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To improve the light-scattering capacity of transparent conductive oxide (TCO) films without increasing in surface roughness, we formed a stacked nanoparticle layers of ZnO (NP-ZnO) and TiO2 (NP-TiO2), which serve as light-scattering and surface-modifying layers, respectively, between the glass substrate and low-resistive TCO layers. The stacked TCO/NP/glass (NP-TCO) substrate indicated strong light-scattering capacity, with a relatively low surface root mean roughness of approximately 10nm. In addition, the haze value of NP-TCO substrates increased with an increase in the component particle diameter of the underlying NP-ZnO layers from about 20% (33nm) to 40% (96nm) at a wavelength of 550nm. [doi:10.2320/matertrans.M2014208]

In doped Si nanowires (SiNWs) boron (B) atoms segregate to the surface oxide layers during thermal oxidation, while phosphorus (P) atoms preferentially pile up in Si crystalline regions close to the Si/SiO2 interface. Here we report on micro-Raman scattering and electron spin resonance (ESR) measurements showing that B atoms can be stabilized at the crystalline Si core region in codped SiNWs with average diameters of 20–30 nm because of the strong interaction between B and P atoms during thermal oxidation below 800 °C. Theoretical calculation clearly demonstrated the effect of B–P pairing, which can stabilize the B atoms in the Si side. In the B–P pairing configuration, dopant passivation—beyond simple compensation—occurs, making the impurities electrically inactive.

Recrystallization of silicon nanowires (SiNWs) after ion implantation strongly depends on the ion doses and species. Full amorphization by high-dose implantation induces polycrystal structures in SiNWs even after high-temperature annealing, with this tendency more pronounced for heavy ions. Hot-implantation techniques dramatically suppress polycrystallization in SiNWs, resulting in reversion to the original single-crystal structures and consequently high reactivation rate of dopant atoms. In this study, the chemical bonding states and electrical activities of implanted boron and phosphorus atoms were evaluated by Raman scattering and electron spin resonance, demonstrating the formation of p- and n-type SiNWs.

*1 Institute of Applied Physics, University of Tsukuba, 1-1-1 Ten'nodai, Tsukuba, Ibaraki 305-8573, bk200820379@s.bk.tsukuba.ac.jp *2 Graduate School of Information Sciences and Electrical Engineering, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395,Japan *3 Department of Health Sciences, School of Medicine, Kyushu University, 3-1-1 Maidashi, Fukuoka 812-8582, Japan *4 Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki305-8565, Japan

Silica glass can be ablated using focused laser plasma soft X-rays. The ablation technique enables us to fabricate trenches with a width as narrow as 50 nm. In the present paper, we have investigated the nano-ablation process. The soft X-ray irradiation causes a silica surface broken into almost atomic species. Ionic species have kinetic energies higher than that gained by heating to the boiling point. We measured the ablation depth as a function of the soft X-ray fluence. The depth analysis revealed that soft X-rays are absorbed in a silica surface with an effective absorption depth of 10 nm. The result indicates that the energy density of the soft X-rays per unit volume at the threshold fluence is comparable to that required for breaking silica glass into atomic species. Further, the results suggest that ablation occurs before diffusion of absorbed energy into the surrounding region. In addition to energy absorption, repulsive forces between ionic species may cause ablation of the silica surface by soft X-ray irradiation. These properties of soft X-ray ablation may make possible the nano-ablation of silica glass. © 2011 Wiley Periodicals, Inc. Electron Comm Jpn, 94(9): 30–35, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10354

We are investigating the micromachining of silica glass using extreme ultraviolet (EUV) light from laser-produced plasma (LPP). The present investigation deals with the ablation using EUV light of around 13.5 nm and 11 nm generated in laser-produced Sn plasma and Xe plasma, respectively. CO2 laser and Nd:YAG lasers were used as pump lasers. The EUV radiation was focused on the surface of silica glass through the meshed mask using an ellipsoidal mirror coated with gold film. The results showed that the silica glass was successfully ablated by EUV light from the Nd:YAG-LPP. The maximum ablation rate was 42 nm per shot for 11-nm light, and 25 nm per shot for 13.5-nm light. On the other hand, EUV radiation from the CO2-LPP did not ablate the silica glass. This is presumably caused by the lower EUV irradiation intensity due to the long pulse duration of the CO2 laser.

