Pore formation in YBCO films deposited by a large-area pulsed laser

Home

Search

Collections

Journals

About

Contact us

My IOPscience

Pore formation in YBCO films deposited by a large-area pulsed laser deposition system

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2004 Supercond. Sci. Technol. 17 1253 (http://iopscience.iop.org/0953-2048/17/11/005) View the table of contents for this issue, or go to the journal homepage for more

Download details: IP Address: 140.116.114.92 This content was downloaded on 13/03/2016 at 22:50

Please note that terms and conditions apply.

INSTITUTE OF PHYSICS PUBLISHING

SUPERCONDUCTOR SCIENCE AND TECHNOLOGY

Supercond. Sci. Technol. 17 (2004) 1253–1260

PII: S0953-2048(04)78767-9

Pore formation in YBCO films deposited by a large-area pulsed laser deposition system K Develos-Bagarinao, H Yamasaki, Y Nakagawa and K Endo National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan E-mail: [email protected] (Develos-Bagarinao)

Received 1 April 2004 Published 18 August 2004 Online at stacks.iop.org/SUST/17/1253 doi:10.1088/0953-2048/17/11/005

Abstract The surface morphology of YBCO films deposited by a large-area PLD system is characterized by pores of varying depths and sizes. The presence of these pores has been correlated to the yttrium-rich composition in the films. In-depth studies on the interplay of parameters including oxygen background pressure, target–substrate distance, film composition, and target stoichiometry on the porosity of the YBCO films deposited on LAO (100) substrates have been conducted. Off-stoichiometry in the composition of the films has been attributed to the scattering of the ablated species with the oxygen ambient. The relative excess of yttrium in the films causes the segregation of yttrium-rich secondary phases, one of which has been identified for the first time as barium yttrium oxide (BaY2 O4 ). From structural characterizations, it has been found that the nucleation and enlargement of the barium yttrium oxide phase obstructs the YBCO growth and eventually causes the pores in the film to develop. Considering the deposition conditions which either promote or hinder the formation of this secondary phase during YBCO growth, a reasonably good control of pore size and density has been achieved.

1. Introduction Pulsed laser deposition (PLD) is a versatile method to fabricate large-area YBCO films with highly uniform critical current densities (Jc ), a very important parameter for utilization in high-power applications such as fault current limiters [1]. However, due to the inherent nature of the laser ablation process, YBCO films produced by the PLD method are beset with surface imperfections such as outgrowths [2], droplets [3], and even pores. In fact, as observed through scanning electron microscopy (SEM), our large-area YBCO films are typically characterized by pores of various depths and sizes. Pores change the effective cross section of the sample for current transport. Thus, while the Jc values typically exceed 1 MA cm−2 for these films, increasing the connectivity and homogenizing the surface quality of the film are expected to further improve Jc distribution. Moreover, the effective surface 0953-2048/04/111253+08$30.00 © 2004 IOP Publishing Ltd

area of the film is greatly increased by these pores, which could lead to faster film degradation due to unavoidable exposure to air and humidity. On the other hand, pores in the films appear to be beneficial in relieving the accumulated stress, thus leading to a significantly higher critical film thickness on CeO2 buffered sapphire substrates [4]. It is therefore imperative that the mechanism for pore formation be understood in order to efficiently control pore size and density to suit the type of YBCO film desired. In this study, we investigate in detail the interplay of factors which affect porosity in the films. In particular, we study the correlation of the following parameters: (1) oxygen background pressure (P), (2) target– substrate distance (D), (3) target stoichiometry, and finally (4) film composition. Furthermore, we conduct a thorough analysis of the film microstructure in order to elucidate the mechanism for pore formation.

