Biaxially aligned YBCO film tapes fabricated by all pulsed laser deposition

Biaxially aligned yttria-stabilized zirconia (YSZ) films on Ni-based alloy substrates were realized with high deposition rate of 0.5 μm min⁻¹ by the inclined substrate deposition (ISD) technique without ion beam assistance. The microstructure of YSZ was examined to study the growth mechanism of biaxial alignment by ISD. Columnar structures toward the plasma plume suggested a self-shadowing effect in the ISD process. To raise I_c values, YBCO thickness was increased up to 5 μm. Thick YBCO films with high J_c values were realized on the ISD-grown YSZ. Long YBCO tapes with biaxial alignment were successfully fabricated using continuous pulsed laser deposition and a high I_c value of 37.0 A (77.3 K, 0 T) at a 75 cm voltage tap spacing was achieved.     

Preparation of in-plane textured buffer layers for YBa2Cu3Oy film growth by modified bias sputtering

A modified bias sputtering technique has been proposed to grow in-plane textured yttria-stabilized zirconia buffer layers on polycrystalline metallic substrates for deposition of YBa₂Cu₃O_y films. The principle of developing an in-plane texturing by this technique is basically the same as that of ion beam assisted deposition; an in-plane texturing occurs by off-normal ion beam bombardment because of the higher sputtering yields of all orientations other than the channelling direction. In our process, however, a flux of energetic particles impinging on the growing film is generated using specially devised negatively biased electrodes installed in a magnetron sputtering system instead of a separate ion-source in IBAD. So far an X-ray phi-scan width of 18° was attained for YSZ films on Hastelloy tapes. Epitaxial YBCO films grown on these buffer layers using pulsed laser deposition showed the J_c’s exceeding 10⁵ A cm⁻² (77 K, 0 T). In this paper, we present variation of the bias sputtering technique also used to obtain the textured films on large area substrates. Although the proposed process offers a very convenient method to grow textured films, more efforts must be made to increase growth rates of the films (currently ∽0.1 nm s⁻¹) for large-scale applications of YBCO films.     

Growth of high quality YbBa2Cu3O7−δ thin films by pulsed laser deposition

YbBa₂Cu₃O_7−δ (Yb-123) films are deposited for the first time using Pulsed Laser Deposition (PLD) method at three different substrate temperatures, viz. 675°C, 700°C and 725°C. Films are characterized using XRD, dc electrical resistivity, critical current density (J_c) and microstructural study by Atomic Force Microscopy (AFM) techniques. It is found that 700°C is the optimum growth temperature for growing high quality Yb-123 films. The best T_c and J_c values obtained at optimum growth conditions are 88 K and 2.6×10⁶ A cm⁻² at 77 K, respectively. AFM photographs provide evidence in confirming the relation between growth temperature and superconducting properties.     

Melt texturing of YBCO for high current applications

In recent years significant progress has been made in J_c enhancement in high T_c superconductors using melt texturing techniques. Among the many melt texturing methods and modifications, seeding and directional solidification techniques offer extensive control over the location of the growth front and the growth direction over long distances during melt texturing which makes these techniques most attractive from the standpoint of long conductor fabrication. These processes have the capability of producing J_cs of about 45 000 A cm⁻² across single domains of YBCO. A novel variant of the conventional melt texturing process called the liquid phase removal method provides a means to improve the grain boundary coupling in melt textured bulk polycrystalline HTS. Grain boundaries in samples processed by this technique with misorientation angles as high as 54° have demonstrated J_cs as high as 18 000 A cm⁻². Recent developments in texturing of RE-123 compounds (Nd and Yb) at high growth rates give promise for considerable reduction in processing times in directional solidification. Texturing has been observed even in samples processed at rates as high as 100 mm h⁻¹. With these advances in melt texturing methods, utilization of bulk HTS in practical applications such as high capacity current leads etc., appears to be a distinct possibility of the near future.     

