Metalorganic Deposition Research Articles

Effect of Yb Substitution on Microstructure and Superconducting Properties of Y1−xYbxBa2Cu3O7−δ Films

The purpose of this study was to improve the critical current density (Jc) of YBa2Cu3O7−δ (YBCO) superconducting films at high magnetic fields. Thin Y1−xYbxBa2Cu3O7−δ (Y1−xYbxBCO) superconducting films on LaAlO3 substrates were prepared by modified low-fluorine metal organic deposition method (LF-MOD) in order to investigate the effect of Yb substitution on the microstructure and superconducting properties of a YBCO-coated conductor. In this study, single-phased, c-axial-aligned Y1−xYbxBCO films were obtained. Yb doping had a little effect on the critical temperature (Tc) and transition width (ΔT) values of Y1−xYbxBCO films. An abnormal phenomenon was observed that the Jc of Y1−xYbxBCO films at low field was smaller than that of the pure YBCO film, but the Jc of Y1−xYbxBCO films at high field was much greater than that of the undoped YBCO film. Compared to pure YBCO film, the Jc values of Y0.75Yb0.25BCO and Y0.25Yb0.75BCO films were enhanced by a factor of 8 at 8 T, 65 K, which meant that chemical substitution of Yb into the Y site could improve flux pinning significantly under the high magnetic field in Y1−xYbxBCO coated conductors.

GaN-on-Si Quasi-Vertical Power MOSFETs

We demonstrate the first GaN vertical transistor on silicon, based on a 6.7- $\mu \text{m}$ -thick n-p-n heterostructure grown on 6-inch silicon substrate by metal organic chemical-vapor deposition. The devices consist of trench-gate quasi-vertical metal–oxide–semiconductor field-effect transistors with a 4- $\mu \text{m}$ -thick drift layer, exhibiting enhancement-mode operation with a threshold voltage of 6.3V and an ON/OFF ratio of over 108. A high OFF-state breakdown voltage of 645 V along with a specific ON-resistance of $6.8~\textsf {m}\Omega \cdot \textsf {cm}^{{\textsf {2}}}$ were achieved thanks to the high-quality 4- $\mu \text{m}$ -thick GaN drift layer, presenting a relatively low defect density and very high electron mobility (720 cm $^{{\textsf {2}}}/\textsf {V}\cdot \textsf {s}$ ). This excellent performance represents a major step toward the realization of high-performance GaN vertical power transistors on low-cost silicon substrates.

Thickness Effect on Operational Modes of ZnGa2O4 MOSFETs

The device mechanism from depletion to enhancement mode for ZnGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> metal-oxide semiconductor field effect transistors (MOSFETs) grown on the sapphire substrate by metalorganic chemical-vapor deposition was studied. It was found that the thickness of the ZnGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> thin-film would affect the operational mode of the MOSFETs. Under the low-voltage operation (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> = 0.5 V), the transistors exhibited a high on/off ratio from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> , low subthreshold swing from 150 to 330 mV/dec, high field-effect mobility from 4.2 to 0.054 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V-s and threshold voltages from -17.8 to 4.1 V (using constant current = 1 nA). These electrical properties all depend on the thickness of the ZnGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> thin-film transistors. Finally, the e-mode ZnGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> thin-film transistor with off-state breakdown voltage over 400 V is fabricated.

Use of Laser Lithography for Striating 2G HTS Conductors for AC Loss Reduction

A scalable laser lithographic process has been used to striate second-generation high-temperature superconducting coated conductors manufactured with a rolling-assisted biaxially textured substrate and metal organic deposition process by American Superconductor Corporation to reduce ac losses for electric power applications. The process involves laser patterning a resist coating and etching, which can be scaled up for commercial production. The results show that the critical current, $\boldsymbol{I}_{\boldsymbol{c}}$ , is not degraded for striation widths greater than 150 μm and that the ac losses for the 150-μm wide striations are reduced from the unpatterned 1-cm wide conductor by a factor of 15 for a 60-mT perpendicular peak magnetic field at 50 Hz.

