Epitaxial growth mechanism of La2Zr2O7 thin film on metal substrate

We have studied the growth behavior of pyrochlore structure La₂Zr₂O₇ (LZO) thin films with different thickness on biaxial texture NiW substrates prepared by metal?Corganic deposition (MOD) method. The structure and morphology of LZO films were investigated by x-ray diffraction (XRD) and scanning electron microscopy (SEM). Our results show that the surface morphology of LZO films varies with changes in the surface coverage ratio of the metal substrates after crystallization in argon?Chydrogen atmosphere. The degree of texture of the LZO films reaches a maximum when the film thickness increases to 37 nm, where the NiW substrate surface is just fully covered. The complete coverage of the substrate surface together with the enhanced texture suggests that there exists a possible connection between the surface morphology and the texture development of the films prepared by MOD. Information on the nucleation and growth of oxide thin films on metal substrates with biaxial texture is important for understanding the mechanism of buffer layer formation and also for optimizing the preparation conditions for high-temperature superconducting coated conductors.     

Microstructural changes induced by the pyrolysis step on epitaxial La2Zr2O7 thin films grown by metalorganic decomposition

La₂Zr₂O₇ (LZO) thin films are used as buffer layers in second generation high Tc superconductor tapes. The microstructure of LZO films grown by metalorganic decomposition is characterized by the presence of nanovoids throughout the whole thickness of the films. We introduced an out-gassing plateau under vacuum during the pyrolysis process to decrease the size of voids. The temperature of this plateau was determined by Fourier transformed infrared spectroscopy, electron back scattering diffraction and X-Ray diffraction characterizations. The dwelling time was also varied. Transmission Electron Microscopy (TEM) studies revealed that a high heating ramp in combination with a less than an hour pyrolysis plateau decreased pore size. The deposition rate during dip-coating was also decreased to enhance out-gassing at the plateau. Successive LZO layers were deposited and energy filtered TEM images at C K-edge were performed to identify the role of carbon in the nucleation mechanisms.     

Photoluminescent and electroluminescent characteristics of various Bi-activated niobate-based oxide phosphor thin films

The photoluminescent (PL) and electroluminescent (EL) characteristics in the thin films of various Bi-activated niobate-based oxide phosphors have been investigated. LaNbO₄:Bi, GdNbO₄:Bi and YNbO₄:Bi phosphor thin films were prepared on thick BaTiO₃ ceramic sheet substrates by either a conventional or a combinatorial radio frequency magnetron sputtering deposition followed by postannealing at a high temperature. Blue PL emissions were observed from Bi-activated niobate-based oxide phosphor thin films postannealed at a temperature above approximately 800 °C in air or a pure Ar gas atmosphere. The excitation spectra of the blue PL emissions as well as the blue emission spectra observed from LaNbO₄:Bi, GdNbO₄:Bi and YNbO₄:Bi phosphor thin films exhibited an increasing red-shift in that order. In addition, blue, whitish blue-green or blue-green emissions were observed from thin-film electroluminescent devices fabricated with a LaNbO₄:Bi, a GdNbO₄:Bi or a YNbO₄:Bi thin-film emitting layer, respectively. However, the EL emission spectra observed from these TFEL devices exhibited a red-shift relative to the PL emission spectra observed from the respective phosphor thin films. The resulting difference between the observed PL and EL emission spectra may be explained by the 6 s²-6 s6p transitions in Bi³⁺ ions. In particular, the long-wavelength bands observed in the EL emissions may be attributable to the selection rule resolved by a high electric field applied to the phosphor thin films.     

Optical properties of zinc oxide thin films doped with aluminum and lithium

Aluminum doped Zinc Oxide (AZO) and Lithium doped Zinc Oxide (LZO) thin films are obtained by Pulsed Laser Deposition (PLD) method. These films are characterized by using Spectroscopic Ellipsometry (SE), X-ray Diffraction (XRD) and Photoluminescence (PL). By modeling the ellipsometry spectra we get the dielectric functions, the optical band gap E_g, and the electrical properties. Our results show the influence of the processing parameters on the optical and structural properties of doped ZnO thin films. The post-annealing treatment applied to AZO thin films, changes strongly the optical properties, by lowering the resistivity and red-shifting the band gap.     