Metal–organic chemical vapor deposition as an emerging technique for the fabrication of YBa2Cu3Ox tapes

The deposition of YBa₂Cu₃O_x thin films on metal substrates is currently being investigated as the basis for future tape applications. The YBa₂Cu₃O_x coatings, discussed in this report, are deposited by metal–organic chemical vapor deposition (MOCVD). MOCVD is a versatile technique with many opportunities in terms of reaction chamber design and shapes of substrates to be coated. MOCVD equipment for fabrication of YBa₂Cu₃O_x coatings on long length, flexible metal tapes is currently being developed. Tapes of various materials, such as polycrystalline stainless steel, have been investigated. The growth of biaxially aligned buffer layers between the stainless-steel tapes and YBa₂Cu₃O_x coatings is necessary, to prevent interdiffusion, to reduce interfacial reactions and to allow the succeeding growth of aligned YBa₂Cu₃O_x thin films. The superconducting properties of the resulting YBa₂Cu₃O_x/buffer/substrate thin film systems, T_c≈90 K and J_c>10⁵ A cm⁻², are very promising. the major drawbacks in MOCVD of YBa₂Cu₃O_x thin films are the thermo-chemical properties of MOCVD precursors. For example, the traditional solid precursors have limited volatility. The development of new precursor delivery systems using precursor solutions and new solid and liquid precursors have resulted in improved process reliability. Finally, in the standard MOCVD process, which is thermally activated, the reported growth rates are approximately 0.5 μm h⁻¹. An increase by one order of magnitude can be achieved in a photo-assisted MOCVD process.     

Long lengths of silver-clad Bi2223 superconducting tapes with high current-carrying capacity

Long lengths of silver-clad (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) high-T_c multifilamentary tapes were produced using the powder-in-tube (PIT) technique followed by a thermomechanical process. The relationships between microstructure and electrical, magnetic and mechanical properties of the heat treated tape were evaluated from the critical current density measurements, irreversibility magnetic field determination and mechanical bending tests. Emphasis was stressed on the J_c behavior in magnetic fields at different temperatures. A J_c of 10,000 A/cm² at 77 K in a zero field for a 10 m tape and 75,000 A/cm² at 23 K in a field of 3 T for a short tape was achieved. The results obtained showed that Bi2223/Ag high-T_c composite tapes are a potential alternative to conventional low-T_c superconductors in magnetic levitation (MAGLEV) applications.     

YBCO/YSZ coated conductors on flexible Ni alloy substrates

A coating system for the deposition of in-plane oriented yttria-stabilized zirconia (YSZ) template films on 1 cm wide flexible metal substrates is presented. In static mode, the system is capable of producing high quality template films on 20 cm substrate lengths. In a continuous coating mode, the system is capable of producing good quality template films on 1.1 m substrate lengths. Superconducting YBa₂Cu₃O_7−δ (YBCO) films subsequently deposited onto these template films have demonstrated critical currents (I_c) of 200 A (1.5 cm length), 70 A (12 cm length) and 4 A (1 m length).     

Study of the correlation between structural and photoluminescence properties of CdSe thin films deposited by close-spaced vacuum sublimation

CdSe polycrystalline films were deposited by a close-spaced vacuum sublimation method at different substrate temperatures (T_s) using glass slides as substrates. At T_s≤673 K the films have a structure with strong dispersion of grain size (d) (from 0.1 to 0.3 μm). In this case the layer-by-layer mechanism determines the growth process of the layers. For T_s=873 K they have a columnar-like structure with a clear growth texture and the average grain size d=3–4 μm. The films obtained at T_s>473 K are n-type and only correspond to a single wurtzite phase. The crystallites are preferentially oriented with the (102) planes parallel to the substrate. At lower temperatures the films are bi-phase. The microstress level in CdSe films obtained at Т_s=873 K (0.5×10⁻³) is considerably smaller than for the films deposited at Т_s=773 K (4.0×10⁻³). Increase of the value of T_s improves the stoichiometry of CdSe films. Analysis of the low-temperature photoluminescence (PL) spectra let us determine the nature and energy of point and extended defects in the investigated films. It was shown that the films contain Na(Li) and P residual impurities. The results of the structural and PL measurements showed that the CdSe polycrystalline films are of fairly good crystal and optical quality for T_s=873 K and can be suitable for various applications.     

Large area Y1Ba2Cu3O7-δ thin film on LaAlO3 substrate deposited by pulsed laser deposition

Double-sided deposition of large-area Y₁Ba₂Cu₃O_7-δ thin films was investigated. The films were deposited on LaAlO₃ substrates, with a diameter of 3 in, using pulsed laser deposition-equipment with an off-axis geometry. For homogeneity in film thickness and superconducting properties, the substrates were set on a heater that was arranged parallel to the plasma plume axis; the substrates were rotated during film deposition. Uniformity of thickness was better than ±3% across the 3 in diameter film. High c-axis orientation was confirmed by X-ray diffraction (XRD) measurement, and the J_c distribution measurements showed more than 10⁶A/cm² (77.3 K) in the whole area of the double-sided 3 in film.     

