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Boron (B) or phosphorus (P) doped silicon nanowires (SiNWs) were synthesized by laser ablation. Local vibrational modes of B were observed in B-doped SiNWs by micro-Raman scattering measurements at room temperature. Fano broadening due to a coupling between the discrete optical phonon and a continuum of interband hole excitations was also observed in the Si optical phonon peak for B-doped SiNWs. An electron spin resonance signal due to conduction electrons was observed only for P-doped SiNWs. These results prove that B and P atoms were doped in substitutional sites of the crystalline Si core of SiNWs during laser ablation and electrically activated in the sites.

Spectra of light transmitted through SiO2 specimens were measured during irradiation of amplified 800 nm ultrashort (400 fs) laser with a repetition rate of 100 kHz. The spectra exhibit a broad dispersion including a peak at around 450 nm, and the peak wavelength and the intensity are found to significantly depend on the irradiation time. Positively charged oxygen vacancies (E′ center) and oxygen-deficiency centers (ODCs) with neutral charge were observed by electron spin resonance (ESR) and photoluminescence (PL), respectively, after irradiation. The formation of the E′ center and the subsequent transformation to ODCs significantly depend on the laser power density. The correlation between the intensity of the peak at around 450 nm and the ESR signal intensity of the E′ center, and also the observation of PL spectrum due to ODCs suggest that self-trapped excitons exhibiting the peak at around 450 nm are created through multi-photon absorption, followed by the formation of the E′ center, and this finally leads to the formation of ODCs.

We have developed a technique to detect Si nanoparticles selectively and to measure size in situ. Applying the technique, we have investigated formation process of Si nanoparticles after pulsed laser ablation of Si targets in Ar gas. Time-resolved photoluminescence (PL) spectroscopy revealed that PL only from Si nanoparticles is observed below 2.4 eV while PL from Si nanoparticles as well as defects in SiO2 is observed above 2.4 eV. Therefore, Si nanoparticles can be detected selectively by excitation light with a photon energy below 2.4 eV. It is found that the onset of the PL from Si nanoparticles is delayed by approximately 0.3 ms from that of the defects and smaller Si nanoparticles. A size can be estimated by a band gap, which is roughly equal to the lowest photon energy at which Si nanoparticles can be excited. Thus, we estimated the sizes of growing Si nanoparticles.

We have investigated excitation of Er3+ ions via energy transfer from Si nanocrystallites embedded in SiO2 films. The Er-doped films were fabricated using a laser ablation technique. We found that a photoluminescence (PL) excitation spectra of Er3+ ions coincides with that of Si nanocrystallites. Thus, it is evident that Er3+ ions are excited via the luminescent singlet state in Si nanocrystallites. Furthermore, we obtained the results that support the energy transfer mechanism. PL intensity of Er3+ ions increases with Er concentration while that of Si nanocrystallites decrease inversely. PL intensity of Er3+ ions increases with temperature from cryogenic to room temperature under photo-excitation at power density higher than 110 mW/cm2. The increase is characteristic of the luminescent state in Si nanocrystallites but not any state in Er3+ ions.

We have investigated the formation process of silicon nanoparticles after laser ablation of silicon targets in argon gas. The nanoparticles exhibit bright photoluminescence in the visible wavelength range and can be applied to opto-electronic devices. In order to observe silicon nanoparticles, we have developed a decomposition method. The nanoparticles were probed by detecting light emission resulting from decomposition using a second laser. This method enables us to observe nanoparticles that cannot be observed directly by the methods applied so far. We have observed that the nanoparticles grow in time periods of 1.0–1.8 ms following ablation in Ar gas at 5 Torr when Si targets are ablated at 5 J/cm2 with a pulse width of 7 ns. The nanoparticles begin to grow above ablation spots slightly apart from the targets just after thermalization of the plume. We also found that the growth is delayed at higher fluxes of ablation laser light.

We observed very intense and highly reproducible photoluminescence (PL) spectra for SiOx films obtained by laser ablation of Si targets in 50-mTorr oxygen gas followed by proper annealing. It was found that the PL peak continuously changes from 1.4 eV at the center of the samples to 1.8 eV at the sample edge. The optimum values of the oxygen component in SiOx was x=1.3-1.4 and the optimum annealing temperature was 1000 °C for intense PL. From transmission electron microscopy images of annealed films, Si nanocrystallites are found to be formed in the matrix of SiO2 grown from the SiOx and have diameters of 2–3 nm. These indicate that a high density of Si nanocrystallites with diameters of 2–3 nm in the SiO2 phase are probably responsible for the PL and that the Si nanostructure is well formed from the as-deposited, metastable SiOx (x=1.3-1.4) films by annealing at 1000 °C.

