N. Okubo

Ion-induced frequency shift of ∼1100 cm−1 IR vibration in implanted SiO2: Compaction versus bond-breaking

Abstract Ion irradiation induces a frequency shift of Si–O–Si stretching vibration (ω4 mode, ∼1100 cm−1) in SiO2. Since the penetration depth of IR light is much larger than the projectile range of low-energy heavy ions (∼200 keV or less), conventional IR reflection spectroscopy detects a smaller shift than the true one. The near-surface sensitive reflection spectroscopy (NSSRS) was applied using a suitable sample structure, and obtained the true shift of ∼44 cm−1. The coherent two-layer reflection model, with the true shift value, well reproduces the spectra of the irradiated SiO2, and indicates that the ion-induced ω4 peak shift obtained with NSSRS has the same origin with the ion-induced 1040 cm−1 shoulder obtained with conventional spectroscopy. The present results indicate that the ω4 peak shift is ascribed not to the compaction, but to the Si–O–Si bond-breaking.

High-Current Heavy-Ion Accelerator System and Its Application to Material Modification

A high-current heavy-ion accelerator system has been developed to realize intense particle fluxes for material modification. The facility of a tandem accelerator attained 1 mA-class ion current both for negative low-energy ions and positive high-energy ions. A negative ion source of the key device is of plasma-sputter type, equipped with multi-cusp magnets and Cs supply. The intense negative ions are either directly used for material irradiation at 60 keV or further accelerated up to 6 MeV after charge transformation. Application of negative ions, alleviating surface charging, enables us to conduct low-energy high-current irradiation to insulating substrates. Since positive ions over MeV are irrelevant to the Coulomb repulsion, the facility as a whole meets high-current irradiation into insulators over a wide energy range. Application of high flux ions provides technological merits not only in efficient implantation but also in different material kinetics. Other characteristics of the system are co-irradiation of intense laser and in-situ detection of kinetic processes. For the material modification, we present nanoparticle fabrication in insulators, and synergistic phenomena by co-irradiation of ions and photons.

Enhancement of metal-nanoparticle precipitation by co-irradiation of high-energy heavy ions and laser in silica glass

Abstract Simultaneous laser irradiation under ion irradiation is conducted to control nanoparticle precipitation in amorphous (a-)SiO 2 . Copper ions of 3 MeV and photons of 532 nm by Nd:YAG laser are irradiated to substrates of a-SiO 2 . The ion dose rate and total dose are set at 2–10 μA/cm 2 and 3.0xa0×xa010 16 –3.0xa0×xa010 17 ions/cm 2 , respectively, and the laser power density is 0.05–0.2 J/cm 2 pulse at 10 Hz. The laser is simultaneously irradiated with ions in the co-irradiation mode, and the result is compared to that in the sequential and ion-only irradiation. Cross-sectional TEM of the irradiated specimens is conducted after measuring optical absorption spectra. In the case of co-irradiation of intense laser power and high dose (0.2 J/cm 2 pulse and 3.0xa0×xa010 17 ions/cm 2 ), Cu nanoparticles precipitate much more extensively than in the sequential irradiation, increasing both the particle diameter and the total Cu atoms in the nanoparticles. The optical absorption spectra show a surface plasmon peak of the nanoparticles. The precipitation enhancement in the co-irradiation mode suggests that the electronic energy is absorbed by the dynamic electronic states and promotes the Cu precipitation via enhancing the atomic migration.

Ion-induced metal nanoparticles in insulators for nonlinear optical property

Abstract Metal nanoparticles embedded in insulators exhibit a strong surface resonance and have been studied for optical switching. In this study, negative Cu ions of 60 keV were implanted into a-SiO2, MgOxa0·xa02.4(Al2O3) and LiNbO3 at 10–50 μA/cm2 to 1xa0×xa01017 ions/cm2. The resultant nanoparticle morphology was studied by cross-sectional TEM and shown to depend on the substrate species. The a-SiO2 showed the formation of spherical Cu nanocrystals of ∼10 nm. The MgOxa0·xa02.4(Al2O3) suppressed particle coarsening even at high dose rates, sustaining crystallinity of the lattice. On the other hand, the LiNbO3 exhibited non-spherical Cu nanocrystals of ∼10 nm. Ion-induced photon spectroscopy was applied to monitor the ion–substrate interactions from outside of the substrates. The non-linear optical properties were evaluated by a pump–probe method around the plasmon energy of about 2 eV. Although LiNbO3 exhibited a sub-picosec non-linear response, ion-induced photon spectroscopy revealed Li-atom release to the vacuum under ion implantation, influencing the Cu nanoparticle formation.

Microstructural changes in silicon thermal oxide induced by high-flux copper negative-ion implantation

Silicon-thermal-oxide (a-SiO2) films of 115 nm thick on Si wafers are implanted by high-flux Cu negative-ions of 60 keV. Optical reflectivity measurements show that the Si substrates are transformed into an amorphous-like state after the implantation. The “amorphization” is ascribed to recoil atoms, not directly to the incident Cu ions. The crystallinity is recovered after a heat treatment at 900°C for 1 h in Ar gas. The interference fringes in the reflectivity spectra shift to high energy side at and the higher fluxes, indicating decrease of the oxide thickness. At the same fluxes, AFM observation shows a prominent change of surface morphology and a large increase of step-height at implanted–unimplanted area boundaries, indicating enhanced sputtering under high-flux implantation.