Chemical solution deposited lanthanum zirconium oxide thin films: Synthesis and chemistry

Pyrochlore lanthanum zirconium oxide (LZO) thin films textured along 〈4 0 0〉 are synthesized using lanthanum acetate hydrate, zirconium propoxide, propionic acid, acetic acid glacial, and methanol as precursors. The materials growth and chemistry are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal gravimetric analysis (TGA). The formation of inkjet printed LZO films on Ni-5%W tape is found to be based on the decomposition of the LZO precursor solution. In the annealing process, Zr metal–oxides bonds are first eliminated between 150 and 250 °C, while carboxylates from precursors remain in LZO after the annealing carried out at 900 °C for an hour. Annealed LZO films have dense and smooth structure that are composed of nanoparticles sizing 10–15 nm and some pinholes sizing 25–35 nm accounted for less than 0.1% of the area are observed.     

Color control of emissions from rare earth-co-doped La2O3:Bi phosphor thin films prepared by magnetron sputtering

Multicolor photoluminescence (PL) and electroluminescence (EL) were observed from developed Bi- and rare earth (RE)-co-doped La₂O₃ (La₂O₃:Bi,RE) phosphor thin films. The phosphor thin films were prepared with various contents of co-doped RE, such as Dy, Er, Eu, Tb and Tm, by a combinatorial radio frequency magnetron sputtering deposition method. The obtainable luminance in EL and PL intensities changed considerably as the kind and content of RE were varied. Color changes from blue and blue-green to various colors in PL and EL emissions, respectively, were obtained in postannealed La₂O₃:Bi,RE phosphor thin films: films prepared by co-doping Bi at a constant content with various REs at varying levels of content. However, all of the observed emission peaks in PL and EL from La₂O₃:Bi,RE phosphor thin films were assigned to either the broad emission originating from the transition in Bi³⁺ or the visible emission peaks originating from the transition in the co-doped trivalent RE ion. The difference between PL and EL characteristics in La₂O₃:Bi,RE phosphor thin films is mainly attributed to the difference in the excitation mechanism.     

Enhanced visible light response of ZnO porous thin film by post-annealing treatment

ZnO thin films were synthesized by a facile electrodeposition method in the aqueous solution. Porous ZnO thin films with wurtzite structure could be achieved by mean of annealing treatment. The growth mechanism of the porous ZnO thin film was discussed. The intensity of the E ₂ mode in the ZnO thin film, which represents crystalline quality of the thin film increases with the increasing of annealing temperature. Optical properties indicate that annealing temperature has strong effect on the optical band gap value and defect concentrations. Both the green and yellow emissions corresponding to respective oxygen vacancies and oxygen interstitials can be identified. The results show that ZnO annealed at 400 °C exhibits a significant photocurrent density enhancement which is about 18 times larger than that of the as-deposited ZnO thin films. The mechanism of the enhanced photoresponse for the ZnO thin film has been discussed in detail.     

Luminescent properties of BaWO4 films prepared by cell electrochemical technique

Highly crystallized BaWO₄ films (45 × 30 mm²) have been prepared by cell electrochemical technique at room temperature. X-ray diffraction measurement reveals that the as-prepared BaWO₄ thin films have a scheelite structure. Under cell conditions, due to the decrease of crystal size and disappearance of large clusters, films' homogeneity has been improved markedly. With the UV excitation the peculiar and tunable luminescence of BaWO₄ films has been observed. Investigations uncover that exciting energies can be transferred among different emission centers, and that the yellow emission is probably relevant to the vacancies of Ba in the outermost layers of films.     

