Shinji Yae

Metal-assisted etching (metal-assisted chemical etching) is an efficient method to fabricate porous silicon (Si). When using platinum (Pt) particles as metal catalysts in metal-assisted etching, a composite porous structure of straight macropores formed beneath the Pt particles and a mesoporous layer formed on the entire surface of Si can be fabricated. The formation mechanism of the composite structure is still open to discussion. We previously demonstrated that the ratio of mesoporous layer thickness to macropore depth showed a large value (approximately 1.1) in the case of highly-doped p-Si. In this study, we investigated the composite structure formation by using p-Si substrates with different doping densities and etching solutions with different concentrations of hydrogen peroxide (H₂O₂). There was not significant difference in the structures formed on low- and moderately-doped Si, despite the large difference in doping density. The ratio of mesoporous layer thickness to macropore depth increased within the range approximately from 0.1 to 0.4 with increasing the H₂O₂ concentration in the case of low- and moderately-doped Si, but it did not change in the case of highly-doped Si. We discussed the observation results based on the spatial distribution of hole consumption and the band structures at Pt/Si and Si/electrolyte interfaces.

This is the first report that applied LIBS to the analysis of electropolishing solution. Quantitative analysis of Nb dissolved in the solution was demonstrated by using porous silicon as the sample loading substrate.

Platinum (Pt) is one of the interesting catalysts in metal-assisted etching (metal-assisted chemical etching) of silicon (Si). The Pt-assisted etching induces not only the dissolution of Si under the Pt catalysts but also the formation of mesoporous layer on the Si surface away from them. In this work, we etched n-Si and p-Si by using patterned Pt films with a diameter of 5 μm and an interval of 50 μm. For both the cases, the Si surface under the Pt catalysts was selectively etched and macropores with a diameter of 5 μm were formed. The macropores formed on n-Si were deeper than those formed on p-Si. The mesoporous layer was observed only around the macropores on n-Si, while it was observed over the entire surface of p-Si. We also measured the open circuit potential of Si in the etching solution. The positive shift of potential of n-Si by the Pt deposition was smaller than that of p-Si except for the initial stage of etching, which can be explained by the polarization characteristics. We discussed the etching behavior of n-Si and p-Si on the basis of the results of structure observation and electrochemical measurements.

Strong correlations were found between underwater LIBS signals and bubble collapse time. Signal fluctuation caused by the repeated irradiation at a fixed position was successfully reduced by the normalization with bubble collapse time.

Surface and outgassing properties of 304 stainless steel samples were studied after electropolishing by a wiping method (WiEP) using felt that is attached to a cathode electrode and impregnated with an electrolyte. Surface morphology observed with an atomic force microscope suggests that WiEP yields a smoother surface with fewer pits compared with the conventional electropolishing method of immersing the samples in an electrolyte. The thickness of the oxide layer after either of the electropolishing processes was 3-4 nm as estimated by x-ray photoelectron spectroscopy, transmission electron microscopy, and energy-dispersive x-ray spectroscopy. Furthermore, no significant difference was found in the chemical state of the surface and oxide film in the two cases. Thermal desorption spectroscopy of the samples revealed that the amount of desorbed H2O and H2 was significantly low in the case of WiEP. The low outgassing was attributed to the formation of a smooth and dense oxide film on the sample surface after electropolishing by WiEP.

The existing states of co-deposited hydrogen in copper films electrolessly deposited from an ethylenediaminetetraacetatic acid complex bath were investigated using thermal desorption spectroscopy. Small desorption peaks at 500-700 K and large desorption peaks at 900-1300 K were observed in the as-deposited copper film, and these were attributed to the break-up of vacancy-hydrogen clusters and the desorption of molecular hydrogen from nanovoids, respectively. Moreover, an unknown desorption peak appeared at around 390 K. The total amount of desorbed hydrogen from the copper film in atomic ratio was 1.2 × 10−2, most of which was non-diffusible hydrogen desorbed above 800 K. A large number of nanovoids distributed in the grains were observed in the copper film. After 7 days, no significant change in hydrogen desorption above 500 K was observed, while the unknown desorption peak around 390 K disappeared. The as-deposited copper film showed lattice expansion and compressive residual stress, which were reduced after 7 days. Therefore, we attributed the unknown desorption peak appearing at around 390 K to the desorption of hydrogen atoms trapped by regular interstitial sites in the copper lattice.

