparticles (core) with gold nanoparticles (shell) configuration

Herein, we used a simple yet novel approach to create and characterize silicon nanoparticles Si NPs/core, gold nanoparticles AuNps/shell hybrid structure. The meso porous silicon layer was synthesized using a laser-supporting etching process with 50 mW/cm2 and 405 nm laser power density and illumination wavelength in a Teflon cell comprising a 1:3 volumetric ratio of 40 % HF in ethanol with etching current density of 25 mA/cm2 for 15 min. The SiNps/AuNps hybrid structure was synthesized by a simple ion reduction process of gold ions via dangling bonds of SiNps in an aqueous solution of HaUcl4 of 0.001 M for 5 min. The characterization of SiNps and SiNps/AuNps hybrid structure were investigated using FE-SEM, TEM, XRD and PL measurements. The results showed that the pores diameter ranged from 5 nm to 50 nm, while the average diameter of the SiNps core layer and AuNps shell layer are about 59.5 nm and 38 nm respectively. The surface morphology of the SiNps characterizes the synthesized hybrid structure; the PL emission of hybrid structure has a high intensity with blue shifting towards short wavelength compared with SiNps. Deposition of AuNps on SiNps led to synthesis more chemical stable hybrid structure with specific increase in the surface area.This resulting hybrid structure can be adoptedin various types of plasmonics and gas sensors in addition to optoelectronics devices.

Enhancing photodetection performance through laser fluence control: SnO2 nanostructured deposited on porous silicon

In this study, we have covered a porous silicon (PS) layer with tin dioxide (SnO2) nanostructured via a pulsed laser deposition process to produce a broadband photoresponse device. The impact of laser fluence on the optical, electrical, and structural characteristics of the fabricated photodetector was identified. The optical features analysis indicated that the SnO2 films covered on porous silicon have a direct energy gap varying from 3.62 to 3.67 eV as a function of laser fluence. The X-ray diffraction investigation proved that the SnO2 nanostructure has a tetragonal rutile nature. The Field-emission scanning electron microscopy emphasised that produced SnO2 nanostructures have a mean size of 81.76–98.68 nm, and nanoparticle agglomeration was remarked. The photoluminescence emissions of SnO2 nanostructure placed on porous silicon displayed a variety of overlapping intensities in the near-ultraviolet/visible spectrum between 369 and 506 nm, while the intense red band decreased in intensity and shifted towards longer wavelengths. Fourier-transformed infrared spectroscopy investigation indicates the existence of the Sn-O-Si bond in the formed structure.The dependence of the optoelectronic characteristics of PS/n-Si and SnO2/PS/n-Si photodetector on laser fluence has been performed. Produced heterojunctions have remarkable rectification properties with ideality factors ranging from 2.27 to 2.91, relying on the laser fluence. The spectral responsivity analysis showed that SnO2/PS/n-Si photodetectors exhibit three response bands at ∼366 nm, ∼621 nm, and ∼806 nm, regarding the highest responsivity of 0.21 A/W, 0.33 A/W, and 0.43 A/W, respectively. The findings suggest that SnO2/PS/n-Si heterojunction photodetectors are suitable for developed ultraviolet–visible light detection enhanced photodiodes that are cost-effective and convenient for use in wearable and portable devices without expensive extra gadgets.

Growth mechanism of bismuth-filled nanoporous silicon with improved structural stability and low thermal conductivity