Printed in the UK

1253

K Develos-Bagarinao et al

Table 1. Films and the deposition parameters. LAPx Dx films were deposited using a laser fluence of ∼2.4 J cm−2 and a deposition temperature of 700 ◦ C, while NLA Px Dx films were deposited using a laser fluence of ∼7.4 J cm−2 and deposition temperature of 750 ◦ C. OLA Px Dx films were deposited using the same parameters as NLA Px Dx films, but using a Ba/Cu-rich, off-stoichiometric YBCO target. Samples LA Px Dx /NLA Px Dx x = 1, 2, 3

P, oxygen-background pressure (mTorr)

D, target–substrate distance (mm)

d, film thickness (nm) LA Px Dx

NLA Px Dx

LA P1 D1 /NLA P1 D1 LA P1 D2 /NLA P1 D2 LA P1 D3 /NLA P1 D3 LA P2 D1 /NLA P2 D1 LA P2 D2 /NLA P2 D2 LA P2 D3 /NLA P2 D3 LA P3 D1 /NLA P3 D1 LA P3 D2 /NLA P3 D2 LA P3 D3 /NLA P3 D3 OLA P1 D1 OLA P1 D3

175 175 175 150 150 150 125 125 125 175 175

142 122 112 142 122 112 142 122 112 142 112

190 400 — — 380 430 170 270 440 — —

200 260 370 210 280 320 210 290 440 — —

2. Experimental details Details of the film preparation are described elsewhere [4]. Simply, a large-area PLD system designed to deposit films up to 5 inch in diameter was used in conjunction with KrF excimer laser sources. The parameters varied in this experiment are listed in table 1. Three sets of films had been prepared and are referred to as LA Px Dx , NLA Px Dx , and OLA Px Dx , where Px and Dx are notations for the oxygen background pressure and target–substrate distance, respectively. Here, x = 1, 2, 3 are indices used to specify the values of P and D used. Films denoted by P1 , P2 , and P3 were deposited at 175, 150, and 125 mTorr, respectively; while films denoted by D1 , D2 , and D3 were deposited at target– substrate distances of 142, 122, and 112 mm, respectively. LA Px Dx films were deposited using 450 mJ/pulse (LambdaPhysik Compex 205, maximum output of 600 mJ/pulse in constant energy mode) and deposition temperature of 700 ◦ C (programmed via temperature controller; as measured by a pyrometer, the substrate temperature is approximately 100 ◦ C higher than the set value at the controller). NLA Px Dx films were deposited using 600 mJ/pulse (Lambda-Physik LPX 305i, maximum output of 900 mJ/pulse in constant energy mode) and deposition temperature of 750 ◦ C. The reason for the change in deposition temperature between these two sets of films will be discussed in section 4. The estimated fluence for the Compex 205 laser was ∼2.4 J cm−2 , while for the LPX 305i laser the fluence had a higher value of ∼7.4 J cm−2 because of the more focused spot size. Finally, OLA Px Dx films were deposited using the same parameters as NLA Px Dx films, except that an off-stoichiometric, bariumand copper-enriched YBCO target with a nominal composition of YBa2.3 Cu3.45 Oy was used. A repetition rate of 25 Hz was employed in the deposition of all films. In this study, 1×1 cm2 YBCO films were deposited on (100) LaAlO3 substrates. The films were characterized by SEM, transmission electron microscopy (TEM, JEM 2000EX), operated at an accelerating voltage of 200 keV, and x-ray diffractometry (XRD). The total composition of the films was measured by inductive coupled plasma atomic emission spectroscopy (ICP-AES). For further 1254

identification of the secondary phase in the films, selected area electron diffraction (SAED) and energy dispersive xray analysis (EDX) in conjunction with scanning transmission electron microscopy (STEM) was utilized.