Structural and superconducting characteristics of YBa2Cu3O7 films grown by fluorine-free metal-organic deposition route

Microstructure and superconducting performance of YBa2Cu3O7 (YBCO) films deposited on LaAlO3 single crystal (LAO) substrates by a fluorine-free metal-organic deposition (FF-MOD) technique, have been studied by means of X-ray reciprocal space mapping (RSM), cross-sectional transmission electron microscopy (TEM) and magneto-optical (MO) imaging. Combining the X-ray diffraction and the TEM cross-sectional analysis, it is revealed that stacking faults, i.e. YBa2Cu4Ox intergrowths, and ab-plane twins are main defects in the FF-MOD YBCO films. Due to the highly epitaxial growth mechanism related to transient liquid phase, the LAO twinned substrate structure is also inherited in the FF-YBCO films. The low-density planar defects containing dislocations parallel to c-axis result in stripy patterns observed in the MO images. For comparison, the low-fluorine (LF) MOD film show a texture mosaic spread in the ab plane and is little influenced by the LAO twinning underneath, implying the severe structural disorder most likely associated with the large amount of small-angle grain boundaries. Moreover, the higher density of stacking faults was also detected by XRD θ−2θ scan in the LF-MOD film. It is suggested that associated partial dislocations formed at the boundary between the stacking faults and YBCO matrix act as strong linear (or dot) pinning centers. These structural characteristics are well in line with the better superconducting performance of the low fluorine-MOD film, in particular under external magnetic field at 77 K. This work offers an in-depth insight into the correlation between the microstructure and superconductivity in the MOD YBCO films.

INVESTIGATION OF ZnO NANO STRUCTURES FABRICATED ON AL2O3 SUBSTRACTES BY MOCVD

High quality ZnO nanostructures were grown directly on sapphire subtracts by metal organic vapor deposition. By changing the growth conditions, the different nano structures (nanorods,nanotubes, nanowalls, nanonetworks) can be selectively. These nanostructures were all epitaxially grown and had the same epitaxial relationship with respect to the substrate. Mechanisms corresponding to different nanostructures were discussed. The nanostructures exhibited stable excitonic states at room temperature, and the emission due to exciton-exciton scattering was observed

Tuning nanoparticle size for enhanced functionality in perovskite thin films deposited by metal organic deposition

Because of pressing global environmental challenges, focus has been placed on materials for efficient energy use, and this has triggered the search for nanostructural modification methods to improve performance. Achieving a high density of tunable-sized second-phase nanoparticles while ensuring the matrix remains intact is a long-sought goal. In this paper, we present an effective, scalable method to achieve this goal using metal organic deposition in a perovskite system REBa2Cu3O7 (rare earth (RE)) that enhances the superconducting properties to surpass that of previous achievements. We present two industrially compatible routes to tune the nanoparticle size by controlling diffusion during the nanoparticle formation stage by selecting the second-phase material and modulating the precursor composition spatially. Combining these routes leads to an extremely high density (8 × 1022 m−3) of small nanoparticles (7 nm) that increase critical currents and reduce detrimental effects of thermal fluctuations at all magnetic field strengths and temperatures. This method can be directly applied to other perovskite materials where nanoparticle addition is beneficial.

Molybdenum-containing amorphous metal oxide catalysts for oxygen evolution reaction

Amorphous mixed metal oxide oxygen evolution reaction catalysts containing Mo, Co, and Fe were synthesized by a low energy required photochemical metal-organic deposition (PMOD) process. The composition of the ternary amorphous metal oxide catalyst film can be precisely controlled as to its precursor solution, and there was no phase segregation observed. The electrocatalytic activity of this ternary catalyst system was carefully mapped out as contour plots to illustrate the kinetics parameters such as onset potential, Tafel slope as well as the overall performance, i.e. the potential required to reach the benchmark current density. In this study, adding higher valence state material, Mo, into catalytic active metal oxides, such as CoOx and FeOx, to form Co50Mo50Ox and Fe50Mo50Ox could highly increase the catalyst stability. More importantly, the ternary metal oxide Fe10Co25Mo65Ox showed the advantage of the relatively low onset potential (1.52 V), low Tafel slope (∼31 mV/dec), lowest potential to reach the benchmark 1 mA/cm2 and a better stability for the oxygen evolution reaction in Mo/Co/Fe system.

