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We review recent works on 1) an isotope effect of penetration of Hydrogen(H) and Deuterium(D) into Silicon through Si/SiO2 interface and 2) H effects for Si nanostructures, as well as summary of hydrogen states in crystal Si. In particular, a new filtering effect of H and D isotope atoms was found for penetration process into crystal silicon (Si) through the interface between a Si and SiO2 native oxide layer. More H atoms are introduced into Si than D for mixing gases. This phenomenon can be tentatively explained in terms of an isotope filtering model of H and D via an intermediate cluster state formed at the interface between the native SiO2 layer and crystal Si. Hydrogen passivation of interface defects is shown to be needed for knowing proper impurity doping effects, and electronic, optical, and magnetic properties in Si nanostructures.

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.

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 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.

The formation mechanism of interstitial H2 in crystalline silicon was investigated by varying the dopant concentration and hydrogenation temperature. At low temperatures such as 125°C, the Raman peak of the interstitial H2 is observed only in heavily doped n-type silicon, suggesting that the formation of H2 includes a metastable donor-hydrogen complex as the precursor and that H2 formation out of two isolated hydrogen atoms is hindered by the Coulombic repulsion between two equally charged atoms at this temperature. At moderate temperatures such as 235°C, the interstitial H2 is formed also for intrinsic and p-type silicon. The absence of the dopant-concentration dependence in p-type silicon indicates that the H2 is created from two hydrogen atoms, at least one of which is neutralized by capturing a thermally excited electron. The formation of the interstitial H2 competes with that of extended planar defects (platelets) at high temperatures such as 305°C.

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.

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 time-and spatially-resolved properties of laser ablated carbon, boron and silicon particles were measured by LAPXAS (Laser Plasma Soft X-ray Absorption Spectroscopy). The maximum speed of positively charged ions is higher than those of neutral atoms and negatively charged ions. The spatial distributions of the laser-ablated particles in the localized rare gas environment were measured. In helium gas environment, by the helium cloud generated on the top of ablation plume depressed the ablation plume. There is no formation of silicon clusters till 15μs after laser ablation in the argon gas environment.

Hydrogen molecule has been observed in the hydrogen-atom-treated silicon crystal using Raman spectroscopy. Trapping site and the vibrational frequency of H2 in the silicon crystal have been studied with theoretical calculations. The ab initio molecular orbital theory and the density-functional-theory calculations have been performed with a Si10H16 cluster model. The calculations show that the trapping site of H2 in the silicon crystal is the tetrahedral interstitial site and the calculated vibrational frequency agrees reasonably with the experiment.

We have performed (1) time-resolved soft X-ray absorption measurements on a time scale up to 15 μs after pulsed-laser ablation of silicon (Si) in an ambient Ar gas with a transient pressure of about 10 Torr and (2) investigation of the correlation between 1.6-eV photoluminescence (PL) from Si nanocluster-based films and the surface oxidation. No soft X-ray absorption lines corresponding to Si clusters were observed, indicating that it is likely that after 15 μs, significant clustering takes place and then Si nanoclusters can grow. From experiments involving hydrogen termination and hydrogen atom treatment in addition to natural oxidation, it is found that the 1.6-eV PL originates from the interface of a Si nanocluster core and a surface oxide layer and occurs without hydrogen atoms.

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 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.

We have investigated the Raman spectra of hydrogen molecules in heavily doped n-type crystalline silicon exposed to hydrogen atoms at various substrate temperatures. A significant hydrogenation-temperature dependence has been observed in the intensity and the shape of the vibrational Raman line of hydrogen molecules. The vibrational Raman line of hydrogen molecules is as broad when measured at 90 K as at 300 K. The results indicate that hydrogen molecules are formed effectively at higher temperature than the temperature appropriate for the platelet formation, and trapped in various interstitial sites in crystalline silicon.

We have calculated potentials of hydrogen molecules in the silicon clusters (Si 10 H 16 ) by the ab initio Hartree-Fock method. A tetrahedral site for the hydrogen molecule is a stable trapping site and the calculated vibrational frequency of the hydrogen molecule is 4470 cm −1 , which agrees resonably with the experimentally obtained frequency of H 2 in silicon crystal.