Silica glass can be ablated using focused laser plasma soft X-rays. The ablation technique enables us to fabricate trenches with a width as narrow as 50 nm. In the present paper, we have investigated the nano-ablation process. The soft X-ray irradiation cause silica surface broken into almost atomic species. Ionic species have kinetic energies higher than that gained by heating up to the boiling point. We measured ablation depth as a function of soft X-ray fluence. The analysis of the depth revealed that soft X-rays are absorbed in silica surface with a effective aborption depth of 10 nm. The result leads to that the energy densty of the soft X-rays per unit volume at the threshold fluence is comparable to that required for breaking silica glass into atomic species. Futher, the results suggests that ablation occurs before diffusion of absorbed energy into the surroudning region. In addition to the energy absorption, repulssive force between ionic species may cause ablation of silica surface by soft X-ray irradiation. These properties of soft X-ray ablation may achieve nano-ablation of silica glass.

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.

We have investigated nanomachining of inorganic materials using laser plasma soft X-rays. The soft X-rays were generated by irradiating Ta targets with pulsed Nd:YAG laser light. The laser plasma soft X-rays were focused using an ellipsoidal mirror, which is designed so as to focus soft X-rays at around 10 nm efficiently. The focused soft X-rays were incident to the surfaces of inorganic materials such as silica glass, LiF, CaF2, and LiNbO3. It is found that these materials are ablated by soft X-ray irradiation. In particular, silica glass can be ablated at rates of 0.2 to 150 nm/shot, which can be controlled by the intensity of soft X-rays. It is remarkable that silica can be ablated smoothly with a roughness of 1 nm after 10 shots of soft X-ray irradiation at a rate of 50 nm/shot. In order to demonstrate nanomachining of silica glass, we used contact masks on top of silica glass plates fabricated by electron beam lithography technique. The silica glass plates were irradiated with laser plasma soft X-rays through the windows of the contact masks. We found that nanofabrication of trenches with a width of 70 nm was performed clearly. © 2008 Wiley Periodicals, Inc. Electron Comm Jpn, 91(4): 45– 51, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10080

The formation and annihilation of hydrogen (H)-related complexes were investigated in boron (B)- or phosphorus (P)-doped Si treated with high concentration of atomic H. The passivation and reactivation process of dopant carriers were significantly different between the p-type and n-type specimens. The differences are explained by the stable sites of the H atoms in the p-type and n-type specimens and, in turn, by the formation of H-related defects: i.e., H multiple trapping centers are formed by bond-breaking due to H atoms only in p-type B-doped Si. The formation of such defects retards the reactivation of B dopants by annealing.

Hydrogen passivation of phosphorus (P) donors and defects in P-doped silicon nanowires (SiNWs) were investigated by electron spin resonance (ESR) at 4.2 K. P doping was performed during the synthesis of SiNWs by laser ablation. Phosphorus doping into substitutional sites of crystalline Si in SiNWs was demonstrated by detection of an ESR signal with a g-value of 1.998, which corresponds to conduction electrons in crystalline Si. ESR signals related to defects in surface oxide layer and at interface between surface oxide and crystalline Si core were observed. These P donors and the defects were partially passivated by hydrogen and oxygen atoms as seen in bulk Si.

The effect of phonon confinement and impurity doping in silicon nanowires (SiNWs) synthesized by laser ablation were investigated. The diameter of SiNWs was controlled by the synthesis parameters during laser ablation and the subsequent thermal oxidation. Thermal oxidation increases the thickness of the SiNWs’ surface oxide layer, resulting in a decrease in their crystalline Si core diameter. This effect causes a downshift and asymmetric broadening of the Si optical phonon peak due to phonon confinement. Boron doping was also performed during the growth of SiNWs. Local vibrational modes of boron (B) in silicon nanowires (SiNWs) synthesized by laser ablation were observed at about 618 and 640 cm–1 by Raman scattering measurements. Fano broadening due to coupling between discrete optical phonons and the continuum of interband hole excitations was also observed in the Si optical phonon peak. These results prove that B atoms were doped in the SiNWs.