Photochemical metal organic deposition of FeO x catalyst on BiVO 4 for improving solar-driven water oxidation efficiency

Abstract Scheelite-monoclinic BiVO4 serving as a photoanode for photoelectrochemical water oxidation is known for its fast electron–hole recombination and slow kinetics, thus limiting its efficiency. In this study, a highly active oxygen evolution reaction catalyst, FeOx, is successfully deposited on BiVO4 by a simple and low energy needed photochemical metal organic deposition (PMOD) process under ambient environment. During the PMOD process, metal organic precursor goes through ligand-to-metal charge transfer, triggers by UV irradiation, and then reacts with oxygen to form a thin amorphous FeOx catalyst layer. This FeOx coated BiVO4 photoanode boosts the hole injection efficiency to ∼100% which is almost triple comparing to the bare BiVO4. Furthermore, with FeOx layer, the onset potential shifts cathodically for about 0.4 V and the photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) reaches 1.1 mA/cm2 which is 2.5 times of a bare BiVO4 photoanode. The stability of the photoanode is also improved. The FeOx coated BiVO4 demonstrates a substantial enhancement of the incident photon to current efficiency (IPCE) and the absorbed photon to current efficiency (APCE) due to its ability in increasing the hole injection efficiency and oxygen evolution reaction (OER) kinetics.

Effects of Plasma Parameter on Morphological and Electrical Properties of Superconducting Nb-N Deposited by MO-PEALD

This paper describes the deposition of superconductive Nb-N thin films in a metal-organic plasma-enhanced atomic layer deposition process using (tert-butylimido)-tris (diethylamino)-niobium and hydrogen plasma as precursors. In extension of our previous work, which investigated the possibility to deposit superconducting Nb-N, we systematically investigated the influence of different plasma parameters on superconducting and morphological properties of the niobium nitride thin film formed during the process. An initial increase of the duration of the plasma dose led to higher transitions temperatures and critical current densities, the optimum being a plasma dose time of 50 s. By decreasing plasma pressure, the resistivity at room temperature decreased, while the transition temperature increased. In addition, Nb-N thin films were deposited onto several substrates such as silicon, thermally grown silica, magnesium oxide (MgO), and r-plane sapphire. ${\rm{T}}_{{\rm{C}}}$ values from 6.2 K up to 14 K were achieved independently of the substrate materials. However, films deposited on MgO showed lower ${\rm{T}}_{{\rm{C}}}$ values. X-ray photoelectron spectroscopy measurement revealed the presence of niobium nitride but also of niobium oxide and oxy-nitride components in the films as well as the existence of a high amount of incorporated carbon impurities. X-ray diffraction measurements revealed two significant reflexes, which could be attributed to niobium nitride only. No crystalline niobium oxide or niobium oxynitride was detected. Thus, the films consisted of a matrix of polycrystalline Nb-N and amorphous or microcrystalline grains of different niobium oxide and oxynitride phases. Due to the fact that the deposited material showed superconductivity especially for ultrathin layers with thicknesses in the nanometer range, these films may be suitable for superconducting nanowire single photon detectors.

