Setsuo Nakao

Visible photoluminescence in Si+‐implanted silica glass

We have investigated visible photoluminescence excited by Ar‐ion laser (488 nm, 2.54 eV) at room temperature from Si+‐implanted silica glass, as‐implanted and after subsequent annealing in vacuum. We found two visible luminescence bands: one peaked around 2.0 eV, observed in as‐implanted specimens and annealed completely after heating to about 600 °C, the other peaked around 1.7 eV observed only after heating to about 1100 °C, the temperature at which Si segregates from SiOx. It was found that the 2.0 eV band anneals parallel to the E’ centers, as detected by electron spin resonance studies. It was also found that Raman lines around 520 cm−1, due to Si—Si bonds, grow and that interference patterns are induced by annealing Si+‐implanted silica glass. Based on these studies, we ascribe the 2.0 eV band to the electron‐hole recombination in Si‐rich SiO2 and the 1.7 eV band to the electron‐hole recombination in the interface between the Si nanocrystal and the SiO2 formed by segregation of crystalline Si from SiOx.

Visible photoluminescence in Si+‐implanted thermal oxide films on crystalline Si

We have investigated visible photoluminescence excited by Ar ion laser (488 nm, 2.54 eV) at room temperature from Si+‐implanted thermal oxide films grown on crystalline Si wafer, as‐implanted and after subsequent annealing in vacuum. We found two types of visible luminescence bands similar to those of silica glasses; one band is observed in as‐implanted specimens and disappears after heating to about 600 °C, and the other band is observed only after heating the specimens to about 1100 °C. Though the shapes of these luminescence spectra are different from those having been observed in Si+‐implanted silica glass, the origins of these bands are the same as in silica glass. We discuss the similarities and the differences of luminescence bands in Si+‐implanted silica glasses and thermal oxide films grown on crystalline Si.

Wavelength tuning of surface plasmon resonance using dielectric layers on silver island films

A simple and effective method was demonstrated to tune surface plasmon resonance (SPR) wavelength of silver island films by introducing a dielectric medium to vary interisland dielectric constant. The medium, either overcoating or sandwiching the silver islands, can make a dramatic effect on the tunability. We had extended the SPR wavelength tunability of standard silver island film substrates from 432 to 482 nm to 506 to 1310 nm using two dielectric layering methods. A generalized Maxwell–Garnett theory was applied to interpret these results.

Tungsten doping into vanadium dioxide thermochromic films by high-energy ion implantation and thermal annealing

Abstract Doping of W into VO 2 was done by high-energy (1.0 MeV) ion beam implantation and thermal annealing for the first time. The implantation was performed with different ion doses of 0.86, 2.18 and 6.12×10 15 ions/cm 2 on a VO 2 polycrystalline film sputter-deposited on Si. The film was characterized by Rutherford backscattering spectroscopy, atomic force microscopy and spectrophotometry. Deterioration of thermochromism and change in surface morphology after implantation, resulting from the irradiation damage such as amorphization, were observed and the extent increased with ion dose. Recrystallization of the implanted film and recovery of thermochromism were obtained by annealing at 400°C in air for 60 min. The W doping resulted in a substantial reduction in the critical temperature τ c for the phase transition of VO 2 .

Deposition of diamond-like carbon films using plasma based ion implantation with bipolar pulses

Plasma based ion implantation (PBII) with bipolar pulses has been proposed to improve a dose uniformity in an ion implantation on a three-dimensional target. A pulsed glow discharge plasma is produced around the target by a positive pulse at a gas pressure less than ∼0.5 Pa, and then ions are implanted into the target from all sides by the subsequent negative high-voltage pulse. It has been shown that ions produced by the positive pulse have been implanted effectively with the negative pulse. The PBII with bipolar pulses is applied to DLC coatings. A carbon mixing layer in the substrate surface is formed by the implantation to improve the adhesion of DLC films. Internal stress of DLC films slightly decreases with increasing frequencies of positive pulse. Moreover, it is shown that the PBII with bipolar pulses is possible to use for inner coating of DLC films on a stainless-steel pipe.

