Key Engineering Materials Vol. 301

Abstract: Thin film heaters made of Ca-doped lanthanum chromite (LaCrO3) are fabricated by means of RF magnetron sputtering. Thin films of Ca-doped LaCrO3 show different heating characteristics under dc current drive for different purities of alumina substrate. In the case of a low purity (95.3%), LaCrO3 thin film heaters showed an increase of resistance with time. On the other hand, in the case of high purity (approximately 99.5%), the resistance was almost constant. It is speculated that impurities such as SiO2 contained in an alumina substrate enhance the migration of Ca atoms through the Pt electrode and increase the resistance because of the decrease of Ca atoms in the thin film. It is considered that the increase of resistance is caused by such a reaction.

Abstract: Copper aluminum disulfide (CuAlS2) powders were synthesized in an evacuated ampoule at elevated temperatures. X-ray diffraction analysis revealed that the powders heated at temperatures higher than 800oC were single-phase CuAlS2. In the cathodoluminescence (CL) spectra measured at room temperature, the powders heated at temperatures higher than 600oC exhibited a visible emission peak at approximately 1.8 eV and a distinct ultraviolet emission peak at 3.45 eV. The powder heated at 700oC showed the maximum intensity of ultraviolet emission which is considered to be associated with excitons.

Abstract: Beta gallium oxide (β-Ga2O3) crystals were grown on β-Ga2O3 ceramics heated by electric current under vacuum at various ambient temperatures. From cathodoluminescence at room temperature, emission peaks at 2.9 and 3.5 eV were clearly observed. With increasing ambient temperature, the relative intensity of ultraviolet emission (3.5 eV) to blue emission (2.9 eV) showed a peak at 400oC. These results suggest that the ambient temperature during the electric current heating of β-Ga2O3 ceramics in vacuum is one of the most effective factors for the ultraviolet emissive β-Ga2O3 crystal.

Abstract: ZnO films doped with Li and Cl atoms have been prepared by organometallic chemical vapor deposition method. In photoluminescence spectra of the doped ZnO films, strong emission appears at 376 nm which is slightly longer than that of free-excitons. As excitation intensity increases, a peak wavelength of the emission is shifted toward a shorter value. The emission near band-gap edge is assigned to radiative recombination via Li-Cl pairs.

Abstract: In order to clarify the fundamental luminescent mechanism of undoped and Er-doped ZnO thin films synthesized by sputtering method, cathodoluminescence (CL) from the samples formed on several kinds of substrate were measured. There was no explicit peak identified with luminescence from ZnO crystal defects in undoped sample, on the contrary, three sharp luminescent peaks were observed in the case of Er-doped ZnO film due to the internal transition of the additive Er ions in the CL spectrum. The mechanism was investigated in comparison with photoluminescence (PL).

Abstract: This paper reports on the formation of film structures and the highly improved photovoltaic output current of the lead lanthanum zirconate titanate (PLZT) employed. The photovoltaic effect of ferroelectrics has the advantage of its simple mechanisms of non-bias applications which are indispensable for semiconductor p-n junctions. But the output current of PLZT bulk is too low for use as a device current source. The PLZT film structure exhibited μA output current upon light illumination. The photovoltaic current of the PLZT film was more than 102 times than that of bulk PLZT. These differences are due to the characteristics of the design of the film including the configuration of the electrode. The PLZT film also has the advantage of easy output control and suitability for use on Si. Results show that the photovoltaic effect of the ferroelectric film is useful as the current source for micro-electro-mechanical systems (MEMS).

Abstract: Praseodymium (Pr3+)-doped BaTiO3 was synthesized by a high concentration sol-gel process and sintered at different temperatures from 800oC to 1200oC. The emission was due to intra-4f transitions of Pr3+ which were mainly from the excited states 3P0 and 1D2 to the ground states of 3H4 of Pr3+, respectively. Enhancement of the luminescent properties of Pr-doped BaTiO3 was also investigated via adding Mg. For BaTiO3: Pr phosphor added with 0.5 mol% Mg, the emission intensity was about 60 times higher than that of samples without Mg addition. The emission enhancement may caused by the improved crystallinity, the charge compensation and the change of the substitution site of Pr3+ with the addition of Mg.

Abstract: Two-dimensional Pb0.865La0.09Zr0.65Ti0.35O3 (PLZT) photonic crystals with hexagonal arrays of air holes on (001) Pt-sputtered magnesium oxide substrates were fabricated using a sol-gel process with resist molds. The PLZT photonic crystals have a thickness of 200 nm, and the period of hexagonal array and the radius of air holes were 400 or 450 nm and 135 or 165 nm, respectively. The PLZT photonic crystals were primarily orientated along the c-axis. The strain hysteresis loop of the photonic crystals suggested that the photonic crystals are ferroelectric. Several peaks within range of wavelength between 500 nm and 1000 nm appeared in the optical reflection spectra from the cleaved facet of the photonic crystals. The peaks have polarization dependence and the frequency bands of the each peak were close to that of photonic band gaps (PBGs) expected by the calculations.

Abstract: The radiation of X-rays using the electric charge generated by a temperature change of a LiNbO3 single crystal has been investigated. When the LiNbO3 single crystal was placed in a conductive cylindrical pipe made of graphite, aluminum or copper, the photon flux of the radiated X-rays was dependent on the type of conductive material used. Depending on the type of materials on which the crystal was placed, the photon flux of the radiated X-rays increased with decreasing work function of the material.

Abstract: We successfully controlled nanoscale morphology of ZnO, SnO2 and TiO2 grown in aqueous solution systems. Nanograins, nanorods and nanosheets of the metal oxides were obtained with variation of the preparation conditions including the concentration of the precursor reagents, pH and addition of various organic molecules and inorganic ions. The morphological evolution of wurtzite-type ZnO crystals from hexagonal nanorods to nanoplates and nanosheets was induced by the coexistence of phosphate anions and specific carboxylates. An increase in the concentration of SnF2 and pH changed the nanoscale morphology of crystalline SnO2 from grains into rods, flakes and sheets. Nanorods of anatase and rutile were deposited in the solutions of TiF4 and TiOSO4 by controlling pH and addition of urea. Nanosheets of titanate were prepared with ammonia anions in agar gel containing TiF4 and easily transformed into nanosheets of anatase.