Low-temperature synthesis of KTa0.55Nb0.45O3 powders and thin films via metal-organic solution

Potassium tantalite niobate powders and thin films were synthesized via metal-organic solution. The synthesized powders and thin films have a tetragonal phase with the perovskite structure. The powders show a cubic shape with sizes in the range of 120–230 nm. The emission spectrum of powders shows emissions peaked at 413, 451 and 468 nm, related to the electron-hole recombination of localized exciton Ta⁴⁺–O^1? and Nb⁴⁺–O^1? in a regular octahedron (TaO₆ and NbO₆). The polarization-electric field hysteresis loop of thin films is rectangular. The current–voltage characteristic of thin films shows a feature of linear to nonlinear transition, which is related to the space-charge-controlled mode of electrical conduction in solids.     

Émission jaune dans la cathodoluminescence des couches minces de ZnS/Cu, Cl

We have observed a yellow emission in the cathodoluminescence spectrum of ZnS/Cu, Cl thin films annealed at 250°C. The association Cu-Cl responsible for this emission is transient and unstable, since annealing at 400°C transforms the yellow centre into blue and green centres.

Thin films appear to be a useful form in which to study and follow the formation of luminescent centres.     

On the origin of blue-green luminescence in spray pyrolysed ZnO thin films

Origin of the well-known blue-green emission of spray pyrolysed ZnO thin films has been discussed on the basis of variation of the properties due to different treatment of the samples such as ion beam irradiation and doping. 120 MeV Au ions and 80 MeV Ni ions were used for ion beam irradiation while indium was used for doping studies. It was assumed that, Au ions might have produced huge amount of defects while the defects might be less in Ni irradiated films due to the difference in energy and mass number. Photoluminescence studies of pristine sample showed only one emission at 517 nm at room temperature while the irradiated films showed a decrease in intensity of this emission. Indium doping also reduced the intensity of this emission. But additional emissions were also observed in these films. Based on the observations, we proposed that the blue-green emission was due to the transition from conduction band to the level due to oxygen antisite (O_Zn).     

Optical and structural properties of Mg-ion implanted GaN nanowires

Mg⁺ ions (60 keV) were implanted into GaN nanowires (NWs) with total fluxes of 5 × 10¹²-5 × 10¹⁴ cm⁻² followed by thermal annealing at 700 °C in N₂ ambient. Transmission electron microscopic images showed amorphous layer formation and defect accumulation in the higher dose Mg-implanted GaN NWs after annealing. Photoluminescence spectra (300 K) of the annealed Mg-implanted GaN NWs exhibited near-band-edge (NBE) emission, donor-acceptor pair (DAP) emission, and defect-related yellow luminescence. With increasing dose, the NBE and DAP emissions are red shifted. Similar phenomena were observed in samples implanted with Ar to produce similar amounts of lattice disorder. The NWs show a much higher sensitivity to defect accumulation than GaN thin films.     

Reel-to-Reel Deposition of Reactively Sputtered LaZrO Buffer Layers on Textured NiW Tapes for YBaCuO Coated Conductors

A biaxially textured buffer layer of LaZrO (LZO) was continuously grown on Ni-5-at%W (NiW) tapes using a reel-to-reel DC reactive sputtering technique. X-ray diffraction measurements revealed that the LZO layer deposited at water vapor partial pressures of 2.5×10^?2 Pa and the temperature of 800?°C are biaxially textured with the misorientation as low as 4.7°. AFM measurements show that the layer??s surfaces are homogeneous and flat with the RMS value less than 5 nm. YBa₂Cu₃O_7??? (YBCO) superconducting films grown on the single LZO buffer layer exhibited the critical temperature (T _c ) of 89 K with a narrow transition width less than 2 K. For comparison, another Ce₂Y₂O₇ (CYO) layer grown using DC reactive sputtering was added on the top of the LZO layer. The architecture YBCO/CYO/LZO/NiW exhibited better texture and higher T _c such as 90 K with ??T _c of 1.5 K, demonstrating the availability of a novel simplified buffer layer stack of CYO/LZO for superconducting coated conductors.     