Optical Properties of Vanadium Dioxide Film during Semiconductive–Metallic Phase Transition

The optical constants of vanadium dioxide (VO2) films were determined at visible and near-infrared wavelengths at various temperatures during a semiconductive–metallic phase transition by ellipsometric analysis with Lorentz-oscillator formulae. The reversible changes in optical constants against temperature due to thermochromism were observed at around 70 °C. The wavelength dispersions of the optical constants were well expressed by the sum of three oscillators and their oscillating energies were attributed to photon-excited transitions. The variation in band structure during the phase transition was monitored, and the relationship between the band structure and optical properties was discussed.

Localized epitaxial growth of α-Al2O3 thin films on Cr2O3 template by sputter deposition at low substrate temperature

Low-temperature growth of α-Al2O3 films by sputtering was studied with x-ray diffraction and high-resolution transmission electron microscopy (HRTEM). Pure α-Al2O3 film was formed at 400 °C using Cr2O3 as template, whereas amorphous or θ-Al2O3 was formed without Cr2O3. HRTEM revealed localized epitaxial growth of α-Al2O3 on Cr2O3 with the relationship [011]Al2O3/[011]Cr2O3, suggesting the importance of Cr2O3 as a structural template for the growth of α-Al2O3, in addition to other contributions such as good stoichiometry, low sputter pressure, and low deposition rate under optimized deposition conditions. Successful growth of α-Al2O3 by sputtering at 400 °C or below makes the film widely applicable to even glass substrates.

Correlation of microstructure and photoluminescence for nanometer-sized Si crystals formed in an amorphous SiO2 matrix by ion implantation

We have investigated microstructure and optical properties of thermal oxide films grown on crystalline Si wafer, modified by 1 MeVSi+-implantation and subsequent annealing at 1100 °C to segregate Si nanocrystals in an amorphous SiO2 matrix. Transmission electron microscopy indicates the formation of Si nanocrystals near the range of implanted Si ions in SiO2 after annealing and the growth in average size of Si crystals with increasing annealing time. Photoluminescence spectroscopy indicates that the luminescence intensity grows and then decreases as the annealing time increases, without changing the shape of the luminescence spectra. These results indicate that the photons are absorbed by Si nanocrystals with modified band-gap governed by the quantum confinement effects, and the emission of photons is not due to direct electron-hole recombination inside Si nanocrystals, but is related to defects, probably at the interface between Si nanocrystals and SiO2.

W-doped anatase TiO2 transparent conductive oxide films: Theory and experiment

W-doped anatase TiO2 films were deposited on glass substrate by magnetron cosputtering. The minimum resistivity, 1.5×10−2 Ω cm, for Ti1−xWxO2 film (x=0.063) was obtained. X-ray photoelectron spectroscopy analysis shows W incorporated in the Ti lattice position is mostly in the W6+ state. Theoretical calculations based upon the density-functional theory were applied to analyze the electronic structure and conducting mechanism. The strong hybridization of Ti 3d states with W 5d states is the dominate factor to cause the shifting in Fermi level into conduction band. Our results suggest that tungsten is a favorable dopant to form TiO2-based transparent conducting oxide materials.

Electrical properties of SrTiO3 thin films on Si deposited by magnetron sputtering at low temperature

Deposition of SrTiO3 (STO) thin films by radio-frequency magnetron sputtering in an ultrahigh vacuum system at a low substrate temperature (∼200 °C) was performed in order to produce high-quality STO/p-Si (100) interfaces and STO insulator layers with dielectric constants of high magnitude. The STO films were identified as polycrystalline by x-ray diffraction, and were approximated with a layered structure according to the best fitting results of raw data from both Rutherford backscattering spectroscopy and variable angle spectroscopic ellipsometry. Room-temperature current–voltage and capacitance–voltage (C–V) measurements on Al/STO/p-Si diodes clearly revealed metal–insulator–semiconductor behavior, and the STO/p-Si interface state densities were of the order of 1011 eV−1 cm−2. The dielectric constant of the STO film was 65, and the dielectric loss factor varied between 0.05 and 0.55 for a frequency range of 1 kHz–10 MHz. For a 387 nm thick STO film, the dielectric breakdown field was 0.31 MV cm−1, and ...