Influence of oxygen partial pressure adopted in separate heat-treatment steps on the epitaxy of LZO film

The influence of oxygen partial pressure on epitaxy is systematically studied in La2Zr2O7 (LZO) thin films fabricated by metal organic deposition (MOD) on yttria stabilized zirconia (00l) single crystal substrates. The results show that oxygen partial pressure affects not only the epitaxial growth but also the morphology evolution of film. It is observed that the texture sharpness of LZO film improves while its surface flatness becomes worse with increasing oxygen partial pressure during the whole heat-treatment process. Further studies illustrate that high oxygen partial pressure at pyrolysis step favors the texture sharpening of LZO film, which is contributed to the complete decomposition of organic constitution and the decrease of residual carbon in oxide film. However, low oxygen partial pressure at crystalline step helps the improvement of surface flatness for LZO thin film. It is supposed to be linked with the decrease of oxygen vacancy defects and the increase of grain size along with the replenishment of oxygen content at the crystallization stage. Tailoring oxygen partial pressure at separate heat-treatment steps is important to improving texture sharpness and surface flatness of LZO buffer film as long as the oxidation of metallic substrate can be avoided in coated conductors.

Effects of Mn3O4 precipitates on the vibrational properties of epitaxial Ca-doped LaMnO3 films

This work presents a systematic study by Raman spectroscopy of the vibrational properties of La0.7Ca0.3MnO3 (LCMO70/30) thin films epitaxially grown on LaAlO3 single crystalline substrates. The epitaxial LCMO70/30 thin films were prepared using a metal organic deposition process with thermal annealing at 800 °C, 900 °C and 1000 °C, with thicknesses ranging from 20–80 nm. The evolution of the Raman modes versus film thickness and annealing temperature have been investigated. The Raman spectra of the obtained films are found to be mainly dominated by three Raman lines at 225, 440 and 661 cm−1. The first two Raman lines are assigned to the vibrational modes of the LCMO70/30 films. However, by combining magnetization, x-ray diffraction and transmission electron microscopy with Raman spectroscopy, we concluded that the 661 cm−1 Raman line is due to the existence of a precipitate of a Mn3O4 phase in the LCMO70/30 films.

Effect of lanthanum addition on the fabrication of REBa2Cu3Oy (RE=Gd, Sm and Dy) thin films by metal organic deposition using fluorine-free solutions

The effects of lanthanum (La) additions of 0.5–10 mol% on the superconducting properties and crystal growth of REBa2Cu3Oy (REBCO, RE = Gd, Sm, and Dy) films fabricated by the fluorine-free metal-organic deposition method using 2-ethylhexanate solutions were investigated. La-doped REBCO films showed the decrease in the number density of holes on the film surface and the increase in the XRD peak intensity of the REBCO superconducting phases. La-added GdBCO with La contents of 0.5–3 mol%, and SmBCO and DyBCO films with La contents of 0.5–10 mol% showed improved critical current density (Jc) at the self-magnetic field compared to the pure films. For the GdBCO film, the Jc value at the self-magnetic field increased from 1.32 to 2.38 MA/cm2 upon La addition.

Properties of Na&lt;sub&gt;0.5&lt;/sub&gt;Bi&lt;sub&gt;0.5&lt;/sub&gt;TiO&lt;sub&gt;3&lt;/sub&gt;-SrTiO&lt;sub&gt;3 &lt;/sub&gt;Ferroelectric Thin Films Prepared by a Modified MOSD Process

0.82NBT-0.18ST and 0.85NBT-0.15ST thin films have been prepared on Si substrates by a modified metalorganic solution deposition process. To achieve films with better ferroelectric properties, three main items have been changed in the process. Then the crystal structures, surface microstructures, hysteresis loops, fatigue curves and capacitance-voltage curves of the films were measured. It can be found that NBT-ST films can crystallize well after annealing at 650 °C for 5 minutes and have smooth surface microstructures. The 0.85NBT-0.15ST thin films exhibit better well-defined hysteresis loops than 0.82NBT-0.18ST, with a remnant polarization of 1.1 mC/cm2 and a coercive fields of 44.2 kV/cm. The clockwise C-V curves show that they have a desired polarization-type switching mode. The memory window of 0.82NBT-0.18ST thin film is about 1.8V, and that of 0.85NBT-0.15ST thin film is about 2.5V.

