Aqueous HF Solution Research Articles

Simultaneous gold deposition and formation of silicon nanowire arrays

A method is presented for simultaneously producing oriented silicon nanowire arrays and depositing gold on a silicon substrate from aqueous HF solution containing KAuCl 4 near room temperature. Scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray were used to characterize the morphologies, structure and compositions of the samples. Experimental results show that these silicon nanowires are single crystals and the growth of gold dendrites plays an important role in the formation of silicon nanowire arrays. Possible mechanism was proposed to explain the formation of silicon nanowire arrays based on local passivation of the silicon dissolution induced by the growth of Au nuclei.

Mechanism of Macropore Formation in Anodized p-Type Silicon

The mechanism of macropore formation in anodized p-type Si was studied using an aqueous HF solution. The depth profile of the anodized surface and the dissolution valence were investigated by varying anodic time. The dissolution valence increased at the initial stage of anodization and decreased after macropore formation. The experimental results are explained with the electrochemical reaction model, which describes that F- and OH- react with Si mainly at the bottom and the top of the macropore, respectively.

Anodic silicon etching; the formation of uniform arrays of macropores or nanowires

Macropore formation on p-type Si in the dark has been studied on 6-inch wafers in aqueous HF solution. More than 10 9 pores with a diameter of 2.5 μm and a depth of 100 μm are obtained in a single etch step. These pores can be used as a template for the fabrication of high-density MOS capacitors. If the current density is close to the characteristic peak for the anodic etching of Si in HF solutions, nanowires are obtained. The length of these wires can reach values up to 100 μm, while their width can be as small as 30 nm. This method is a suitable way to the high-volume production of nanowires.

FTIR spectroscopic studies of the stabilities and reactivities of hydrogen-terminated surfaces of silicon nanowires.

Attenuated total reflection Fourier transform infrared (FTIR) spectroscopy was used to characterize the surface species on oxide-free silicon nanowires (SiNWs) after etching with aqueous HF solution. The HF-etched SiNW surfaces were found to be hydrogen-terminated; in particular, three types of silicon hydride species, the monohydride (SiH), the dihydride (SiH(2)), and the trihydride (SiH(3)), had been observed. The thermal stability of the hydrogen-passivated surfaces of SiNWs was investigated by measuring the FTIR spectra after annealing at different elevated temperatures. It was found that hydrogen desorption of the trihydrides occurred at approximately 550 K, and that of the dihydrides occurred at approximately 650 K. At or above 750 K, all silicon hydride species began to desorb from the surfaces of the SiNWs. At around 850 K, the SiNW surfaces were free of silicon hydride species. The stabilities and reactivities of HF-etched SiNWs in air and water were also studied. The hydrogen-passivated surfaces of SiNWs showed good stability in air (under ambient conditions) but relatively poor stability in water. The stabilities and reactivities of the SiNWs are also compared with those of silicon wafers.

Processing of nano-holes and pores on SiO 2 thin films by MeV heavy ions

Scanned beams of 0.1 MeV/u 197Au ions were employed for the bombardment of silicon oxide films thermally grown on silicon (1 0 0) substrates. Subsequently the films were etched in aqueous HF solution (1% and 4%) for various times and at different temperatures. Scanning force microscopy and transmission electron microscopy images of etched films reveal conical holes with diameters from 20 to 350 nm, depending on the HF concentration and etching time. For a fixed etching temperature, hole size increased roughly linearly with etching time, and for a fixed etchant concentration, it varied exponentially with etching temperature. These porous surfaces were tested for trapping of nano-particles, as templates for deposition of porous films, and for localized etching of the Si substrate.

A way to obtain visible blue light emission in porous silicon

Strong blue emission around 435 nm from porous silicon (PS) embedded in Pb(Zrx,Ti1−x)O3 (PZT) is realized, which is stable and clearly visible to the naked eye even after exposure in an ambient atmosphere for a year. The sample is obtained by hydrothermal etching of (100)-oriented single crystal silicon wafers in an HF aqueous solution, followed by pulsed laser ablation deposition of a PZT thin layer on the surface of the PS. The electrons and holes excited from silicon nanocrystallites could be separated by the PZT-induced electrostatic field and migrate to the boundaries of PS/PZT along opposite directions, respectively. Therefore, the blue emission is interpreted in terms of the recombination of excited carriers from the silicon nanocrystallites in the interfacial region of PS/PZT.