Effect of Gd addition on the superconducting properties of (Nd-Sm-Eu)123 system

Bulk (Nd, Sm, Eu)_1-xGd_xBa₂Cu₃ O_y superconductors in which the Gd content x ranged from 0 to 0.25, were grown by the oxygen-controlled-melt-growth (OCMG) method. X-ray diffraction results show that the main phase is RE123 with a small amount of the second phase. SQUID measurements on the oxygenated samples showed that a small amount of Gd123 addition to (Nd, Sm, Eu)123 increased T_c and made the superconducting transition sharper. All the samples, except the Gd-free one, exhibited the secondary peak effect in the magnetization curves, which shows that the field-induced pinning is active, like the other RE123 fabricated by the OCMG process. The critical current density also increased with Gd addition, however, no systematic change was observed with the amount of Gd addition. A peak J_c value of 60,000 A/cm² was recorded with an applied field of 1.2 T (H//c) at 77 K. The results suggest that, in the (Nd, Sm, Eu)123 system the addition of Gd is effective in achieving high T_c and a large J_c.     

Pulse electrodeposited copper indium sulfide films

Copper indium sulfide (CISu) films were deposited by the pulse galvanostatic deposition technique at different duty cycles. The films are polycrystalline with peaks corresponding to the chalcopyrite phase of CISu. The grain size and surface roughness increased from 10 to 25 nm and 0.85 to 2.50 nm respectively with increase of duty cycle. Optical band gap in the range of 1.30–1.51 eV was observed for the films deposited at different duty cycles. Room temperature resistivity of the films is in the range of 0.1–3.67 Ω cm. Photoconductivity measurements were made at room temperature. Photocurrent spectra exhibited maximum corresponding to the band gap of copper indium sulphide. CdS/CuInS₂ fabricated with CISu films deposited at 50% duty cycle have exhibited a V_oc of 0.62 V, J_sc of 16.30 mA cm^?2, FF of 0.71 and efficiency of 7.16%.     

Structural and magnetic properties of silver doped melt-textured YBCO prepared in a solar furnace

Magnetic measurements and structural investigation have been performed on melt-textured YBCO and AgYBCO HTS. The “sun” technique produces very dense YBCO (ρ=5.86 g cm⁻³) and AgYBCO [ρ=6.36 g cm⁻³; 10% b/w (by weight) silver. This technique renders samples with a large volume fraction of the Y₂BaCuO₅ (211) phase. The material is characterized by very high J_c values, as compared with bulk polycrystalline YBCO prepared by other methods. This feature is attributed to the enhanced amount of 211 particles which serve as pinning centers. Additional significant densification of the structure due to silver incorporation is obtained, and a reduction of the size of 211 inclusions is also observed. Silver doped samples show “butterfly”-like hysteresis loops at relatively high temperatures (T≥60K). This feature is probably associated with oxygen deficiency which arises from the slower oxygen diffusion into silver doped samples. J_c values enhancement was obtained in silver doped “sun” samples at high temperatures (T≥60K) and fields of 20–30 kOe. The temperature dependence of effective activation energy of pinning, U_eff, was measured for YBCO and AgYBCO materials. U_eff is higher in silver doped samples in the high temperature region T≥60K.     

Growth of highly (1 1 1) oriented CuIn0.75Al0.25Se2 thin films

CuIn_0.75Al_0.25Se₂ thin films prepared onto glass substrates at T_S=573 K were single phase, nearly stoichiometric and polycrystalline with a strong (1 1 1) preferred orientation showing sphalerite structure. The results of X-ray diffraction and electron diffraction studies are compared, interpreted and correlated with micro-Raman spectra. The optical absorption studies indicated a direct band gap of 1.16 eV with high absorption coefficient (>10⁴ cm^?1) near the fundamental absorption edge.     

