Features Of Obtaining Superconducting Layer In Second-Generation High-Temperature Superconducting Tapes Pulsed Laser Deposition Method
Attempts to create industrial devices using high-temperature superconductors (HTSC) have begun soon after discovery of a compound from the class of high-temperature superconductors by K. Muller and H. Bednorz in 1986 . Currently, in terms of technology, due to its performance characteristics (critical current value, strength, behavior in magnetic fields), the second-generation superconducting tapes, intended for manufacturing power cables, current limiters, generators, transformers and superconducting magnets [2–4], are of the most interest. They represent multilayered tapes, including metal substrate, buffer, superconducting and protective layers (Fig. 1).
The superconducting layer is a polycrystal with high mosaic structure degree; critical current value is characterized by both intragranular and intergranular critical currents. Intragranular current is equal to the critical current in single crystal, while intergranular current strongly depends on the angle of misorientation of the neighboring grains, and this is what determines the critical current of the second-generation high-temperature superconductor tape, so it is necessary to create a tape with high texture sharpness (<00l>) to create tapes with high electrical characteristics. Currently, there are two basic approaches to creating textures in HTSC tapes. The first one is to use a biaxially textured metal backing strip obtained by cold deformation (rolling) followed by texturing annealing [5, 6]. The second one is the formation of a textured buffer layer with an assist ion beam on a polycrystalline metal tape [7–9].
The complex four-component oxides YBa2Cu3O7-x and GdBa2Cu3O7-x are most common materials for superconducting layer in second-generation HTSC tapes. To obtain high-quality second-generation tapes, it is necessary to form a highly textured superconducting layer with strictly controlled stoichiometry. Therefore, the main methods of forming a high-temperature superconducting layer in second-generation tapes are as follows:
• metal-organic chemical vapor deposition (MOCVD);
• reactive co-evaporation (RCE);
• pulsed laser deposition (PLD).
MOCVD is a method of metal-organic chemical vapor deposition. The first results on the deposition of HTSC coatings by this method were obtained in 1988 . The films obtaining occurs during substance chemical reaction on the hot substrate surface. The precipitated material is in the gaseous state and is mixed with an inert carrier gas.
Chemical deposition methods have a high rate of synthesis of films, which is their main advantage in comparison with physical deposition methods. The main drawback of this method is complexity of obtaining a film with the required stoichiometry, as well as the need for the use of expensive high-purity raw materials.
Currently, this method is used for production of tapes by American companies SuperPower (up to 350 A/cm, IBAD) and Americansuperconductor (up to 300 A/cm, IBAD).
Today, the most productive (up to 6 nm/s)  is the method of reactive co-evaporation followed by deposition and chemical reaction (RCE-DR) (Fig. 2). It involves rapid target evaporating by an electron beam at low temperature and low oxygen partial pressure, followed by annealing in an oxygen medium.
The main disadvantage of this method is complex equipment, it is necessary to create a differential pumping system, since evaporation of metals occurs under high vacuum conditions, and low vacuum is observed in the deposition zone.
Currently, this method is used for production of tapes by American company SuperconductorTechnologiesInc (up to 500 A/cm, IBAD) and Korean company SuNam (up to 500 A/cm, IBAD, GdBCO).
Pulsed laser deposition (PLD) makes it possible to obtain high-quality films from a wide range of materials with a high degree of correspondence between the formed films stoichiometry and the target material composition, which is especially important in the deposition of multicomponent materials, including YBa2Cu3O7-x (YBCO). PLD installation includes a laser (typically, excimer one operating at wavelength of 193, 248, 308 or 351 nm, less often fourth harmonic solid state Nd : YAG with wavelength of 266 nm), a vacuum chamber, an optical system and gas supply system. The main disadvantage of this method is low deposition rate. A strong increase in the power of the laser radiation does not increase the performance of the process (it only increases the droplet phase, thereby degrading the quality of the film) therefore, in order to increase the performance, a multiplume-multiturn PLD (MP-MT PLD) is used by splitting one powerful beam or by installing several laser emitters (Fig. 3) .