We have investigated the fabrication of two types of Er-doped silicon oxide films. The films were prepared by ablating a Si target covered with a thin Er metal layer, by Nd:YAG laser light at 50 mJ/pulse in 40 Torr O2 gas. After depositing the Er-dispersed SiOx (x ~ 1.4) films, the films were annealed in Ar gas. We found that Er-doped films deposited at (a) 4 J/cm2 and (b) 100 J/cm2 have the optimum annealing temperatures of 600°C and 900°C, respectively. Furthermore, we found that Er-doped films deposited at 4 J/cm2 exhibit much more intense light emission at 1.5 µm than those deposited at 100 J/cm2. For the Er-doped films deposited at 100 J/cm2, it is evident that electron-hole pairs are generated in Si nanocrystallites precipitated in a SiO2 film and that recombination energy is transfered to Er3+ ions that emit 1.5 µm light, via the lowest luminescent state in Si nanocrystallites.

We have investigated micromachining of a variety of materials by irradiation with laser plasma soft X-rays (LPSXs) at around 10 nm. The pulsed LPSXs were generated by irradiation of a Ta target in a vacuum chamber with Nd:YAG laser light at 532 nm, with a pulse duration of 7 ns, at a fluence of ~104 J/cm2. The LPSXs were focused on the surfaces of samples using an ellipsoidal mirror that is designed so that LPSXs at around 10 nm are focused efficiently. We found that quartz glass plates are ablated by LPSX irradiation at a typical rate of 48 nm/shot. Furthermore, the ablated regions have smooth surfaces with a roughness less than 10 nm after 10 shots of LPSX irradiation. It is demonstrated that quartz glass is machined with a lateral resolution higher than 100 nm. In addition to quartz glass, the LPSX processing can be applied to micromachining of a variety of materials such as Pyrex, CaF2, LiF, LiNbO3, Si and silicone.

We have investigated nanomachining of inorganic materials using laser plasma soft X-rays. The soft X-ray was generated by irradiating Ta targets with pulsed Nd:YAG laser light. The laser plasma soft X-rays were focused using an ellipsoidal mirror, which is designed so as to focus soft X-rays at around 10 nm efficiently. The focused soft X-rays were incident to the surfaces of inorganic materials such as silica glass, LiF, CaF2 and LiNbO3. It is found that these materials are ablated by soft X-ray irradiation. In particular, silica glass can be ablated at rates of 0.2-150 nm/shot, which can be controlled by the intensity of soft X-rays. It is remarkable that silica can be ablated smoothly with a roughness of 1 nm after 10 shots of soft X-ray irradiation at a rate of 50 nm/shot. In order to demonstrate nanomachining of silica glass, we used contact masks on top of silica glass plates fabricated by electron beam lithography technique. The silica glass plates were irradiated with laser plasma soft X-rays through the windows of the contact masks. We found that nanofabrication of trenches with a width of 70 nm are performed clearly.

The formation of hydrogen (H)-related complexes was investigated in boron (B)-doped Si treated with high concentration of H. The isotope shifts of H-related Raman peaks by replacement of H to deuterium and 10B to 11B clearly showed the formation of the B–H complexes in which H directly bonds to B in Si. The results of the resistivity measurements suggested that the B acceptors are passivated via the formation of the B–H complexes, as well as the well-known passivation center in B-doped Si, namely, H–B passivation center.

The phonon confinement effect was investigated by Raman measurements for Si nanowires (SiNWs) synthesized by laser ablation of a Si target with nickel (Ni) catalyst and for SiNWs thermally oxidized at 700–1000 °C. The Si optical phonon peak for SiNWs, unlike that for bulk Si, showed a downshift and an asymmetric broadening. Thermal oxidation caused a further downshift and broadening. These phenomena can be explained by the phonon confinement effect due to the decrease in the diameter of the Si core of the SiNWs. It was additionally found that excess oxidation caused an upshift of the optical phonon peak due to compressive stress.

