Takeshi Okato

Catalyst-free growth of high-quality ZnO nanorods on Si(100) substrate by two-step, off-axis pulsed-laser deposition

We demonstrate the growth of ZnO nanorods on a Si(100) substrate through a two-step process, annealing and off-axis pulsed laser deposition (PLD), without a catalyst. ZnO powder was first dispersed on the Si substrate, and annealed to form seeds for ZnO nanorod growth for 2.5 h. In the second step, ZnO was deposited on the seeds by PLD for 1 h. It was experimentally confirmed that the surface morphology after annealing acts as growth nuclei of nanorods. Controlling growth parameters during deposition enables the adjustment of the sizes of nanorods, namely, diameter and length. The diameters of the grown nanorods are 50–700 nm and the lengths are 2–10 µm. Cathode luminescence (CL) spectra were used to evaluate the states of defects within the ZnO nanorods. According to the CL measurement results, the thinnest nanorod arrays were found to have fewer defects, and more defects were introduced as nanorods became thicker.

Fabrication and characterization of ZnO nanorods by pulsed laser deposition method through catalyst-free process

The ZnO nanorod possesses large surface area, high aspect ratio and quantum confinement effect. Therefore, the ZnO nanorod would be a candidate for a gas sensor, dye-sensitized solar cell, etc. For device applications, it is very important to control the growth of ZnO nanorods. Pulsed-laser deposition (PLD) is an effective method to grow ZnO nanostructures. In this paper, we have fabricated the ZnO nanorods on Si substrate through a two-step process without a metal catalyst. As for a first step, ZnO powder dispersed on Si substrate is thermally annealed in order to fabricate ZnO seed layer. The seed acts as a catalyst of the ZnO nanorod growth, and is found to be zinc silicate (112) by XRD measurement. Secondly, ZnO is deposited on the seed layer by PLD at an argon pressure of 10-2 Torr. The length of nanorods is up to 4 μm with a typical diameter of 100 nm. The CL emission spectra are observed and the existence of defects within the ZnO nanorods has been identified. By controlling the growth parameters, high-quality nanorods without defects were fabricated by this two-step PLD method.

Cathodoluminscent properties of pulsed laser deposited Er,Yb co-doped Y2O3 thin films

Thin Er3+, Yb3+ co-doped Y2O3 films have been grown on (001) SiO2 substrates by pulsed laser deposition from ceramic targets. The influence of the active ion concentration and processing parameters on the structure, cathodoluminescent (CL) spectra and optical properties of the films have been investigated. The films were prepared in an oxygen atmosphere at pressures of 0.01 and 0.1 mbar and at 500°C substrate temperature. Blue, green and red light emissions centred at 408 nm, 562 nm and 660 nm, respectively, and an infrared (IR) spectrum centred at 1.54 µm were measured. The intensities of the CL peaks strongly depend on the Er and Yb ion concentration in the films and their ratio. An increase in Yb content at a fixed Er concentration leads to a decrease in the CL intensity in the visible range. It is higher for the films deposited at 0.1 mbar, which correlates with the better crystallinity obtained. Despite their better crystallinity, the films prepared at 0.01 mbar have a slightly higher optical transmission in the visible and near-IR range, which corresponds to the smoother surface and lower absorption measured.

Nitrogen-doped TiO 2 thin films grown on various substrates

We demonstrate the effect of N-doping in various phases, where N-doped states, bandgap shifts, and photocatalytic efficiencies are determined. The N-doped TiO2 films were grown by pulsed-laser deposition using TiON targets. The crystal structures were analyzed using x-ray diffraction and Raman spectroscopy. The crystalline phases of TiO2 were artificially controlled by choosing appropriate substrates. The anatase and rutile were epitaxially grown on (100) LaAlO3 and (001) sapphire substrates. Rutile-anatase mixtured phase were grown on soda lime glass substrates. We here note that N-concentration strongly depends on the growth temperature, so that we kept the growth temperature at 300 °C in order to fix the N concentrations for respective specimens. Chemical bonding states of N within the matrix were investigated by x-ray photoelectron spectroscopy. The optical absorption and bandgap were measured using UV-VIS spectrometer. The photocatalytic activity of the films was evaluted by measuring the decompositon rate of methylene blue solution with the visible light illumination.

Fabrication of Waveguides by Laser Deposition

We presented the growth of Nd:KGW waveguides on Si substrates. The waveguide structure was realized by the CeO2 buffer layer, and the as-grown Nd:KGW films were optically active. We also demonstrated the nozzle-gas-assisted PLD (NGA-PLD) method for efficient film oxidation. These results are applicable to the general problems of PLD, and are of great interest.

