The influence of 173 MeV Xe-ion irradiation on the microstructure of YBa2Cu3O7 thin films

Physica C 179 ( 1991 ) 75-84 North-Holland

The influence of 17 3 MeV Xe-ion irradiation on the microstructure of YBa2Cu307 thin films H. W a t a n a b e a,b, B. Kabius a, K. U r b a n a, B. Roas c.d, S. Klaumtinzer e and G. Saemann-Ischenko d a b c a c

Forschungszentrum Jiilich GmbH, Institutfur Festkorperforschung, Postfach 1913, W5170 Jfilich, Germany On leave from Kyushu University, Research Institute for Applied Mechanics, Kasuga, Fukuoka, Japan Siemens AG, Erlangen, Germany Universitiit Erlangen, Physikalisches Institut, Erlangen, Germany Hahn-Meitner-lnstitut, Berlin, Germany

Received 27 May 1991

YBa2Cu307 thin films grown epitaxially on [001 ] SrTiO3 substrates have been irradiated at a temperature of 77 K with 173 MeV Xe-ions up to a dose of 3 × 10 t6 m-.2. A study of the irradiated films by transmission electron microscopy revealed amorphous channels with diameters between 2 and 4 nm. The density of these channels depended on the irradiated dose. The twin structure of the orthorhombic phase of YBa2Cu307 was found to disappear for high irradiation doses. It is concluded that the amorphous regions were introduced along the paths of Xe-ions as a results of electronic excitation. These regions can act as pinning centres for magnetic flux lines.

1. Introduction

For many technical applications of high-temperature superconductors it is important that the critical current density (Jc) at which the superconductivity breaks down remains high even when a magnetic field is applied. Recently, it was found that Jc of YBa2Cu307 in the presence of a magnetic field is enhanced by irradiation with neutrons [ 1,2 ] and ions [ 3,4 ]. This enhancement was explained by pinning of the magnetic flux lines at radiation-induced crystal defects or zones of structural disorder. These defects can act as pinning centres if their size is of the same order of magnitude as the coherence length, which is estimated to be of the order of 0.4 nm along and 4 nm perpendicular to the crystallographic c-axis [5-71. For the study of defect-related properties, ion irradiation has been used extensively in many materials. In such experiments the size and the density of the irradiation-induced defects can easily be controlled by choosing a proper value of the particle energy. In most cases, however, information on the el'-

feet of irradiation was obtained by means of measurements of the electrical resistivity or magnetic properties. For an understanding of the effect of radiation-induced defects on the pinning of flux lines, it is advantageous to obtain direct information about the sample microstructure. Therefore, the aim of the present study was to investigate the mechanisms of defect formation in YBa22CuaO7 by 173 MeV Xe-ion irradiation by means of transmission electron microscopy and to characterize the defect structure responsible for the enhancement of Jc in the presence of a magnetic field.

2. Experimental Thin-film samples of YBa2Cu307 of about 0.3 ~tm in thickness were prepared on [001 ] SrTiO3 substrates using laser ablation [ 8 ]. During film deposition, the substrate temperature and oxygen pressure were kept at about 1075 K and 0.4 mhar, respectively. In this way high-quality epitaxially grown films were obtained with the c-axis parallel to

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H. Watanabe et al. / 173 M e V Xe-ion irradiation on YBa2Cu307 thin films

the substrate normal. A detailed study of their superconducting properties has been published elsewhere [ 8 ]. The resistivity measurements yielded zero resistance at 90 K. At 77 K the critical current density in zero magnetic field was 2.2 × l0 j° A / m 2. The samples were irradiated at 77 K with 173 MeV Xe-ions up to a dose of 3.0 × 1016 m -2. The direction of the ion beam was parallel to the c-axis. Using the T R I M computer code [ 12 ] the range of the ions was calculated to be about 17 ~tm, i.e. much longer than the film thickness. The concentration of Xe deposited in the YBaECU307 film is therefore negligible. After irradiation, samples for electron microscopy were prepared by ion milling in a liquid-nitrogen cooled stage using a 5 keY Ar-ion beam (angle of incidence 18 ° ). Diffraction-contrast experiments and high-resolution lattice imaging were carried out using a JEOL 4000 FX electron microscope. In order to reduce the temperature rise resulting from elec-

tron beam heating and to avoid possible additional radiation damage by electrons, the electron flux was kept low during observation.

