松本 歩

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.

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.

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.

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>

<p>The first report on general corrosion during metal-assisted etching of silicon.</p>

© 2016 Elsevier B.V. All rights reserved. We investigate the possibility of on-site multi-element analysis of part-per-million order Cu2 + and Zn2 + in an aqueous solution by underwater laser-induced breakdown spectroscopy (underwater LIBS) of a Pt electrode on which Cu2 + and Zn2 + are electrodeposited in advance. Atomic emission lines of Cu and Zn are simultaneously observed. To apply a calibration-free LIBS (CF-LIBS) approach to the present system, we estimate the Zn/Cu ratio in the laser ablation plasma from the emission spectra. Although the Zn/Cu ratio in the deposit agrees with the ratio in the solution, the ratio in the plasma considerably deviates from the ratio in the deposit. We demonstrate the possibility of quantitative analysis even in such a case by correcting the deviation using a simple model considering heat propagation and thermal evaporation during liquid-phase laser ablation.

© 2015 American Chemical Society. We investigate the transfer of the dissolved species into the plasma during laser ablation in liquid. Laser ablation of a Cu target in LiCl + NaCl aqueous solutions and emission spectroscopy of the plasma are performed. Emission lines of Li and Na atoms are observed. Atomic density ratio NLi/NNa in the plasma estimated from the emission spectra agrees with the concentration ratio CLi/CNa in the solutions. This suggests the phase explosion of water due to the strong interaction between the extremely high-temperature matter ablated from the target and the surrounding water. This causes a cavitation bubble and the transfer of the dissolved species from the solution into the bubble without any element selectivity.

© 2015 American Chemical Society. We propose a technique of on-site quantitative analysis of Zn2+ in aqueous solution based on the combination of electrodeposition for preconcentration of Zn onto a Cu electrode and successive underwater laser-induced breakdown spectroscopy (underwater LIBS) of the electrode surface under electrochemically controlled potential. Zinc emission lines are observed with the present technique for a Zn2+ concentration of 5 ppm. It is roughly estimated that the overall sensitivity over 10 000 times higher is achieved by the preconcentration. Although underwater LIBS suffers from the spectral deformation due to the dense plasma confined in water and also from serious shot-to-shot fluctuations, a linear calibration curve with a coefficient of determination R2 of 0.974 is obtained in the range of 5-50 ppm.