3. Results 3.1. Effect of deposition parameters on pore formation 3.1.1. Oxygen background pressure (P). For convenience, we shall refer to the films as either belonging to the LA Px Dx or the NLA Px Dx set, where x = 1, 2, 3 (refer to table 1). Figure 1 shows the representative SEM images of the LA Px Dx films. For clarity, the SEM images are arranged such that films deposited at the same target–substrate distance (D) are in the same row, and those deposited at the same P are in the same column. In general, the LA Px Dx films exhibit large, partly disconnected islands on the surface subtended by pores. The pores in this set are irregularly shaped, with sizes ranging from submicron to micron, and occupy a substantial area on the surface of the film. We also see a marked reduction in the sizes of the pores when P is decreased, as can be observed by comparing LA P1 D1 (figure 1(a)) and LA P1 D3 (figure 1(c)). In contrast, the NLA Px Dx films are more connected and possess smoother morphology, as can be observed in the SEM images of figure 2. The images in figure 2 are arranged in the same pattern as in figure 1. The pores in this set are rounder in shape and much smaller in comparison to LA Px Dx films. Moreover, we also observe quite a number of outgrowths on the surface of the NLA Px Dx films. The presence of these outgrowths may be attributed to the higher laser fluence which can dislodge larger globules from the target and contribute to higher droplet density [5, 6]. The outgrowths appear to occur more at reduced P, such as displayed in figure 2(b). Furthermore, as can be observed by comparing figures 2(c) and (d), we also see a reduction in pore size and density when P is decreased. We define pore fraction as the percentage of the surface area taken up by the pores (only the pores observed on the surface of the film) with respect to the total area of the film

Pore formation in YBCO films deposited by a large-area PLD system

Figure 1. Scanning electron micrographs of (a) LAP1 D1 , (b) LA P3 D1 , (c) LA P1 D3 , and (d) LAP3 D3 . (P1 = 175 mTorr, P3 = 125 mTorr, D1 = 142 mm, D3 = 112 mm.)

Figure 2. Scanning electron micrographs of (a) NLA P1 D1 , (b) NLAP3 D1 , (c) NLA P1 D3 , and (d) NLA P3 D3 . (P1 = 175 mTorr, P3 = 125 mTorr, D1 = 142 mm, D3 = 112 mm.)

as imaged on an SEM micrograph. The pore fraction of the films was determined via image analysis software (Ultimage 2.6J) and plotted with respect to oxygen background pressure (P) in figure 3(a). Generally, the LA Px Dx films exhibit much higher pore fractions than NLA Px Dx films. Furthermore, the former demonstrate a dramatic decrease in pore fraction with decreasing P. A similar relation can be observed in the latter, albeit at a much lesser degree since pore density in this set is markedly lower by comparison. We determined the ratios of Ba/Y, Cu/Y, and Cu/Ba from ICP-AES analysis. Deviations from the ideal values such as expected from a perfectly stoichiometric YBCO film will indicate the relative excess or deficiency in any of the atomic

concentrations. In LA Px Dx films, the relative concentrations of barium and copper increased with the decrease in P, as illustrated in figure 3(b) for a target–substrate distance of D1 . In comparison to LA Px D1 films, the relative concentrations of barium and copper are higher in the composition of the NLA Px D1 films, although the dependence on P is not very significant as depicted in figure 3(b). Still, a slight increase in barium and copper concentration in the NLA Px Dx set is evident at other target–substrate distances D2 and D3 (not shown). 3.1.2. Target–substrate distance. Interestingly, pores form anew when D is decreased, as can be readily seen in the SEM 1255


Oxygen background pressure (mTorr) 180 12

160

140

Target-substrate distance (mm) 120

Pore fraction (%)

LAPxD1 NLAPxD1 LAPxD2 NLAPxD2 LAPxD3 NLAPxD3

8

4

150

(a)

140

130

120

110

10

Pore fraction (%)

(a)

1 LAP1Dx NLAP1Dx LAP2Dx NLAP2Dx LAP3Dx NLAP3Dx

0.1 Ba/Y-LAP1Dx Ba/Y-NLAP1Dx Cu/Y-LAP1Dx Cu/Y-NLAP1Dx Cu/Ba-LAP1Dx

0

(b) 3

Cu/Y Ba/Y-LAPxD1 Ba/Y-NLAPxD1 Cu/Y-LAPxD1

Cu/Y-NLAPxD1 Cu/Ba-LAPxD1 Cu/Ba-NLAPxD1

(b)