3. Results 3.1. Unirradiated samples

Figure 1 shows a conventional bright-field image (a) and the corresponding lattice fringe picture (b) of an unirradiated YBa2Cu307 sample. The substrate layer was removed by ion milling. The viewing direction is parallel to the c-axis. The pictures show the twin structure characteristic of the orthorhombic phase. The width of the twin lamella is about 50 nm. The twin angle as measured from the electron diffraction pattern is 2 ° . These results are in accordance with data found in the literature [9,10].

Fig. 1. Diffraction-contrast image (a) and corresponding lattice fringe picture (b) of unirradiated YBa2Cu30~ taken along the c-direction. Microtwins can be seen along two mutually perpendicular directions.

H. Watanabe et al. / 173 MeV Xe-ion irradiation on YBa2Cu307 thin films

77

Fig. 2. Diffraction-contrast image in cross-section of YBa2Cu307 irradiated with 173 MeV Xe-ions at a dose o f 1.5 × 10 t6 m -2.

3.2. Irradiated samples A cross-sectional low-magnification view of an irradiated specimen, including the substrate, is presented in fig. 2. The irradiation dose was 1.5× 1016 m -2. The arrows indicate the direction of the Xeions. It is evident that the local microstructure depends on the depth coordinate x measured from the ion-entrance surface on the left-hand side. Four damage regions can be distinguished: (1) inside the superconductor ( x > 0 . 3 ~tm) faint lines of dark contrast parallel to the irradiation direction are observed; (2) in the substrate near-surface region (0.3 ~tm < x < 5 l~m), again lines dark contrast parallel to the irradiation direction occur; (3) in an intermediate region in the substrate (5 ~tm < x < 10 ~tm), no prominent effects due to irradiation can be observed; (4) for x > 10 lxm (later on called peak-damage region) a high density of small defects is found, which can be identified as dislocation loops. Figure 3, which is rotated by 90 ° with respect to

Fig. 3. Diffraction-contrast image o f the interface o f YBa2CU3OT, irradiated 173 MeV Xe-ions at a dose o f i .5 × l 0 t6 m -2.

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H. Watanabeet al. / 173 MeV Xe-ion irradiation on YBazCu30z thinfilms

fig. 2, shows the interface between the superconductor (top) and the substrate (bottom) at higher magnification. The ion tracks can be recognized as long irregular channels of dark contrast. Figure 4 shows the corresponding lattice fringe picture. In this case the tracks give rise to faint contrast variations which are again quite irregular. Figure 5 (a) shows a diffraction-contrast image of the superconductor layer (substrate removed by ion milling) taken along the c-axis direction. The sample was irradiated with a dose of 3.0× 10 IS m -2. The shape of the twin boundaries has become irregular. In addition we find a high density of dark dots which are due to lattice defects with a strain field around them. They represent the ion tracks seen along the irradiation direction. The corresponding high-magnification lattice fringe picture (b) indicates that these defects consist of amorphous regions (two ex-

amples indicated by arrows) with diameters between 3 and 7 nm. Figure 6 shows diffraction patterns before and after irradiation. The pattern of the unirradiated sample (a) shows the characteristic splitting of the diffraction spots typical of the twin structure of the orthorhombic phase. In the diffraction pattern of the sample irradiated at a dose of 2.0× 10 ~6 m -2 (b) the splitting of diffraction spots is significantly reduced. 3.3. Dose dependence o f the superconductor damage structure and properties No twin structure could be found in samples irradiated with a dose higher than 2 × 1016 m -2. Figure 7 shows the dose dependence of the microstructure in the a-b plane. The increase in size of the amorphous regions can be understood as a result of overlapping of individual ion channels at higher densities. This is confirmed by fig. 8 which depicts the dose dependence of the areal number density of amorphous regions as function of ion dose (full line). The dashed line gives the density calculated making the assumption that a single amorphous region is produced per ion. While at low doses the density measured is almost equal to the calculated one, it is, due to overlapping of the damage regions, considerably lower at higher doses. Figure 9 depicts the dose-dependence of the transition temperature (To) taken from ref. [ 3 ] and the volume fraction of amorphous material as determined from the lattice fringe pictures making the assumption that the ion tracks have a columnar structure whose diameter is independent of the depth in the sample. While for doses up to about 2.0× 1016 m -2 Tc decreases at an almost constant rate to about 70 K, it falls to 40 K between 2.0 × 1016 and 3.0 × 1016 m-2.

