A study of the fabrication and characterisation of high temperature superconductor YBa2Cu3O7 thin films

Transcription

1 University of Wollongong Thesis Collections University of Wollongong Thesis Collection University of Wollongong Year 2006 A study of the fabrication and characterisation of high temperature superconductor YBa2Cu3O7 thin films Aihua Li University of Wollongong Li, Aihua, A study of the fabrication and characterisation of high temperature superconductor YBa2Cu3O7 thin films, PhD thesis, The Institute for Supeconducting and Electronic, University of Wollongong, This paper is posted at Research Online.

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3 Chapter I: Literature review I. 1. Introduction Since the discovery of high-temperature superconductivity (HTS) in cuprate oxide superconductors [1-3], such as La-Sr-Cu-O (LASO), Y-Ba-Cu-O (YBCO), Bi-Sr-Ca- Cu-O (BSCCO), Tl-Ba/Sr-Ca-Cu-O (TBCCO), and Hg-Ba-Ca-Cu-O (HBCCO), extensive studies have been carried out worldwide to elucidate the mechanisms of superconductivity and to develop the applications of those HTS compounds. Great attention has been particularly paid to the research and development of superconducting thin films due to the following motivations. One is the potential of HTS thin films to give insight into the fundamental mechanisms governing HTS [4]. The other involves the various electronic applications that high-quality thin films might enable, including low-loss microwave cavities, filters, bolometers, various superconducting two-and three-terminal devices, flux transformers, and dc and rf superconducting quantum interference devices (SQUIDS) [5]. For large current applications, HTS tapes and wires are very desirable. The first generation of superconducting tapes is based on Ag sheathed Bi-Sr-Ca-Cu-O tapes or wires fabricated using the powder-in-tube technique. However, the BSCCO/Ag tapes and wires suffer serious degradation in critical current density in high magnetic field and have a high fabrication cost, hindering their practical application. From the viewpoint of practical applications, YBa 2 Cu 3 O 7 (Y123) has been the focus of thin film studies, as it is non-toxic and has a high T c of 90 K, with excellent ability to 1

4 carry a high superconducting critical current in high magnetic fields. The second generation of HTS Y123 tapes, also called Y123 coated conductors, which are based on thin film technology has been further triggering HTS thin film studies worldwide. Despite the extensive investigations made on Y123 films or coated conductors, some fundamental issues have to be further studied on the correlation between crystal microstructures or defects and flux pinning, which limits critical current density, and engineering technology suitable for the fabrication of long length Y123 coated conductors. The work of this thesis is focused on fundamental studies of both the fabrication and characterization of Y123 thin films grown on various substrates by chemical and physical techniques, with an emphasis on the improvement of critical current density through optimizing the processing conditions and nanopartical doping. The thesis is organized into three major parts. The first part reviews the major work relating to the fabrication and characterization of Y123 films from published papers. The details about the fabrication and characterization of Y123 films grown by various physical and chemical methods are presented in second and third parts. I.2. The structural and physical parameters of YBa 2 Cu 3 O 7-x Knowledge of the YBa 2 Cu 3 O 7-x crystal structure is crucial to the understanding of transport properties, thin film growth, and the discussion of practical electronic devices made from this superconductor. The fully oxidized compound YBa 2 Cu 3 O 7, which is 2

5 superconductive around 93K, has an orthorhombic unit cell as shown in Fig. 1, with the lattice paremeters a = 3.82 Å, b = 3.89 Å, and c = Å along the (100), (010), and (001) directions. The crystal structure and the lattice constants change with the oxygen deficiency x, as well as the superconducting critical temperature, as shown in Fig. 2. Two features of the crystal structure are particularly relevant to charge transport in this material. These are the CuO 2 planes formed by the Cu2, O2, and O3 atoms, and the CuO chains comprising the Cu1 and O1 atoms. Fig. 1. The structure of YBa 2 Cu 3 O 7-x. 3

6 Fig. 2. (a) Temperature dependence of normalized resistivities for a Y123 film with various oxygen contents. (b) Critical temperature as a function of the oxygen content [6]. The existence of the CuO 2 planes is a common feature in all the high-t c superconducting cuprates, and is believed to be essential for superconductivity in this class of materials. The CuO1 chains, on the other hand, are found in some materials only. The chains are thought to contribute to superconductivity by supplying charge carriers to the CuO 2 planes. Structural elements acting as charge reservoirs for the CuO 2 planes are found in every copper-oxide high-tc superconductor. 4

7 The layered structure of the copper oxide superconductors leads to strong anisotropy in the normal and superconducting properties. The anisotropy produces dramatic differences in properties along the ab plane and the c-axis. The strong uniaxial anisotropy can be seen in the normal state resistivities and in the superconducting upper critical fields. See Figs As the ab plane carries much great critical current density than the c-axis, thin films or coated conductors must be fabricated with a strong ab texture. Fig. 3. The temperature dependence of resistivity along the ab and out of the ab plane of a Y123 single crystal [7]. 5

8 Fig. 4. Upper critical magnetic field vs temperature of a Y123 single crystal for field in the ab plane (open circles) and perpendicular to the ab plan (closed circles). The dashed line is the calculated critical field using the slope near T co of the perpendicular data [8]. I.3. Thin film growth techniques It should be mentioned that the definition of thin and thick films is not based on film thickness. Films are characterized more correctly by the type of material transport and 6