Recent experiments have shown that Si nanostructures can be formed in a matrix Of SiO2 by laser-pulse irradiation, with appealing applications in nanotechnology. We hereby present first principles simulations that provide a microscopic insight into the underlying mechanism, showing how electron excitations weaken Si-O bonds in SiO2, dislodging O atoms and allowing the formation of stable Si-Si structures below the melting temperature. (c) 2006 Elsevier B.V. All rights reserved.

We have investigated ablation of silica glass using pulsed laser plasma soft X-rays. For generating a plasma that emits pulsed soft X-rays, a Ta target was irradiated with focused 532 nm Nd:YAG laser light with a pulse duration of 7 ns at a fluence of 10 4 J/cm 2 . The soft X-rays were focused on the surfaces of silica glass using a Au-coated ellipsoidal mirror. The energy density of the soft X-rays can be roughly estimated to be 0.1 J/cm 2 , which is sufficiently enough for heating silica glass beyond the boiling point. We found that silica glass is smoothly ablated at 40 nm/shot. The processes induced by X-ray irradiation have been discussed based on transient-absorption measurements after X-ray irradiation.

Effects of amorphous carbon (a-C) thin film coating over carbon nanotubes (CNTs) on the field emission characteristics have been studied. Large-area surface-wave plasma (SWP) device was used both for growth of multi-wall CNTs on Ni-evaporated Si substrate and for deposition of a-C thin layer over CNTs. Field emission characteristics of the a-C film-coated CNTs prepared with different CVD durations were investigated. Experimental results show that the emission performance was significantly improved by coating a-C film typically with a thickness of 0.6–1 μm, that is, the ignition voltage for electron emission was reduced from 240 V for the sample of CNTs only to 110 V for that of CNTs deposited with a-C film during 60 min. The surface morphology has been also studied using the field emission type scanning electron microscopy (FE-SEM). It was deduced that the reduction of ignition voltage was due to the field enhancement originated from grain shape structures of the surface after a-C film deposition.

We measured real-time spectra of light transmitted through SiO2 specimens during irradiation of amplified ultrashort laser. The real-time spectra exhibit a peak at around 400 nm. The observation and identification of defects were performed by measurements of electron spin resonance (ESR). Both dependences of the peak at around 400 nm on irradiation time and laser power are in good agreement with those of the ESR signal intensity of positively charged oxygen vacancies (E′ center: O3≡Si·). This strong correlation shows that self-trapped excitons are created followed by the formation of the E′ center and finally that of ESR inactive centers, namely, oxygen-deficiency centers (ODCs: O3≡Si-Si≡O3 or O3≡Si:).

Er-doped SiO2 thin films including Si nanocrystallites were fabricated by laser ablation of Si targets covered with Er thin films in O2 gas and subsequent thermal annealing. Photoluminescence measurements were performed at temperatures from 10 K to 295 K. The optimum condition was found to be an O2 gas pressure range of 40–50 mTorr. We found that the temperature dependence of Er photoluminescence intensity is governed by power density of excitation laser light. Furthermore, Er photoluminescence is more intense at 295 K than that at 10 K at power densities higher than 480 mW/cm2. These results confirm that Er atoms are excited by energy transfer from photo-excited Si nanocrystallites. On the other hand, photoluminescence intensity of Er-doped Si nanoparticles grown in gas phase gradually decreases with increasing temperature.

Laser ablation using nanosecond pulsed laser light includes some important transient processes during and after laser irradiation. In this paper, the optical absorption of semiconductors and the energy relaxation of excited electrons and holes are briefly described, followed by rapid phase transformation such as transient melting and vaporization which lead to laser ablation of materials. Initial processes of vaporization of solids due to laser ablation have been investigated by several time-resolved measurements. Here we describe a method of time-resolved soft X-ray absorption spectroscopy measurement to show an example of the initial plume produced by laser ablation of Si. This technique gives us information of the time dependence of ejection of Si atoms and Si ions (Si+˜Si4+), and their time-dependent spatial profiles. In addition to nanosecond pulsed laser ablation, characteristics of femtosecond pulsed laser ablation is also discussed, which has been very important to applications.

