Tetsuo Sakka

Laser ablation at solid–liquid interfaces: An approach from optical emission spectra

The emission spectra from the solid–liquid interface irradiated by a pulsed laser were studied. The solid target used in this study was graphite and boron nitride, and the liquid in which the target was immersed was water, benzene, n-hexane, and carbon tetrachloride. The results showed strong continuous spectrum immediately after a pulse shot, whereas after ≈100 ns later from the irradiation it was greatly reduced, and instead, the emission from small molecules dominated the spectra. The line spectra of small molecules observed in the later time range indicate the chemical reaction between the ablated species and the species originated from the liquid molecules. The intensity of the continuous spectrum was very prominent compared to what has been observed for solid–gas interfaces. This is due to rapid electron ion recombination or bremsstrahlung due to highly confined interface plasma.

Oxidation of Porous Silicon under Water Vapor Environment

The oxidation process of porous silicon in wet air at 323 K was investigated by infrared spectroscopy, particularly focusing on the behavior of the stretching vibrations of Si-H. The fine structure of the absorption bands were clarified with help of vibrational analysis by ab initio molecular orbital calculations. Absorption bands at 2150-2300 cm -1 are assigned to the Si-H vibrations due to OSiH 3 , O 2 SiH 2 , and O 3 SiH in order of increasing frequency, respectively. The presence of H 2 O causes Si-Si bond breaking with dissociative adsorption of H 2 O, resulting in an increase in the amount of Si-H species, which is readily oxidized. The bond breaking causes a prominent enhancement of O 3 SiH in the oxidized states. This contrasts with the behavior of the oxidation in dry air where the introduction of oxygen keeps the amount of Si-H species constant in the course of the oxidation and the growth of O 2 SiH 2 and OSiH 3 in addition to O 3 SiH

Hydrogen in Porous Silicon: Vibrational Analysis of SiH x Species

To assign infrared (IR) absorption bands related to Si-H vibrations in porous silicon, harmonic frequencies for model clusters were calculated by an ab initio molecular orbital (MO) method using a 6-31 G * basic set. Vibrations associated with a group of interest can be obtained by terminating the cluster with pseudoatoms of a heavy hydrogen isotope in the calculations. The calculated results reproduce the measured IR spectrum accurately, indicating that the isolation technique is effective, although the localization for some deformation modes, particularly the rocking modes, is insufficient at lower frequencies. It is concluded from the results of the calculated and measured spectra that there are clustered states such as (SiH 2 ) n and (SiH) 2 in addition to the monomer states, SiH x , and that absorption bands at 600 to 700 cm -1 are due to the vibrations of the SiH x bending or deformation mode rather than the Si-Si lattice vibration

Metal Deposition onto a Porous Silicon Layer by Immersion Plating from Aqueous and Nonaqueous Solutions

Immersion plating of metals (Ag, Cu. Ni) onto a porous silicon (PS) layer from aqueous and nonaqueous solutions has been studied. The modified PS layers after the immersion plating were analyzed by X-ray diffraction and X-ray photoelectron spectroscopy. Fourier transform infrared spectroscopy and scanning electron microscopy were also performed to investigate the structural changes and microstructure of PS samples after the plating process. In booth solutions, the deposition of metal oxidizes PS simultaneously to SiO 2 . The different deposition behaviors are discussed in terms of different rest potentials of PS in these solutions and electrode potential of each metal. Immersion plating in nonaqueous organic solutions shows that a trace of residual water affects the metal deposition. Based on the results obtained, the mechanism of metal deposition is proposed. The metal deposition proceeds by nucleation and growth via the local cell mechanism. It is also found that metal deposition proceeds very differently on Si wafer and PS surfaces. The different deposition behaviors on both surfaces are discussed.

Use of a long-duration ns pulse for efficient emission of spectral lines from the laser ablation plume in water

The effect of pulse duration upon the line profile of Cu I emission observed by laser ablation of a copper metal plate immersed in water has been examined. By irradiating a pulse with the duration longer than 40 ns the spectral profile with clear narrow emission lines of Cu atoms is obtained, while the emission spectra always suffer from broadening and self-absorption by the irradiation of the 20 ns pulse for the ablation. The results show that the use of a long-duration pulse enables in situ elemental analysis of the solid surface in contact with a bulk liquid.