4. Discussion

4.1. The depth dependence o f the ion damage Fig. 4. Lattice fringe picture of the interface of YBa2Cu307irradiated 173 MeV Xe-ionsat a dose of 1.5 × 10t~ m-2. Arrows indicate ion tracks. Their contrast is relatively weak since, along the viewingdirection, regions of perfect lattice overlap with the amorphous regions.

4.1.1. General There are two mechanisms for interaction of ions with solid matter [ 11 ], nuclear collisions resulting in ion energy loss at a rate ("stopping power" ) ( d E /

H. Watanabe et al. / 173 M e V Xe-ion irradiation on YBa2Cu307 thin films

79

Fig. 5. Diffraction-contrast image (a) and lattice fringe picture (b) o f YBa2Cu307 with 173 MeV Xe-ions at a dose of 3.0X 10 Is m -2. The arrows indicate amorphous regions.

Fig. 6. Diffraction patterns taken along the c-direction of an unirradiated sample (a) and of sample (b) irradiated with a dose of 2.0× 1016 m-2. The arrow indicates diffraction-spot splitting due to the twin structure.

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H. Watanabe et al. / 173 M e V Xe-ion irradiation on YBaeCu$Oz thin films

Fig. 7. Lattice fringe pictures taken along the c-axis show the dose dependence of damage formation in the a - b plane. d X ) n , where E is the ion energy, and electronic interaction with stopping power ( d E / d x ) e. The results of theoretical calculations obtained from the TRIM computer program [ 12 ] are depicted in fig. l 0. Inside the superconductor ( x < 0.3 I~m) the electronic

stopping power is about two orders of magnitude higher than the nuclear stopping power. The depth dependence of the damage production by atom displacements in nuclear collisions (in units of displacements per atom, dpa) and the distribution of

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H. Watanabeet al. / 173 MeV Xe-ion irradiation on YBazCuj07 thin films

81

the form of dislocation loops [ 14-16 ]. As a consequence the character of the damage produced by energetic ions changes with penetration depth. At low depths and high ion energies electronic stopping is dominant favouring amorphization while at great depths nuclear stopping is dominant favouring pointdefect cluster formation.

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In this region ( x < 0.3 ~tm), the microstructure of the YBa2Cu307 film shows amorphous regions connected with a strain field around them. These regions can be attributed to the path of the Xe-ions. Figure 9 shows that for a dose of 1.5 × 10 ~6m -2 about 15% of the atoms are displaced, i.e. are located within amorphous regions. This is higher by a factor of about 400 than the calculated concentration of atoms displaced in nuclear collisions (fig. 11 ). From this we conclude that under the conditions employed in our experiments the injected ions dissipate their energy in the YBa2Cu307 film mainly by interaction with target electrons and that the amorphous zones are produced as a result of electronic excitation. 4.1.3. Substrate near-surface region

This region coincides with the region where the lines of dark contrast perpendicular to the irradiated surface can be observed (figs. 2-4). The T R I M code [ 12 ] does not allow the difference in damage pro,1.5

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82

H. Watanabe et al. / 173 MeV Xe-ion irradiation on YBa2Cu30z thin films

duction between YBa2Cu307 and SrTiO3 to be treated quantitatively. The calculated value of electronic and nuclear stopping power is essentially the same for both materials. The value of (dE/dx)e, about 1.5X 104 eV/nm, is almost constant up to a depth of about 5 ~tm. Although we did not study the structure of the damaged regions in SrTiO3 in detail we presume that the lines of dark contrast in the substrate near-surface region also represent amorphous regions along the ion paths resulting from energy deposition by electronic excitation. 4.1.4. Intermediate region in the substrate

As show in fig. 2, no irradiation damage can be observed in this region (5 t x m < x < 10 Ixm). Figure 10 indicates that ( d E / d x ) e decreases gradually with increasing depth while (dE/dx)n increases. From the absence of visible damage we conclude that the electronic stopping power of the Xe ions is no longer sufficient to produce amorphous zones along the ion tracks. On the other hand, the nuclear stopping power is not yet high enough to produce a sufficient concentration of point defects whose agglomeration could result in visible defects. 4.1.5. Substrate peak-damage region