3

Cu/Ba-NLAP1Dx Ba/Y-OLAP1Dx Cu/Y-OLAP1Dx Cu/Ba-OLAP1Dx

Cu/Y

2.5

2

1.5

Ba / Y

Ratios

Ratios

2.5

2

Ba/Y

Cu/Ba

1.5

Cu/Ba

D1 = 142 mm

1 180

160

140

120

Oxygen background pressure (mTorr) Figure 3. (a) Plot of pore fraction with respect to oxygen background pressure for LA Px Dx and NLA Px Dx films. (b) Corresponding elemental ratios Ba/Y, Cu/Y and Cu/Ba as a function of oxygen background pressure for D1 = 142 mm; dashed lines refer to the ideal values derived from a 1–2–3 stoichiometry.

images of figures 1(c) and (d), as well as in figures 2(c) and (d). For clarity, the plot of pore fraction with respect to D in a logarithmic scale is shown in figure 4(a), and here we observe that decreasing D enhances porosity in the films. We note that decreasing D inevitably allows more material to reach the substrate at a given time, and the grown films will become thicker. On the other hand, we see in figure 4(b) that the barium and copper concentrations with respect to yttrium also increase when D is decreased. This is normally what we would expect, based on the effect of oxygen as discussed in section 3.1.1. It follows that barium and copper would be more concentrated at shorter distances, because these would be less scattered at distances closer to the target. However, unlike the observation in section 3.1.1 where the increase in barium and copper concentrations is accompanied by a reduction in pore fraction, here we find that the most porous films deposited at the shortest D are the ones that seem to contain more barium and copper. This apparent contradiction to the results in section 3.1.1 is an indication that another mechanism for pore formation must also be considered. This will be discussed further in section 4. 1256

1 150

P1 = 175 mTorr

140

130

120

110

Target-substrate distance (mm) Figure 4. (a) Plot of pore fraction with respect to target–substrate distance for LA Px Dx and NLA Px Dx films. (b) Corresponding elemental ratios Ba/Y, Cu/Y and Cu/Ba as a function of target–substrate distance for P1 = 175 mm; dashed lines refer to the ideal values derived from a 1–2–3 stoichiometry.

3.1.3. Effect of target stoichiometry. Within the range of parameters employed in the deposition of the YBCO films, use of the stoichiometric target has always yielded films which are yttrium rich, i.e., the total Ba/Y and Cu/Y ratios still fall below the ideal stoichiometric ratios. Previous results have suggested that increasing the barium and copper at the source, i.e., using an off-stoichiometric target, would be able to significantly offset the effect of scattering and yield a film with composition closer to the ideal stoichiometry. As evidenced by the SEM images in figure 5, the surfaces of the films are devoid of any pores, although outgrowths are particularly numerous in these films compared to those which were deposited from a stoichiometric target. In particular, we see a significant increase in the size and number of these outgrowths at D3 = 112 mm, and the morphology is very similar to the YBCO films which are deposited using a standard PLD system (D ∼ 60 mm). Using the off-stoichiometric YBCO target further improves the microstructure and superconducting properties of the YBCO films; details of this study are reported in


Figure 5. Scanning electron micrographs of films deposited using an off-stoichiometric YBCO target: (a) OLAP1 D1 ; (b) OLA P1 D3 . No pores were detected in these films.

and OLA P1 D3 films and included in the plot of figure 4(b). Notice that the Ba/Y ratio is now well above the ideal value of 2, while that of Cu/Y still falls below the ideal value but is already significantly higher than any of the films grown from the stoichiometric target. 3.2. Analysis of pore microstructure by TEM

Figure 6. (a) TEM micrograph of a planar YBCO film at low magnification. This film was deposited at P1 = 175 mTorr and D3 = 112 mm. Precipitates are observed as black patches on the surface; the pore areas are denoted by ‘1’ and ‘2’. (b) TEM micrograph of the same YBCO film at high magnification. Small inclusions of ∼10–50 nm2 in size are densely distributed throughout the surface. Two different precipitates distinguished by their moiré patterns are labelled ‘A’ and ‘B’.