We have investigated the chemical reaction of Si nanoparticles with H2 and O2 gases in gas phase after laser ablation of Si targets in Ar and Ne gases. First, we observed time-resolved photoluminescence (PL) from Si nanoparticles in pure Ar gas. The formation of Si nanoparticles begins after the thermalization of Si plasma on a time scale of 1 ms. The energy dissipation of electronic system govern the formation of Si nanoparticles. It was clearly observed that Si nanoparticles grow up to 1.8 ms. Based on the formation dynamics, we observed chemically modified Si nanoparticles using the time-resolved PL method. We found that Si nanoparticles react with hydrogen and oxygen atoms dissociated in the laser plasma. The hydrogenation results in PL at 550 nm and vibronic lines in the wavelength range of 650–800 nm disappear. Oxidation results in PL quenching when O2 partial pressure is higher than 1 Pa. Thus, we have demonstrated chemical modification of Si nanoparticles.

Droplet-free deposition of Si nanoparticle films has been studied applying time-resolved imaging of Si nanoparticles formed by laser ablation of Si targets in Ar gas. We found that Si nanoparticles can be deposited not only on substrates facing to the targets but also on substrates placed beside the target. We further confirmed using a scanning tunneling microscope (STM), Si nanoparticles with sizes of 5–8 nm are deposited on substrates placed beside the target and using a scanning electron microscope (SEM) on the substrates, no droplets are observed.

We fabricated P-doped Si nanocrystallites embedded in SiO 2 films and have investigated by photoluminescence (PL) spectroscopy and electron spin resonance (ESR) spectroscopy. The films were fabricated by annealing of P-doped SiO x films that were deposited by laser ablation of P 2 O 5 -coated Si targets in O 2 gas. Visible PL from nanocrystallites is enhanced at room temperature by 10 times as intense as undoped ones. ESR spectroscopy revealed that deposition at high energy density is effective for defect-free deposition. Furthermore, P atoms are found to be doped in the crystallites and electron–hole pairs bound to the P atoms are suggested to be responsible for the enhanced PL. In addition, we found a P-related center located at interface region between P-doped films of Si nanocrystallites and Si substrates, with a large hyperfine splitting of ∼80 G observed at 20 and 40 K.

We report a systematic investigation on the Fermi-level dependence of the formation of two different types of hydrogen molecules in silicon. Hydrogen molecule at the tetrahedral (T) site (H 2 (T)) is formed only in heavily doped n-type below 200°C, but is observed both in the n- and p-type above 200°C. The temperature dependence is due to the increase of the thermally activated carrier concentration. The possible precursors of the H 2 (T) are H + (BC) and H 0 (BC) in the p-type, and H − (T) and H 0 in the n-type. The Fermi level dependence of the formation of the hydrogen molecule trapped in platelets (H 2 (p)) above 250°C is very different from that below 250°C. The result suggests that platelets are formed from hydrogen-complexes other than the H 2 * above 250°C.

Optical properties of Al- and Ga-doped ZnO layers have been studied in the spectral range from 1.5 to 5.4 eV using a four-zone null spectroscopic ellipsometer and in the spectral range from 0.5 to 6.5 eV using near-normal incidence reflectivity measurements. The layers were prepared by RF magnetron sputtering onto (1 1 2 0) oriented single-crystal sapphire substrates. Al- and Ga-doping gives rise to a shift of the fundamental absorption edge from 3.4 to 3.7 eV. The model dielectric function (MDF) based on an excitonic structure derived by Tanguy [Phys. Rev. B 60 (1999) 10660] was completed by the Sellmeier and Drude terms. The Drude term describes a free-electron contribution originating from presence of the dopant. Spectroscopic ellipsometry and reflectometry are very sensitive to a surface roughness. The surface roughness was modeled by a surface layer of the Bruggeman effective medium and by diffraction theory.

