Masahiro Okuda

Low resistivity transparent conducting Al-doped ZnO films prepared by pulsed laser deposition

A series of AZO films were grown on glass substrates by a method of pulsed laser deposition using a split target divided into AZO (Al2O3: 1 wt.%) and AZO (Al2O3: 2 wt.%). The film deposition took place at a substrate temperature of 230 °C within a magnetic field applied perpendicularly to the plume. To suppress the droplet generation caused by the intense laser energy, the laser energy density was reduced to 0.75 J/cm2 (15 mJ). For an approximately 280-nm thick-AZO film grown at a target-to-substrate distance of 25 mm, we obtained the lowest resistivity of 8.54×10−5 Ω·cm and an average transmittance of more than 88% in the visible range. In the cross-sectional TEM observation and XRD spectra, the regularity in the crystal growth was generated immediately from the interface between the substrate and the film.

Electrical and Optical Properties of GexSe1-x Amorphous Thin Films

Measurements of electrical and optical properties have been made on GexSe1-x amorphous thin films of about 1 µm thickness. From the experiment of electrical conductivity at various temperatures, two processes of the d.c. conductivity in GexSe1-x films are observed in plots of ln σ against 1/T. The thermoelectric power exhibits a maximum value of p-type conduction at pure Se and a change of the sign from positive to negative at 0.4<x<0.5. Measurements of the photoconductivity and the absorption spectra have been made at room temperature. It is found that the energy gap is decreased with increasing additions of the Ge content. An energy band model of amorphous GexSe1-x films has been proposed for consistent explanations of experimental data.

Polarized Memory Effect Observed on Se-SnO2 System

The polarized memory effect (the memory in which switching between ON and OFF states depends on the polarity of the bias voltage) on the Se-SnO2 system is described. This effect is observed on the samples with Se films 300~5000 A in thickness. The ratio of the resistance of OFF state ROFF to that of ON state RON is about three orders of magnitude. In the case of the film with thickness 103 A, the threshold voltage from OFF to ON states Vth is 15 V, and the reverse voltage from ON to OFF states Vr is 3.5 V.

Photovoltaic Effect Observed on the Construction of Metal-Amorphous InxSe1-x Thin Film–SnO2 System

Photovoltages as a function of light intensity and wavelength have been studied on the system of metal-amorphous InxSe1-x thin film-SnO2, where the metal electrodes of Au, Sb, Al, Ag, In, and Cu were used. For all except the Sb electrode, the system had a positive photovoltage at short wavelength and a negative one at longer wavelength. Among these electrodes, the construction having the top electrode of Sb exhibited the best photovoltaic characteristic; next was the system with the Au electrode. The photovoltage increased first linearly and then logarithmically with the photon flux. A considerable photovoltage was obtained for a contact with x=0.2. A tentative band model for a consistent explanation of the experimental data is postulated.

Highly conducting transparent indium tin oxide films prepared by pulsed laser deposition

Abstract In 2 O 3 doped with 5 wt.% SnO 2 [indium-tin oxide (ITO) (5 wt.%)] films were deposited on glass substrates by pulsed laser deposition using an ArF laser (λ=193 nm). In all experiments, repetition rates of 1∼50 Hz, an energy density of 6 J/cm 2 , and an ablation time of 30–900 s were used. A lowest resistivity of 8.45×10 −5 Ω cm and an optical transmittance of more than 80% in the visible range of the spectrum were obtained for ITO (5 wt.%) films of approximately 300-nm-thickness fabricated at a substrate temperature of 400 °C and oxygen pressure of 10 Pa. Smooth surfaces with an average surface roughness of 1.26 nm were observed by field-emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM).

Transient photocurrent in amorphous selenium and nematic liquid crystal double layers

Transient photocurrents in amorphous selenium (a‐Se) and nematic liquid crystal (NLC) double layers have been investigated to elucidate the charge carrier transport process in NLC. It is shown that the transient photocurrents obtained in the experiment are space‐charge limited currents induced by the delayed charge injection from the a‐Se layer to the NLC layer. The mobile charge carriers are found to be positive ions with the drift mobility of 3.5×10−6 cm2/V s at 303 K and the ionic radius of 0.32 nm in 4‐cyano‐4′‐alkyl‐biphenyl. The origin of the ions is briefly discussed.