Currently, this method is used for tape production by German company Bruker (up to 350 A/cm, IBAD), Russian company SuperOx (up to 500 A/cm, IBAD, GdBCO) and Korean company Fujikura (up to 550 A/cm, IBAD, GdBCO).
This paper, jointly with RDC "Kurchatovsky Institute", a superconducting layer in second-generation HTSC tapes was obtained by pulsed laser deposition to reach the critical current value to 247 A.
In order to measure critical current (IC), four-contact method was applied. The critical current was determined using the criterion of 1 µV/cm. The measurements were carried out at a temperature of T = 77.4 K. Film surface morphology control was carried out with the help of scanning electron microscopy (SEM) method by Quanta 3D200i, FEI Co. Diffraction spectra were recorded with BRUKER D8 ADVANCE powder diffractometer in Θ–2Θ mode in Bragg-Brentano geometry. The texture sharpness was determined by the pole figures profile according to FWHM value (full width at half maximum) of the texture maximum captured by RigakuSmartlab diffractometer. The substrate texture sharpness in plane was 6.0°. The α-phase percentage calculation in the films was carried out according to formula (200) / ( I (006) + I (200)) · 100%. The thickness of the superconducting coatings was measured with BrukerDektak XTL contact profilometer.
Metal textured substrate consisting of Ni alloy alloyed with 5at%W (by EVICO, 10 mm wide and 69 µm thick) was used as substrates as well as buffer architecture NiW / Y2O3 / YSZ (ZrO2 + 8% Y2O3) / CeO2 , widely distributed for this type of substrates .
The growth of buffer and superconducting layers was carried out by pulsed laser deposition method on a tape-winding multiplume-multiturn PVD installation T1000. It was equipped with XeCl excimer laser source Coherent LEAP 130 (308 nm) with a power output of 650 mJ, a pulse repetition frequency up to 200 Hz and a pulse duration of 22 ns. In this installation, a multiplume-multiturn deposition system was implemented. The system is equipped with a rotating mirror, which allows linear scan target surface, creating sequentially up to 4 consecutive focused spots (energy density at the target of up to 4 J/cm2). The buffer layers were deposited at a heater temperature of 970 °C with laser radiation energy of 600 mJ. Y2O3 seed bed was deposited in a reducing medium (Ar + 5% H2 gas mixture) at a pressure of 2 mTorr with a repetition rate of 100 Hz. Its thickness was 200 nm. The barrier layer YSZ (100 Hz, 150 nm) and the final CeO2 (25 Hz, 70 nm) were deposited in an oxygen medium at a pressure of 10 mT. The sharpness of the texture of CeO2 layer was 6.4°.
HTSC deposition was performed out of YBCO stoichiometric target in an oxygen environment at a pressure of 100 mT by four beams of laser radiation with energy of 420 mJ, the pulse repetition frequency was 100 Hz. The heater temperature has a feedback (7 thermocouples are installed), and the set temperature is maintained. The superconducting layer was sprayed onto the tape in the regime of tape rewinding from the coil to the coil. YBCO film 150 nm thick is formed in one turn of the tape through the deposition are. The protective silver layer is formed by evaporation (1–1.5 µm thick), then the films are annealed in oxygen medium for 2 hours at a heater temperature of 600°C.
DISCUSSION OF THE RESULTS
It is known that crystalline and morphological defects accumulate with increasing thickness of HTSC layer, and a transition to α-oriented growth (h00) occurs, which adversely affects the electrical characteristics of the tape . This may be due to a decrease in the temperature on the tape surface due to a change in the blackness of the film . One of the methods to combat this phenomenon is the increase in temperature during the growth of the film. Therefore, a series of experiments was performed to determine the optimal growth temperature of the HTSC layer at all stages of thick superconducting film formation.
First, the growth temperature of the initial layer, 300 nm thick (890°C) (Fig. 4) was determined. Fig. 4 shows the values of the critical current, and the percentage of α-phase in the films is calculated. With such HTSC layer thickness, weak dependence of the critical current on the growth temperature near the optimum is observed. SEM images (Fig. 5) show smooth films with no significant defects (except droplets which are associated with low target density).