Dendrite‐Assisted Growth of Silicon Nanowires in Electroless Metal Deposition

AbstractThis article concerns the detailed investigations on the silver dendrite‐assisted growth of single‐crystalline silicon nanowires, and their possible self‐assembling nanoelectrochemistry growth mechanism. The growth of silicon nanowires was carried out through an electroless metal deposition process in a conventional autoclave containing aqueous HF and AgNO3 solution near room temperature. In order to explore the mechanism and prove the centrality of silver dendrites in the growth of silicon nanowires, other etching solution systems with different metal species were also investigated in this work. The morphology of etched silicon substrates strongly depends upon the composition of the etching solution, especially the metal species. Our experimental results prove that the simultaneous formation of silver dendrites is a guarantee of the preservation of free‐standing nanoscale electrolytic cells on the silicon substrate, and also assists in the final formation of silicon nanowire arrays on the substrate surface.

Formation of new crystal faces on TiO2 particles by treatment with aqueous HF solution or hot sulfuric acid

We have demonstrated that new crystal faces are generated on anatase and rutile TiO2 particles by means of chemical etching in aqueous hydrofluoric acid or hot sulfuric acid. In the treatment with aqueous hydrofluoric acid, the {112} face of anatase particles and the {021} face of rutile particles are newly formed. When treated with hot sulfuric acid, anatase particles exposed the {122} face and rutile particles exposed the {001}, {010}, {021} and {121} faces. In both cases, anatase particles are etched at a higher rate than rutile particles. The etched particles are expected to show photocatalytic properties unique to the crystal faces. For example, the {112} face of anatase particles is demonstrated to be active in the oxidation of Pb2+ ions.

Anodisation-related structural variations of porous silicon nanostructures investigated by photoluminescence and Raman spectroscopy

Porous silicon layers (PSLs) were prepared in ethanoic and aqueous HF solutions using the conventional electrochemical etching technique. The microscopic structure of the PSLs has been studied by polarised photoluminescence (PL), Raman scattering and atomic force microscopy (AFM). We found that parallel ( I //) and perpendicular ( I ⊥) polarised PL present quite different line shapes. By assuming PS as a mixture of quantum wires (QWs) and quantum dots (QDs) nanocrystallites having specific mean diameter sizes and proportions, we give an explanation to the polarised PL behaviour. This PS modelling enable us to obtain calculated Raman spectra that fit very well the experimental ones for PSLs formed in ethanoic HF solution. AFM images of PSLs prepared in pure aqueous HF solution show morphological inhomogeneities as anodisation time increases, leading to non-conventional and reproducible PL and Raman behaviours. The latter PL behaviour was explained as being due to the non-uniformity of etching in aqueous HF, while that of the Raman spectra was described by introducing an amorphous phase beside the QWs and QDs contribution.

Selective Electroless Plating of Copper on (100)-Oriented Single Crystal Silicon Surface Modified by UV-Induced Coupling of 4-Vinylpyridine with the H-Terminated Silicon

A Sn-free electroless plating process for producing patterned and adherent copper deposits on (100)-oriented single-crystal silicon substrates is described. Pristine and resist-patterned Si(100) substrates were etched, initially, by aqueous HF to produce the hydrogen-terminated silicon surfaces (H−Si(100) surfaces). The H−Si(100) surfaces were further functionalized by the UV-induced reactive coupling of 4-vinylpyridine (4VP). The composition and topography of the modified Si(100) surfaces were characterized by X-ray photoelectron spectroscopy and atomic force microscopy, respectively. The coupled 4VP layer exhibited good stability in boiling chloroform, boiling water, sonicating dichloromethane, aqueous solution of HF and aqueous solution of KOH. Not only did the 4VP layer provide chemisorption sites for the palladium complexes without the need for prior sensitization in SnCl2 solution during the electroless plating process but it also served as the adhesion promotion layer and diffusion barrier for the ...

Synthesis of Large-Area Silicon Nanowire Arrays via Self-Assembling Nanoelectrochemistry

Large-area silicon nanowire arrays have been prepared on a silicon wafer without the use of a template. The simple method, which can be carried out near room temperature, involves the nanoelectrochemistry of silver nanowires in an aqueous HF solution containing silver nitrate. This technique may be generally applicable to other semiconductors and metals. The Figure shows nanodendritic silicon wires.