Micro-Textured Milky ZnO:Ga Thin Films Fabricated by Pulsed Laser Deposition Using Second-Harmonic-Generation of Nd:YAG Laser.

Approximately 800-nm-thick ZnO films doped with 4 wt% Ga2O3 (GZO films) have been deposited on glass and quartz substrates by a pulsed laser deposition method using Second-Harmonic-Genreation (SHG) of Nd:YAG laser (λ=532 nm). In all experiments, a repetition rate of 10 Hz, energy densities of 0.1–2.5 J/cm2, and irradiation times of 2–3 h were used. Scanning Electron Microscopy (SEM) observations of the film surface revealed that there were micro-textured structures (0.5–1 µm in diameter), surrounded by small rugged and irregular structures (250–400 nm in size). It was recognized from XRD spectra that a strong peak of (103) planes reflected from the micro-textured milky surfaces is dominant. A haze ratio of 46.6 % was obtained at a wavelength of 550 nm for approximately 800-nm-thick GZO (4 wt%) films grown at a substrate temperature of 300 °C in oxygen with a flow rate of 3 sccm. The lowest resistivity of 2.33×10-4 Ωcm and the lowest sheet resistance of 2.72 Ω/sq were obtained for these GZO (4 wt%) films.

Surface Flatness of Transparent Conducting ZnO:Ga Thin Films Grown by Pulsed Laser Deposition

ZnO films doped with 4, 7 and 13 wt% Ga2O3 (GZO films) have been deposited on glass and quartz substrates using a pulsed laser deposition technique with an ArF laser (λ=193 nm). In all experiments, a repetition rate of 10 Hz, an energy density of 1 J/cm2, and an irradiation time of 20-30 min (12000-18000 shots) were used. An optical transmittance of more than 90% was obtained in the visible region of the spectrum for 150-200-nm-thick GZO (7 wt%) films deposited at substrate temperatures of 200-300° C. The lowest resistivity of 2.08×10-4 Ω⋅cm and the lowest sheet resistance of 14.5 Ω/sq were obtained for GZO (7 wt%) films grown at a substrate temperature of 200° C. It was found from AFM images that there were minute irregularities (50-70 nm in size, average roughness 0.8 nm) on the surfaces of GZO (7 wt%) films grown at substrate temperatures of 25-300° C.

Transient charging current in nematic liquid crystals

Transient charging currents in nematic liquid crystals have been measured with a single‐ and a double‐voltage pulse technique to elucidate the role of impurity ions incorporated in the materials in the transient charging processes. In the single pulse experiment, a peak in the current transient is observed in the nematic phase. It is shown that occurrence of the peak is due to the alignment of the director of the liquid crystal molecules in the direction of applied electric field. In the double pulse experiment, a voltage pulse (prepulse) is applied to the liquid crystal before the measurement of the transient current. It is observed that the application of the prepulse significantly alters the shape of the current. The changes in the current can be explained by considering the impurity ion distribution, modified by the prepulse, in the nematic liquid crystals. It is concluded that the double pulse experiment is a powerful tool for the examination of the influence of impurity ions in liquid crystals on th...

Pulsed Laser Deposition of Transparent Conducting Indium Tin Oxide Films in Magnetic Field Perpendicular to Plume

In2O3 doped with 5 wt% SnO2 (indium-tin oxide(ITO)(5 wt%)) films have been deposited on glass and quartz substrates in a magnetic field generated from three pieces of rare–earth permanent magnets (1.25 T flux density) placed at every 120° angle to surround the plume produced by pulsed laser deposition using an ArF laser (λ=193 nm). In all experiments, a repetition rate of 10 Hz, an energy density of 1.5 J/cm2, and an irradiation time of 20–30 min (12000–18000 shots) were used. The lowest resistivity of 7.2×10-5 Ωcm and an optical transmittance of more than 90% in the visible region of the spectrum were obtained for approximately 30-nm-thick ITO (5 wt%) films deposited at a substrate temperature of 300°C in oxygen with a flow rate of 2 sccm. Very smooth surfaces with an average surface roughness of 0.61 nm were observed by scanning electron microscope (SEM) and atomic force microscope (AFM).