Nano porous silicon (PS) is a strong candidate for thermoelectric application, but it shows low structural stability and is prone to collapse. To enhance the structural stability of PS and offer a new way to improve its electrical conductivity, PS was prepared by electrochemical etching and Bi nanowires were fabricated in the pores of PS by electrodeposition. In addition, for studying the effect of structures on the properties of Bi-filled PS composites, aperiodic single-layer (SL) structure, and periodic rugate (Rgt) structure and Bragg structure PS samples were prepared by changing the etching current waveform and etching time. The results show that the initial Bi deposition position and filling fraction of Bi nanowires in PS templates can be precisely controlled. Since Bi nanowires have significantly different continuity in Rgt and Bragg structures compared with SL structures, we proposed two growth mechanisms of Bi nanowires, including the axis-growth mechanism and transverse-axis-growth mechanism, with the supports of the experimental results of XRD, SEM, and EDS.The room temperature thermal conductivities of pristine and Bi-filled PS layers are in the ranges of 4.5 ∼ 5.0 and 4.2 ∼ 6.0 W･m−1K−1, respectively, about 1/10 of the original unetched silicon wafer. No apparent increase of thermal conductivity was observed in any Bi-filled composites. In addition, the thermal conductivities of PS layers with Bragg structure, including pristine and Bi-filled PS composites, are significantly lower than those of the other two structures.SEM images also demonstrates that Bi greatly improved the mechanical property of PS composites, especially the periodic structures composites. This study firmly demonstrates that Bi-filling in PS with periodic Bragg structure is practical to obtain nanocomposites with low thermal conductivity and improved structural stability. It offers a new dimension to optimize the property of PS for thermoelectric application by pore configuration design and controllable nanowire Bi filling.

Temperature Etching and Metallic Agent Concentration Effect on Structure, Morphology and Wettability of Silicon Nanowires

Abstract. Structural, Morphologycal and Wettability of SiliconNanowires (SiNWs) elaborated using Ag assisted electroless chemical etching are investigated. Prior the etching, Ag nanoparticles (AgNPs) were deposited at room temperature in a HF/AgNO3 solution with different concentration of AgNO3. The XRD spectra of the Ag NPs deposit show a good crystallinity. The effects of temperature etching bath and concentrations of AgNO3 on the etching process were examined. The morphological study, performed using a Scanning Electron Microscopy (SEM), shows porous silicon layer of 2µm for the lower temperature etching. For 25°C, perpendicular silicon nanowires about 15µm were formed. For the higher etching temperature (50°C), the silicon nanowire about 50 nm in diameter and 50µm in length were formed. The impact of Ag concentration on the SiNWs formation is examined in the second part of the present work. It is shown that the etching depth decreases as the Ag concentration decreases with values of 2.8 μm and 2 μm for concentrations of 0.025M and 0.0125M, respectively. The hydrophobicity of the samples was monitored by measuring the contact angle between a drop of water and the sample surface. It was established that the morphology is strongly influenced by etching conditions and their wettability changes from superhydrophilic to hydrophobic. FTIR analysis confirms the oxide-free silicon nanowires.

Porous silicon layer decorated with PbS nanoparticles by SILAR method for enhanced photocatalytic degradation of amido black dye

Porous silicon layer decorated with PbS nanoparticles by SILAR method for enhanced photocatalytic degradation of amido black dye

Design of porous silicon /PECVD SiOx antireflection coatings for silicon solar cells

The meso-poreux porous silicon layer (PS) has become an interesting material owing to its potential applications in many fields including optoelectronics and photovoltaics. PS layers were grown on the front surface emitter n+ of n+ -p mono-crystalline Si junction. The thickness of the PS formed and the porosity were measured by an ellipsometer as a function of time duration of anodization, and the variation law of the PS growth kinetics is established. Single layers PS antireflection coating (ARC) achieved around 9% effective reflectivity in the wavelength range between 400 and 1000nm on junction n+ -p solar cells. To reduce the reflectivity and improve the stability and passivation properties of PS ARC, the design of PECVD oxide silicon layers were investigated. SiOx layers of thickness of 105nm deposited on PS ARC showed a decrease of ~3.8% in the effective reflectivity, compared to the single layer PS ARC, improve the reflectivity of 55%. Voc measurements were carried out on all the samples by suns-Voc method and showed an improvement of the quality of the passivation brought by the oxide layer. Using the experimental reflectivity results and taking into account the passivation quality of the samples, the PC1D simulations predict an enhancement of the photogenerated current exceeding 49%.