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.

Chemical reaction of laser-ablated silicon particles with oxygen gas diluted with a helium buffer gas is found by means of soft X-ray absorption spectroscopy. In order to make clear the dynamics of silicon laser plume and the mechanism of the chemical reaction, we measured time-resolved spatial distribution of light emission from the laser-ablated particles as well as time- and space-resolved emission spectra.

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.

Two kinds of Er-doped Si-based films have been fabricated by laser ablation. One was grown in solid phase and the other was done in gas phase. For the solid-phase growth, we deposited Er-dispersed SiOx films by laser ablation and annealed them at 1000 °C for forming SiO2 films including Er-doped silicon nanocrystallites. Photoluminescence (PL) measurements were performed at temperature from 6 to 300 K. We successfully fabricated Er-doped films with intense 1.5 μm PL and without thermal quenching by the solid-phase growth. For the gas-phase growth, Er atoms were probably doped in Si nanoparticles during they are growing in Ar gas. The PL intensity gradually decreases with increase in temperature, suggesting that the sites of Er are an important factor for restraining thermal quenching.

We have investigated the formation dynamics of silicon nanoparticles after laser ablation of silicon targets in H 2 gas and O 2 gas diluted with Ar gas by 2D imaging of time-resolved photoluminescence (PL). PL intensity from silicon nanoparticles is found to be enhanced in H 2 gas with partial pressures up to 400 Pa. In O 2 gas, the PL intensity drastically decreases with increasing partial pressures. A broad band green PL indicates that hydrogenated Si nanoparticles are formed in the H 2 gas.

We found a route for synthesizing hydrogenated Si nanoparticles with 550 nm photoluminescence. The hydrogenated Si nanoparticles were synthesized by laser ablation of Si targets in H2 gas diluted with 5 Torr Ne gas. The photoluminescence, which was measured at 90 ms after laser ablation, is most enhanced in H2 gas at a partial pressure of 1 Torr under pulsed-laser excitation at 355 nm. The enhanced photoluminescence is ascribed to hydrogenated Si nanoparticles. The time-resolved photoluminescence spectroscopy revealed that the hydrogenated Si nanoparticles are stable against photoexcitation, while photodesorption of Si2 from Si nanoparticles was observed in Ne gas without H2 gas.

Gaining an understanding the dynamic behaviors of dopant atoms in silicon nanowires (SiNWs) is the key to achieving low-power and high-speed transistor devices using SiNWs. The segregation behavior of boron (B) and phosphorus (P) atoms in B- and P-doped SiNWs during thermal oxidation was closely observed using B local vibrational peaks and Fano broadening in optical phonon peaks of B-doped SiNWs by micro-Raman scattering. Electron spin resonance (ESR) signals from conduction electrons were used for P-doped SiNWs. Our results showed that B atoms preferentially segregate in the surface oxide layer, whereas P atoms tend to accumulate in the Si region around the interface of SiNWs. The radial distribution of P atoms in SiNWs was also investigated to prove the difference segregation behaviors between of P and B atoms.

application/pdf, We have investigated the dynamic formation of silicon nanoparticles after pulsed-laser ablation of a silicon target into argon gas, in order to fabricate the nanoparticles that exhibit visible photoluminescence with higher efficiency. The nanoparticles growing in argon gas were detected by measuring plasma emission resulting from decomposition of the nanoparticles by a second pulsed-laser beam. It was directly observed that the nanoparticles grow on the time scale of 1 ms.

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.