Wehaveinvestigatedthethermalbehaviorofhydrogenmoleculestrappedbymultivacanciesincrystallinesiliconbyperformingthermalannealing.ThehydrogenmoleculeshadtwoRamancomponentswithdifferentannealingstages.Theiractivationenergiesforannihilationwere0.7 0.3and0.5 0.2eVforthemaincomponentandtheshoulder,respectively.Theformerwasattributedtothepotentialbarrierforahydrogenmoleculetoescapefromamultivacancytrap,andthelattertomovefromametastablesitearoundthemultivacancytraptothemoststablesite.Annealingat6108CsuggestedthatrelativelylargemultivacanciessuchasV 6 andV 10 arepossibletrappingcentersofthehydrogenmolecules.# 2001ElsevierScienceB.V.Allrightsreserved. PACS: 61.72.JiKeywords: Hydrogenmolecule;Multivacancy;Annealingbehavior;Activationenergy 1. IntroductionHydrogenisanimportantimpurityinsemicon-ductorsbecauseofitsabilitytochangeelectricalpropertiesofhostmaterialsdrastically[1].Theo-retical calculations predicted that a hydrogenmoleculeisoneofthefavorableconfigurationsincrystalline silicon. Three different configurationsof hydrogen molecules have been found experi-mentally by means of vibrational spectroscopysuchasRamanscatteringandinfraredabsorption.Murakamietal.revealedtheexistenceofhydro-genmoleculesincrystallinesiliconin1996.TheyobserveditsvibrationalRamanlineat4158cm

We have investigated the formation of silicon nanoparticles after pulsed-laser ablation of a silicon target into argon gas at pressures of 2-10 Torr. Growing process of silicon nanoparticles was observed by measuring light emission resulting from decomposition of the nanoparticles by using a second pulsed-laser. It was directly observed that the nanoparticles are formed on the time scale of 1 ms, depending on the ambient gas pressure. This information is expected to be quite helpful for growth control and for chemical surface-modification by using pulsed-laser light or by exposure to a pulsed reactive-gas jet. The nanoparticles exhibit visible photoluminescence after deposition. Effect of chemical modification to photoluminescence have been investigated. Brighter luminescence have been observed from silicon nanocrystallites precipitated in SiO2 films during annealing of SiOx films deposited using a laser ablation technique.

The Raman line of a new hydrogen molecule at approximately 3820 cm −1 has been observed for the first time in silicon after Si + ion implantation with proper doses, followed by hydrogen atom treatment. The assignment was confirmed by isotope shifts to 2770 cm −1 for D 2 molecule and to 3353 cm −1 for HD molecule. Both the ion-dose dependence and hydrogenation-temperature dependence of the Raman intensity of the H 2 molecules correlate with those of the intensity of peaks of Si–H stretching observed at 1957±1.8 cm −1 and at approximately 2185 and 2210 cm −1 . We propose a model where the hydrogen molecule corresponding to the 3820 cm −1 vibrational line is trapped in or adjacent to small H-terminated multivacancies.

We have investigated quantitatively the effect of crystal disorder on the formation of hydrogen molecule in crystalline silicon. Disordered crystalline silicon was obtained by implanting crystalline silicon with 200 keV Si + with doses ranging from 10 13 to 10 16 Si cm −2 . The optical phonon line broadened and downshifted with increasing ion dose, corresponding to an increase in crystal disorder and decrease in phonon correlation length. The implanted samples were then treated with a remote downstream hydrogen plasma at 250°C. The vibrational Raman line of hydrogen molecule at around 4160 cm −1 decreased monotonically in its intensity with decreasing phonon correlation length. The results suggest that hydrogen molecules are formed preferably in well-ordered parts of Si lattice.