Enhanced low-temperature critical current by reduction of stacking faults in REBCO coated conductors

The effect of stacking faults (SF) on flux pinning and critical current (Ic) in rare earth based coated conductors was investigated. The SF density in YBa2Cu3O7−δ (YBCO) films with and without Dy addition, produced by metal organic deposition, was modified by altering the oxygenation temperature. A detailed microstructural analysis of the coated conductors was performed by x-ray diffraction, scanning and transmission electron microscopy and energy dispersive spectroscopy, and the observed defect population was correlated with both the self-field and in-field Ic. We report that the best self-field Ic was obtained for samples having a low SF density, in spite of the SF being effective flux pinning defects at 77 K for magnetic fields applied within the ab plane. We also show that the SF have no observable flux pinning effect at low temperatures. This study demonstrates that for devices operated at low temperatures, the elimination of SF in the conductor wires is essential to attain higher Ic.

Artificial Multilayers of REBaCuO/YBaCuO and YBCO/CuO(Y2O3) for High-Performance Long Tapes of Coated Conductors Derived by Metal Organic Solution Deposition

Thick superconducting multilayer architectures consisting of alternate superconducting and superconducting layers DyBCO/YBCO and alternate superconducting and insulating layers YBCO/CuO(Y 2 O 3 ) are developed on buffered Hastelloy tapes using a metal organic deposition process. It is revealed that multilayer intermittent structures for superconducting layers are effective to avoid the presence of microcracks that occur with increasing thickness, which are frequently observed in monolayer films of REBa 2 Cu 3 O 7-δ (REBCO, RE = Y, Gd, or other rare earth elements). This paper shows that the thicknesses of DyBCO/YBCO multilayer films can reach 2.8 μm with a homogeneous, dense and crack-free surface morphology. The out-plane FWHM of the YBCO (005) peaks and the critical current value reach 1.1° and 365 A/cm (77 K, self-field) for such a thick film consisting of DyBCO/YBCO multilayers. It is also revealed that the introduction of a CuO layer is effective to avoid the presence of a-axis grains and voids of the YBCO films, which are frequently observed in multilayer films. Note that the introduction of CuO shows little negative influence on the texture of multilayer structure, especially for the bottom CuO. However, the in-plane texture deteriorates seriously for Y 2 O 3 /YBCO/YBCO multilayer film. The critical current density of YBCO films with middle CuO and bottom CuO are 2.0 MA/cm 2 and 2.5 MA/cm 2 (77 K, self-field), respectively. The microstructure optimization including the texture, uniformity, crack, defect, and porosity in the multilayered films is probably the reason for the enhancement of superconducting performance.

Fabrication and Excitation of a Model Magnet Using Coated Conductors for Spiral Sector FFAG Accelerators

With the aim of verifying the winding technology required for fixed-field alternating gradient (FFAG) accelerator magnets, and also to verify the magnetic field accuracy as compared with the designed field distribution, a reduced-size test magnet, referred to as a “model magnet,” has been developed. The model magnet was designed to incorporate multiple REBa2Cu3Ox (REBCO) coils with characteristic shapes, namely negative bend parts and 3-D winding shapes, derived from a full-scale spiral-sector FFAG accelerator design which was accomplished in this study. The coils were wound using 4-mm-wide REBCO-coated conductors made by the ion-beam-assisted deposition metal organic chemical-vapor deposition method, and the total length of the conductor for the model magnet was about 2000 m. After winding, the coils were installed in heat-conduction shells made of an aluminum alloy and were directly cooled by a Gifford-McMahon (GM) cryocooler in a vacuum vessel. It was essential to wind the REBCO coils without degradations, such as local delamination of the superconducting layer, because the voltage generated at such degraded parts frequently increases during operation, and eventually results in thermal runaway. The superconducting properties of all coils were verified before and after installing them in the aluminum shells. This paper describes the assembly of the REBCO coils, as well as testing procedures and test results of the model magnet.

