Microstructure, flux pinning and critical current density in YBa2Cu3O7−δ films grown by laser ablation

Microstructure, flux pinning and critical current density in YBa2Cu3O7−δ films grown by laser ablation

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Microstructure, flux pinning and critical current density in YBa2Cu3 07 _ 6 films grown by laser ablation D. S. M i s r a a n d B. D. P a d a l i a Physzcs Department, HT, Powat, Bombay 400 076 (Indta)

S. P. Pai a n d R. P i n t o Tata Institute of Fundamental Research, Colaba, Bombay 400 005 (Indta)

S. B. Palmer Physics Department, Unwerstty of Warwtck, Coventry CV4 7AL (UK)

(Recewed July 20, 1993, accepted October 27, 1993)

Abstract Thin films of YBa2Cu307_,~ (YBCO) are deposited by laser ablation at substrate temperatures (Ts) ranging from 575 to 850 °C. The microstructure and growth behaviour of the films are found to be a sensitive function of T,. We also find that the critical current density (arc) of the films is strongly related to the microstructure. It is proposed that the boundaries between a-axis and c-axis grains may act as pinning centres.

1. Introduction Amongst all the techniques currently employed for growing in situ thin films of YBa2CuaO7_ 6 (YBCO), laser ablation enjoys a prominent status. Thin films of YBCO grown in situ by laser ablation show critical current density (Jc at 77 K and in zero magnetic field) of >t 106 A cm -2 on a routine basis [1]. The cause of the high value of Jc is, however, poorly understood. It has often been assumed [2] that a large number of pinning centres have to be present in the thin films to accommodate high values of J¢. Indeed, the microstructure of films prepared by laser ablation as observed by the transmission and scanning electron microscopic (TEM and SEM) techniques shows a variety of inhomogeneities [3-10] which can act as pinning centres. The fact that high J¢ values are observed routinely [2] in laser ablated films may be related to the presence of a larger number of pinning centres (or inhomogeneities) in the films. Various inhomogeneities in the films as observed by TEM, SEM and energy dispersive X-ray analysis (EDAX) are the fine precipitates of CuO [3], Y2BaCuO 5 [4], Y20 5 [5], YBa3CH2OT_x [6], point defects and dislocations [7], stacking faults [8], twin boundaries [9] and a-axis intergrowths [10]. Which of the above is responsible for the pinning in YBCO films is not yet clear. The study of rnicrostructure and growth behaviour of YBCO films should, therefore, lead to a better understanding of the pinning. We describe here

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in detail the results of a study of the growth behaviour and microstructure of YBCO films grown at various substrate temperatures and correlate these with transport Jc measurements. Magnetisation in field cooled (FC) and zero field cooled (ZFC) states for some samples is also reported.

2. Experimental details Thin films of YBCO (thickness ,~0.5 Inn) were deposited by laser ablation. The deposition rate of the films was ~ 1.5 A, s -1. An XeC1 excimer laser (2 = 308 nm)was used for the ablation work. A few samples were also grown using a KrF laser beam (2 = 248 nm). A sintered pellet of YBa2Cu307_ 6 was used as the target. The laser beam was focused onto the target so as to generate a laser fluence of ~2 J cm -2. The target was rotated during ablation to maintain uniformity of the films. Single crystals of SrTiO3(100) were used as substrates and Were held onto a heated block. For a few films, MgO single crystals with (100) orientation were also used as substrates. The substrate temperature (Ts) was measured by connecting a tiny thermocouple to the top surface of the substrates. An optical pyrometer was also used to measure Ts occasionally. The partial pressure of the oxygen during the growth was approx. 200 mTorr. The growth chamber was filled with oxygen to approx. 500 Torr after the deposition and the samples were subsequently cooled at a rate approx. 20 °C min -1. Resistivity and transport

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D. S. Mtsra et al. / Mtcrostructure and c r m c a l current denstty o f YBa 2 Cu 3 0 7 _ ~ f i l m s

J¢ measurements were performed using the d.c. four probe method. FC and ZFC magnetisation measurements were carried out in a SQUID magnetometer. The microstructure of the YBCO films was studied using a JEOL scanning electron microscope.