Spatial population distribution of laser ablation species determined by self-reversed emission line profile

We propose a method for determining the spatial distribution of population densities for the species in laser-produced plasma. Our method relies on the parameter fittings of the experimentally observed self-reversed emission profiles to the model which is based on the calculation of one-dimensional radiative transfer. Employed parameters in the model represent spatial distribution of emitters, absorbers, and plasma free electrons. Since the density of plasma electrons has a spatial dependence, Stark shifts and broadenings are incorporated in a position-sensitive manner. After a general description of the method, we have specifically applied it to the laser-ablated Al plasma, where Al(I) 2P∘–2S emission line is employed for the analysis. In this specific example, we find that the accuracy of the fittings is significantly improved due to the presence of two emission lines originating from the fine structure, i. e., 2P1/2∘–2S1/2 and 2P3/2∘–2S1/2. In particular, the depth of the self-reversed structure turns ...

Metal deposition into a porous silicon layer by immersion plating: Influence of halogen ions

Metal deposition into a porous silicon (PS) layer by immersion plating has been studied. Ag and Cu were found to deposit on PS, while Ni was found not to deposit. The dependence of the amount of Cu deposition on Cu2+ concentration, halogen ion concentration, and immersion time was investigated using chelatometric titration. Copper deposition from halide solutions exhibited an unusual behavior; the amount increased with increasing concentration, then decreased, and no copper deposited at high concentration. This is because adsorption of chloride and bromide ions inhibits the copper deposition process. We have also discussed the metal deposition mechanism on the basis of x-ray diffraction, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy measurements. They revealed that metal deposition occurred simultaneously with the oxidation of silicon to SiO2. Copper crystals 30–80 nm in diameter deposited on the oxidized PS surface rather than on the unoxidized PS surface.

Structural change in p-type porous silicon by thermal annealing

The morphological change of p-type porous silicon during annealing has been investigated. The x-ray diffraction (XRD) pattern was composed of a sharp Bragg reflection peak and a diffuse scattering. The diffuse scattering is not related to the presence of the amorphous phase. The shape of the XRD pattern started to change at an annealing temperature as low as 400 °C, and the 2θ angle of the sharp peak varied at a temperature as low as 350 °C. These changes at low temperatures seem to be closely related to the desorption of hydrogen and the resultant change of the dangling bond density in porous silicon. The molecular orbital calculations also support the participation of dangling bonds in the structural reorganization in the surface region.

Rotational spectra and temperature evaluation of C2 molecules produced by pulsed laser irradiation to a graphite–water interface

Temperature of an ablation plume produced by pulsed laser irradiation to a graphite target submerged in water was evaluated as a function of time by analyzing the emission spectra of C2 molecules. The method is based on the determination of rotational temperature from the rotational spectra of the (0,0) Swan band of C2 molecules. In the time range shorter than 1000 ns from the ablation laser pulse, the rotational temperature of ∼6000 K was obtained. After 1000 ns it decreases rapidly, in contrast to the temperature obtained for the irradiation in air, where the decrease of the temperature is rather slow. The linewidth obtained as one of the fitting parameters suggests the high density and high pressure of this region. Temporal behavior of the laser ablation plume in water is discussed.

Changes in the environment of hydrogen in porous silicon with thermal annealing

Changes in hydrogen environments in porous silicon prepared from a p-type silicon wafer with thermal annealing were studied to make an exact assignment of the infrared absorption spectrum and to clarify the fine structures of hydrogen in porous silicon. Annealing makes it possible to desorb the SiH 3 and SiH 2 states from porous silicon. The results, with the help of the vibrational analysis by molecular orbital calculations, revealed the following. First, an absorption band at 2100 cm -1 is composed of seven fine structures and their assignment is clarified. Second, the wagging mode of SiH 2 contributes an absorption band at 667 cm -1 , and finally it is confirmed that the strongest absorption band at 626 cm -1 belongs to the bending vibration of SiH. Further, oxidation behavior under wet environments of porous silicon in which hydrogen is partially desorbed is compared with that without the desorption. Acceleration of the oxidation is explained in terms of strained Si-Si bonding.