Electroless deposition of metal particles on silicon (Si) followed by the metal-assisted etching (metal-assisted chemical etching) is a simple way to fabricate Si nanostructures. A composite porous structure consisting of straight macropores and a mesoporous layer can be created by platinum (Pt)-particle-assisted etching. In this work, we studied the composite structure formation on a highly-doped p-Si (ca. 5 × 1018 cm−3) in comparison with a moderately-doped p-Si (ca. 1 × 1015 cm−3). The composite structure drastically changed: the ratio of mesoporous layer thickness to macropore depth increased to 1.1 from 0.16 by using the highly-doped Si instead of the moderately-doped Si. The open circuit potential of Si in the etching solution shifted to the positive direction by the Pt deposition. The potential shift of highly-doped Si was smaller than that of moderately-doped Si, which can be explained by the polarization characteristics. We calculated the band bending in Si by using a device simulator that reproduced the conditions of Pt-particle-assisted etching. The results indicated that, in the case of highly-doped Si, the consumption rate of positive holes at the Si surface away from the Pt particles increases due to the tunneling effect, which is consistent with the thick mesoporous layer formation.

<p>The LIBS signal of the dry residue from a small amount of liquid sample is significantly enhanced by using a porous silicon substrate produced by gold-nanoparticle-assisted etching.</p>

Metal-assisted etching is a promising technique for microfabrication of semiconductors. In this method, porous silicon (Si) can be produced with a very simple procedure, and various nanostructures can be designed by changing the catalyst patterns. The kind of metal catalysts is one of the key factors to control the porous structure. In this work, we performed the etching of n-type Si (100) in a hydrofluoric acid solution containing hydrogen peroxide in the dark using silver, gold, and platinum particles electrolessly deposited at a constant coverage, and demonstrated the difference in the porous structures obtained for the different kind of metal catalysts. By comparing the mass loss of substrates with the depth of pores formed under the metal particles, we found that general corrosion occurred on the top-surface of the Si substrate around the metal particles even under the dark condition. The general corrosion depended on the metal species and it was explained by the formation and dissolution of a mesoporous layer. The kind of metal catalysts influences the dissolution of the Si surface not only under the metal catalysts but also between them.

Metallisation of silicon carbide (SiC) wafers is a key technology for producing efficient power devices. Conventional autocatalytic electroless deposition cannot produce adherent metal films directly on SiC substrates. The authors applied their recently developed surface-activation process for electroless metal-film deposition on silicon wafers to SiC wafers. Gold nanoparticles were produced on 4H-SiC substrates by displacement deposition after immersing the substrates in a tetrachloroauric(III) acid solution that includes hydrofluoric acid or potassium hydroxide. The size and the particle density of the deposited gold are changed with deposition parameters such as the surface condition of the substrates, the solution composition, and the UV-light illumination. The gold nanoparticles work not only as catalysts to initiate autocatalytic electroless deposition but also as binding-points between the metal film and the SiC surface. Adherent and uniform nickel-phosphorus alloy films are produced on such SiC substrates by autocatalytic electroless deposition without any further treatments.

Electrolessly deposited metal films on silicon substrates using gold nanoparticles as catalysts yield much higher adhesion than that by using palladium or silver nanoparticles. In this study, the structure of the gold nanoparticles is analyzed by X-ray diffraction and transmission electron microscopy. Single crystalline gold nanoparticles are epitaxially deposited on silicon substrates and silicon-gold alloy is formed at the interface. Those epitaxially grown gold nanoparticles cause the high adhesion of electrolessly deposited nickel films on silicon substrates.

The electrodeposition of Pt from aqueous solutions of K2PtCl4 (Pt(II)), H2PtCl6 (Pt(IV)), and a mixture of Pt (II) and Pt(IV) was studied using the electrochemical quartz crystal microbalance (EQCM) method. Pt deposition and cathode current flow began at the same potential in the Pt(II) solution. On the other hand, in the Pt(IV) solution, the cathode current increased at a more positive potential followed by Pt deposition at a more negative potential than in the Pt(II) solution. This difference in the potentials is due to the reduction reaction of Pt(IV) to Pt(II). Thus, Pt deposition in the Pt(IV) solution occurred in two potential ranges. In the first range, which was more positive than the second one, Pt was deposited via the reduction of Pt(II) to Pt(0). In the second range, direct deposition from Pt(IV) to Pt(0) proceeded, but was followed by hydrogen adsorption, which inhibited further Pt deposition. (C) 2015 Elsevier Ltd. All rights reserved.

