Hiroshi Ebisu

Refractive index and degree of inhomogeneity of nanocrystalline TiO2 thin films: Effects of substrate and annealing temperature

Nanocrystalline TiO2 thin films, deposited on single crystal Si (100) substrates under different temperature conditions by the sol–gel dip coating method, have been investigated for their optical properties using ultraviolet-visible spectroscopic ellipsometry. A gradual increase in refractive index, n, with increasing annealing temperature up to 600 °C, and thereafter a sharp increase in n at 800 °C of annealing temperature have been observed. For the heat-treated and low temperature (400 °C) annealed films, n is found to be higher at the film–substrate interface than at the film surface and the refractive index gradient slightly increases for the annealed sample. However, for the 600 °C temperature annealed film, the refractive index gradient significantly decreases and the film appears to be almost homogeneous. These results are in sharp contrast with those for the films deposited on a vitreous silica substrate where n was found to be higher at the film surface than at the film–substrate interface and t...

Phosphorus doping and defect studies of diamond-like carbon films by pulsed laser deposition using camphoric carbon target

Abstract Phosphorous is doped in diamond-like carbon (DLC) films by pulsed (XeCl) laser deposition technique using a camphoric carbon (CC) target, obtained from burning camphor (C10H16O), a natural source. The activation energy of undoped film is approximately 0.17 eV and increased to approximately 0.23 eV for the film deposited from the target containing 1% P. With a further increase of P content, the activation energy decreases to approximately 0.12 eV for the film deposited from the target containing 7% P. Study of activation energy reveals that the Fermi level of the carbon film moves from the valence band edge to near the conduction band edge through the mid-gap. The Tauc gap, conductivity, activation energy together with electron spin resonance (ESR) spectroscopic studies reveal successful doping of P in the films deposited from target containing up to 5% P upon modifications in the gap states.

Optical and structural properties of nitrogen doped amorphous carbon films grown by rf plasma-enhanced CVD

Abstract Nitrogen doped hydrogenated amorphous diamond-like carbon thin films have been synthesized by radio frequency (rf) plasma-enhanced chemical vapor deposition (PECVD) using CH4 as the source of carbon and with different N2 flow rates (N2/CH4 gas ratios between 0 and 3), at 300 K. The dependence of optical and structural properties/modifications on the nitrogen incorporation was investigated using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-visible spectroscopy and electron spin resonance (ESR) measurements. It was found that the configuration of nitrogen atoms incorporated into an amorphous carbon network gradually changed from nitrogen atoms surrounded by three (σ bonding) to two (π bonding) neighbor carbons with increasing nitrogen flow rate. The Tauc optical gap was reduced from 2.6 to 2.0 eV with increasing nitrogen flow rate. The ESR spin density and the peak-to-peak linewidth increased drastically with increasing nitrogen flow rate. Significant structural modifications were observed above a gas ratio N2/CH4 of 1.

EPR Study of Cr3+ Centres in Rb2ZnF4, Rb2CdF4 and Cs2CdF4 Crystals

Results of EPR measurements on Cr 3+ centres in Rb 2 ZnF 4 , Rb 2 CdF 4 and Cs 2 CdF 4 are reported for crystals co-doped with Li + ions. Due to charge compensation some of Cr 3+ ions are associated with a cation vacancy (V M ) or a Li + ion at the nearest divalent-cation site in the c -plane. The fine structure parameters D and E are interpreted by two uniaxial spin-Hamiltonian terms along the c -axis of the host crystal and along the direction of the Cr 3+ –V M or Cr 3+ –Li + pair. The validity of this interpretation is discussed by the superposition model.

Effect of radio frequency power on the properties of hydrogenated amorphous carbon films grown by radio frequency plasma-enhanced chemical vapor deposition

We have studied the influence of radio frequency (RF) power on material properties of hydrogenated amorphous carbon (a-C:H) films prepared by RF plasma-enhanced chemical vapor deposition (CVD). The RF power has a significant impact on film properties such as electron spin resonance (ESR) spin density, the optical band gap, chemical bonding and photoluminescence (PL). Raman spectroscopy, infrared (IR) absorption and ESR measurements reveal that the residual defect density (ESR spin density) increases with increasing RF power due to the increase of sp2 C–Hx components. We obtained an ESR spin density as small as 8 ×1016 spins/cm3 at a low RF power, which is comparable to that of high-quality amorphous silicon films. Plasma optical emission spectroscopy indicates that the intensity ratio (IC–H/IH) between hydrogen peaks (IH) and hydrocarbon peaks (IC–H) significantly decreases with increasing RF power. The optical band gap, obtained from UV-visible spectroscopy, varies from 3.0 to 1.9 eV with increasing RF power. The broad emission band centered around 2.4 eV was observed in the PL spectra of the samples deposited at lower RF powers. We found a 0.5–0.3 eV Stokes shift by comparing the PL emission and the absorption coefficient spectrum from the films deposited at ≤100 W RF powers, whereas, we failed to observe a clear Stokes shift from the films deposited at >100 W RF powers. A weak correlation between the PL peak and the optical band gap is demonstrated.

