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.
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.
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.
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.
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.
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.
Silicon substrate is very important for integrated opto-electronic devices applications; because it is widely used the ULSI industries and allowing technically matured processing. However, the large refractive index of Si does not allow for an optical waveguide structure on it by direct growth. Nd:KGW waveguide laser is also a very promising device, which has not only a high stimulated emission cross-section as the laser crystal, but also a high 3rd nonlinear susceptibility. Here we will present our recent results of pulsed laser deposition (PLD) of Nd:KGW thin films on (100) Si substrate by introducing (100) CeO2 buffer layer. The waveguide structure is achieved by the lower refractive index of CeO2. It is well known that films containing alkali metal fabricated with single crystal targets have lower alkali metal concentrations than the stoichiometric target. One solution is the use of K-rich ceramic targets instead to prevent lack of the K during deposition. In this paper, we demonstrate the K compensation and high quality films growth from the viewpoints of crystallinity and optical properties by use of K-rich target. Moreover, we will investigate the required thickness of CeO2 buffer layer and prove the validity of results using a numerical analysis.