9 deposition method. Thin film techniques consist of the transport and deposition of atoms and/or molecules. On the other hand, thick film techniques consist of the transport and deposition of agglomerates or particles. All the physical and chemical vapor deposition techniques, as well as electrodeposition, may be characterized as thin film techniques, while spray pyrolysis, plasma spray, and screen printing would be termed thick film techniques. I.3.1. Chemical deposition I Metalorganic chemical vapor deposition (MOCVD) MOCVD stands for Metal-Organic Chemical Vapour Deposition. The principle of MOCVD is quite simple. Atoms forming the film composition are combined with complex organic gas molecules and passed over a hot semiconductor wafer. The heat breaks up the molecules and deposits the desired atoms on the surface, layer by layer. By varying the composition of the gas, you can change the properties of the crystal at an almost atomic scale. This technique can grow high quality semiconductor layers (as thin as a millionth of a millimetre), and the crystal structure of these layers is perfectly aligned with that of the substrate. The MOCVD approach has advantages over other methods because of its flexibility in growing precision controlled layers for special applications, as well as its ability to be scaled up to industrial-scale production with relative ease. 7

10 MOCVD is an important process for producing superconducting thin films [9-11]. Films grown by MOCVD have excellent superconducting properties, except for a rougher film surface and larger surface resistance than the best films grown by other techniques. Because of the advantages of MOCVD, such as the capacity for large-area growth and high growth rates, and familiarity in industry, this technique is promising for YBCO coated conductors. I MOD-Solution methods Solution deposition has been widely used to fabricate high T c superconducting films. Solution deposition is a fast, cost-efficient method to ensure extensive ranges of film composition, and it is easy to achieve stoichiometric control of complex mixed oxides. The most frequently used solution-preparation approaches can be divided into three groups: sol-gel processes [12] that use 2-methoxyethanol as a reactant and solvent; metalorganic decomposition (MOD) approaches [13-15] that use large, waterinsensitive carboxylate compounds; and hybrid processes that use chelating agents such as acetic acid, or ciethanolamine to reduce alkoxide reactivity. The precursor films are usually deposited by spin-coating, dip-coating, or print coating methods, so that it is easy to make uniform large area films. The precursor films are very porous because a large fraction of the precursor volume is eliminated during pyrolysis as volatile hydrocarbon gas or water vapour, or both. There are many cracks left in the precursor films caused by tensile stresses after the films significantly decrease 8

11 their volume. Post-treatment can improve the density and surface morphology, but it is hard to obtain films as good as with vacuum deposition. So solution deposition methods are not commonly used to deposit the superconducting films that are used in electronic applications. However, the MOD solution method, having the advantages of low cost and easy control in large scale production, has been widely used to fabricate Y123 coated conductor on a large scale with excellent superconductivity. Part of this thesis work focuses on studies of Y123 thin films fabricated using the MOD technique. I Liquid phase epitaxy (LPE) Liquid phase epitaxy (LPE) is a method of depositing films from the liquid phase, either from solution or from melt. It is a method similar to the Czochralski method and to the top seeded solution growth method [16, 17], with a substrate introduced vertically from above and axially rotating under isothermal conditions. As a robust and cost-saving method LPE is a widespread production process in the semiconductor industry. An absence of structural defects in highly perfect LPE- grown films results in weaker flux pinning, with typical J c (77K) values of to 10 4 A/cm 2. additional pinning can be introduced by growing on a substrate possessing a large lattice mismatch, such as MgO [18]. The defects introduced by the lattice mismatch increase the pinning, with J c (77K) 10 5 A/cm 2 in zero field and increased pinning evident at high fields. The LPE technique has been used for the deposition of Y123 thick films of superconducting yttrium barium copper oxide (YBCO). The LPE of YBCO has reached 9

12 a level allowing the development of techniques for the manufacturing of thick films for high current applications. I.3.2. Physical methods I Magnetron Sputtering Sputtering equipment is simple, and the cost is low, with a process that is easy to control. HTS thin films can be deposited either by direct current (DC) or radio frequency (RF) sputtering from single or multiple targets comprising elemental metals, alloys, unreacted mixtures of oxides, or sintered superconducting phase. Sputtering is a vacuum process used to deposit very thin films on substrates for a wide variety of commercial and scientific purposes. It is performed by applying a high voltage across a low pressure gas to create a plasma, which consists of electrons and gas ions in a high energy state [19]. During sputtering, energized plasma ions strike a target, composed of the desired coating material, and cause atoms from that target to be ejected with enough energy to travel to, and bond with, the substrate. Magnetron sputtering has been widely used for Y123 thin film preparation. The Y123 films made using this technique have the unique feature of spiral growth patterns. The critical current density of this Y123 thin film is smaller than that of film prepared by pulsed laser deposition. I Pulsed laser deposition (PLD) 10