another paper [7]. From ICP analysis, the elemental ratios of Ba/Y, Cu/Y, and Cu/Ba were obtained for the OLA P1 D1

From XRD measurements, we have verified that the YBCO films are all c-axis oriented, and no a-axis-related peaks were observed. Figure 6(a) shows a micrograph of a planar TEM sample of a YBCO thin film at low magnification. This film was deposited at P1 = 175 mTorr and D3 = 112 mm. Many black patches with different sizes can be seen in the micrograph; these are small precipitates on the surface of the film. Furthermore, two irregularly shaped white patches of submicron size are identified as pores in the film. The typical twinned structure of orthorhombic YBCO with twin boundaries along the {110} planes is prominent throughout the surface. However, pores block and change the width and orientation of the twin structures as illustrated in the inset for the case of the pore labelled ‘1’. The planar view image at higher magnification in figure 6(b) shows a large number of densely distributed precipitates with sizes between 10 and 50 nm in diameter. These precipitates are embedded in the YBCO matrix and do not appear to disrupt the continuity of the film. We can also distinguish two different kinds of precipitate based on the different moiré fringe spacings; for comparison we label these as A and B in the micrograph. However, further characterization is necessary at this stage to unambiguously identify the phases corresponding to these precipitates. The pore microstructures are illustrated in the crosssectional TEM micrographs of the same YBCO film in figure 7. Five of these pores clearly originate from easily distinguishable secondary phase occlusions which grew within the YBCO film. The occlusions have sizes of approximately 0.1–0.2 µm in diameter, although the resulting pore sizes depend on the relative position of the occlusion from the YBCO surface. In the case of pore ‘2’, for instance, the longitudinal shape of the occlusion and its closer position to the surface resulted in a larger pore than the others. In the case of pore ‘5’, the occlusion has nucleated and grown relatively far from the surface; the pore structure was initially large at the bottom, but slowly decreased in width towards the surface as the surrounding YBCO attempted to grow over it. The occlusions appear to have nucleated sometime during deposition, away from the YBCO–LAO interface. Examination of other TEM 1257


Figure 7. Cross-sectional TEM micrograph of a YBCO film showing the pore microstructures. The pores emanated from occlusions within the film.

(c) (a)

(d) (b)

Figure 8. (a) EDX spectrum from the secondary phase occlusion. (b) Cross-sectional STEM micrograph showing a pore microstructure; circles denote the analysed areas belonging to the YBCO matrix (labelled ‘1’) and the secondary phase occlusion (labelled ‘2’). (c) and (d) Corresponding SAED patterns of the analysed areas denoted in the STEM micrograph. Figure 8(c) is indexed along the [010] zone axis of YBCO, while figure 8(d) is indexed along the [001] zone axis of BaY2 O4 .

micrographs of YBCO films with much larger, micron-sized pores revealed even more extensive occlusions that permeated the thickness of the film. For a definite identification of the secondary phase, spot analyses of the occlusions were performed by STEM/EDX. A typical EDX spectrum is shown in figure 8(a), showing signals from only yttrium (Y), barium (Ba), and oxygen (O). Figure 8(b) shows the STEM image of a cross-section of the YBCO film including the pore microstructure. The analysed areas are indicated by labels ‘1’ and ‘2’ in the STEM. The corresponding SAED patterns of the YBCO film and the secondary phase are displayed in figures 8(c) and (d), respectively. The SAED pattern in figure 8(c) is indexed along the [010] axis of YBCO. From comparison of the dspacings calculated from the electron diffractogram and the 1258

lattice constants of the possible compounds, it was verified that the best match is BaY2 O4 (barium yttrium oxide). BaY2 O4 is orthorhombic in structure, with lattice constants a = 10.39 Å, b = 12.11 Å, and c = 3.45 Å [8]. The corresponding SAED pattern is indexed along the [001] zone axis of BaY2 O4 as depicted in figure 8(d).