Then, the growth temperature of the film 750 nm thick, which was 900 °C (see Fig. 4), was determined. With such superconducting film thickness, the critical current depends strongly on the crystalline quality of the film. At a low deposition temperature, the film already consists of α-oriented crystallites by quarter, which negatively affects the critical characteristics. However, as the growth temperature increases, no increase in the critical current occurs, despite a decrease in the α-phase content. SEM images (Fig. 6,7) show that with increasing thickness of the HTSC layer, the surface relief develops and the films become less smooth.
The next stage was a further increase in the substrate temperature with an increase in the thickness of the deposited film (a superconducting film up to 750 nm was deposited under the optimal regimes found earlier). An increase in the film thickness up to 1500 nm made it possible to obtain a critical current equal to 147 A (Fig. 8) (the diagram shows the heater temperature with the growth of 6–10 layers for 1500 nm film and 10–13 layers for 1950 nm film), but a further increase in thickness to 1950 nm has only led to a drop in the critical current. SEM images (Fig. 7) show completely overgrown films; the appearance of α-oriented crystallites (in the form of rectangles) is seen on the surface.
The results obtained show low values of electrical characteristics of superconducting films. Simultaneous formation of α-oriented crystallites and fused regions in thick films may indicate non-uniform distribution of the heater temperature. Therefore, the temperature profile along the heater was measured (Fig. 9). The temperature in the film deposition area turned out to be lower than at the periphery of the heater, which adversely affects the epitaxial growth of the film: when the tape moves, the formed film leaves the growth area, enters the area with a higher temperature, and therefore it is partially melted and recrystallized with arbitrary orientation.
After increasing the distance X between the heater and the screen by 15, 20, 25 and 30 mm (Fig. 3), the temperature profile was significantly leveled, but it is not possible to obtain a linear distribution due to design features, and despite the small temperature difference, it was possible to significantly increase the electrical parameters of the superconducting coatings. Thus, HTSC layer as thick as 750 nm, a critical current of 150 A was reached, which in the previous factory configuration of the growth chamber could not be achieved for 1950 nm film (Fig. 10). For 1500 nm coatings formed at a constant temperature, the maximum current is 166 A, which is slightly higher than the value for 750 nm coatings. In such a film, the fraction of α-oriented crystallites is large, and therefore, periodically increasing the temperature of the heater directly in the growth process, it is possible to improve the crystalline perfection of the superconducting film being formed. Thus, with an increase in temperature by 45 °C, the critical current rises to 218 A. A further increase in the film thickness up to 1950 and 2250 nm yields a current value of 243 and 247 A, respectively. For films 750 nm thick, the increase in current was 63%, for 1500 nm – 48%, and for 1950 nm – 73%.
The surface of films 750 and 1500 nm thick, obtained with a lowered thermal screen (x + 30), has a less developed morphology (Fig. 10). On a 1500 nm film, separate α-oriented crystallites are observed, in contrast to the film as thick as 1950 nm. The current-voltage characteristic of a high-temperature superconductor tape with the highest current is shown in Fig. 12. As can be seen from the figure, this technology helped to obtain critical current values of 247 A.
The modernization of the growth chamber made it possible to equalize the temperature of the heater in the direction of the belt motion, which provided an improvement in the crystalline quality of the superconducting coatings formed, which in turn led to an increase in electrical characteristics up to 70%.
The deposition regimes of long-dimensional superconducting YBCO films of various thicknesses on NiW substrates with high electrical characteristics have been mastered. A slight change in the standard factory design allowed to increase the critical current of superconducting coatings by 70% (up to 247 A). Similar results have been achieved for domestic target production at A.A.Bochvar All-Russian Scientific Research Institute for Inorganic Materials. However, the fundamental problem of the critical current density drop with an increase in the coating thickness of the coating cannot help in obtaining single-layer films with a critical current of more than 300 A on NiW substrates. Earlier, it was shown the possibility of increasing the electrical characteristics of HTSC tapes due to the formation of multilayer epitaxial structures YBa2Cu3Ox-interlayer-YBa2Cu3Ox , so the next stage of work will be the creation of long-length multilayer HTSC tapes with high electrical parameters.