Metal-assisted chemical etching of silicon is an electroless method that can produce porous silicon by immersing metal-modified silicon in a hydrofluoric acid solution without electrical bias. We have been studying the metal-assisted hydrofluoric acid etching of silicon using dissolved oxygen as an oxidizing agent. Three major factors control the etching reaction and the porous silicon structure: photoillumination during etching, oxidizing agents, and metal particles. In this study, the influence of noble metal particles, silver, gold, platinum, and rhodium, on this etching is investigated under dark conditions: the absence of photogenerated charges in the silicon. The silicon dissolution is localized under the particles, and nanopores are formed whose diameters resemble the size of the metal nanoparticles. The etching rate of the silicon and the catalytic activity of the metals for the cathodic reduction of oxygen in the hydrofluoric acid solution increase in the order of silver, gold, platinum, and rhodium.

Metal-enhanced HF etching of Si is an electroless method used to produce porous Si. Such etching generally uses not only metal-modified Si but also an oxidizing agent, such as hydrogen peroxide or metal ions. Pd exhibits high activity in enhancing the HF etching of Si without an oxidizing agent even under dissolved-oxygen-free and dark conditions. Electrolessly deposited Pd particles on n-type Si enhance the HF etching of Si but produce no porous layer. Patterned Pd films localize the etching under the boundary of the Pd deposited areas, and thus Pd can produce a microetch pattern on Si with a simple immersion in the HF solution. This etching reaction is explained by electron injection into the conduction band of Si due to the Pd-enhanced anodic oxidization of Si with water and the cathodic hydrogen evolution on Pd with the injected electrons.

We investigate the nucleation behavior in the electroless displacement deposition of metal particles (Pt, Rh, Pd, Cu, Ag, and Au) onto n-Si wafers from a metal-salt solution containing HE The particle density of metals varies widely from 106 (pt) to loll (Au) cm(-2), depending on the kind of metal. Deposited metals can be classified into two types of nucleation behavior. One consists of the platinum group elements, including Pt, Rh, and Pd, which display lower particle densities than elements of the other group and depend on the type of pretreatment of the n-Si wafer, and thus the surface conditions of Si. The second group consists of the copper group elements, including Cu, Ag, and An, which display higher particle density than the first group and are independent of pretreatment. The size of deposited particles decreases from hundreds nm to tens nm as the particle density increases. Moreover, the displacement deposition of the Pt and Ag particles onto n-Si are in progressive and instantaneous nucleation modes, respectively. (C) 2007 Elsevier Ltd. All rights reserved.

Microcrystalline silicon (mu c-Si:H) thin films, which are prospective low-cost semiconductor materials, are used as photoelectrodes for the direct conversion of solar energy to chemical energy. An n-type microcrystalline cubic silicon carbide layer and an intrinsic mu c-Si:H layer are deposited on glassy carbon substrates using the hot-wire cat-CVD method. The mu c-Si:H electrodes are modified with platinum nanoparticles through electroless displacement deposition. The electrodes produce hydrogen gas and iodine via photoelectrochemical decomposition of hydrogen iodide with no external bias under solar illumination. Surface modification with platinum nanoparticles and surface termination with iodine improve the conversion efficiency. (c) 2006 Elsevier B.V. All rights reserved.

Antireflection of silicon (Si) surface is one key technology for the manufacture of efficient solar cells. Metal particle enhanced HF etching is applied to produce uniform antireflecting porous layer on multicrystalline Si wafers that cannot be uniformly texturized by anisotropic etching with an alkaline solution. Fine platinum (Pt) particles are deposited on multicrystalline n-Si wafers by electroless displacement reaction in a hexachloroplatinic acid solution containing HF. Both macroporous and luminescent microporous layers are uniformly formed by immersing the Pt-particle-deposited multicrystalline Si wafers in a HF solution. The reflectance of the wafers is reduced from 30% to 6% by the formation of porous layer. The photocurrent density of photoelectrochemical solar cells using porous multicrystalline n-Si has a 25% higher value than non-porous Si cells. (c) 2005 Elsevier Ltd. All rights reserved.