Photon irradiation effects under ion implantation into insulators and applications to optical material processing

Abstract Photon irradiation effects during heavy-ion implantation have been studied to distinguish and control electronic excitation effects on surface modification of insulators, particularly on nanoparticle formation in insulators. Photons of a sub-gap energy (2.3 eV) and high-energy Cu ions were applied to substrates of a-SiO 2 (KU-1). The Cu 2+ ions of 3 MeV irradiated the specimens at 10 μA/cm 2 to 3×10 17 ions/cm 2 , and the 532 nm-YAG laser, either simultaneously or sequentially, irradiated them at 0.2 J/cm 2 · pulse , 10 Hz, for a period corresponding to the ion dose. Atomic-force microscopy and cross-sectional transmission electron microscopy were conducted to study the surface- and bulk morphology, respectively. The ion-alone irradiation smoothed the surface, up to the high dose. Intense photon irradiation under ion implantation significantly roughened the surface and concurrently promoted nanoparticle formation. Optical absorption spectra showed that simultaneous irradiation decreased radiation-induced defects for the low dose and enhanced surface plasmon of nanoparticles for the high dose. The results indicate that intense photon irradiation, coexistent with heavy-ion irradiation, excites transient electronic states and/or implanted atoms and result in surface desorption, defect annihilation and nanoparticle formation. The in-beam photon irradiation is suitable for optical material processing, both for the matrix and nanoparticles.

Laser-induced bleaching of insulators under MeV heavy-ion implantation

High-energy ion implantation is one of the unique methods to fabricate nano-scale structures, taking advantage of the spatial controllability and the non-equilibrium atomic injection. Metal-ion implantation into a transparent insulator creates a metal nanoparticle composite, which is promising as a nonlinear optical material with ultrafast response. Radiation damage of substrates and/or nanoparticles, which is inherent in ion implantation, is a drawback for optical performance of the nanoparticle composites. It is desired to annihilate radiation damage without melting the matrix. We have applied laser irradiation of sub-gap energy during heavy-ion implantation. Copper ions of 3 MeV and laser of a sub-gap energy (2.3 eV) irradiated insulators of a-SiO2 and spinel MgO•2.4(Al2O3). The dose rate varied up to 10 (mu) A/cm2 for Cu ions of 3 MeV and up to 0.2 J/cm2•pulse at 10 Hz for YAG-SHG laser. Only when ions and photons were simultaneously irradiated at the higher photon intensity (> 0.1 J/cm2•pulse), the insulators were effectively bleached in the optical absorption spectra. As well as the bleaching, precipitation enhancement and atomic desorption took place. The results indicate importance of dynamical electronic excitation during ion irradiation and that the photon irradiation enhances atomic displacements either at the surface or in the bulk.

Co-irradiation effects of intense heavy ions and photons on surface modification of insulators

Abstract Co-irradiation effects of ions and photons on insulators have been studied to explore electronic excitation effects on surface modification. Ion implantation of MeV–Cu ions and green-laser irradiation was conducted to a-SiO2 (820 ppm OH − ) and spinel MgO·2.4(Al2O3). The dose rate varied from 2 to 10 μA/cm 2 for Cu ions of 3 MeV and from 0.05 to 0.2 J/cm 2 · pulse at 10 Hz for YAG–SHG laser, for a period corresponding to 3×10 16 ions/cm 2 . Atomic-force microscopy (AFM) was conducted to evaluate the surface morphology. Optical absorption was measured in a photon energy range from 0.5 to 6.5 eV. Single irradiation, either Cu ion-alone or laser-alone, caused few discernible changes of surface morphology. Co-irradiation of ions and photons at high power densities caused significant changes in the surface texture with an increase in surface roughness. Optical absorbance of the co-irradiated specimens also showed a change larger than that of sequentially irradiated ones. The results indicate that nonlinear synergistic effects are induced by the co-irradiation and may develop to a new modification method.

Surface smoothening and compaction of silica glass under dynamic negative ion mixing

Abstract A dynamic mixing method has been developed to fabricate thick insulator films containing metal nanoparticles. The fabrication process consists of negative ion mixing/implantation and vacuum deposition of the matrix material. Negative Cu ions of 60 keV irradiated a silica substrate at a dose rate 5–30 μA/cm2 and a simultaneous evaporation of silica glass, at an evaporation rate 0.2–0.4 nm/s, produced thick films up to 500 nm. The surface of the samples was greatly smoothened by the dynamic negative-ion mixing (DNIM) method, as compared to that of evaporated films without ion irradiation. The frequency of the Si–O–Si stretching vibration mode of the DNIM samples became smaller than that of the silica glass substrates. Optical absorbance of the samples, particularly around Cu plasmon resonance, was significantly dependent on the Cu dose rate. The results indicate that the ion co-irradiation induces not only surface smoothening but also a compact network of the SiO2 matrix.

Spatial control of nanoparticle structures using dynamic processes under high flux Cu− implantation

Negative Cu ions of 60 keV, at high dose rates, have been applied for nanoparticle fabrication in insulators, using both conventional implantation and dynamic negative ion mixing (DNIM). Intense ion beams cause significant in-beam rearrangements of implants, i.e., spontaneous precipitation and depth-directional atomic migration. These dynamic processes are applicable for spatial control of nanoparticle structures. The spatial control is also possible by changing substrates, i.e., amorphous and crystalline SiO2 and a spinel crystal of MgAl2O4. The other approach for the spatial control is to use a DNIM method with dynamic growth, which presents a thick uniform structure of nanoparticles.