13 The pulsed laser deposition (PLD) process gained prominence when it was found to be the most convenient and efficient technique for the synthesis of high-t c superconducting thin films [20, 21]. For the process of making YBCO thin films using PLD, a pulsed laser strikes a solid bulk YBCO target. Some of the target materials are removed, escaping in the form of a plume. Part of the plume comes in contact with the surface of a heated substrate kept a few centimeters away from the target. The plume, which consists of the building blocks of the YBCO lattice, covers the substrate. The result is the fabrication of a thin film of YBCO material with the same chemical structure as the target. PLD offers numerous advantages, including film stoichiometry close to that of the target, low contamination levels, high deposition rate, and nonequilibrium processing. The basic experimental design for thin film deposition by laser ablation is similar to any other physical vapor deposition process. Namely, the apparatus includes a vacuum chamber, a substrate holder with precise temperature control, and source materials (target). Fig. 1 (in Chapter 3) shows a schematic diagram of a typical PLD system. It comprises of a deposition chamber, fitted with a resistive sample heater, and a 6 target manipulator. A KrF excimer laser, with a wavelength of 248 nm is used as the ablating power source. The laser beam is focused onto the rotating target by a fixed-beam optical train. The laser fluence is varied by either varying the laser output energy or by focusing the beam. The quality of the Y123 film or coated conductor made by the PLD technique is the best among all the Y123 films produced by all other chemical or physical methods. 11

14 I.4. Substrates selection I.4.1. Main requirement for the substrate A number of issues are critical in substrate selection for thin-film growth, regardless of the details of the film to be produced. Chemical compatibility: one of the first issues that must be dealt with in determining the suitability of a substrate for a HTS film is the chemical compatibility of the two materials. This is true whether or not the film is epitaxial. Ideally, there should not be any chemical reactions between the film and substrate. The constraint is especially severe for HTS films since these materials are reactive with many substrates that might otherwise be good candidates. Regardless of the specific film growth method used, the substrate must be unreactive in the oxygen-rich ambient required for growth and processing. Thermal-expansion match: most combinations of substrates and films will be more or less mismatched in regard to thermal expansion. This may result in loss of adhesion or film cracking during thermal cycling. A good thermal-expansion match is necessary, whether or not one is dealing with an epitaxial system. 12

15 The best HTS films grown to date, as determined by a multitude of metrics, including critical current density, morphology, and stability over time, are epitaxial on their substrates. Epitaxial growth requires the controlled crystallographic orientation of the film with respect to the substrate. In general, this necessitates matching the film and substrate with respect to lattice parameters, atomic positions, crystallographic orientation, etc. The better the match of all these parameters, the more likely highquality epitaxial growth is to occur. For high frequency applications, the substrate should have low dielectric loss. The main requirements for polycrystalline substrates relate to chemical stability under the deposition conditions, surface roughness and mechanical strength. These substrates are used in conjunction with buffer layers deposited in a biaxially textured form by pulsed laser deposition, with the required texture being formed by the ion beam, in the so-called ion beam assisted deposition (IBAD). Uniaxially aligned Y-123 films (c-axis substrates) are easily grown on polycrystalline substrates, but the superconducting properties are heavily degraded at grain boundaries. Weak coupling occurs due to grain misorientations. If the angle θ defines the misorientation angle between adjacent grains in the basal plane, and the angle φ represents the misorientation angle between grains in the direction perpendicular to the basal plane, work by Dimos [22] showed that even if 13

16 samples can be fabricated with almost perfect c-axis alignment (uniaxially aligned, φ 0), misorientation in the basal plane between adjacent grains will limit J c. By increasing the orientation both normal to and within the basal plane (biaxially aligned structure), an increase in current density can be obtained compared to unbiaxially aligned substrates [22] Cube texture can be produced in many fcc metals and alloys of medium-high stacking fault-energy (SFE) with excellent precision and sharpness. The cube texture can be found in copper, aluminium, silver, gold, nickel, iron-nickel ( > 30% nickel) alloys, and iron-nickel-copper alloys. The cube texture in these materials is often so strong that it represents a single crystal, as the individual recrystallised grains are so closely oriented to the ideal orientation [23]. The number of conflictiing experiments and different theories that exist on the formation of cube texture in fcc metals shows that the origin of cube texture in these metals is still a subject of controversy [24]. A detailed review of nucleation, recrystallisation, and texture formation in polycrystals can be found in reference [25]. I.4.2. Other requirements for the substrate Although a highly oriented substrate is critical for coated conductor fabrication, a number of other substrate requirements must be met. 14

17 1) Surface roughness. In order to successfully deposit the buffer and superconducting layers, the surface of the substrate must be smooth and flat. Any surface irregularities on the substrate will be duplicated in the buffer and superconducting layers, resulting in degraded superconducting properties. The principal factors affecting the surface quality of the substrate have been identified as rolling damage (function of roll roughness) and grain boundary grooving during annealing [26]. The surface smoothness can be enhanced by mechanical and/or electrolytic polishing prior to deposition of the buffer and superconducting layers, although this may contaminate the surface and introduce strain into the grains. Mechanical polishing alone can produce satisfactory surface smoothness. In the production of Iinconel and Hastelloy substrates, Willis [27] obtained an average surface roughness of nm at the conclusion of thermomechanical processing, which was reduced to 2-5 nm by mechanical polishing. Mechanical polishing can cause contamination of the surface by means of polishing artifacts, and strain can be introduced into the grains due to plastic deformation of the abrading particles. The outermost layer of an abraded surface is equivalent to a material which has been severely cold rolled (99% reduction). This fragmented layer is composed of very fine sub-grains and contains a high density of defects [28]. The orientation of these subgrains is found to result in characteristic textures [29]. 15

