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Pure GdBa 2 Cu 3 O 7− δ (GdBCO) films were deposited on (1 0 0)-oriented LaAlO 3 (LAO) substrates by photo-assisted metal organic chemical vapor deposition (PhA-MOCVD) technique. The effects of substrate temperature ( T s ) and oxygen partial pressure (Po 2 ) on microstructure, growth rate and superconducting critical current density ( J c ) were investigated. A dense and no grain boundary visible, single-crystal-like cross-sectional morphology was observed. For the GdBCO film sample obtained at T s of 810 °C and Po 2 of 4 Torr, the full width at half-maximum were 0.08° and 0.41° for out-of-plane and in-plane orientations, respectively. Such low values were similar to that of single crystal GdBCO. Optimally processed GdBCO samples exhibited J c of 2.5 MA/cm 2 at 77 K in self-field. A relatively high growth rate of 0.104 μm/min for the GdBCO film is realized by the PhA-MOCVD technique.

Normally, surface of c -axis oriented YBCO films grown by MOCVD are found to be populated by micron-sized outgrowths with elemental compositions such as Ba–Cu–O and Cu–O. For both low and high current applications with the YBCO films, these surface outgrowths should be eliminated. In this study, it is found that by using a simple processing technique of decreasing appropriately the sublimation temperature of the Cu-based precursor, the surface outgrowths can be essentially subdued. At the same time it is qualitatively predictable that J c of these smoother YBCO films will be lowered. However, experimentally, it is found that at appropriate YBCO film growth conditions, the lowered J c value is acceptable for most applications. Actually, a surface outgrowth covered YBCO film made by photo-assisted MOCVD, with film composition Y:Ba:Cu as 1:1.2:1.5, has its J c being 3.4 MA/cm 2 (0 T , 77 K). As composition of YBCO film is changed to 1:0.9:1.2 by lowering the sublimation rate of the Cu-based precursor, then there are no more outgrowths on surface, with its J c being 2.6 MA/cm 2 .

It is well-known that there is a relatively small temperature window for the growth of epitaxial and c -axis oriented YBCO thin films by various methods. In this study, this window was more precisely examined for YBCO film growths by photo-assisted MOCVD. At specified total reactor pressure of 4.6 torr, and specified respective partial pressures for the oxidizing agents, O 2 and N 2 O at 1.7 torr and 1.1 torr, the film growths were tested at several substrate temperature ( T s ) points from 780 °C to 864 °C. The film growth time for each test was 3 min. As to compositional purity, purity in c -axis orientation, and crystal structural quality of these tested films by using photo-assisted MOCVD technique, it was found that for the growth of purely c -axis oriented YBCO thin films with high crystalline quality, the appropriate growth temperature range (or “ T s window”) was from about 800 °C to 830 °C, at relevant pressures as specified above. T c and J c of a typical YBCO film sample grown within this “ T s window”, i.e., 810 °C, were found as 90 K and 1.32 MA/cm 2 (77 K, 0 Oe) respectively by magnetization measurements. It is noticeable that this film, and other purely c -axis oriented YBCO films are all single-crystal like and dense, with no SEM visible grain boundary or void. The causes of precipitates found on top surface of these c -axis oriented YBCO films are also discussed.

High-quality superconducting YBa2Cu3O7−δ (YBCO) thick films with thickness over 2 μm were grown on LaAlO3 (LAO) substrates by photo-assisted metalorganic chemical vapor deposition (PhA-MOCVD) with a single liquid precursor delivery system. The YBCO thick films fabricated are single-crystal like with no visible grain boundaries or voids. Surface and interface microstructures of YBCO thick films have been systematically investigated by scanning electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction. Large precipitates were observed on the surface of these films. These precipitates are mostly composed of crystalline BaCu3O4, along with some polycrystalline and amorphous YBCO phases, and were found to exist only on top of the YBCO films, with no penetration into the films. The high quality of the prepared YBCO thick films was confirmed further by the results of X-ray diffraction and Tc/Jc measurements. Thus, these thick YBCO films prepared by PhA-MOCVD have promising application potential in fields such as coated conductors.

High quality superconducting YBa 2 Cu 3 O 7− x (YBCO) films were deposited on LaAlO 3 ( 1 0 0 ) substrates at a high growth rates of 0.2–1 μm /min with high reproducibility by a novel photo-assisted metalorganic chemical vapor deposition process with single liquid precursor delivery technique. The photo irradiation technique can dramatically increase the growth rate and the single liquid precursor delivery technique can significantly improve the process reproducibility. X-ray diffraction analysis indicated that single phase, c -axis oriented and well ab-aligned YBCO films were high reproducibly prepared. The as-grown YBCO films showed critical temperatures ( T c ) of 91 K with the transition width ( ΔT c ) less than 1 K, and self-field critical current densities ( J c ) of 4.6×10 6 A/cm 2 at 77 K.

YBa 2 Cu 3 O 7-x films have been deposited by photo-assisted MOCVD at commercially viable rates on both single crystal oxide substrates and atomically textured metallic substrates. CeO 2 and YSZ buffer layers have also been developed by photo-assisted MOCVD for deposition on metallic substrates at rates approaching 1 μm/min. The resulting buffer layers show good duplicate of the moderate atomic order of the underlying metallic substrates, and the subsequent YBCO layers show similar moderate atomic order with resulting J c ∼ 5 x 10 5 A/cm 2 .

Intrinsically, the thin-film preparation technique of metalorganic chemical vapor deposition involves a large number of control variables which are especially critical in work with quaternary or higher high- T c materials. Thus, effective methods are needed to optimize variable settingsin the preparation of high- T c films. A matrix experimental plan and robust design methodology has been employed for this purpose. The first design phase was based on existing knowledge regarding growth temperature, pressure and annealing temperature for photo-assisted preparation of YBCO thin films on LaAlO 3 (100) substrates. A battery of only nine experiments plus three confirmation depositions were then used to optimize the process control parameters of precursor oven temperature, carrier gas (Ar) flow rate, and the flow rates of two oxidizing gases, O 2 and N 2 O, with respect to critical current density ( J c ) of the films. J c (77 K, zero field) values for deposited YBCO films (with thickness in the 0.4–0.6 μm range) were found to be improved from about 1 to 3 × 10 6 A/cm 2 . J c for a thinner sample (0.15 μm) reached 5.0 × 10 6 A/cm 2 . It was found that under these processing conditions the most significant process control parameter affecting J c is the flow rate of O 2 , or more precisely, the oxidizing agent flow-rate ratio of O 2 and N 2 O. Analysis of the variance of the J c data indicated that there likely exist interactions among some of the control variables, most obviously between the flow rates of O 2 and N 2 O.

The growth of purely a -axis oriented YBCO thin films on LaAlO 3 (100) substrates with no template layer addition has been accomplished by photo-assisted MOCVD. In addition to the requirement of lattice-matched substrates, the difficulty of growth of purely a -axis oriented YBCO films with this processing technique has been overcome by utilizing three appropriate processing/growth factors: substrate temperature ( T s ), total partial pressure of oxidizing gases O 2 and N 2 O ( P (O 2 + N 2 O)), as well as high film growth rate. A high growth rate (up to 7000 a/min) has been accomplished through MOCVD growth assisted by photo-irradiation with tungsten-halogen lamps. Variance of the three factors leads to tailored growth of either a -axis, c -axis or [110] oriented growth for YBCO. The growth of a -axis YBCO thin films under different precursor flow rates also affects the film morphology.