3. Results and discussion

Figure 1 shows micrographs of the thin films of YBCO grown at varying T~. T~ was varied between 575 and 850 °C. The films whose mlcrostructures are shown in Figs. l(a) and l(b) were deposited onto SrTiO3(100) substrates at 575 and 625 °C respectively. In contrast, Figs. l(c) and l(d) correspond to films grown onto MgO(100) substrates. The micrograph of a thin YBCO film grown by sputtering onto SrTiO3(100) substrate and post annealed at 920 °C in partial pressure of oxygen is also shown (Fig. l(f)) for the purpose of comparison Several interesting features in the microstructures of the in situ grown YBCO films (Figs. l(a)-l(e)) can be








Fig 1 Micrographs o f the Y B C O film grown at (a) T, = 575 °C, (b) T s = 625 °C, (c) Ts = 675 °C, (d) T s = 725 °C, (e) T, = 850 °C, and (f) grown at 27 °C and annealed at 920 ':C Arrowheads m (a) and (b) m a r k rectangular grains. Square-shaped m e s h structure m marked by an arrow in (c). a-Axis grains are marked with letter a in (c) and (d) Background grams are marked with letter b m (c) and (d). For explanation, refer to the text


noted. The micrograph of the film grown at 575 °C (Fig. l(a)) consists of fine grains (approx. 100 nm size) embedded in large grains. Spherical particulates typical of laser ablation [11] are also seen. Figure l(b) shows a micrograph of YBCO film grown at 625 °C. The microstructure of the films grown at 625 °C is similar to that of the 575 °C film, but the presence of rectangular grains is more marked in Fig. l(b) (indicated with arrow). Further, the intensities of XRD peaks arising due to (OOL), (100) and (200) reflections are significantly enhanced in the films grown at 625 °C (Fig. 2(b)). This implies that the crystallisation of YBCO grains begins at Ts < 625 °C. As Ts is increased further (Ts = 675 °C for the film in Fig. l(c)), the crystallisation of YBCO grains is nearly complete. The randomness in the shape of the grains is absent in Fig. l(c), and a-axis and c-axis oriented grains coexist (the a-axis and c-axis grains having the c-axis of the unit cell of YBCO is parallel and perpendicular to the plane of the substrate respectively). This is evident from XRD results (Fig. 2(c)) where peaks due to reflections from the (200) plane along with (OOL) planes can be seen. A salient feature of the mlcrostructure of the films grown at 675 °C (Fig. l(c)) is the appearance of the squareshaped mesh structure. Each mesh (marked with an arrow in Fig. l(c)) having the shape of a square has four grains (marked with letter a) which are connected as sides. These grains are different from the grams appearing in the background (marked with letter b). We believe that the grains connected at the sides of the mesh and appearing in the background are a-axis and c-axis oriented respectively. As the growth rate [12] of a-axis grains is higher than that of c-axis grains, the a-axis grains appear as surface outgrowth. Outgrowths of a similar nature have been observed by other authors on the surface of YBCO films [12, 13]. Another noteworthy feature of the microstructure of the films grown at 675 °C is that although the surfaces of the films contain large numbers of a-axis oriented grains, the ratio of the intensities of XRD peaks corresponding to (200) and (006) reflections is much smaller. This indicates that the bulk of the film has c-axis oriented grains. It can be seen from the microstructure of the film deposited at T ~ - 7 2 5 ° C (Fig. l(d)) that the square-shaped mesh structure has vanished completely leaving behind predominantly c-axis oriented film. A few a-axis oriented grains (marked with letter a) do however remain embedded in the c-axis oriented film. Figure l(e) shows the microstructure of a film grown at Ts ~- 850 °C. As is evident, melting of the YBCO grains has taken place. The microstructure of the postannealed film (Fig. l(e)) is similar to that of polycrystalline sintered bulk samples. The results of X-ray diffraction (XRD) measurement on the films (Figs. l ( a ) - l ( d ) ) are shown in Fig. 2.