18 Electrolytic polishing can also be used to improve the surface smoothness. The surface to be electrolytically polished must be as flat as possible. This flatness requirement generally dictates that the sample must be subjected to some form of mechanical polishing prior to electrolytic polishing. The rate of material removal during electrolytic polishing is typically 1µm min -1, so long polishing times would be required to remove the damage introduced by the prior mechanical polishing step [28]. This means that electrolytic polishing itself does not ensure the production of an artifact free surface. Also, electrolytically polished surfaces will usually be contaminated by both the formation of a thin film due to the anodic surface as an essential part of the process and the possibility of absorption of complex ions, either formed in the viscous layer during polishing or present in the polishing solution itself [28]. It is anticipated that the surface roughness should not be greater than 1/10 of the buffer layer thickness (5-10 nm). 2) Mechanical properties. The comparatively strong, ductile metal substrate must support the more brittle oxide superconductor [30]. The coated conductor must have sufficient mechanical strength to allow fabrication into superconducting devices. Much of this mechanical strength is imparted by the substrate. Many applications do create significant mechanical stress on the coated conductor, as it forms an integral part of the design, incorporating cooling and other structural elements. In a study of Inconel/YSZ/Y-123 superconductors performed by Thieme et al [31]. it was shown that at 77 K up to 0.5% strain does not change J c by more than 3%. This 16

19 strain dependence is better than the tensile stress dependence of J c when the samples are bent at room temperature [31]. 3) Grain size. It is desirable to have as large a grain size as possible, as grain boundaries degrade superconducting properties. It is also desirable for the grain size formed during cold rolling and annealing to remain stable during further processing. There are many possible substrate materials, including silver, nickel, copper, and alloys of these materials. Whilst all of these metals develop cube texture from thermomechanical processing, they all offer different mechanical, physical, and chemical properties. I.4.3. Metallic substrates I Textured Ag The early work involving the epitaxy of Y-123 films vapour deposited onto bulk single crystals of silver is covered in the work of Budai et al [32]. Hence efforts were directed towards producing a substrate material with a sharp biaxial texture using rolled silver, since silver is one of the few metals with benign interactions with high temperature superconductors [33]. Some success has been achieved using cold rolling and recrystallisation [34] and hot rolling and recrystallisation [26,34]. It has been found that Y-123 grows with a single orientation on the (110) plane of silver, so that the ideal recrystallisation texture would be one which orients this plane parallel to the surface of the tape [26,32,34]. Typical rolling textures in silver 17

20 correspond to a brass texture having orientations of {110}<112> and {236}<385> [33]. Under controlled deformation rates, annealing conditions, and initial billet oxygen content, several researchers have obtained the {110}<110> texture in silver [26,34] and silver alloys [34]. However, there is some twinning present, which decreases J c. Since the twins mostly likely nucleate and recrystallise from stacking faults in the as rolled silver, improved texture should be possible by increasing the stacking fault energy of the silver through doping additions and oxygen control [34]. Alloying is also necessary to increase the mechanical strength of the tape [35,26]. The {001}<100> cube texture has been obtained in silver by Doi et al. [36] by warm rolling and subsequent annealing at o C. These tapes have been referred to as cube-textured silver tape (CUTE tapes) and have been manufactured in 80 m lengths with grain boundary misorientation angles of only a few degrees. Further work is still required with silver substrates to sharpen the texture, reduce the twin component and increase mechanical properties [26,34,35]. I Textured nickel substrates In order to avoid the problems associated with texturing silver directly researchers have performed thermomechanical processing of fcc base metals such as nickel [33,35,37-39]. 18

21 A biaxially textured buffer layer can be easily grown on the surface of nickel or nickel alloy tapes with the {001}<100> orientation. The development of nickel substrates was rapidly advanced by Goyal et al. [40], who fabricated long lengths of superconducting tape, referred to as rolling-assisted-biaxially-textured-substrates (RABiTS TM ). Work by Truchan et al. [41] showed that nickel sheets uniformly rolled to > 95% reduction developed a sharp cube texture for all heat treatments between 400 and 1000 o C, showing a slight improvement with longer annealing times. Recrystallisation studies on cold-rolled pure nickel performed by Makita et al. [42] showed that the primary recrystallisation texture produced during high temperature annealing of heavily cold rolled nickel sheets was an extremely sharp cube texture, independent of initial grain size and depth from the sheet surface. The sharpness of the cube texture is found to increase with higher percentage rolling reductions [43]. Texture studies performed by Petrisor et al. [43] revealed that the {001}<100> cube texture can be easily developed in Ni-V alloys (up to 11 at.% vanadium) by a cold rolling process followed by a thermal recrystallisation treatment. The technological issues concerning nickel substrates are related to the mechanical strength and magnetism of nickel. Various approaches to the fabrication of stronger substrates with reduced magnetism have been suggested, including composite structures and alloys [35]. I Textured copper substrates 19

22 After heavy deformation of copper the primary texture is {112}<111> with small traces of {001}<100>, but on annealing it is the {112}<111> texture which is the host for the cube texture, which is nucleated by the traces of cube texture in the as-deformed material. The first requirement for a strong annealing texture is thus a minor texture component accompanying the major deformation texture [40]. Little work to date has focused on developing coated conductors using cube textured copper substrates. However, researchers have highlighted the possibility [44]. For biaxially textured substrates, the following specifications can be used as a guideline [45]: {001}<100> cube texture fraction, as determined from the normalized X-ray pole figure, >97%; {001}<100> cube texture fraction variation along the lengths, as determined from the normalized X-ray pole figure, < 2%; {122}<212> twin-of-cube texture fraction, as determined from the normalized X-ray pole figure, < 2%; Out-of-plane texture variation, as determined by X-ray θ - 2θ scan, (111)/(200) < 1%; Out-of-plane texture variation, as determined by X-ray ϕ scan, FWHM < 8 o ; In-plane texture variation, as determined by X-ray ϕ scan, FWHM < 10 o ; 20