Low density of defect states in hydrogenated amorphous carbon thin films grown by plasma-enhanced chemical vapor deposition

The density of electronic defect states in most forms of amorphous carbon deposited at room temperature is found so far to be very high (1018–1022 spins cm−3). In this letter, we demonstrate that the radio-frequency plasma-enhanced chemical vapor deposited hydrogenated amorphous carbon (a-C:H) thin film exhibits the lowest spin density of the order of 1016 cm−3, investigated by using electron spin resonance (ESR) spectroscopy, a very promising reproducible result comparable with high-quality a-Si:H. In addition, the optical gap of a-C:H has been tailored between a wide range, 1.8–3.1 eV. The ESR spectra of all the films reveal a single Lorentzian line whose linewidth ΔHpp varies strongly with the optical gap. Also, there is a strong dependence of spin density on the optical gap, and we show that this dependency is a direct result of structural changes due to sp3/sp2 carbon bonding network.

Anomaly in the fine-structure splitting of EPR spectra of Cr3+ centres in Tl2ZnF4 crystals

EPR measurements have been made at room temperature on as-grown single crystals of Tl2ZnF4. For crystals doped with Cr3+, signals from the Cr3+ centres with monoclinic (centre II) and orthorhombic (centre III) symmetries are observed together with ones from the uncompensated tetragonal centre (centre I). On the basis of spin-Hamiltonian separation analysis, the centre II is ascribed to a Cr3+ ion substituted for a Zn2+ site with a vacancy at its nearest Tl+ site. The centre III is ascribed to a Cr3+ ion associated with a vacancy at the nearest Zn2+ site. Anomalies were revealed in the separated parameters b2a(1) for the centres II, III and in b2a(2) for the centre III, which have about double the magnitude of those in the other layered perovskite fluorides Rb2ZnF4, K2ZnF4 with different monovalent cations. These anomalies may be due to some effect of the nearest and the next-nearest Tl+ ions.

EPR study of Fe3+−VCd and Fe3+−Li+ centres in RbCdF3 and CsCdF3 crystals

EPR measurements on Fe 3+ centres in RbCdF 3 and CsCdF 3 for crystals co-doped with Li + ions have been made in the cubic phase of host crystals. In both the crystals spectra from two kinds of Fe 3+ tetragonal centres associated with Cd 2+ vacancy (V Cd ) and Li + ion at the nearest-divalent-cation sites have been observed. From obtained fine structure parameters in the spin Hamiltonian the metal-ligand distances have been determined by the superposition model. The fine structure parameters have been explained well by the obtained metal-ligand distances, which show the contraction ordistortion of fluorine octahedra surrounding Fe 3+ ions.

Photocatalytic Activity of Titanium Dioxide Coated with Apatite

Titanium dioxide coated with apatite (AP–TiO2) was produced by stirring commercial TiO2 (ST-01) into a simulated body fluid (SBF). Radicals generated at the surface of photoaccelerated ST-01 and AP–TiO2 were analyzed by electron spin resonance (ESR) spectroscopy in a spin-trapping experiment. During photoirradiation for 0–60 s, photogenerated [DMPO–OH] for ST-01 and AP–TiO2 increased with irradiation time. On the other hand, during photoirradiation for 60–500 s, [DMPO–OH] for ST-01 decreased but [DMPO–OH] for AP–TiO2 did not. On the basis of these phenomena, it was suggested that DMPO trapped photogenerated OH• and was converted into DMPO–OH which then decomposed to H2O2 upon further photoirradiation. The decomposition of DMPO–OH and generation of H2O2 were prevented by apatite at the surface of TiO2. Additionally, the adsorptive nature of AP–TiO2 for methylene blue and photocatalytic activity for bleaching were improved by apatite at the surface of TiO2.

Host Lattice Effects in the EPR Spectra of Cr3+-Li+ and Cr3+-VMg Centres in A2MgF4 (A=K, Rb) Crystals

EPR measurements on Cr 3+ centres in K 2 MgF 4 and Rb 2 MgF 4 for crystals co-doped with Li + have been made at room temperatures. In both crystals spectra for Cr 3+ centres associated with a cation vacancy (V Mg ) and a Li + ion at the nearest Mg 2+ site in the c plane have been observed. The obtained fine structure parameters D and E are interpreted by two uniaxial spin-Hamiltonian parameters D 1 and D 2 using the separation method. The systematic relations among the tetragonal Cr 3+ -V M and Cr 3+ -Li 1 centres in cubic perovskite fluorides are discussed by the separated parameters D 2 . The relation of the separation method to the Rudowiczs net charge compensation model is also discussed.