In this region ( x > 10 lxm) a high density of dislocation loops is observed. The formation of dislocation loops by irradiation is a very common phenomenon in many materials. The loops are planar aggregates of point defects produced in nuclear collisions [ 17 ]. Indeed, as shown in fig. 11, the value of point defect production by nuclear collisions is about one order of magnitude higher in this region than in the near-surface region. Thus, we can conclude that in our experiments we have evidence for both kinds of damage production mentioned above, i.e. by electronic excitation at depth values up to about 5 ~tm and by nuclear collisions at depths higher than about 10 ~tm. Recently, it was reported [ 18 ] that discontinuous amorphous tracks occur in bulk YBaECU307samples irradiated with 3.5 GeV Xe-ions. Such tracks have also been observed in several magnetic oxides [ 19,20] under GeV heavy-ion irradiation. In fact, also in our case the ion tracks are not entirely continuous. In fig. 2 the tracks exhibit granular contrast and in figs. 5 and 7 the diameter of the elementary

amorphous zones varies between about 3 and 7 nm. 4.2. Twin structure

In addition to the formation of amorphous tracks, the twin structure characteristic for the orthorhombic phase becomes irregular and is eventually destroyed by irradiation. The fading of twin contrast has also been observed after electron irradiation [ 13,21,22 ]. The twins themselves are thought to be the result of a tetragonal to orthorhombic phase transformation upon cooling. This transformation is due to ordering of oxygen atoms along the Cu-O planes, which results in a small difference between the a and b lattice parameters. In the case of electron irradiation point defects are produced homogeneously in the specimen and in the same way oxygen atom disordering should be homogeneous. In contrast, in the present study the visible damage was very localized and the contribution of nuclear collisions is extremely small compared to the case of electron irradiation. It was reported that, in the case of electron irradiation, the fading of twin structures began at a dose of about 0.1 dpa [ 21 ]. On the other hand, in the present study the disappearance of the twin structure can already be observed at a dose of 4.0 × 10-4 dpa. Therefore, the vanishing of the twin structure cannot result from nuclear collisions. On the other hand, the irradiated samples show a strain field around the amorphous regions (fig. 5 ( a ) ) . This strain field may be responsible for an overall change in the lattice parameters. This view is supported by the observation reported in the literature that the c-axis parameter increases during Xeion irradiation [ 23 ]. Therefore, we presume that, due to the mechanical strains introduced by the amorphous regions, the YBa2Cu307 samples become tetragonal. 4. 3. Effect o f irradiation on electrical properties

It has been reported that radiation-induced point defects increase the electrical resistivity and give rise to a decrease of the critical temperature. The reduction of Tc is strongly dependent on the type and energy of the irradiation particles and the type of specimen (bulk or thin-film) [24-26]. As described in section 3.3., at an initial stage of irradiation, Tc de-

H. Watanabe et al. / 173 M e V Xe-ion irradiation on YBa:Cu30z thin films

creases linearly with increasing irradiation dose. At this stage we also observe a close-to-linear increase in the density of amorphous regions. Nevertheless is is unlikely that, in the low-dose range, the amorphous regions are as such responsible for Tc reduction. We suggest that the mechanical strains and the resulting structural changes in the crystalline material surrounding the amorphous ion tracks are responsible for Tc reduction. The dose necessary to reduce the zero resistance temperature of the specimen to 40 K is 3.0× 10 ~6 m -2. AS reported before [23], this value is smaller by a factor of 103 than the dose needed to achieve a comparable T~ reduction by irradiation with 25 MeV O-ions [27]. In that case the specimen showed no prominent amorphization along the path of the ions. This can be explained by a lower electronic stopping power. Indeed for 25 MeV O-ions, (dE/dx)e is lower by about one order of magnitude than for 173 MeV Xe-ion irradiation [ 28 ]. The enhancement of J~ in the presence of a magnetic field is observed only in the case of low-dose irradiation, i.e. up to about 1.5×1015 m -2 [23]. Under these conditions the volume fraction of amorphous material is only about 3%. In addition, the small reduction of T¢ indicates that in between the amorphous ion tracks and the surrounding area of structurally (but invisibly) changed material there must be a large volume fraction of undamaged material. The increase in J¢ in the presence of a magnetic field can thus be attributed to flux-line pinning at individual ion tracks. At higher doses the effect of additional pinning centres is overcompensated by the decrease of J¢ resulting from the reduction of T~.

Acknowledgements The authors would like to thank Dr. L. Schultz and B. Hensel for stimulating discussions and Prof. E. Kuramoto for the TRIM-program calculations.

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