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Fig. 2. X-ray diffractmn (XRD) plots o f YBCO films grown at (a) 7", = 575 °C, (b) 7", = 625 °C, (c) T, = 675 °C and (d) T, = 725 °C. The verUcal scales o f (a) and (b) have been kept the same for comparison. X R D peaks arising due to substrates are also shown.

TABLE 1 Superconducting properties o f the YBCO films Sample no

Substrate temp., T~ (°C)

T~(zero) (K)

J~ (at 77 K. B = 0)

l(a) l(b) l(c) l(d) l(e) l(f)

575 625 675 725 820 ~920 ~

87 89 90 90 82 85

2.5 x 103 A crn -2 6 × I0 a A crn -~ 104 A cm -2 106 A c m -e 109 A c m - : 102 A cm -2

Intense X R D peaks arising due to the reflections from [OOL], (100) and (200) planes of YBCO, emphasising the oriented nature of the films, can be clearly seen. As is evident from Fig. 2, the intensity of X R D peaks arising due to [OOL] reflections is stronger for the films grown at 625 °C as compared with 575 °C (Figs. 2(a) and 2(b)). This implies that the films grown at 625 °C contain a larger number of oriented crystallites than the 575 °C films. The absence of X R D peaks corresponding to (100) and (200) reflections in Fig. 2(d) implies that the films grown at 725 °C are highly c-axis oriented. Superconducting properties (T~ and J~) of the thin films of YBCO whose m_icrostructures are shown in Fig. t are listed in Table 1. The films prepared at 575 °C show zero resistance temperature (To(zero)) ~ 87 K and Jr -~ 2.5 x 103 A cm -2. A source of oxygen ions (d.c. plasma) was used only during the growth of YBCO films at T~ = 575 °C and was found to be essential to the inducement of a high T~(zero) in the films. Details of the oxygen ion source and its influence on the








SO ~00 Temperoture (K)

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Fig. 3 MagncUsatlon of the film grown at T, = 575 °C, in FC and ZFC states, as a function o f temperature.

superconducting properties (To and J¢) of the YBCO films grown at T, = 575 °C are describod elsewhere [ 14]. As shown in Fig. l(a), at T, = 575 °C the erystatlisation of YBCO grains is starting and the film contains a few small rectangular grains embedded in bi88er grains. Results of magnetisation m e a s u r e r s of the films grown at T~ ~ 575 °C in a magnetic field of intensity H = 20 Oe using a SQUID magnetometer are shown in Fig. 3. The sample follows different magnetisation curves for field cooled (FC) and zero fic~d cootvd (ZFC) states. In the field cooled (FC) state the _~ampt¢ is cooled m the prcsenoe of magnetic field (H = 20 Oe) and the magnetisation is measured as a function of t c m p ~ t a r e ( M vs. T). To measure the magnct'~tion in ZFC state, the sample is initially cooled to the lowest temperatm-e and the magnetisation is measured as the sample is heated through T~(zero). Similar results for M vs. T measurements in FC and ZFC states have been obtained in polycrystalline [2] and single crystal [15] YBCO and have been interpreted in terms of a supexconducfing glass model. Results of M vs. T measurements suggest that weak links (of similar nature to those in potycrystatline YBCO) are pre-scnt at the grain b o u n d a g ~ / o f films grown at 575 °C. This is supported by the low J~ values ( "-'2 x 103 A c m -2 at 77 K) in the films. Although X R D results show the oriented nature of the films (Fig. 2(a)), it is quite likely that the grains are just beginning to align at 575 °C and the weak links are not completely eliminated. Films deposited at Ts = 625°C show similar microstructure, the only difference being that the large grains are reduced in number and the grains are increasingly of rectangular shape.