The interface between metallic materials, especially titanium, and living cells (osteogenic cell) was investigated using a high-resolution transmission electron microscope, in order to establish the mechanism of the bio-compatibility for metals. At the titanium/cell interface there is an intermediate layer whose thickness is several ten nm involving not only metallic elements but also cell original elements. The intermediate layer does not have long/short range order or is rather complex structure. It is concluded that this intermediate layer is a mixture of titanium oxide and organic materials, which precipitates during cell culture. This precipitation of mixture is one of the advantages for titanium's biocompatibility. It is not cleared that calciumphosphate, one of the main composition of bone, precipitation on the surface on the material has any responsibility for bio-compatibility.

Applying laser ablation technique, we have synthesized two types of SiO 2 films that included nanometer-sized Si particles. One is synthesized by alternative deposition of Si nanoparticle layers and SiO 2 layers. The synthesized film exhibits red photoluminescence (PL) with a peak energy below 1.5 eV. The other is a SiO 2 film in which Si nanocrystallites precipitate. The films were synthesized by annealing at 1000°C of SiO x films which are formed by laser ablation in diluted O 2 gas. The films exhibit PL with peak energies of 1.5 eV, 2.2 eV and 2.7 eV. We find that there is a narrow range of composition for efficient 1.5-eV PL.

We have investigated direct nanomachining of inorganic materials using laser plasma soft X-rays. Soft X-rays (X) were generated by irradiation of Ta targets (Ta) with 532 nm Nd:YAG laser light(Y) with a pulse duration of 7 ns, at an energy density of about 10/sup 4/ J/cm/sup 2/. Under the condition, Ta plasma emit soft X-rays at around 10 nm. The soft X-rays were focused on samples (Q) with a spot size of 150 /spl mu/m, using an ellipsoidal mirror (M) that we designed so as to focus soft X-rays at around 10 nm efficiently. It was confirmed that SiO/sub 2/ has optical absorption at around 10 nm. We found that synthetic quartz glass, fused silica, Pyrex, HF, CaF/sub 2/, Al/sub 2/O/sub 3/ and LiNbO/sub 3/ can be machined smoothly by irradiation with laser plasma soft X-rays. Typically, quartz glass is ablated at 40 nm/shot, and has a surface roughness less than 10 nm after 10 shots. In order to investigate lateral resolution, we fabricated a WSi contact mask with 200-nm-pitch line-and-space patterns on quartz glass. We found that quarts glass plates can be machined at a resolution less than 100 nm. In conclusion, we have established a technique for nanomachining a wide variety of inorganic materials using laser plasma soft X-rays at a precision less than 100 nm.

We have found that hydrogen exists in molecular form in crystalline silicon treated with hydrogen atoms in the downstream of a hydrogen plasma. The vibrational Raman line of hydrogen molecules is observed at $4158{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for silicon samples hydrogenated between 180 and 500 \ifmmode^\circ\else\textdegree\fi{}C. The assignment of the Raman line is confirmed by its isotope shift to $2990{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for silicon treated with deuterium atoms. The Raman intensity has a maximum for hydrogenation at 400 \ifmmode^\circ\else\textdegree\fi{}C. The vibrational Raman line of the hydrogen molecules is broad and asymmetric. It consists of at least two components, possibly arising from hydrogen molecules in different occupation sites in crystalline silicon. The rotational Raman line of hydrogen molecules is observed at $590{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}.$ The Raman band of Si-H stretching is observed for hydrogenation temperatures between 100 and 500 \ifmmode^\circ\else\textdegree\fi{}C and the intensity has a maximum for hydrogenation at 250 \ifmmode^\circ\else\textdegree\fi{}C.

Hydrogen molecules are formed in the crystalline silicon treated with hydrogen atoms at substrate temperatures between 180 and 500°C. The vibrational Raman line (Q1) of hydrogen molecule in the hydrogenated crystalline silicon is observed at around 4160 cm-1. The line shape changes with hydrogenation temperature, and is composed of at least two components. The two components are ascribed to hydrogen molecule in two different interstitial sites in crystalline silicon. Each component is much broader than that of hydrogen molecules in gas phase. The line broadening can not be explained by collisional or motional broadening, but is attributed due to the configurations of hydrogen molecule with different orientations in the crystalline silicon.