Temperature dependent optical properties of UV emitting γ-CuCl thin films

In this work, ultra violet emitting properties of RF sputtered CuCl thin films were studied as a function of temperature in the region of 15-300 K. Deposition parameters were optimised to obtain good optical quality films exhibiting an intense and sharp Z₃ free exciton emission (at 383 nm) with an FWHM of ∼ 74 meV at room temperature. At lower temperatures, there are two other emissions observed at 375 and 390 nm, and they are identified as Z₁₂ and I₁ exciton lines, respectively. The free exciton emission is extremely stable at room temperature and it was found to be less influenced by the exciton-phonon interaction compared to the bound exciton emission which dissociates after 40 K. The Z₃ free exciton line exhibits a slight blue-shift on increasing the temperature. The temperature dependence of exciton line shift, line broadening and emission intensity are discussed in detail.     

Mg-Al共掺杂ZnO薄膜的制备及其光学特性

采用溶胶-凝胶（sol—gel）旋涂法在载玻片上制备了不同A1掺杂量的Mg—Al共掺杂ZnO薄膜．在室温下利用X射线衍射仪（XRD）、扫描电子显微镜（SEM）和光致发光（PL）谱仪等手段分析了Mg—Al共掺杂Zn0薄膜的微结构、形貌和发光特性．XRD结果表明Mg．AI＆掺杂zn0薄膜具有六角纤锌矿结构;随着Al掺杂量的增加,共掺杂薄膜呈C轴取向生长．由SEM照片可知薄膜表面形貌随Al掺杂量的增加由颗粒状结构向纳米棒状结构转变．透射光谱表明共掺杂薄膜在可见光区内的透射率大于50％,紫外吸收边发生蓝移．在室温下的PL谱表明Mg—Al共掺杂zn0薄膜的紫外发射峰向短波长方向移动：Al掺杂摩尔分数为1％和3％的Mg—Al共掺杂ZnO薄膜的可见发射峰分别为596nm的黄光和565nm的绿光．黄光主要与氧间隙有关,而绿光主要与氧空位有关．     

Fabrication and photoluminescence of thin films of lanthanide–polyoxometalates grafted on colloid silica sphere

In this paper, two lanthanide–polyoxometalates Na₉LnW₁₀O₃₆ (Ln=Eu, Dy, LnW₁₀) were grafted on 400 nm amine-functionalized spherical Stöber silica. The monolayer thin films of hybrid particles were fabricated on quartz by spin-coating method. The hybrid particles and thin films obtained were characterized by IR, UV–vis spectra, scanning electron microscopy, transmission electron microscope and luminescent spectra, respectively. The hybrid particles show strong luminescence which could be seen by naked eyes. The excitation spectra of hybrid particles and thin films show both abroad ligand to metal charge band and the excitation lines of rare earth ions. The transition ⁵D₀→⁷F₀ could be seen in the emission of hybrid EuW₁₀/SiO₂ spheres and thin films, which could not be found in spectrum of EuW₁₀ solid. It is noticed that the intensity ratio of red (⁵D₀→⁷F₂) to orange (⁵D₀→⁷F₁) of Eu³⁺ and the yellow to blue (⁴F_9/2→⁶H_13/2: ⁴F_9/2→⁶H_15/2) of Dy³⁺ in LnW₁₀/SiO₂ particles and thin films are quite different from those of LnW₁₀ solids. The different shapes and ratios between characteristic emissions of hybrid particles and thin films indicated the various symmetry of sites occupied by Ln³⁺ ions.     