Development of highly conducting n-type micro-crystalline silicon oxide thin film and its application in high efficiency amorphous silicon solar cell

Wide band gap and highly conducting n-type nano-crystalline silicon film can have multiple roles in thin film solar cell. We prepared phosphorus doped micro-crystalline silicon oxide films (n-μc-SiO:H) of varying crystalline volume fraction (X_c) and applied some of the selected films in device fabrication, so that it plays the roles of n-layer and back reflector in p-i-n type solar cells. It is generally understood that a higher hydrogen dilution is needed to prepare micro-crystalline silicon, but in case of the n-μc-SiO:H an optimized hydrogen dilution was found suitable for higher X_c. Observed X_c of these films mostly decreased with increased plasma power (for pressure<2.0 Torr), increased gas pressure, flow rate of oxygen source gas and flow rates of PH₃>0.08 sccm. In order to determine deposition conditions for optimized opto-electronic and structural characteristics of the n-μc-SiO:H film, the gas flow rates, plasma power, deposition pressure and substrate temperature were varied. In these films, the X_c, dark conductivity (σ_d) and activation energy (E_a) remained within the range of 0–50%, 3.5×10⁻¹⁰ S/cm to 9.1 S/cm and 0.71 eV to 0.02 eV, respectively. Low power (30 W) and optimized flow rates of H₂ (500 sccm), CO₂ (5 sccm), PH₃ (0.08 sccm) showed the best properties of the n-μc-SiO:H layers and an improved performance of a solar cell. The photovoltaic parameters of one of the cells were as follows, open circuit voltage (V_oc), short circuit current density (J_sc), fill-factor (FF), and photovoltaic conversion efficiency (η) were 950 mV, 15 mA/cm², 64.5% and 9.2% respectively.     

Temperature effect on the physical properties of CuIn11S17 thin films for photovoltaic applications

CuIn₁₁S₁₇ compound was synthesized by horizontal Bridgman method using high-purity copper, indium and sulfur elements. CuIn₁₁S₁₇ thin films were prepared by high vacuum evaporation on glass substrates. The glass substrates were heated at 30, 100 and 200 °C. The structural properties of the powder and the films were investigated using X-ray diffraction (XRD). XRD analysis of thin films revealed that the sample deposited at a room temperature was amorphous in nature while those deposited on heated substrates were polycrystalline with a preferred orientation along the (311) plane of the spinel phase. Ultraviolet–visible (UV–vis) spectroscopy was used to study the optical properties of thin films. The results showed that CuIn₁₁S₁₇ thin films have high absorption coefficient α in the visible range (10⁵–10⁶ cm⁻¹). The band gap E_g of the films decrease from 2.30 to 1.98 eV with increasing the substrate temperature (T_s) from 30 to 200 °C. We exploited the models of Swanepoel, Wemple–DiDomenico and Spitzer–Fan for the analysis of the dispersion of the refractive index n and the determination of the optical constants of the films. Hot probe method showed that CuIn₁₁S₁₇ films deposited at T_s=30 °C and T_s=100 °C are p-type conductivity whereas the sample deposited at T_s=200 °C is highly compensated.     

Mixing Y(RE)BCO (RE=Nd,Sm) superconductors by crystal pulling method

Y_1−xRE_xBa₂Cu₃O_7−δ [Y(RE)BCO or REsBCO] superconductors were prepared by the crystal pulling method. The RE mixing content x in the crystal can be effectively controlled by the processing temperature. With an increase in processing temperature, the RE concentration in the liquid increased, which resulted in the higher RE substitution content in single crystals. The higher critical temperatures T_c above 92 K could be obtained by an optimized oxygenation treatment. The T_c value of Y(RE)BCO tends to be insensitive to the growth atmosphere of the oxygen partial pressure under a certain limit of the RE content. The Y_.727Nd_.273Ba₂Cu₃O_7−δ and the Y_.941Sm_.059Ba₂Cu₃O_7−δ samples show values of the critical current density J_c are about 2×10⁴ Acm⁻² at 1.2 T and 2.8×10⁴ Acm⁻² at 1.1 T for H//c, respectively, indicating that mixing REs 123 have obvious effects on J_c–H curves since RE ions are likely to substitute at both Y²⁺ and Ba²⁺ sites.     