4. Discussion 4.1. Correlation between porosity and composition In general, NLA Px Dx films are less porous and contain more barium and copper in the composition than LA Px Dx films. Both the laser fluence and the deposition temperature were varied between these sets of films. While this paper does not


seek to elucidate directly the relationship between deposition temperature and porosity, based on a few samples deposited, porosity appears to be enhanced at higher temperatures [9]. We believe that the formation of BaY2 O4 must be enhanced with increasing deposition temperature, hence the increased porosity. However, since we wanted to demonstrate that a very smooth film can be achieved within the range of the varied parameters (refer to figure 1(b)), we chose a lower deposition temperature for La Px Dx samples. Such a drastic change in morphology (i.e., from very porous to very smooth) with variation in the oxygen background pressure is more obvious in this set of films than any other. A film deposited at the same deposition conditions as La P3 D1 but using a higher deposition temperature of 750 ◦ C is not entirely smooth, but still exhibits small pores on the surface. In contrast, within the range of parameters investigated, NLa Px Dx samples are intrinsically smoother and contain much smaller pores—properties which we attribute to the increased laser fluence. We purposely increased the deposition temperature for these samples in an attempt to enhance porosity. As it turned out, even if a higher deposition temperature was used, the NLa Px Dx samples remained smoother than La Px Dx films—further lending support to our assumption that laser fluence must have exerted a greater influence on the porosity and composition. Hence, it is supposed that the use of a higher laser fluence value in NLA Px Dx films must have increased the relative amount of barium and copper in the films. A very interesting result was observed by Venkatesan et al [10] in a standard PLD system (D = 3 cm), where they concluded that there are two distinct components in the angular distribution of composition of YBCO films: one a cos θ component, a result of evaporation, the other a highly forward directed component, a result of the secondary ejection process. The evaporated component is nonstoichiometric, whereas the forward-directed component has a composition close to that of the pellet. This forward-directed component increases with the laser fluence in comparison with the evaporated component [10]. If such a similar behaviour is obeyed in our large-area PLD system, in a similar manner, it is possible that the increase in the stoichiometric component due to higher fluence has led to higher concentrations of barium and copper in the NLA Px Dx films. In general, the behaviour of these films with respect to the variation in oxygen background pressure is a manifestation of the scattering effect caused by interaction with oxygen. Foote et al [11] have observed a preferred scattering of copper and barium in oxygen ambient, presumably because copper is the lightest element, and barium species possess large cross-sections for oxygen scattering. Furthermore, plasma species containing yttrium may be typically heavier than those containing barium, thereby further contributing to the excess of yttrium arriving at the substrate [11]. They have also observed that barium and copper deficiency is enhanced with increasing target–substrate distances. In the large-area PLD set-up, typically long target–substrate distances are necessary, thus resulting in deficiency of barium and copper in the films. For a fixed target–substrate distance, reducing the oxygen background pressure results in less scattering of barium and copper, thereby increasing the relative barium and copper concentrations in the films. Correlating these results to the pore