Blue-violet phosphate phosphor thin films for EL

Photoluminescence (PL) and electroluminescence (EL) in blue-violet emission were observed in newly developed phosphate phosphor thin films such as Ba₃(PO₄)₂:Eu and Ba₃(PO₄)₂:Ti. These phosphate phosphor thin films were first deposited on thick BaTiO₃ ceramic sheets by r.f. magnetron sputtering using powder targets and then post-annealed in various atmospheres. Blue-violet PL and EL emissions were obtained in Ba₃(PO₄)₂:Eu and Ba₃(PO₄)₂:Ti phosphor thin films that were deposited in an Ar + H₂ (10%) gas atmosphere and then post-annealed above about 900 °C in an Ar + H₂ (10%) gas atmosphere. In particular, the EL observed in Ba₃(PO₄)₂:Eu thin films exhibited two peaks, a red emission peaking at about 615 nm and a blue-violet emission peaking at about 420 nm. A luminance of 2.0 cd/m² in blue-violet emission was obtained in a thin-film EL device using a two step post-annealed Ba₃(PO₄)₂:Eu thin-film emitting layer: step 1, post-annealing at 1000 °C in air for 1 h, and step 2, post-annealing at 1000 °C in an Ar + H₂ atmosphere.     

Microstructure modification of La2Zr2O7 buffer films for coated conductors by metal organic decomposition

La₂Zr₂O₇ (LZO) film directly deposited on Ni-5 at.%W by a chemical solution technique, metal organic decomposition (MOD), indicated a poor texture characteristic, which would result in high-angle grain boundaries in subsequent YBa₂Cu₃O_7?δ (YBCO) associated with weak-link behavior. Different ultrathin MOD-LZO, Y₂O₃, and CeO₂ seed layers (~several nanometers) with various annealing temperature were inserted to improve the crystallographic alignment. The relation between the texture and annealing temperature was systematically investigated. A CeO₂ seed layer allows us to grow high quality LZO epitaxial films with values of full width at half-maximum around 5.61° and 5.13° for the Φ-scan of (222) and rocking curve of (400) LZO, respectively, which is comparable to the microstructure of films grown using physical vapor deposited Y₂O₃ as a seed layer. This buffer template, serving for YBCO coated conductors, could potentially decrease the overall fabrication cost.     

The photoluminescence and optical constant of ZnSe/SiO2 thin films prepared by sol–gel process

In this paper, ZnSe/SiO₂ thin films were prepared by sol–gel process. X-ray diffraction results indicate that the phase structure of ZnSe particles embedded in SiO₂ thin films is sphalerite (cubic ZnS). The dependence of ellipsometric angle ψ on wavelength λ of ZnSe/SiO₂ thin films was investigated by spectroscopic ellipsometers. The optical constant, thickness, porosity and the concentration of ZnSe/SiO₂ composite thin films were fitted according to Maxwell–Garnett effective medium theory. The thickness of ZnSe/SiO₂ thin films was also measured by surface profile. The photoluminescence properties of ZnSe/SiO₂ thin films were investigated by fluorescence spectrometer. The photoluminescence results reveal that the emission peak at 487 nm (2.5 eV) excited by 395 nm corresponds to the band-to-band emission of sphalerite ZnSe crystal (2.58 eV). Strong free exciton emission and other emission peaks corresponding to ZnSe lattice defects are also observed.     

Rapid thermal annealing effects on atomic layer epitaxially grown Zns:Mn thin films

ZnS:Mn thin films were coated on transparent conducting layer indium tin oxide (ITO) 2×2 in. glass substrates by an atomic layer epitaxy process. Grazing-angle X-ray diffraction on thin films shows a phase-pure ZnS with a wurtzite structure oriented along the 002 direction. Photoluminescence and CL measurements were carried out on the films. The emission from ZnS:Mn thin films consists of two strong bands at 515 and 452 nm with an excitation band at 329 nm. For improved brightness, the samples were annealed in a rapid thermal annealing furnace under different gas atmospheres (N₂, O₂ and forming gas) so that the green emission was increased. The green emission is due to donar-acceptor combination. Promising results were obtained when the thin films were annealed in forming gas at 600°C for one minute. Scanning electron microscope micrographs showed that the particles are well crystallized, with a grain size of 0.3–0.5 μm. This paper reports the particle size and morphology on the luminescent characteristics of a Mn²⁺ center in ZnS thin films. Received: 16 April 1999 / Reviewed and accepted: 21 April 1999