Growth features and intergranular connectivity of melt processed YBCO

Large grain melt processed Y-Ba-Cu-O (YBCO) samples have been prepared by seeded and unseeded growth techniques. The current carrying ability within individual grains and across grain boundaries has been investigated and correlated with features in the microstructure of samples fabricated by both techniques. The development of an inhomogeneous, cell-like growth microstructure in seeded samples at distances ≈4 mm from the seed is related to a saturation in inclusion density and volume proportion of Y₂BaCuO₅ (Y211) particles. A significant decrease in critical current density is associated with this change over a wide temperature range. The resistance of high angle c-axis grain boundaries is observed to depend critically on the magnitude of the injection current whereas the intra-granular resistance is not influenced significantly by this variable. J_c of a grain boundary fabricated by unseeded melt growth is estimated to be less than 100 A cm⁻² at 77 K in zero applied field, which is more than two orders of magnitude lower than the intragranular J_c. Field screening measurements suggest that low angle grain boundaries do not form weak links between grains in modest magnetic fields and hence do not present a significant barrier to current flow.     

Properties of fluorine-doped tin oxide films prepared by an improved sol-gel process

Fluorine-doped tin oxide (FTO) films were prepared by an improved sol-gel process, in which FTO films were deposited on glass substrates using evaporation method, with the precursors prepared by the conventional sol-gel method. The coating and sintering processes were combined in the evaporation method, with the advantage of reduced probability of films cracking and simplified preparation process. The effects of F-doping contents and structure of films on properties of films were analyzed. The results showed the performance index (Φ_TC=3.535×10⁻³ Ω⁻¹ cm) of the film was maximum with surface resistance (R_sh) of 14.7 Ω cm⁻¹, average transmittance (T) of 74.4% when F/Sn=14 mol%, the reaction temperature of the sol was 50 °C, and the evaporation temperature was 600 °C in muffle furnace, and the film has densification pyramid morphology and SnO_2−xF_x polycrystalline structure with tetragonal rutile phase. Compared with the commercial FTO films (Φ_TC=3.9×10⁻³ Ω⁻¹ cm, R_sh=27.4 Ω cm⁻¹, T=80%) produced by chemical vapor deposition (CVD) method, the Φ_TC value of FTO films prepared by an improved sol-gel process is close to them, the electrical properties are higher, and the optical properties are lower.     

Epitaxial growth of nonpolar GaN films on r-plane sapphire substrates by pulsed laser deposition

Single-crystalline nonpolar GaN epitaxial films have been successfully grown on r-plane sapphire (Al₂O₃) substrates by pulsed laser deposition (PLD) with an in-plane epitaxial relationship of GaN[1-100]//Al₂O₃[11-20]. The properties of the ~500 nm-thick nonpolar GaN epitaxial films grown at temperatures ranging from 450 to 880 °C are studied in detail. It is revealed that the surface morphology, the crystalline quality, and the interfacial property of as-grown ~500 nm-thick nonpolar GaN epitaxial films are firstly improved and then decreased with the growth temperature changing from 450 to 880 °C. It shows an optimized result at the growth temperature of 850 °C, and the ~500 nm-thick nonpolar GaN epitaxial films grown at 850 °C show very smooth surface with a root-mean-square surface roughness of 5.5 nm and the best crystalline quality with the full-width at half-maximum values of X-ray rocking curves for GaN(11-20) and GaN(10-11) of 0.8° and 0.9°, respectively. Additionally, there is a 1.7 nm-thick interfacial layer existing between GaN epitaxial films and r-plane sapphire substrates. This work offers an effective approach for achieving single-crystalline nonpolar GaN epitaxial films for the fabrication of nonpolar GaN-based devices.     

Effects of Ga concentration on structural and electrical properties of screen printed-CIGS absorber layers on polyethylene terephthalate

Copper indium gallium diselenide (CIGS) films were deposited as an absorber layer on polyethylene terephthalate (PET) substrates by a screen printing technique using CIGS ink with a Ga content ranging from 0.3 to 0.6. The melting point of PET substrate is 254.9 °C; the average transmission in the visible (400 nm–800 nm) for PET substrates is greater than 85%. Effects of Ga content of the CIGS absorber layer on structural and electrical properties of the CIGS films were studied. The lattice parameters, a and c for all CIGS films were decreased with increasing Ga content. At room temperature, Hall mobility and charge-carrier concentration of the CIGS films varies from 97.2 to 2.69 cm² V⁻¹ s⁻¹ and 9.98×10¹⁶ to 3.23×10¹⁸ cm⁻³, respectively.