Boron (B) doped silicon nanowires (SiNWs) were synthesized by laser ablation. Local vibrational modes of B in SiNWs were observed by micro-Raman scattering measurements at room temperature. Broadening due to a coupling between the discrete optical phonon and a continuum of interband hole excitations was also observed in the Si optical phonon peak. This is called Fano broadening. These results prove that B atoms were doped in substitutional sites of crystalline Si core of SiNWs during laser ablation.

Hydrogen passivation effect on the enhancement of photoluminescence (PL) of Er ions in SiO2films contained Si nanocrystallites (nc-Si) has been investigated. Er-doped SiO2films were fabricated by laser ablation of Er-deposited Si substrate in oxygen gas atmosphere. The PL intensity of Er ions and nc-Si were increased by hydrogen gas treatments, while ESR signal intensity of residual defects located at the interfaces between nc-Si and SiO2was decreased. These results indicate that hydrogen passivation of residual defects is useful for the enhancement of the Er PL.

Size control of silicon nanowires (SiNWs) synthesized by laser ablation of a Si target with nickel (Ni) as catalysts were investigated. The diameter of SiNWs decreased with decreasing synthesis temperature and content of Ni catalyst. Gradual down shift and asymmetric broadening of Si optical phonon peak depending on the diameter of SiNWs were observed for continuously, thermally oxidized SiNWs and SiNWs with smaller diameters formed at lower temperature. This can be interpreted by the phonon confinement effect. On the other hand, further thermal oxidation produced an upshift of the optical phonon peak. This is considered to be due to compressive stress since this stress was relieved by removing the surface oxide layers formed around the SiNW cores, resulting in a downshift of the optical phonon peak.

Utilizing laser ablation of Si targets, nanoparticles can be cleanly formed in rare gas. In order to fabricate nanoparticles with well-defined structures such as those whose surfaces are chemically modified, it is important to investigate the formation process of the nanoparticles. We have developed a decomposition method for measuring time-resolved spatial distributions of nanoparticles in rare gas. Applying this method, we have investigated formation processes of silicon nanoparticles in 2-Torr argon gas. The nanoparticles are found to grow from 300 Ais to 1 ms after the ablation.

Hydrogen molecules have been formed in crystalline silicon at various temperatures by a hydrogen-atom remote treatment. The Raman spectrum of the vibrational lines of hydrogen molecules in crystalline silicon is detected for silicon samples treated at temperatures between 250 and 500° C. The maximum production is obtained at 400° C. The Raman spectrum of hydrogen molecules in silicon observed at room temperature exhibits a frequency shift of around 4158 cm−1 and a very broad half-width of approximately 34 cm−1. Isotope shift also can be observed at around 2990 cm−1 in silicon treated with deuterium atoms at 400° C. The frequency shifts of the observed lines are in close agreement with those reported for molecular hydrogen and deuterium in gas, liquid, and solid phases. We discuss a model for the hydrogen molecule configuration and rule out the possibility of high-pressure hydrogen molecular gas in microvoids in crystalline silicon. These results indicate that hydrogen molecules exist at the tetrahedral interstitial sites in crystalline silicon.

Applying laser ablation technique, we have synthesized two types of SiO2 films that include nanometer-sized Si particles. One is synthesized by alternative deposition of Si nanoparticles layers and SiO2 layers. The synthesized film exhibits red photoluminescence (PL) with a peak energy below 1.5 eV. The other is synthesized by annealing at 1000°C of SiOx films, which are formed by laser ablation in diluted O2 gas. We find that there is a narrow range of composition for efficient red PL. Based on the experimental results, we tentatively discuss a possible model for the origin of the red PL.

Time-resolved Si lattice-temperature measurement has been developed on wide time scale from 10−9 to 100 sec during laser annealing, by utilizing the time-dependent optical interference in Si on sappire. This interference is due to small changes in Si refractive index induced by temporal changes in Si lattice-temperature. For ns–pulsed laser annealing, part of the absorbed photon energy is found to be transferred into lattice (phonons) in a time much shorter than 40-ns pulse duration. A new method using a microscope is demonstrated for time- and space-resolved Si latticetemperature measurements during cw laser annealing.

We report time-resolved X-ray absorption and extended X-ray absorption fine structure (EXAFS) measurements on amorphous silicon under nanosecond pulsed-laser irradiation. Each measurement was performed with one laser shot in the X-ray energy range from 90 to 300 eV. An X-ray absorption spectrum for induced liquid Si (liq*Si) was first observed above an energy density of 0.17 J/cm2. It differs significantly from the spectrum for amorphous Si and characteristically shows the disappearance of the Si-L(II,III) edge structure at around 100 eV. This phenomenon is interpreted in terms of a significant reduction in the 3s-like character of the unfilled part of the conduction band of liq*Si compared to that of amorphous Si. This is the first direct evidence that liq*Si has a metallic-like electronic structure. Timeresolved EXAFS results are also discussed briefly.