We have developed an autocatalytic plating solution for pure nickel deposits using hydrazine as a reducing agent. The solution is characterized by its extended lifetime, high deposition rate and high deposit brightness. In this study, we have found that a simple solution consisting of nickel acetate and hydrazine can deposit black nickel films with stability. The deposition rate was 0.79 nm/sec. The addition of a small amount of formaldehyde to the plating solution gave bright nickel films. The purity of the deposited nickel was higher than 99.7%. Its electrical conductivity increased with purity.

A solution has been developed for autocatalytic (electroless) pure Ni plating using hydrazine as a reducing agent that has three desired characteristics: high stability, a high deposition rate, and high deposit brightness. The solution components are nickel acetate, hydrazine, ethylenediaminetetraacetic acid, lactic acid, and boric acid. Saccharin sodium and formaldehyde are used as additives in some cases. The deposition rate and surface morphology of the films change with the composition of the plating solution. The deposition rate reaches 3.0 nm s(-1). The brightness of the Ni films depends on the surface morphology. The reflectance of the deposited films at a wavelength of 550 nm changes from 0.6% (black) to 54%, the same level as autocatalytic Ni-P alloy films. The total impurity content (C, N, B and S) of the deposited Ni films is less than 0.4 mass%. The lowest value of the total content is 0.12 mass%. Electrical conductivity increases with purity and reaches 11.3 x 10(6) S m(-1), which is equivalent to pure Ni foil with a purity of 99.7%. The deposition rate, surface morphology, brightness, and electrical conductivity of autocatalytic pure Ni films can be controlled by changing the composition of the plating solution.

Porous Si was formed on n-Si wafers, modified with fine Pt particles, by simply immersing the wafers in a HF solution without a bias or an oxidizing agent. The Pt particles were deposited onto n-Si wafers by electrodeposition or electroless displacement deposition. SEM images show that many pores, ranging between 0.1 and 0.8 mum in diameter and covered with a luminescent nanoporous layer, were formed only on the Pt-modified area of the n-Si surface by immersion in 7.3 M HF solution for 24 h. The weight loss of Pt-electrodeposited n-Si wafer was 0.46 mg cm(-2), corresponding to ca. 2 mum in thickness. The weight loss and the structure of porous Si changed with the etching conditions, such as concentration of dissolved oxygen in the HF solution, distribution density of metal particles, and different kinds of metal particles. A photoelectrochemical solar cell equipped with a Pt-particle-modified porous n-Si electrode gave 13.3 mW cm(-2) of maximum output power, which corresponds to a 13% conversion efficiency and is higher than that for the Pt-particle-modified flat n-Si electrode. (C) 2003 Elsevier Science B.V. All rights reserved.

For electroless Co-P based alloy films, corrosion resistance increases with increased W content due to passivity, but decreases with increasing Zn content. The coercivity of the films increases with the Zn content, and is independent of the W content. The measurement of film magnetisation using a magnetic balance is effective for the determination of quantitative corrosion loss of ferromagnetic films, such as Co alloys. The electroless Co-W-Zn-P film, about 3 mum in thickness, containing 1.2 atom % of W and 1.6 atom % of Zn gave high coercivity of 69 kA m(-1) and high corrosion resistance, three times that of Co-Zn-P films. XPS analysis indicated that an oxide of Co and WO3 were formed on, and Zn was eluted from, the surface of the Co-W-Zn-P film by the corrosion. The passive WO3 layer improves the corrosion resistance of films.