D S Mtsra et a l / Mwrostructure and crztwal current density of YBazCu307_a films

In the films grown at Ts = 675 °C, individual large grams of YBCO are not present (Fig. l(c)). Small a-axis oriented grains are, however, clearly seen and their density in the film is ---5 × 1013 m -2. It is noted from Fig. l(c) that the a-axis grains, when present in large numbers, tend to join up with each other and form a square-shaped mesh structure as discussed earlier. It is evident from Fig. l(c) that the plane of each mesh is misoriented by 45 ° with respect to the substrate plane. This misorientation could be responsible for the lower value ofJc ( ~ l04 A cm -2 at 77 K) observed in the films grown at 675 °C (Table 1). In contrast, the density of a-axis grains is much reduced (,,~2 x 10 ~zm -2) in the films grown at 725 °C. The films grown at 725 °C are predominantly c-axis oriented and can support a Jc of approx. 106 A cm -2 as listed in Table 1. Various mechanisms [I 6-18] have been suggested for the pinning of vortices in YBCO films. Spiral growth is observed in YBCO films [19, 20] and the screw dislocations associated with spiral growth have been proposed as a pinning centre. However, the density of dislocations as observed by other workers using STM is too low to explain the high value of Jc in the thin films Intragranular defects such as point defects [7] and stacking faults [8] have also been cited as pinning centres. We believe that the defect structure at the boundaries between the a-axis and c-axis oriented grains can act as a pinning centre. On the interface between a-axis and c-axis grains, the ordering of the oxygen atoms will be disturbed and may lead to the formation of the tetragonal phase of YBCO. The thickness of the tetragonal phase on the interface would be of the order of 10 A (approximately the length of the c-axis of the unit cell), which is nearly equal to the dmmeter of the vortices. The above contention is supported by the results of high resolution TEM which indicate that the width of the a-axis and c-axis grain boundaries is of the same order [21]. The superconducting order parameter will be suppressed in this region and would lead to the pinning of the vortices. Evidence of the presence of a thin nonorthorhombic layer (thickness ~ 10 A,) at the twin boundaries in the a - b plane of YBCO has been found by others [22]. However, if the density of the a-axis grains is high the morphology of the films will change as shown in Fig. l(c), resulting in deterioration of J¢. In contrast, low density of a-axis grains will result in too few grain boundaries to support high J~ in the YBCO films. An optimum density ( ~ 10~2m -2 as observed in Fig. l(d)) of a-axis grains may therefore be necessary to support high J~ in the films [10]. The energy (Up) of the vortices pinned on the boundaries between a-axis and c-axis grains can be estimated using the equation Fp =


where Fp is the total pinning force per unit length acting on all the pinned vortices, and B, in the absence of applied magnetic field, is the self field generated by the critical current. B has been estimated [23] to be 100 mT for the YBCO films with Jc ~ 106 A cm -2 at 77 K. The length and width of a-axis grains are approx. 300 nm and 75 nm respectively (Fig. l(d)). Using the fact that the minimum separation between the pinned vortices is approx. 10/~ (coherence length, 3, in a - b plane) the total number of pruned vortices (No) may be estimated. The force on an individual vortex is "fp"=Fp/No=Up/(~ ). This gives Up~10meV at 77 K for the vortices whose length is equal to the thickness of the film. The value of Up is calculated only to a first approximation, and a direct estimate using an independent measurement is much desired. However, we find it to be of the same order of magnitude as values obtained by others [2, 24]. Much higher values of Up have been obtained using transport measurement by Zhu et al. [25].

4. Conclusions In summary, we have made a careful investigation of the microstructure of thin YBCO films grown at various substrate temperatures. We find that the critical dilute Jc current density of the films is a sensitive function of the microstructure, and propose that the boundaries between a-axis and c-axis grains may act as pinning centres. The pinning energy of the vortices is estimated to be approx. 10 meV.

Acknowledgment One of us (DSM) is indebted to the University of Warwick for providing some of the samples which were used for the present study.

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D S Mlsra et a/ /Mwrostructure and cr __a./ current densttv of YBa: Cu3O- ,,/lhn~

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