Monolayers of simple organic molecules on silicon studied by surface tools

AbstractSimple organic molecules such as 1‐octadecene, 1‐tetradecene, 1‐dodecyne and 1,13‐tetradecadiene have been grafted to hydrogenated B‐doped silicon (100) surfaces. Such substrate has been obtained by etching commercial native oxide‐capped flat crystal silicon wafers with procedures involving dilute HF aqueous solutions, but also by hydrogenation with a new method involving treatment of the wafer in an oven with H2 gas. The functionalization of such a surface has been done by using described wet methodologies involving pure liquids or solutions with thermal activation of the hydride. Contact angle, ESCA, atomic force microscopy (AFM) and time‐of‐flight secondary ion mass spectroscopy (ToF‐SIMS) measurements have been carried out to characterize the above systems. Particular emphasis is devoted to the interpretation of the ToF‐SIMS spectra, seldom used until now to characterize organic monolayers. Copyright © 2002 John Wiley & Sons, Ltd.

Nano-scale anodic oxidation on a Si( [formula omitted]) surface terminated by bilayer-GaSe

Nano-scale oxide patterns were fabricated on a bilayer-GaSe terminated Si(1 1 1) surface using an atomic force microscope (AFM) in air. The Si(1 1 1) surface regularly terminated by bilayer-GaSe is very stable in air, although it can be oxidized through the electrochemical reaction when positive sample bias voltage is applied between the surface and a conductive cantilever tip of AFM. It has been revealed that higher sample bias voltage, slower tip velocity and/or higher ambient humidity produce wider and/or thicker oxide protrusions. Then, nano-scale oxide patterns as narrow as 50 nm have been successfully drawn on the terminated surface by adjusting these experimental conditions. These oxide lines can be etched away by dipping the sample into aqueous HF solution, and nano-scale grooves can be fabricated on the bilayer-GaSe terminated Si(1 1 1) surface.

X-ray diffractometry of Si epilayers grown on porous silicon

X-ray double-crystal diffractometry was used to measure lattice deformation of porous silicon (PS) and Si epitaxial layers grown on PS. PS layers 1–10 μm in thickness and 15-65% in porosity were formed by anodization of n +-type Sb doped Si wafers in a 12% HF aqueous solution. Lattice deformations of both PS and epitaxial layers are shown to strongly depend on PS porosity. Grown on uniform PS 40–60% in porosity, the epilayers, single-crystal as they are, display high lattice deformation and defect density. Epilayers grown on two-layer PS are comparable with the films grown on the n +-type single-crystal Si substrate.

Improvement mechanism of photoluminescence in iron-passivated porous silicon

Taking into account the H 2 generation during porous silicon (PS) formation, a special hydrothermal etching process was designed and carried out in HF aqueous solution containing Fe 3+ ions. Several chemical reactions were found to take place on the surface of Si nanocrystallites, iron ion termination of Si dangling bonds, and formation of an outer layer of α-Fe 2O 3, which result in a stronger and more stable photoluminescence (PL) band peaking at 670 nm in comparison with normal PS. The improved PL was interpreted in terms of the decrease of the density of Si dangling bonds due to iron ion termination, and outer α-Fe 2O 3 layer protecting the inner c-Si from being oxidized.

AFM investigation of selective etching mechanism of nanostructured silica

We have developed the atomic force microscope (AFM) for in situ observation of SiO 2 etching in the HF aqueous solution. This work is devoted to the AFM investigation of etching of the SiO 2 surface layer modified by high-dose Fe + ion bombardment. Such samples have a two-phase nanostructure (SiO 2 containing buried Fe nanoparticles). Formation and dissociation of nanorelief during etching was observed in situ with AFM. The computer animation of this phenomenon was made with morphing of the experimental AFM images. The additional in situ registration of the optical absorption of Fe nanoparticles during etching, ex situ AFM and ferromagnetic resonance measurements enable us to propose that observed morphology transformation takes place because the etching rate of Fe nanoparticles is much higher than the SiO 2 etching rate. This etching mechanism was used for computer simulation of the AFM image transformation during etching of the model samples with buried nanoparticles. Good correlation of simulated and experimental AFM images and the corresponding surface roughness parameters vs. etching time confirms that this structural model and the mechanism of selective etching are correct.