23 Average roughness, as determined by AFM, R a < 25 nm. I.5. Main results from the literature The major results on the fabrication and characterization of Y123 thin films grown on different substrates using various techniques have been summarized in several review articles published in 1996 [46], 1997 [47], 1998 [20], and 2000 [48]. This section of the literature review only focuses on the main results from published work since Although the thesis work focuses on fundamental studies of fabrication and characterization of Y123 thin films grown on various substrates by chemical and physical technique with an emphasis on the improvement of critical current density through optimizing the processing conditions and nanoparticle doping, a general review is given on most of the Y123 films grown on various substrates. The terminology coated conductor is another name given to Y123 thin films that are grown on metallic substrates using thin film technologies that are exactly the same as those used for Y123 thin films grown on various insulating oxide substrates. Therefore, the review covers both Y123 films grown on insulating substrates and Y123 films/coated conductors grown on metallic substrates. I.5.1. Fabrication of YBa 2 Cu 3 O 7-x coated conductors Coated conductors are a type of superconductor fabricated by epitaxial vapour deposition. A ribbon-like metallic substrate is used as support. The polycrystalline substrate could be non-textured, or could be cube-textured to decrease misorientation of 21

24 the polycrystalline film in the basal plane, and is approximately 0.1 mm thick. One or two buffer layers are often employed to prevent poisoning of the superconducting phase by the underlying substrate. The thickness of the buffer layers ranges from 50 nm to several microns. As superconducting film approximately 400 nm thick is deposited on top of the buffer layer/layers. Finally, the superconducting film is covered with a protective metallic layer 1-5 µm thick. Manufacturing superconducting tapes by this method should allow long lengths to be fabricated for use in power applications such as motors, generators, transformers, magnets, power transmission, and energy storage. Much research has been focused on producing long lengths of superconducting tape for power applications such as transmission lines, transformers, motors, generators, etc. [44,49-51] The brittle nature of superconducting oxide materials coupled with difficult processing techniques has proven a challenge to superconductor tape production. The powder-in-tube (PIT) technique was the first successful processing method for making long lengths of bismuth-strontium-calcium-copper oxide superconductors. This process has reached its limit in terms of the maximum current density obtainable at a particular applied field for the length of wire that could be produced [33]. The next advance in superconductor tape technology came with the deveplpment of Y- 123 coated conductors. In order to minimize the decrease in current density caused by grain misorientation, there are two approaches: (1) the use of a randomly oriented substrate, onto which biaxially textured buffer and superconducting layers are grown; (2) the use of a cube-textured (also called biaxially textured) substrate onto which epitaxial 22

25 buffer and superconducting layer are grown using vapour deposition. Under certain growth conditions, the resultant YBa 2 Cu 3 O 7-x superconducting film does not suffer percolative current flow as in PIT processed superconductors. Also, the process can be scaled up to produce the long lengths of tape necessary for use in power applications. Many substrate and buffer combinations have been developed. Silver and nickel are common substrate materials, as a strong biaxial texture can be obtained by cold rolling to a large reduction, followed by annealing. Reactions between the substrate and superconducting film during processing require the use of buffer layers, which are metallic (silver, palladium, platinum) or ceramic materials (Y 2 O 3 -stabilised-zro 2, CeO 2, MgO, ZrO 2, SmBa 2 Cu 3 O 7, La 0.7 Sr 0.3 MnO 3 ). A number of techniques are currently used for depositing the protective, buffer, and superconducting layers: sputtering (rf, dc, magnetron) [52,53], pulsed laser deposition (PLD) [37,52-56], ion-beam-assisted-deposition (IBAD) [35,54,56], liquid-phaseepitaxy (LPE) [35,57], vapour-phase-solid (VPS) growth [57], metal-organicdeposition (MOD) [35], electron beam evaporation [56,58,59], inclinedsubstrate-deposition (ISD) [35], and the ink-printed/floating zone heated method [60]. I Protective layer A protective layer is often deposited last to protect the Y-123 layer. The main requirements for the protective layer are that it is chemically compatible with the superconducting film, is easily deposited and protects the surface from mechanical 23

26 damage. The protective layer also allows the attachment of electrical contacts (necessary if ceramic buffer layers are used) and prevents moisture from reacting with the superconducting film. A layer of silver or gold approximately 1-5 µm thick is typical. The final tape is then annealed in flowing oxygen to lower the contact resistance between the protective layer and the Y-123 film, to obtain the orthorhombic (superconducting) phase, and to fully oxygenate the Y-123 [27]. I Y123 thin films or coated conductors grown on various substrates with various buffer layers Buffer layers are required to prevent poisoning of the Y-123 superconducting layer by the underlying substrate material, and to provide an appropriate template for epitaxial growth of the Y-123 layer with an in-plane grain misorientation of less than 8 o. Noble metals such as platinum, palladium, or silver can be deposited onto the biaxially textured substrate at high rates, forming an epitaxial buffer layer [45]. The resulting noble metal film provides the required template for epitaxial growth of the oxide buffer layers or the direct growth of the superconductor [33]. The good chemical compatibility of silver and Y-123 means that the superconducting layer can be deposited directly, without the need for complex buffer layers. However, twinning could occur in the silver buffer layer, which would significantly degrade the superconducting properties. 24