Impact of MeV-Ag ions irradiation of silicon substrate on structural and optical properties of porous silicon

Porous silicon (PSi) was formed by an electrochemical etching of crystalline silicon substrates irradiated with silver (Ag) ion beam with energy of 1.25 MeV. Micro-Raman scattering results reveal that the irradiation process induces amorphization in the silicon substrate. Scanning electron microscopy, energy dispersive X-ray spectroscopy and micro-Raman scattering results show that the irradiation process induces important morphological and structural modifications in the PSi layer. The irradiation process leads to a considerable enhancement in the etching rate, a significant increase in the O/Si ratio and a reduction in the average size of the silicon nano-crystallites (nc-Si) from ∼7.76 nm to ∼2.26 nm in the skeleton of the PSi layer. These changes resulted in an important modification of photoluminescence (PL) properties of the PSi layer. Additional PL peak associated with the ‘F-band’ of the PSi was de-convoluted in the PL spectrum of the PSi layer formed on the irradiated silicon substrate.

Composition-adjustable silicon-germanium alloy films based on porous silicon matrices

Morphology and crystalline structure of silicon-germanium alloys formed by rapid thermal processing of germanium-filled porous silicon layers are evaluated. Two types of porous silicon are employed as matrices for electrochemical pore filling using GeO2 aqueous solutions and subsequently compared, the first one formed by electrochemical anodization and the second by silver-assisted chemical etching of monocrystalline silicon. The resulting alloys’ structure and composition are investigated using scanning electron microscopy, energy-dispersive X-ray analysis, Raman spectroscopy and X-ray powder diffraction. It is shown that by varying the porosity of the initial matrix (by adjusting anodization current density for anodic porous silicon or changing silver deposition time for structures produced by metal-assisted etching) in the range from 55 to 75%, Si1-xGex alloys with germanium fractions of x = 0.31 to x = 0.83 can be formed, as indicated by Raman spectroscopy. It is concluded that composition-adjustable layers of silicon-germanium can be successfully formed on either type of porous silicon layer. While an increase in porosity generally leads to a decrease in silicon fractions in the alloy, the steepness of this effect varies heavily depending on the type of porous matrix used and should be considered independently for anodic porous silicon and silicon nanowires.

Investigations of Nanoscale Columnar AlxGa1-xN/AlN Heterostructures Grown on Silicon Substrates with Different Modifications of the Surface

The growth of nanoscale columnar AlxGa1-xN/AlN heterostructures on the surface of silicon substrates using plasma-activated nitrogen molecular-beam epitaxy was investigated in this work. Silicon substrates include atomic-smooth cSi substrate, Si substrate with a transition layer of porous silicon porSi/cSi and a hybrid substrate involving a silicon carbide layer grown with matched substitution of the atoms on the surface of porous silicon SiC/porSi/cSi. A complex analysis performed using a set of structural and spectroscopic techniques demonstrated that the epitaxial growth of the nuclear AlN layer on all types of the substrates in a N-enriched environment resulted in the formation of AlxGa1-xN/AlN heterostructures with a Ga-polar surface, which was realized only on the SiC/porSi/cSi substrate. The layer of AlxGa1-xN on cSi and porSi/cSi substrates was in the state of disordered alloy with an excess of gallium atom content. It was shown that a great difference in the lattice parameters of a substrate–film pair resulted not only in the appearance of a number of various defects but also in a considerable effect on the chemical process of the formation of the alloys, in our case, the AlxGa1-xN alloy. It was shown that nanoscale columns of AlxGa1-xN formed on SiC/porSi/cSi substrate were inclined relative to the c-axis, which was connected with the features of the formation of a SiC layer by the matched substitution of the atoms on the porous Si substrate, resulting in the formation of the inclined (111) SiC facets at the boundary of the (111) Si surface and pores in Si. Optical studies of the grown samples demonstrated that the optical band-to-band transition for the AlxGa1-xN alloy with Eg = 3.99 eVB was observed only for the heterostructure grown on the SiC/porSi/cSi substrate. A qualitative model is proposed to explain the difference in the formation of AlxGa1-xN layers on the substrates of cSi, porSi/cSi and SiC/porSi/cSi. The results obtained in our work demonstrate the availability of using SiC/porSi/cSi substrates for the integration of silicon technology and that used for the synthesis of nanoscale columnar AlxGa1-xN heterostructures using plasma-activated molecular-beam epitaxy with a nitrogen source.