Improvement of Flux Pinning Properties for Hafnium-Doped Gd123 Thin Films Fabricated by a Fluorine-Free MOD Method

In order to enhance the flux pinning force density $F_{p}$ in magnetic fields for REBa2Cu3O $_{y}$ (RE123)-coated conductors, Hf and Zr doping are the very valid way of introducing the perovskite nanostructure as artificial pinning centers (APCs) into the superconductors. However, the studies for Hf-doped RE123 thin films fabricated by the fluorine-free metal organic deposition (FF-MOD), which is the easiest production process, have not been done yet. In this study, we have successfully fabricated Hf-doped FF-MOD GdBa2 Cu3O $_{y}$ (Gd123) thin films on LaAlO3 substrates and investigated their flux pinning properties. Critical temperature for Gd123 thin films indicated around 92 K, and $T_{c}$ varied little by Hf doping. Hf 10 mol% doped film achieved high critical current densities of 2.72 mA⋅cm −2 at 77.3 K under 0 T, and 0.27 mA⋅cm−2 at 77.3 K under 1 T. With increasing Hf-doping amount, $F_{p}$ gradually increased, and the peak of $F_{p}$ shifted to the high magnetic field side. The elementary pinning force and the effective pinning center density also increased. We believe that effective APCs, probably BaHfO3 are introduced into FF-MOD Gd123 thin films by Hf doping.

Influence of Substrate-Film Reactions on YBCO Grown by Fluorine-Free MOD Route

Recently, fluorine-free metal organic deposition routes (FF-MOD) for growth of YBCO superconducting films have attracted increased attentions. In this paper, a comparison study was performed on the YBCO-Ag superconducting thin films deposited on two types substrates, LaAlO 3 and CSD-Ce 0.9 La 0.1 O 2 -y (CLO)/YSZ, respectively. Although conventional TFA-MOD derived YBCO films exhibit high performance on both substrates, the results vary when using the FF-MOD precursor. SEM and XRD results reveal that c-axis and a/b-axis orientations coexist in the YBCO-Ag films grown on the CSD-CLO/YSZ substrate deposited by the FF-MOD route, while the BaCeO 3 by-product is a dominating phase in the fully reacted film. Based on the structural analysis of the partially converted films, we found that interfacial reactions between the film and the CLO cap layer play an essential role on the epitaxial growth of YBCO-Ag films from the FF-MOD solution. Because of the different chemical reaction path compared to conventional TFA-MOD routes, it seems that the polycrystalline BaCeO 3 formation takes place prior to the YBCO-Ag epitaxial growth associated with the melting process, which results in structural deterioration at high growth temperatures and, therefore, no superconductivity. This study indicates the necessity of further reducing the nucleation temperature of YBCO films processed by FF-MOD routes.

A review of thickness-induced evolutions of microstructure and superconducting performance of REBa2Cu3O7−δ coated conductor

The research and development of high temperature superconducting (HTS) films, especially ReBa2Cu3O7− δ (REBCO or RE123; RE=Y, Gd, or other rare earths) yttrium-based coated conductors, has generated widespread interest for the potential applications of the second generation superconducting films. In view of commercialization, however, the maximum superconducting currents for coated conductors should be increased further. Unfortunately, it has been frequently observed that the average critical current density J c decreases with an increase in film thickness. The thickness effect is still a hurdle for large-scale production, especially in pulsed laser deposition and metal organic deposition processes. An engineering current of more than 1 000 A/cm is desired owing to the high cost of 2G superconducting materials. The present work attempts to review the evolution of various issues subject to the thickness effect, including the microstructure, epitaxial texture, surface roughness, pinning force, oxygen deficiency, residual stress, copper-rich layers, and segregation of elements. Furthermore, recent progress in enhancing the performance of superconductors especially in terms of critical current density is illustrated, such as the use of heavy doping. Further understanding of the thickness effect is extremely important for large-scale commercial development of the second generation high temperature superconductors.