27 For nickel substrates a combination of metal and ceramic buffer layers may be used. A multilayered film architecture is used, comprising Y 2 O 3 -stabilised-zro 2 (YSZ) and CeO 2 on the surface for improvement of the crystallinity of Y-123 and control of the chemical reaction between the metal substrate and the Y-123 film [35]. If exposed to an oxidizing atmosphere, bare nickel tape will form a randomly oriented NiO layer. The CeO 2 layer is first deposited to suppress the formation of NiO. Surface smoothness of ZrO 2 buffer layers [61] and cracking in CeO 2 buffer layers have both been reported [30]. These cracks may be associated with the high deposition temperature of the CeO 2 layer in oxygen. Shi et al. [62] reported a considerable decrease in the temperature of epitaxial deposition of CeO 2 in Ar + 10% H 2, which should also be beneficial for a reduction in the grooving of the grain boundaries in Ni. By controlling the oxidation temperature and oxidation atmosphere, Matsumoto [63] developed an epitaxial relationship between the underlying nickel substrate and the NiO. This technique is called surface-oxidation-epitaxy (SOE). Under certain growth conditions, the NiO layer can grow epitaxially on the textured Ni substrate, but the cracking of the oxide layer is apparently difficult to control [65]. Nd 2 O 3 buffer layers were deposited on textured Ni tapes using a reel-to-reel sol gel process for YBa 2 Cu 3 O 7 x (YBCO) coated conductors [65]. An Nd based precursor solution was prepared using solvent and chelating agents. It was found that the cubic phase transformed to a hexagonal structure at temperature of up to 1000 C. The textured films were grown onto the textured Ni tapes at 1150 C for 10 min under 4% 25

28 H 2 Ar gas flow using modifying triethanol amine. SEM images of the Nd 2 O 3 buffer layer showed crack-free, pinhole-free, dense and smooth microstructure. Mixtures of europium oxide (Eu 2 O 3 ) and ytterbium oxide (Yb 2 O 3 ), (Eu 1-x Y x ) 2 O 3 were investigated as a candidate buffer layer that could have same lattice parameter as Y123 [66]. The mixtures were prepared using metal-organic precursor produced by the sol gel process, and it was found that all the mixed samples were single phase, complete solid solutions, and have same crystal system over the whole range of x. The lattice parameter of mixed (Eu 1-x Y x ) 2 O 3 oxide powders was changed to some value between and Å, which are the lattice parameters of Eu 2 O 3 and Yb 2 O 3, respectively, by changing the ratio of Eu/Yb in the mixture. Phase and lattice parameter analysis revealed that the pseudo-cubic lattice parameter of (Eu Yb ) 2 O 3 is 3.82 Å, which is same as the lattice parameter of YBCO. Textured (Eu Yb ) 2 O 3 buffer layers were grown on biaxially textured-ni (100) substrates. X-ray diffraction (XRD) of the buffer layer showed strong out-of-plane orientation on Ni tape. The omega and phi scans revealed good out-of-plane and in-plane alignments. The full-width-at-halfmaximum (FWHM) values of the omega and ρhi scans of (Eu Yb ) 2 O 3 films were 6.45 and 7.70 o, respectively. SEM micrographs of the films revealed pinhole-free, crack-free and dense microstructures. The texturing influence of the process parameters in sol-gel Tb 2 O 3 buffer layers on textured Ni tapes was studied by Erdal et al. [67]. A solution deposition process was used to grow epitaxial Tb 2 O 3 buffer layers on the Ni tapes for YBCO coated conductors. 26

29 The solution was dip-coated onto the textured Ni substrates by a reel-to-reel sol-gel process. The buffer layers exhibited a strong c -axis orientation on the Ni tape substrate. The textured buffer layers were laid down on the textured Ni tapes at 1150 o C for 10 min under 4% H 2 + Ar gas flow. Images of the Tb 2 O 3 buffer layers showed crack-free, pinhole-free, dense and smooth microstructure. Epitaxial La 2 Zr 2 O 7 (LZO) buffer layers on roll-textured Ni (100) substrates were produced using a solution process for YBCO coated conductors [68]. The LZO precursor solution was prepared by an all alkoxide sol gel route using mixed metal methoxyethoxides in 2-methoxyethanol. The partially hydrolyzed solution was either spin-coated or dip-coated onto the textured Ni substrates. The buffer layer indicated a strong c-axis orientation on the Ni (100) substrate. The LZO (222) pole figure revealed a single cube-on-cube texture. SEM images of the LZO buffer layer showed a dense microstructure without cracks. The YBCO deposited on the sol gel LZO-buffered Ni substrates with sputtered YSZ and CeO 2 top layers had a critical current density of 480,000 A/cm 2 at 77 K and self-field. A 25-nm thick NdGaO 3 (NGO) buffer layer was dip-coated onto STO single crystal from a solution of metal methoxyethoxides in 2-methoxyethanol [69]. High resolution scanning electron microscopy (SEM) of the bare NGO surface revealed ~40 nm diameter pinholes with a number density of ~ /m 2, corresponding to an area fraction coverage of 2.5%, on an otherwise featureless surface. The X-ray diffraction ω and ϕ-scans indicated that the YBCO film was highly oriented with a full-width-half maximum peak breadth of 1.14 for in-plane and 0.46 for out-of-plane alignment, 27