fraction values in figure 3(a), it is evident that yttrium richness, or deficiency in barium and copper, in the total composition of the films is characteristic of films which have a high degree of porosity. However, at this point the actual mechanism for oxygen scattering is not entirely clear for the case of large-area PLD. Unlike in the study of Foote et al [11], we raster the laser beam across the surface of the rotating target (raster length: 50 mm). Furthermore, the substrate is likewise rotated during deposition, so that any angular broadening in the distribution of elements due merely to scattering cannot account for the observed barium and copper deficiency in the total film composition. A more reasonable explanation would be that significant amounts of barium and copper were being scattered to the extent that they do not reach the substrate at all, such that the components arriving at the substrate are entirely nonstoichiometric. Another possibility would be to consider the existence of both stoichiometric and non-stoichiometric components in the film composition, similar to the results reported by Venkatesan et al [10], and that the proportion of these components with respect to each other may be dependent on parameters such as P and D. While this is an interesting issue, this is beyond the scope of the present paper and will be tackled in a more detailed, separate investigation. 4.2. Effect of deposition parameters on BaY2 O4 formation Our results have shown that off-stoichiometry induced by deposition conditions inherent in the large-area PLD system resulted in the formation of secondary phases in the YBCO films. In particular, the excess yttrium resulted in the formation of the barium yttrium oxide (BYO) phase, which consequently disrupts the growth and continuity of YBCO, resulting in the observed pores on the surface. Since barium and copper are easily scattered in oxygen ambient, lowering the background pressure results in higher barium and copper concentrations with respect to yttrium, and consequently the formation of the secondary phases will be minimized. To put it succinctly, the lower the excess yttrium, the less BYO will be formed, and the resulting YBCO film will develop fewer pores. In contrast, decreasing the target–substrate distance D creates more pores on the surface. To explain this behaviour, let us consider three aspects: firstly, the film deposition rate is higher at decreased D (the deposition rate at D3 is almost twice that at D1 ). Based on the scattering mechanism, the relative amounts of barium and copper are also expected to increase at decreased D. However, even at the shortest D that can be set in our PLD system all films have been found to be still yttrium rich—an inherent characteristic of films produced in our PLD system. The observed increase in porosity in YBCO films at shorter D is therefore attributed to the increase in film deposition rate. To verify this, we prepared two YBCO films using different deposition rates but adjusted the deposition time to obtain approximately the same final film thickness for both. Comparison of the morphologies of these films clearly showed higher porosity for the film deposited at higher deposition rate. The increase in film deposition rate must have enhanced the growth of the BYO phase, resulting in higher film porosity. Secondly, large-sized BYO occlusions will have a tendency to grow faster than smaller ones due to the greater 1259


influx of components coming from an increased interface area with the matrix film. Such large-sized occlusions can cause wider and deeper pores to develop in YBCO films. Thirdly, as seen in the cross-sectional TEM micrographs, both the size of the occlusion and its location determine the resultant pore size. Therefore, another factor that may contribute is the location of the BYO phase within the film, and apparently the probability of BYO nucleation increases with film thickness. For instance, a film with 2d thickness may have twice as many BYO nucleation sites compared with a film of only d thickness. To verify this, for a given D, we prepared films with different thicknesses by varying the deposition time. As expected, the film porosity increased with respect to film thickness. In very thick films (>0.5 µm), grains which appear to be BYO phases have grown large enough to become visible within the pore area, as observed in the SEM micrographs. In summary, the cumulative effect of these three aspects is the observed increase in porosity in thicker films deposited at shorter D. The presence of yttrium-rich secondary phases in YBCO is not unexpected. Under well equilibrated and thermally relaxed conditions, a highly crystalline film with almost exact 1–2–3 composition is formed with extra components segregated as secondary phases [12]. Y2 O3 or Y2 BaCuO5 phases can stably co-exist with YBCO and these compounds are frequently observed as impurity phases in YBCO films with yttriumrich composition [13, 14]. Moreover, these phases are usually found on the surface of the film as precipitates. The presence of the barium yttrium oxide phase, on the other hand, was totally unanticipated, particularly because this type of phase is not common in YBCO thin films. To our knowledge, this is the first report on the presence of this phase coexisting in YBCO thin films. However, there is no tie line between BaY2 O4 and the YBCO 1–2–3 phase in the YO1.5 – BaO–CuO pseudoternary phase diagram at 850 ◦ C and 1 atm oxygen pressure (presumably valid to up to ∼300 mTorr oxygen pressure) [15]. Our findings suggest that for the lower pressures and temperatures used in our experiments a tie line may possibly exist between BaY2 O4 and YBCO 1– 2–3. Critical current densities exceeding 1 MA cm−2 at zero field and 77.3 K were obtained even in porous YBCO films on LAO. We believe that, in spite of the presence of pores, as-yet unidentified small and densely distributed precipitates on the surface, as revealed by TEM observations, may act as pinning centres to allow the attainment of a relatively high Jc in the films. However, succeeding studies indicated that the presence of such pores does influence the value and distribution of Jc in large-area YBCO films. As we have demonstrated in another paper [7], by actually reducing the porosity in the films, improvements in Jc value and homogeneity (exceeding 2 MA cm−2 ) could be obtained. Moreover, it is highly plausible that the existence of the pores and the barium yttrium oxide phase generates extended defects throughout the film, contributing to the relieving of the accumulated stress in the film due to thermal shrinkage. This will contribute in part to the attainment of microcrack-free, micron-thick YBCO films, as has been demonstrated on sapphire substrates [4].