High-barrier height Sn/p-Si schottky diodes with interfacial layer by anodization process

We have fabricated the Sn/p-Si Schottky barrier diodes with different surface treatments. Prior to the Sn evaporation on the p-Si(0 0 1), the first kinds of samples consisted of a dip in diluted aqueous HF solution followed by a rinse in de-ionized water (sample 1, SD1), the second kinds of samples several steps of anodization in aqueous KOH solution each followed by a dip in diluted aqueous HF solution and a subsequent rinse in de-ionized water (sample 2, SD2), and the third kinds of samples one anodization step only (sample 3, SD3). We have found the lowest values of both the barrier heights and ideality factors with the diodes of preparation type SD2. The anodization, on the other hand, have increased both, the barrier heights as well as the ideality factors. The extrapolation of the barrier heights versus ideality factors plot to the ideality factor determined by the image force effect have given the laterally homogeneous barrier heights of approximately 0.75 and 0.92 eV for the SD2 and SD3 diodes. Furthermore, we have calculated a mean tunneling barrier height of χ=0.12 eV for the MIS Sn/p-Si diodes with the anodic oxide layer.

Chemical stability of yttria doped zirconia membranes in acid and basic aqueous solutions: chemical properties, effect of annealing and ageing time

Corrosion resistance of 3Y–ZrO 2 microfiltration membrane was evaluated at room temperature and 80°C in aqueous solutions of acids and bases. Chemical properties were investigated in order to explain the chemical attack and the effect of elemental dissolution on the phase transformation. Dissolution of yttria was detected in acid solutions of low pH except in HF solution. In the basic solutions, dissolution of Y and Zr was negligible. Silica dissolved both in acid and basic solutions. As a result of chemical reactions and dissolution of ions, pH after corrosion showed a shift to higher values in acid solutions and to lower values in basic solutions, indicating the amphoteric reactions in 3Y–ZrO 2. Samples corroded in acid and basic solutions at 80°C were annealed at 1200°C after the corrosion test. Phase transformation was completely reversible in the samples tested in basic solutions. The sample in acid solution showed partial re-transformation of the monoclinic phase due to the dissolution of yttria. The effect of corrosion time was studied for aqueous solutions of HF and HCl.

Chemical Etching Characteristics of GaAs(100) Surfaces in Aqueous HF Solutions

Chemical etching characteristics of GaAs(100) surfaces in aqueous HF solutions have been studied using spectroscopic ellipsometry (SE), atomic force microscopy (AFM), X‐ray photoelectron spectroscopy (XPS), and wettability measurements. The SE data clearly indicate that the HF solutions cause the efficient removal of the GaAs native oxide upon immersing the samples in the solutions. When the native oxide is completely etch‐removed, the resulting surface has a roughened overlayer about 1.3 nm thick, which is about five times as large as the AFM rms value (∼0.2–0.3 nm). The discrepancy of this is due to the SE technique being sensitive to both the surface microroughness and the adsorbed chemical species, but AFM is sensitive only to the surface microroughness. The XPS data reveal that etching in the concentrated (50 wt %) solution selectively removes gallium while leaving elemental arsenic on the GaAs surfaces. The XPS data also indicate the presence of fluorine on the HF‐treated surfaces. The as‐degreased surface is hydrophilic. while the HF‐treated surfaces are highly hydrophobic. © 2000 The Electrochemical Society. All rights reserved.

Behavior of fluorine on silicon (100) surfaces etched with NH4F aqueous solutions

The bonding states and the behavior of F atoms on Si(100) surfaces exposed to air after etching with NH4F aqueous solutions were studied using X-ray photoelectron spectroscopy. F1s spectra from the Si surfaces could be separated into two Gaussian curves corresponding to respective Si–F and Si–F2 bonds, similar to the spectra from Si surfaces dipped into HF aqueous solutions. The majority of F atoms were in the Si–F2 bonding state. During the growth of native oxide on Si surfaces exposed to air, F1s peak intensities increased to a maximum and then decreased with increasing exposure time. The binding energies of both separated F1s components shifted toward higher binding energies. These results suggest that F atoms are distributed not only on the Si surface, but also beneath the Si surface, and move to the surface of the native oxide film formed by exposure to air. © 2000 Scripta Technica, Electron Comm Jpn Pt 2, 83(7): 41–47, 2000