30 respectively. The film contained sparse a-axis oriented grains, an appreciable density of (001) stacking faults and apparently insulating second phase precipitates of the type that typically litter the surface of PLD films. All of these defects are typical of YBCO thin films. High-resolution cross-sectional TEM images indicate that no chemical reaction occurs at the YBCO/NGO interface. An YBCO film with a transport critical current density (J c ) value of 1 MA/cm 2 (77 K, 0 T) was grown on a solution deposited NGO buffer layer on (100) SrTiO 3 substrate. An YBCO film with a transport critical current density (J c ) value of 1 MA/cm 2 (77 K, 0 T) was grown on a solution-deposited NGO buffer layer on (100) SrTiO 3 (STO) [70]. The epitaxial relationships are cube-on-cube throughout the structure, when pseudocubic and pseudo-tetragonal unit cells are used to describe the NGO and YBCO crystal structures, respectively: (001) YBCO (001) NGO (001) STO and (100) YBCO (100) NGO (100) STO. Biaxially textured magnesium oxide (MgO) films on metallic substrates were fabricated by inclined substrate deposition (ISD) [71]. The ISD MgO films showed columnar grain structures with a roof-tile-shaped surface. X-ray diffraction and pole figure analysis revealed that the c-axis of the ISD MgO is tilted at an angle with respect to the substrate normal. A full width at half maximum (FWHM) of 10 was observed in the ω-scan for MgO films. Yttria-stabilized zirconia (YSZ) and ceria (CeO2) buffer layers were epitaxially grown on ISD MgO by pulsed laser deposition (PLD) prior to YBCO deposition by PLD. The YBCO films grown on YSZ/CeO 2 buffered ISD MgO 28

31 substrates were biaxially aligned with the YBCO c-axis normal to the substrate surface. A critical current density of J c > A/cm 2 was measured at 77 K in self-field. Biaxially aligned, heteroepitaxial oxide buffer layers and superconducting YBa 2 Cu 3 O 7 x (YBCO) thin films were deposited with PLD on Ni-based cube textured substrates [72,73]. Critical current transport measurements in zero field showed J c values up to 0.7 MA/cm 2. The FWHM values of x-ray ω-scans of the buffer and the YBCO were about 9 and 11, respectively. The oxide buffer was a bilayer, composed of CeO 2, Y 2 O 3, or YSZ in various combinations and thickness. Since the Ni-diffusion barrier efficiency strongly depends on the buffer morphology, detailed SEM and FIB (focused ion beam) studies of the different buffer systems were carried out. They revealed columnar, but dense growth characteristics in the buffer bilayers, but also deficiencies at the substrate surface. These observations indicate that a smooth substrate surface without significant impurities is essential for YBCO coated conductors. Biaxially textured MgO template films have been fabricated on a Ni-based alloy substrate (Hastelloy C276) by inclined-substrate deposition (ISD), using electron beam evaporation, at the high deposition rate of nm/min [74]. Buffer films were subsequently deposited on these template films, and YBCO films were finally deposited by pulsed laser deposition (PLD). Crystal textures of the YBCO films were examined by X-ray as well as pole figure, ω- and ϕ-scans analysis. Good in-plane and out-ofplane textures were observed, with MgO (002) -scan full-width-at-half-maximum (FWHM) of 10.0 and ω-scan FWHM of 5.5, for a film deposited with an incline angle of 55. YBCO films were epitaxially grown on ISD MgO-buffered Hastelloy C276 29

32 substrates by PLD. T c of 90 K with a sharp transition and a transport J c of A/cm 2 were obtained from a 0.5 mm thick YBCO film at 77 K in zero field. YBCO films with high critical current density (J c ) were fabricated on nickel tapes buffered with bi-axially textured NiO prepared by surface-oxidation epitaxy (SOE) [75]. The effects of oxide cap layers, such as YSZ, CeO 2, and MgO, on the SOE-grown NiO were investigated to improve the superconducting properties of the YBCO films on NiO. By inserting a thin MgO cap layer between the NiO layer and the YBCO film, a J c of A/cm 2 (77 K, 0 T) was achieved. This result indicates the potentiality of the SOE method. I.5.2. Y123 thin films grown on insulating single crystal substrates The work on Y123 thin film grown on single crystal substrate before the year 2000 has been reviewed in several review articles [46] and [20]. Most of the work was optimistic that the best growth conditions would achieve high supercurrent and high crystallinity combined with extremely low surface resistance for use in microwave devices. This section only summarizes the main results that have been obtained since 2000 on Y123 thin films on single crystal substrates made using both chemical and physical methods. Surface modifications and structural changes in Y123/MgO thin films prepared by exsitu processing of a spin-coated organic sol gel precursor were studied using time-offlight heavy ion elastic recoil detection analysis (HERDA) [76]. Evidence is provided for a smoothing of the surface of the films accompanied by an amorphisation of the YBCO films in the HERDA spot region. The virgin surface roughness of nm 30