We have investigated responses of polymers to EUV radiation from laser-produced plasmas beyond ablation thresholds and micromachining. We concentrated on fabricate precise 3D micro-structures of PDMS, PMMA, acrylic block copolymers (BCP), and silica. The micromachining technique can be applied to three-dimensional micro-fluidic and bio-medical devices. The EUV processing is a promising to realize a practical micromachining technique. In the present work, we used two EUV radiation sources; (a) Wide band EUV light in a range of 10{300 eV was generated by irradiation of Ta targets with Nd:YAG laser light at 500 mJ/pulse. (b) Narrow band EUV light at 11 and 13 nm was generated by irradiation of solid Xe and Sn targets, respectively, with pulsed TEA CO2 laser light. The generated EUV light was condensed onto the materials at high power density beyond the ablation thresholds, using ellipsoidal mirrors. We found that through-holes with a diameter of one micrometer an be fabricated in PMMA and PDMS sheets with thicknesses of 4-10 micrometers, at 250 and 230 nm/shot, respectively. The effective ablation of PMMA sheets can be applied to a LIGA-like process for fabricating micro-structures of metals for micro- and nano-molds. PDMS sheets are ablated if it is irradiated with EUV light beyond a distinct threshold power density, while PDMS surfaces were modified at lower power densities. Furthermore, BCP sheets were ablated to have 1-micrometer structures. Thus, we have developed a practical technique for micromachining of PMMA, PDMS and BCP sheets in a micrometer scale.

application/pdf, We report the first observation of the vibrational Raman spectrum of hydrogen molecules H2 in crystalline silicon treated with hydrogen atoms at 400 °C. The Raman spectrum of H2 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. An isotope shift was also detected 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 H2 and D2 in the gas, liquid, and solid phases.

Time- and space-resolved characteristics of laser-ablated carbon particles were studied by x-ray absorption spectroscopy. Absorption peaks from the 1s→2p transition of neutral carbon atom ( C 0), C −, C +, and C 2+ ions were observed, An absorption peak from 1s→π* and a broad absorption maximum from the 1s→σ* transition of small clusters were also observed. Neutral carbon, C −, C +, and small carbon clusters were emitted from the sample long after laser irradiation. Spatial distributions of laser-ablated carbon particles in the localized helium-gas environment were measured. In the helium-gas environment, the ablation plume was depressed by the helium cloud generated on the top of the ablation plume.

We have performed x-ray absorption measurements of laser-ablated Si particles in vacuum and in Hegas environment. It is found that the spatial distributions of laser-ablated particles are very different between the two cases. This suggests that a higher-density region of helium gas is formed at the top of the plume of the ablated particles, and free expansion of particles is restrained by this helium cloud. Moreover, the spectrum obtained at 260 ns indicates that it takes more than 260 ns to form Si clusters. It is also demonstrated that Al particles can be sputtered in vacuum by laser-ablated Si particles if an Al plate is placed in front of the plume.

We have studied dynamical mechanism of laser ablation using combined time- and spatially-resolved measurements of laser plasma soft x-ray absorption spectroscopy and visible light emission imaging/spectroscopy. The obtained results indicate that there are no significant formation of silicon clustres and no growth of silicon nanoclusters till 2.55 ms after laser ablation. We have fabricated nanocluster-based silicon films by laser ablation in ambient gases and investigated photoluminescence at room temperature from the films. It is found that the growth of silicon nanocluster-based films with visible light emission can be well controlled by means of novel laser ablation methods.

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 a technique for micromachining inorganic transparent materials. In the technique, patterning and coloration are performed by the direct irradiation of materials with pulsed laser soft x rays and the patterned areas are ablated using ultraviolet laser light. The technique utilizes the high precision of the soft x rays and the high energy density of conventional laser light. For demonstration, we irradiated quartz plates with Ta laser plasma soft x rays. This results in generation of transient surface opaque layers that absorb more than 40% of the 266nm Nd:YAG laser light. Applying the technique, quartz plates are found to be ablated smoothly at 85nm∕shots.