Contribution of the photoluminescence effect of the stain etched porous silicon in improvement of screen printed silicon solar cell performance

In this work, we investigate the potential use of porous silicon (PS) nanostructures stain etched into photovoltaic technology as one possible way to increase the silicon solar cell (SC) efficiency at low cost. The process is based on a combination of texturization and porous silicon formed by metal assisted etching which used the screen printed front grid contact. The photoluminescence (PL), the reflectivity spectra of the PS layers, spectral response and electrical measurements are presented and discussed. As a result, the short circuit current density was improved by more than 21%. We observed an increase in internal quantum efficiency (iQE) in the region where the PL of the PS layers appears and the minority carriers’ diffusion length is enhanced by more than 60 . We report the correlation between the converter PL intensity and iQE and we demonstrate that the photoluminescence properties and the increased photocurrent result in an improvement of the solar cell efficiency.

Comparative studies of nanoscale columnar AlxGa1-xN/AlN heterostructures grown by plasma-assisted molecular-beam epitaxy on cSi, porSi/cSi and SiC/porSi/cSi substrates

Problems of the growth of nanoscale columnar AlxGa1-xN/AlN heterostructures on hybrid substrates involving porous silicon and silicon carbide layers by molecular beam epitaxy technique with plasma-activated nitrogen are discussed in this study. The epitaxial growth of nanoscale columnar AlxGa1-xN/AlN heterostructure is shown to be specified by the layer of silicon carbide, which is formed by atomic substitution technique, and a porous silicon sublayer predetermines the oriented growth of SiC. The performed complex of structural-spectroscopic analysis demonstrated that epitaxial growth of the nuclear AlN layer on all types of the substrates in N-enriched conditions resulted in the formation of AlxGa1-xN/AlN heterostructures with Ga-polar surface. At the same time it was found that the layer of ordered AlxGa1-xN alloy was formed only on the hybrid SiC/porSi/cSi substrate. The layer of AlxGa1-xN on the substrates of cSi and porSi/cSi is present in the state of disordered alloy with an excess content of gallium atoms. Nanocolumns of AlxGa1-xN/AlN grown on the hybrid SiC/porSi/cSi substrate have two types of preferential azimuthal orientation that affects not only their structural and optical properties but also the value of elastic deformation in the heterostructure nanoscale layers. For the first time, azimuthal dependence of intensity of E1(TO) and E2high photon modes was detected in the Raman spectra. The observed periodicity angle in micro-Raman scattering coincides with the characteristic swivel of nanoscale columns of AlxGa1-xN/AlN around c-axis according to the data of XRD pole figure measurements. Results obtained in our work show promising capabilities in the use of SiC/porSi/сSi substrates for integration of silicon technology and technology of synthesis of the nanoscale columnar AlxGa1-xN heterostructures by molecular-beam epitaxy with plasma-activated nitrogen.

Study the Effect of Hydrofluoric (HF) Concentration on the Topography of the Porous Silicon Layer Prepared by Sunlight Photochemical Etching (SLPCE)

Silicon nanocrystals have a vast range of potential applications, from improving the efficiency of solar cells and optoelectronic devices to biomedical imaging and drug delivery, wastewater treatment, and antibacterial activities. In this study a photochemical etching technique was used to create layers of porous silicon on a donor silicon wafer with orientation (111) and resistivity equal to 1‑10 ohm·cm. The process involved focusing sunlight onto the samples using a telephoto lens with a suitable focal length of 30cm and a diameter of 90 mm, which provided sufficient energy to complete the chemical etching. By using a constant etching time of 60 minutes and different concentrations of hydrofluoric acid (ranging from 25% to 40%), layers with varying properties were obtained. The resulting surfaces were studied using the atomic force microscope (AFM), revealing the formation of different nanostructures and particles with varying shapes, sizes, and thicknesses depending on the preparation conditions. The average size of the particles was found to be 90.43nm at a concentration of 40% acid, while decreasing to 48.7nm at a concentration of 25% HF acid.