33 could be reduced to 1 2 nm after bombarding the film for a short time with a pa current. However, large scale surface roughness on the 200 nm scale remained unchanged. It was also observed that the integrity of the superconductor films did not change significantly during HERDA, allowing simultaneous measurements of high resolution Y, Cu, Mg and C depth profiles. It has been demonstrated that the fluorine-based sol gel approach can produce high quality, epitaxial YBCO thin films that exhibit a superconducting J c of over 3 5 MA/cm 2 in self-field, 77 K. Yao et al. [77-80] have developed a new fluorine-free sol gel approach (FFSG) as an effective alternative. The advantages of this new approach is three-fold: (1) no detrimental HF during the processing; (2) the FFSG solution is much less reactive to the buffer layer; and (3) the microstructure of the YBCO thin film is more uniform and denser than what is achieved by the fluorine-based method. Using the new FFSG method, YBCO thin films have been deposited on LAO and YSZ substrates. A high J c on the order of A/cm 2 at 77 K and self-field was reported. Experimental results on film synthesis and superconducting properties were presented. YBCO films were grown on SrTiO 3 (100) and MgO (100) substrates by means of pulsed laser deposition in order to study the flux pinning properties of the films [81]. The field angle dependence of J c (θ) measured at 77 K, where θ is the angle between the magnetic field direction and the film surface direction, was analysed from the normalized J c (θ) and the peak component of J c (θ). 31

34 Large-area pulsed laser deposition (PLD) has reached a state in terms of film quality and reproducibility that now makes possible real applications of PLD-YBCO thin films on both sides of R-plane sapphire substrates as HTSC devices in mobile communication systems. Bandpass filters optimized from PLD-YBCO thin films presently fulfil the requirements of the main national companies that are active in future communication techniques. Ca doping of YBa 2 Cu 3 O 7 is well known to enhance the critical current density across large-angle grain boundaries, for example in bicrystals. Large-area Ca x Y 1-x Ba 2 Cu 3 O 7 films were prepared on 3-in. diameter sapphire wafers using PLD [82]. The microwave surface resistance R s at 8.5 GHz of the Ca-doped YBCO thin films shows a clear reduction (up to 20%) with respect to that of YBCO for temperatures of about K. In addition, the microwave surface resistance R s of Ca-doped YBCO is lower than that of YBCO even for an enhanced microwave surface magnetic field up to about 20 mt from 20 to 40 K. The effect of partial substitution of Y by Ca in YBCO superconducting thin films was investigated [83]. The films were grown on (1 0 0) SrTiO 3 single crystal substrates by pulsed laser deposition (PLD). The Ca doped film were ablated from Y 1 x Ca x Ba 2 Cu 3 O 7 targets with x = 0, 0.05, 0.07, and The dc transport properties of the films in applied magnetic field were analysed to study the role of Ca on the superconducting properties of YBCO films. The irreversibility line for the samples with x = 0 and 5 at.% Ca was derived from the E J curves using a scaling theory for the vortex-glass transition. 32

35 Ca-gels were applied to highly dense YBCO pellets and PLD thin films followed by high temperature post-annealing [84]. Parameters of Ca diffusion in YBCO (diffusion coefficients, activation energy, etc.) as a function of temperature and oxygen partial pressure were determined by depth profiling SIMS. Fast Ca diffusion along the grain boundaries was observed by FIB and SIMS. Ca distribution in the samples, induced by preferential diffusion along the grain boundaries, did not appear to change superconducting properties within the grains but showed increased J c across the grain boundaries. A relatively simple PLD arrangement with a fixed laser plume and rotating substrate, with an offset between the laser plume and the center of the substrate, was employed to deposit laterally homogeneous 4-inch diameter Ag-doped YBCO thin films [85]. With the experience of more than 1000 double-sided 3-inch diameter films a high degree of homogeneity and reproducibility of J c and R s was reached. The extension up to an 8- inch substrate diameter will increase the productivity of the flexible PLD technique considerably. Y123/TiN and Y123/TiO 2 bilayer structures have been prepared on SrTiO 3 (STO) (100) substrates by in situ pulsed laser deposition (PLD) [86]. Sn films were originally intended to serve as the lower contact electrode for studying the c-axis transport properties of YBCO thin films. It was found that, under the optimum conditions for depositing YBCO thin films, the TiN (100) layer was oxidized and transformed into rutile TiO 2 (110) film. On the other hand, pure anatase TiO 2 (001) films were prepared 33

36 by PLD using a rutile TiO 2 (110) substrate as the target. YBCO films grown on both phases of TiO 2 films show virtually the same transport properties as typical good quality single-layer YBCO films. Comparative studies of depositing YBCO films directly onto a dc-sputtered TiO 2 template such as is commonly used in the selective epitaxial growth process have, however, reported the formation of a non-superconducting YBCO top layer. The crystal orientation of Y123 films was found to be changeable from the a-axis to the c-axis by making the films in nitrous oxide (N 2 O) gas during the PLD process [87]. The possible origin of this result is a rise in the surface temperature of the substrate by thermal radiation from the hot filament and the generation of oxygene-related radicals produced by cracking of N 2 O. Temperature measurements revealed that the change in crystal orientation caused by the hot filament could be partly explained by the substrate heating effect. A comparative study on the microstructures in relation to critical current density T c has been made for YBCO films on LaAlO 3 single-crystal substrates by TFA and PLD, respectively [88]. In-plane and out-of-plane texture evaluated quantitatively by the phi and omega scans indicated that the FWHMs of phi and omega scans for the TFA sample were comparable to those of the PLD sample, indicating that TFA films follow the substrate texture precisely. The properties of the samples are almost identical, except for the intensity of the BaCuO 2 Raman peaks. Microstructure observation by SEM showed that TFA films were generally more porous and smaller-grained than PLD samples. Considering these results, the difference in J c values between TFA and PLD samples 34