Fine platinum (Pt) particles were deposited electrochemically on n-type silicon (n-Si) electrodes from an aqueous hexachloroplatinic acid(IV) solution by the single potential step (SPS) and double potential step (DPS) methods. The distribution density of the Pt particles on n-Si was 10(8) cm(-2) for the SPS method, whereas it increased from 10(9) to loll cm(-2) by a shift of the pulse potential at the initial step of the DPS method from -1.0 to -4.0 V versus SCE and remained nearly constant at more negative potentials. The size of the Pt particles enlarged with the charge density passing across the electrode surface at a potential of -0.70 V versus SCE, which was applied throughout for the SPS method and at the second step for the DPS method. Photoelectrochemical (PEC) solar cells equipped with Pt-electrodeposited n-Si electrodes generated open-circuit photovoltages (V-OC) of 0.51-0.61 V, much higher than those for n-Si electrodes coated with continuous Pt layers (ca. 0.2-0.3 V). Solar cell characteristics changed with the pulsed potential and charge density passing across the electrode surface which changed the size and distribution density of the Pt particles. The characteristics were explained well by our previous theory on metal-dot coated n-Si electrodes. (C) 2001 Elsevier Science Ltd. All rights reserved.

Photoelectrochemical reduction of carbon dioxide (CO 2 ) on p-type silicon (p-Si) electrodes modified with small metal (Cu, Ag, or Au) particles has been studied. The electrodes in CO 2 -saturated aqueous electrolyte under illumination produce methane, ethylene, carbon monoxide, etc., similar to the metal (Cu, Ag, or Au) electrodes, but at ca. 0.5 V more positive potentials than the corresponding metal electrodes, contrary to continuous-metal-coated p-Si electrodes. The results clearly show that the metal-particle-coated p-Si electrodes not only have high catalytic activity for electrode reactions but also generate high photovoltages and thus work as an ideal type semiconductor electrode. It is discussed that the CO 2 photoreduction proceeds with an upward shift of the surface band energies of p-Si in order to get energy level matching between the semiconductor and solution reactants, though hydrogen photoevolution occurs without such an upward shift. It is also discussed that the control of surface structure on a nanometer-sized level, as well as on an atomic scale, is important for getting higher efficiencies.

A p-type silicon (p-Si) electrode modified with small metal (Cu, Au and Ag) particles works as an ideal-type electrode for photoelectrochemical reduction of carbon dioxide, producing carbon monoxide, methane, ethylene, etc., with a large photovoltage of 0.5 V. It is discussed that two types of surface-structure control on atomic and nanometer-sized levels are important for getting a high efficiency. (C) 1997 Elsevier Science B.V.

Porous n-Si electrodes, prepared by photoetching in HF under appropriate conditions, have macroporous layers at the surface, consisting of micrometer-sized pores and Si pillars. The wall and top of the Si pillars are further covered with 0.2-0.5-mu m-thick nanoporous layers having nanometer-sized pores. The nanoporous layer can be thinned by immersion in HF. The solar cell characteristics (open-circuit photovoltage V-OC, fill factor, and stability) for the porous n-Si electrodes with Pt coating in 8.6 M HBr/0.05 M Br-2 were improved by thinning the nanoporous layer to an appropriate thickness, although the electrodes with no nanoporous layers gave only poor characteristics. The maximum solar energy conversion efficiency of 14% (V-OC 0.575 V, j(SC) 34.7 mA . cm(-2), and fill factor 0.701) was obtained, which is one of the highest for n-Si photoelectrochemical solar cells. A mechanism for the generation of high V-OC's as well as high fill factors in porous Si-based photoelectrochemical solar cells is discussed including a possibility of a low resistivity of the nanoporous layer for hole transport.

The open-circuit photovoltages (V(OC)s) of photoelectrochemical (PEG) solar cells, equipped with n-Si electrodes modified with ultrafine platinum (Pt) particles, have been investigated as functions of the Pt-particle density and the post-heat-treatment temperature. Langmuir-Blodgett layers of Pt-colloid particles stabilized by polyvinylpyrrolidone (PVP) are used to control the Pt-particle density on the n-Si electrodes. The ideality factor (n) and the dark saturation current density (j(0)) are determined from plots of logarithm of the short-circuit photocurrent density versus V-OC. The n is unity for all the electrodes, independent of both the Pt density and the heat-treatment temperature, indicating that an ideal n-Si/solution junction is formed. The j(0) decreases and, hence, the V-OC increases as the Pt density decreases and also as the post-heat-treatment temperature is decreased. The decrease of j(0) is caused by the decrease in the majority carrier dark current density (j(0n)). In cases where the Pt-modified n-Si electrodes are heat-treated at a low temperature of 150 degrees C or not heat-treated, V-OC is in the range of 0.61 to 0.63 V, independent of the Pt density, indicating that minority carrier controlled solar cells are obtained.