1260

5. Summary and conclusions We have investigated the interplay of oxygen background pressure, target–substrate distance, film composition, and target stoichiometry on the porosity of YBCO films deposited using a large-area PLD system. Yttrium-richness in the films is attributed to the scattering effect of the oxygen ambient on the ablated species. We have identified the origin of the pores to be an yttrium-rich secondary phase, namely, barium yttrium oxide (BaY2 O4 ). Other precipitates, suspected to be of yttrium and copper origin, are also present but are embedded in the YBCO matrix and do not play a role in the pore formation. Barium yttrium oxide resides within the YBCO film as an occlusion and blocks the growth of YBCO, resulting in pores on the surface. The formation of secondary phases is inevitable due to the inherent set-up of the large-area PLD system; however, an approximation of the ideal conditions such as in standard PLD systems can be achieved by the use of a bariumand copper-rich off-stoichiometric YBCO target. Considering the deposition conditions which either promote or hinder the formation of the barium yttrium oxide phase during YBCO growth, a reasonably good control of pore size and density has been achieved.

Acknowledgments This work has been carried out as a part of the Super-ACE project (R&D of fundamental technologies for superconducting AC power equipment) of the Ministry of Economy, Trade and Industry (METI).

References [1] Gromoll B et al 1999 IEEE Trans. Appl. Supercond. 9 656 [2] Proyer S, Stangl E, Berz M, Hellebrand B and Bäuerle D 1996 Physica C 257 1 [3] Strikovsky M D, Klyvenkov E B and Gaponov S V 1993 Appl. Phys. Lett. 63 1146 [4] Develos-Bagarinao K D, Yamasaki H, Nakagawa Y and Yamada H 2003 Physica C 392–396 1229 [5] Misra D B and Palmer SB 1991 Physica C 176 43 [6] Chen LC 1994 Particulates Generated by Pulsed Laser Ablation in: Pulsed Laser Deposition of Thin Films vol 167 (New York: Wiley) chapter 6 [7] Develos-Bagarinao K D, Yamasaki H, Nakagawa Y and Endo K 2004 Physica C at press [8] 2002 JCPDS-International Centre for Diffraction Data PDPDFWIN v. 2.3 [9] Develos K, Yamasaki H, Nakagawa Y, Obara H and Yamada H 2002 unpublished [10] Venkatesan T, Wu X D, Inam A and Wachtman J B 1988 Appl. Phys. Lett. 52 1193 [11] Foote M C, Jones B B, Hunt B D, Barner J B, Vasques R P and Bajuk L J 1992 Physica C 201 176 [12] Gong J P, Kawasaki M, Fujito K, Tsuchiya R, Yoshimoto M and Koinuma H 1994 Phys. Rev. B 50 3280 [13] Scotti di Uccio U, Miletto Granozio F, Di Chiara A, Tafuri F, Lebedev O I, Verbist K and Van Tendeloo G 1999 Physica C 321 162 [14] Selinder T I, Helmersson U, Han Z, Sundgren J E, Sjöström H and Wallenberg L R 1992 Physica C 202 69 [15] Ahn B T, Lee V Y, Beyers R, Gür T M and Huggins R A 1990 Physica C 529