Nitrogen-doped ZnO thin films by use of laser ablation of ZnO(1-x)Nx targets

ZnO is inherently a strong n-type semiconductor due to its intrinsic defects. Among the group V elements (N, As, P, Sb), nitrogen is considered as teh most hopeful dopant for p-type ZnO, because substitute N (N0) is a relatively shallow acceptor. However, technical issues of the low solubility for the desirable defect and compensations from undesirable donor-like defects are imposed on the development of high mobility and low resistivity p-type ZnO. Breaking through these issues is accompanied by the optimization of dopant concentration and reduction of intrinsic defects. In this study, we have investigated the dependence of the nitrogen concentration on its electrical properties. Home-made ZnO1-xNx targets were prepared and used for KrF excimer pulsed-laser deposition (PLD) at precisely controlled growth conditions. Thin films were deposited on c-cut sapphire substrates. The nitrogen concentration was tuned by adjusting the amount of nitrogen in the ablation targets. The film properties were characterized by x-ray diffraction (XRD) and x-ray photoemission spectroscopy (XPS). The electrical properties were measured by van der Pauw method. The as-grown ZnO:N films showed n-type conductivity, however, they were converted to p-type upon post-deposition thermal treatment. Further improvement was demonstrated by introducing a ZnO low-temperature buffer layer which realized the lattice mismatch relaxation.

Fabrication and characterization of hexagonally assembled ZnO and ZnO:N thin films with buffer layer by pulsed-laser deposition

Zinc oxide (ZnO) is a wide band gap (3.37 eV) material and significantly interesting for many applications. Recently, many studies have been directed toward the fabrication of p-type ZnO using the group V elements (N, As, P, Sb). We have fabricated ZnO thin films in nitrogen background gas by the pulsed-laser deposition (PLD), because nitrogen is the most promising dopant. The nitrogen incorporation into the films was confirmed by X-ray Photoelectron Spectroscopy (XPS) analyses for the films grown under the high nitrogen pressures. However, the nitrogen doped films do show the disordered hexagonal microstructures which induce the defects into the crystal resulting from strains and stresses. Therefore, we have introduced the ZnO low-temperature buffer layers (LTBLs) between ZnO thin films and sapphire substrates to reduce the defects. The growth conditions of the ZnO LTBL were experimentally optimized for the first time. Characteristics of ZnO thin films with and without a ZnO LTBL were determined by x-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FE-SEM), and Atomic Force Microscopy (AFM). The electrical properties of the ZnO thin films were measured by the van der Pauw method. As a result, epitaxial lateral overgrowth (ELO) and hexagonally assembled ZnO have been successfully confirmed using LTBL. Nevertheless, the films still show the n-type conductivity, our results clearly demonstrate the advantages of the ZnO LTBL.

Laser ablation of TiO 2-2x N x targets for efficient N-doping into anatase TiO 2 photocatalytic films

Among the well-known photo-catalytic materials, the anatase TiO2 is the most promising in terms of its chemical stability and high reactivity. It is known that the photo-catalytic activity under the visible light irradiation can be enhanced by nitrogen doping into the anatase, because the substitutional nitrogen produces an impurity state which absorbs the visible light. In this paper, we will report on the properties of the nitrogen doped films with different dopant concentrations. The anatase films are prepared by KrF excimer pulsed-laser ablation of TiO2-xNx targets. The films are deposited on the (100) LaAlO3 substrate which has a good lattice matching with anatase (~ 0.2%). First, we discuss the optimization of the growth conditions. To prepare the nitrogen doped anatase thin films, we have developed a low-temperature epitaxy. The growth of anatase-type TiO2 was confirmed using an x-ray diffraction (XRD). The nitrogen incorporation was evaluated by an x-ray photoemission spectroscopy (XPS). The as-grown films have very smooth surface and exhibit good amphiphilic properties. Then, we present the photo-catalytic activity of the films. The nitrogen doping concentration was varied by adjusting the amount of nitrogen in the ablation targets. The photo-catalytic activity was measured by the decomposition rate of methylene blue solution under a fluorescent light illumination.

Fabrication of Nd:KGW waveguides by use of nozzle-gas-assisted PLD method

Nd:KGW [or Nd:KGd(WO4)2] films are grown using the nozzle-gas-assisted pulsed-laser deposition (NGA-PLD) method. A KrF excimer laser is used for the ablation of K-rich ceramic targets and films are deposited on r-cut sapphire substrates. The dependences of the oxygen nozzle gas on the film optical and crystallographic properties are investigated. The Nd:KGW film is colored if the mass flow is not sufficient. The origin of the color is attributed to the oxygen deficiency phase which is confirmed by the optical absorption and x-ray diffraction (XRD) measurements. Highly crystallized Nd:KGW films are grown by NGA-PLD under the optimized conditions. Comparing the films grown by conventional PLD (C-PLD) method, a dramatic improvement in the film surface morphology is chieved with NGA-PLD.

Nd:KGW planar waveguides on silicon substrate for integrated active optical devices

This study presents the fabrication of Nd:KGW films on CeO/sub 2//Si by use of conventional PLD. A further improvement of the Nd:KGW laser waveguide by use of newly developed nozzle-gas-assisted PLD (NGA-PLD) method is presented.