Erbium (Er)- or phosphorus (P)-doped silicon nanocrystallites (nc-Si) buried in SiO2 layer were fabricated by laser ablation and the subsequent thermal annealing; i.e. solid-phase growth. The doping effects have been studied by measurements of temperature-dependent photoluminescence (PL) of Er from 6 K to 300 K and PL measurements of nc-Si at room temperature, as well as electron-spin-resonance (ESR) measurements of P donors in nc-Si. The results demonstrates that the solid-phase growth method can realize an intense 1.5micrometers Er PL without thermal quenching and P-donor doping can be attained in nc-Si. These results suggest that impurity doping is useful for modifying furthermore quantized properties of Si nanostructures and making them more functionally active.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have utilized pulsed-laser ablation in ambient inert gas to synthesize silicon (Si) nanoparticles and to perform surface modification and impurity doping. The dynamical growing process of Si nanoparticles has been investigated by measuring time-resolved light emission induced by the second pulsed-laser irradiation with a delayed time. It was found from the time-resolved measurements that the onset of the formation of Si nanoparticles appears at around 1 ms, on inert gas pressure and laser fluence. We demonstrate that light emission can be controlled by adding hydrogen or oxygen gas to inert gas. It is also demonstrated that the thermal quenching which has been serious problem in Er-doped semiconductors can be removed in 1.54 micrometers photoluminescence of Er-doped Si nanocrystallites. These results suggest that laser ablation is useful not only for synthesis of nanostructured materials such as nanoparticles and nanowires/nanotubes, but for their surface modification and impurity doping that are important techniques for realizing functional nanostructures.

For studying the material properties of nanostructured group IV materials, we have developed a pulsed laser ablation method into inert background gases. SiGe alloy nanocrystallites have possibility of novel band structure engineering by controlling not only compositions but also particle sizes. An ArF excimer laser was focused onto the surface of the powder-sintered SixGe1-x target. During the laser ablation, He gas was introduced into a vacuum chamber and was maintained at a constant pressure. Size distribution of the SixGe1-x ultrafine particles decreases with decreasing composition x under fixed conditions of deposition such as background gas pressure. Raman scattering spectra of the deposited SiGe ultrafine particles show three peaks ascribed to mixed crystalline SiGe after annealing, and the linewidths of the peaks broaden due to the reduced size of the crystallites. The frequencies and intensities of the peaks depend on the composition x. Visible PL spectra have broad peaks from 2.25 eV to 2.10 eV, at room temperature. The peak positions show blue shifts with increasing x. Electroluminescent diodes with the Si0.8Ge0.2 nanocrystallite active region were fabricated, and emit visible high peaked at around 1.8 3V, at room temperature.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.

Time‐resolved X‐ray absorption spectroscopy using laser plasma has been developed to measure the time evolution of the electronic and geometrical structures of laser ablated particles and their spatial distribution. This method is demonstrated to have potential for pulsed‐laser induced, fast‐phase changes like laser annealing and laser ablation. Laser‐ablated particles of silicon and carbon have been investigated by this method to clarify dynamics of ablated particles. We show the obtained results of laser ablation of Si and C, the first experimental determination of Si–L2,3 edges of Si atom and ions, and absorption lines suggesting formation of Si clusters and negative C ions.

We found a new filtering effect of hydrogen (H) and deuterium (D) isotope atoms for penetration process into crystal silicon (Si) through the interface between a Si and SiO2 native oxide layer. More H atoms are introduced into Si than D for mixing gases. It was determined from H and D mixed-atom treatment of Si wafer with the native oxide and the systematic measurements of Raman scattering of hydrogen molecules (H2, HD, or D2) and secondary ion mass spectrometry. This phenomenon can be tentatively explained in terms of an isotope filtering model of H and D via an intermediate cluster state formed at the interface between the native SiO2 layer and crystal Si.

application/pdf, We have investigated phosphorus ion (P+) implantation in Si nanocrystals (SiNCs) embedded in SiO2, in order to clarify the P donor doping effects for photoluminescence (PL) of SiNCs in wide P concentrations ranging in three orders. Some types of defects such as Pb centers were found to remain significantly at the interfaces between SiNCs and the surrounding SiO2 even by high-temperature (1000 °C) annealing of all the samples. Hydrogen atom treatment (HAT) method can efficiently passivate remaining interface defects, leading to significant increase in the intensity of PL arising from the recombination of electron-hole pairs confined in SiNCs, in addition to significant decrease in interface defects with dangling bonds detected by electron spin resonance. From both the results of the P dose dependence before and after HAT, it is found that the amount of remaining defects is higher for samples with SiNCs damaged by implantation with relatively lower P+ doses and then annealed, and that through HAT the observed PL intensity increases surely as the P concentration increases up to a critical concentration. Then it begins to decrease due to Auger nonradiative recombination above the critical concentration which depends on the size of SiNCs. These results suggest an effect of relatively low concentration of P atoms for the enhancement of PL intensity of SiNCs and we present an unconventional idea for explaining it.