The mechanism of generation of high open-circuit photovoltages (V(oc)s) of 0.62-0.63 V for n-Si (similar to 1 Omega cm) electrodes modified with colloidal Pt particles (4 nm in diameter) is investigated by measurements of minority-carrier (hole) diffusion length (L(p)) and temperature dependence of V-oc Langmuir-Blodgett (LB) layers of colloidal Pt particles are used to control the Pt density on n-Si. The L(p) value is determined to be 200 mu m, irrespective of whether n-Si is modified with Pt or not. The temperature dependences of V(oc)s at 203-298 K have been explained well by our previously proposed model. It is shown that heat treatments of the Pt-modified n-Si electrodes increase the area and the width of the direct Pt-Si contacts and thus decrease V-oc, but minority-carrier controlled (ideal) solar cells are obtained if the electrodes are prepared under appropriate conditions. Copyright (C) 1996 Elsevier Science Ltd

Various platinum (Pt) colloid solutions were prepared and used for the modification of n-type silicon (n-Si) electrodes. All the photoelectrochemical (PEC) solar cells, equipped with the Pt-colloid modified n-Si electrodes thus prepared, gave the open-circuit photovoltages (V(oc)) of 0.550 to 0.630 V, much higher than those (0.25 approximately 0.30 V) for n-Si electrodes coated with continuous thin Pt layers, clearly showing the beneficial effect of the discontinuous metal coating. The PEC cells, equipped with the textured-surface n-Si electrodes modified with the Pt colloid particles prepared by Bredig's method, yielded an energy conversion efficiency of 14.9% in a three-electrode configuration. The relation between the V(oc) and the size and distribution of the Pt particles is discussed.

A Langmuir layer of ultrafine platinum particles (2–6 nm in diam) has been developed on a water surface by dropping a Pt colloid solution, prepared by refluxing an ethanol-water (1:1) solution of hexachloroplatinic(IV) acid in the presence of poly(N-vinyl-2-pyrrolidone) as a stabilizer. The layer is transferred onto a single-crystal n-type silicon (n-Si) wafer by the horizontal lifting method. The Pt particles are rather homogeneously scattered on n-Si, and the particle density can be controlled on a nanometer scale by changing the area of the Langmuir layer at the time of transfer. The open-circuit photovoltage (Voc) for photoelectrochemical (PEC) solar cells with such n-Si electrodes is inversely related to Pt-particle density, and reaches 0.635 V, much higher than that for n-Si coated with a continuous Pt layer (ca. 0.30 V) or that for the conventional p-n junction Si solid solar cell of a similar simple cell structure (ca. 0.59 V). This result is in harmony with our previously proposed theory, the above increase in Voc being explained by the decrease in the majority carrier dark saturation current density. © 1994, The Electrochemical Society, Inc. All rights reserved.

We previously made it clear theoretically that semiconductor electrodes coated with minute metal islands were stable and generated very high open-circuit photo voltages (V^c)? and showed that n-type single crystal silicon electrodes coated with minute platinum (Pt) islands actually generated remarkably high V0c of 0. 68 V. In the present paper we have tried to apply this method to amorphous silicon (a-Si) electrodes having n-i junctions (cf. Fig. 1 (b)). The a-Si electrodes were prepared with an rf plasma CVD apparatus, by depositing n-type a-Si layers and then i-type a-Si layers on metal plates or conductive oxide films deposited on glass plates. Four methods (cf. the first column of Table 1) were employed to deposit minute Pt islands. All the a-Si electrodes thus coated with Pt were stable and showed F0o&gt s in the range from 0. 57 to 0. 76 V, much higher than those for a-Si electrodes coated with continuous Pt layers (0.37 to 0.47 V) (Fig. 3 and Talbe 1). The a-Si electrodes, coated with 3-nm thick Pt layers and etched in hot alkali solutions, showed the highest Voc of 0.76 V (Table 1). This VQO is nearly equal to that of an n-i-p junction a-Si solid-state solar cell (0.72 to 0.87 V). These results are interesting in that they point to a new direction of the development of highly efficient photoelectrochemical solar cells using a-Si electrodes. © 1988, The Chemical Society of Japan. All rights reserved.