Yukio Yamamoto

A transparent metal: Nb-doped anatase TiO2

This Letter focuses on the discovery of a transparent conducting oxide (TCO), anatase Ti1−xNbxO2 films with x=0.002–0.2. The resistivity of films with x⩾0.03 is 2–3×10−4Ωcm at room temperature. The carrier density of Ti1−xNbxO2 can be controlled in a range of 1×1019to2×1021cm−3. The internal transmittance for films with x⩽0.03 (40nm thickness) is about 97% in the visible light region. The transport and optical parameters are comparable to those of typical TCOs, such as In2−xSnxO3 and ZnO.

Ta-doped Anatase TiO2 Epitaxial Film as Transparent Conducting Oxide

We present electrical transport and optical properties of Ta-doped TiO2 epitaxial thin films with varying Ta concentration grown by the pulsed laser deposition method. The Ti0.95Ta0.05O2 film exhibited a resistivity of 2.5×10-4 Ω cm at room temperature, and an internal transmittance of 95% in the visible light region. These values are comparable to those of a widely used transparent conducting oxide (TCO), indium tin oxide. Furthermore, this new material falls into a new category of TCOs that utilizes d electrons.

Transport properties of d-electron-based transparent conducting oxide: Anatase Ti1−xNbxO2

The transport properties of a d-electron-based transparent conducting oxide, Nb-doped anatase Ti1−xNbxO2, were investigated as a function of the Nb content x. From optical resistivity spectra, the static effective mass was evaluated to be ∼0.4m0 in the low-carrier-concentration (ne) regime, which is approximately the same as those of conventional transparent conducting oxides (TCOs), and two orders of magnitude smaller than that reported for rutile TiO2. The Hall mobility at room temperature, which is maximized at around x=0.01 (ne∼1021cm−3), was found to be mainly dominated by optical phonon scattering unlike that of other TCOs.The transport properties of a d-electron-based transparent conducting oxide, Nb-doped anatase Ti1−xNbxO2, were investigated as a function of the Nb content x. From optical resistivity spectra, the static effective mass was evaluated to be ∼0.4m0 in the low-carrier-concentration (ne) regime, which is approximately the same as those of conventional transparent conducting oxides (TCOs), and two orders of magnitude smaller than that reported for rutile TiO2. The Hall mobility at room temperature, which is maximized at around x=0.01 (ne∼1021cm−3), was found to be mainly dominated by optical phonon scattering unlike that of other TCOs.

Radiation and photochemistry of onium salt acid generators in chemically amplified resists

The difference in photochemistry and radiation chemistry of sulfonium salt acid generator was investigated by product analysis and time resolved spectroscopic methods for chemically amplified resist application. After KrF excimer laser and electron beam irradiation of sulfonium salt, yields of decomposed products including acid were determined. The ultra fast in-cage reactions after laser irradiation were directly observed by the femtosecond laser flash photolysis method. Intermediates after electron beam irradiation were observed by nanosecond electron beam pulse radiolysis. From both the product analysis and time resolved spectroscopic methods, the contribution of each reaction pathway to acid generation was determined.

(Cd,Zn)S thin films prepared by chemical bath deposition for photovoltaic devices

Abstract The preparation and characterization of (Cd,Zn)S thin films were attempted using two kinds of chemical bath deposition process: all the reactants were mixed either at room temperature (process A) or after preheating the source solutions (process B). The band gaps and the lattice constants of the thin films prepared by process B changed from the values for CdS to those for ZnS, but those of the films prepared by process A were not changed. X-ray diffraction analyses showed that all the peaks from the thin films prepared by process B could be assigned to diffraction lines of wurtzite (Cd,Zn)S. The photoconductivity of the thin films was larger than the dark conductivity, in particular 5×10 3 times larger in the thin films prepared by process B with [Zn]/([Cd] + [Zn]) = 0.9 in the solution.

Preparation and characterization of (Cd,Zn)S thin films by chemical bath deposition for photovoltaic devices

Abstract Thin films of (Cd,Zn)S were prepared by the improved chemical bath deposition process which was optimized with the amounts of ammonia playing an important role, and their structural, electrical and optical properties were investigated for photovoltaic device applications. X-ray diffraction peaks for the deposited thin films were assigned to diffraction lines from wurtzite (Cd,Zn)S. The resistivity of (Cd,Zn)S thin films was in the range of 4–8 × 107 Ω cm. The [Zn]/([Cd] + [Zn]) ratio and the band gaps in the thin films could be controlled by varying the mixture ratio of CdI2 and ZnI2 solutions used as reactants. The variation of band gaps yielded the bowing parameter of 0.724 eV.

Fabrication and characterization of CuIn(SxSe1-x)2 thin films deposited by r.f. sputtering

Abstract Thin films of CuIn(SxSe1−x)2 were prepared by r.f. sputtering from powder targets which were previously mixed from powdered binary compounds. X-ray diffraction analyses showed the films had the single phase chalcopyrite structure, and the lattice parameters (a and c) varied linearly with the increase in sulphur content x from x=0 (CuInSe2) to x=1 (CuInS2). The orientation to the (112) plane of the CuIn(SxSe1−x)2 thin films increases as the sulphur content x increases. From scanning electron microscope images, the grain sizes in the films were found to decrease with the decrease in sulphur content. Optical transmittance measurements showed that bowing behaviour is not, observed in the films.

Characterization of CuInS2 thin films prepared by sputtering from binary compounds

For fabricating more inexpensive thin film solar cells, CuInS2 thin films were prepared by RF sputtering from binary compounds (Cu2S and In2S3). From EPMA analyses, composition of the thin films was varied with changing the mixing ratio ‘x’ (x = [Cu2S][In2S3]). X-ray diffraction studies showed that the thin film sputtered from the target with the mixing ratio of x = 1.5 had a single phase with chalcopyrite structure. In this case, CuInS2 thin films showed higher optical absorption coefficients and a band gap of 1.52 eV, suitable for absorbing incident solar spectrum.

Effect of heating probe on reactively sputtered TiO2 film growth

In this paper we propose a simple method to ionize sputtered neutral Ti and added oxygen gas, by inserting a heating probe into the plasma space. The effect of bias voltage applied to the heating probe on the crystalline structure and optical properties was investigated for the TiO2 films deposited by reactive sputtering using this method. In the films deposited with the heating probe applying positive bias voltage to the substrate, the rutile phase was grown on the Si(111) substrate at a low temperature, and this film showed better optical properties than the films prepared without bias voltage. These changes in crystalline and optical properties were attributed to the ionization and acceleration of Ti and oxygen particles.

Radiation effects on the thermal degradation of poly(vinyl chloride) and poly(vinyl alcohol)

Radiation effects on the formation of conjugated double bonds in the thermal degradation of poly(vinyl chloride) (PVC) and poly(vinyl alcohol) (PVA) were investigated. Thin films of PVC and PVA were either irradiated with γ-rays at ambient temperature (pre-irradiation) and then subjected to thermal treatment, or irradiated at elevated temperatures (in situ irradiation). An extensive enhancement of the thermal degradation was observed for the pre-irradiation of the PVC films, which was more effective than the effect of the in situ irradiation at the same absorption dose. For the PVA degradation, however, the effect of the in situ irradiation was larger than that of the pre-irradiation. The results were explained and related mechanisms were discussed based on radiation-induced chemical reactions and their individual contributions to the thermal degradation behaviors of the two polymers.