application/pdf, Codoping of boron (B) and phosphorus (P) atoms was performed during the synthesis of silicon nanowires (SiNWs) by laser ablation. The observation of a local vibrational mode of B clearly showed B doping in codoped SiNWs, while Fano broadening due to heavy B doping disappeared, indicating compensation by P donors. The electrospin resonance signal of conduction electrons also disappeared due to compensation by B acceptors. These results indicate that codoping of B and P atoms was achieved in SiNWs during laser ablation.

application/pdf, We have developed a robust method for fabricating Si nanoregions in silica glass using femtosecond laser processing. We attained a vivid formation of silicon-rich nanoregions site-selectively generated in SiO2 by irradiation of femtosecond laser pulses to the interface of a SiO2 substrate and deposited aluminum (Al) thin film, where the Al element acts as a gettering site for O atoms. Growth of high-density Si nanocrystals and amorphous Si was observed by transmission electron microscopy in the region that was multiply irradiated with the femtosecond laser. Furthermore, local annealing with a cw laser enhances the Si nanocrystal growth, which was determined by micro-Raman measurements.

application/pdf, Phosphorus (P) doping was performed during the synthesis of silicon nanowires (SiNWs) by laser ablation. At least three types of signals were observed by electron spin resonance (ESR) at 4.2 K. Phosphorus doping into substitutional sites of crystalline Si in SiNWs was demonstrated by the detection of an ESR signal with a g value of 1.998, which corresponds to conduction electrons in crystalline Si, and by an energy-dispersive x-ray spectroscopy spectrum of the P Kalpha line. The ESR results also revealed the presence of defects. These defects were partially passivated by hydrogen and oxygen atoms.

application/pdf, The formation of hydrogen (H)-related complexes and H effects on boron (B) and phosphorus (P) dopants was investigated in B- or P-doped silicon (Si) crystal treated with high concentration of H. The reactivation process of dopant carriers by annealing after hydrogenation was significantly different between the p-type and n-type specimens. The difference is likely to be attributable to the formation of H-related defects based on the stable sites of the H atoms, i.e., complicated H multiple trapping centers are formed by bond breaking due to H atoms in only p-type B-doped Si.

application/pdf, 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. Boron doping was performed during the growth of SiNWs. Fano [Phys. Rev. 124, 1866 (1961)] broadening was also observed in the Si optical phonon peak. These results prove that B atoms were doped in the SiNWs. Hydrogen (H) passivation of B acceptors in the SiNWs was also investigated. A broad peak was observed at around 650–680 cm−1 after hydrogenation, demonstrating that B dopants were passivated by the formation of the well-known H–B passivation centers.

application/pdf, The phonon confinement and self-limiting oxidation effects of silicon nanowires (SiNWs) synthesized by laser ablation were investigated. The size 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, while excess oxidation causes an upshift due to compressive stress. The compressive stress retarded the oxidation of the SiNWs by self-limiting oxidation effect. This result shows that the Si core diameter can be controlled by compressive stress.

application/pdf, A gradual downshift and asymmetric broadening of the Si optical phonon peak were observed by Raman scattering measurements of continuously thermally oxidized silicon nanowires (SiNWs) synthesized by laser ablation. This downshift and broadening can be interpreted by the phonon confinement effect. Further thermal oxidation produced a reverse change; namely, an upshift of the optical phonon peak. This is considered to be due to compressive stress since this stress was relieved by removing the oxide layers formed around the SiNW cores, resulting in a downshift of the optical phonon peak.