SIMS study of deuterium trapping and migration in a YBa2Cu3O7−δ thin film

Nuclear Instruments and Methods in Physics Research B 99 (19951627-631

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Beam Interections with Materials 8 Atoms

ELSEVIER

SIMS study of deuterium trapping and migration in a YBa,Cu,O,_, thin film Yupu Li a,* , J.A. Kilner a, T.J. Tate b, M.J. Lee b, Y.H. Li a, P.G. Quincey a Department

’

of Materials and Centre for High Temperature Superconductivity, Imperial College of Science, Technology and Medicine, London S W7 ZBP, UK b Department of Electronic and Electrical Engineering and Centre for High Temperature Superconductivity. Imperial College of Science Technology and Medicine, London SW7 ZBT, UK ’ National Physical Laboratory, Teddington, Middlesex TWI 1 OLW, UK

Abstract Ion implantation

and secondary ion mass spectroscopy (SIMS) have been used to study the behaviour of trapping and migration of deuterium (D or 2H) in a c-oriented YBCO film (800-900 nm) from room temperature to 350°C. The D was introduced, at room temperature, by ion implantation at 50 keV to a dose of 1 X lOi ions cm-‘. Pieces of the as-implanted samples were annealed in a flowing oxygen ambient using a conventional annealing furnace. D concentration depth profiles were obtained on an Atomika 6500 SIMS instrument, using a 10 keV Cs+ primary beam. It has been found that 1) OH shows a declining “diffusion-controlled” distribution to a depth of N 150 nm within the as-received YBCO film; 2) some of the implanted D must bond with oxygen; 3) the implanted D is a fast diffuser in the YBCO film.

1. Introduction

together with the SIMS OH/OD plantation and annealing.

Hydrogen ion implantation has been employed to induce controllable irradiation damage into high T, superconductor (HTSC) materials for studying the mechanisms of superconductivity and material modification [l-6]. A study of the behaviour of trapping and migration of hydrogen in HTSC is beneficial to fully understand hydrogen irradiation effects on HTSC materials. It has been reported that material such as YBCO could be degraded in humid atmospheres as well as in water due to chemical reactions which involve (OH) species as the product [7,8]. In the latter case, the measurement of hydrogen and (OH) depth profiles will provide more information to understand corresponding mechanisms and monitor the degree of such reactions. Secondary ion mass spectroscopy (SIMS) provides ideally practical technique to synchronously analyse both hydrogen and (OH) depth distributions in HTSC films. Certainly, nuclear reaction analysis could be another powerful technique to analyse hydrogen depth distribution in HTSC materials. In this paper, the general behaviour of D trapping and release from D implanted YBCO film will be discussed,

* Corresponding 584 3194.

author. Tel. t44

071 589 5111, fax +44 071

depth profiles

after im-

2. Experimental The sample investigated in this work was a YBCO film deposited onto (100) LaAlO, by DC sputtering at 840°C using the inverted cylindrical magnetron method [9]. The film was 800-900 nm thick, with a superconducting transition temperature of 90.5 K (R = 01, as determined from an inductive method [lo]. X-ray diffraction (XRD) showed that the YBCO grows primarily with the c-axis perpendicular to the substrate surface (i.e a c-axis oriented film). The c-axis lattice constant $nd inferred oxygen content (7 - S > are equal to 11.67 A and 6.95 [ll], respectively. Irradiation was performed with 50 keV D+ to a dose of 1 X 1016 cm-’ at room temperature, and at 7” to the surface normal to minimise channelling effects. Deuterium was chosen to help in the subsequent SIMS analysis. After implantation the sample was sub-divided into w 2 mm X 3 mm pieces and annealed for 60 min in a tube furnace, in a flowing oxygen ambient. The anneals were done at various temperatures between 175°C and 350°C. Sequential isochronal annealing was performed on the sample at 175°C for 60 min + 250°C for 60 min + 350°C for 60 min. For the sequential isochronal annealing sample, after each of the anneal steps the sample was cooled to room temper-

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K Li et al. / Nucl. Ins@. and Me&. in Phys. Res. B 99 (1995) 627431 ionisation yield (i.e. the number of those ejected particles carrying a charge), f is the instrumental transmission factor and C is the concentration of the species under study. These measurements were terminated at the interface between the film and the substrate, where there is a small step for the 160 signal and a big step for both “A1 and 63Cu signals, mainly due to the change of S and C. For all three oxygen isotopes, S, y, and f are the same. The measured intensity ratio, i.e. Z(1sMass)/[Z(‘6Mass) + Z(‘8Mass)], at the end of the depth profiles, is very close to 0.2% (the natural “0 isotopic abundance). Here, during the calculation of the intensity ratio, the contribution of 170 is ignored, due to the natural isotopic abundance of 0.04% is only one fifth of that for 180. The contribution of both the ?160HH) and ‘8(‘70H) signals is too small to be significant. Therefore, we consider the measured intensity ratio represents the 180 isotopic ratio. The corresponding intensity ratio of Z(17Mass)/[Z(‘6Mass) + Z(17Mass) + Z(‘8Mass)], taken at the end of the profiles, is about twice as large as the value of the “0 isotopic abundance. It is hard to say if the higher background of the mass 17 signal contains some “(160H) species, because during SIMS analysis the background of (OH) or (CO) may be dependent on the experimental conditions such as the analysis chamber pressure. However, in this study, when we talk about the interference of the “(160H) signal to the “0 signal, we mean that the higher background of the mass 17 signal has been subtracted.

ature for analysis, and then annealed again to the next higher temperature. Depth profiles of the retained D after implantation and after each of the post-implantation annealing steps were obtained on an Atomika 6500 SIMS instrument. The samples were sputtered using a 10 keV Cs++ primary beam at normal incidence to produce a 350 pm X 350 p,m square crater. Electronic gating was used to minimise crater edge effects and since the substrate is an insulator, charge compensation was achieved with a 1.5 keV electron beam. Further details on the SIMS analysis of D+ implanted YBCO/LaAlO, samples can be found in our previous publications [12,13]. The non-implanted, as-implanted, and the annealed samples were also characterised by ac susceptibility to determine T, [lo] and X-ray diffraction using a Cu target and Ka irradiation on a Philips PW1730 instrument.

3. Results and discussion

3.1. SIMS depth profiles Figs. la, lb and lc) show the SIMS depth profiles from the as-implanted sample, the (175°C for 60 min) annealed sample, and the (175°C for 60 min +25O”C for 60 min) annealed sample, respectively. Negative secondary ions were monitored at masses 2 (D), 16 (160), 17 (“0, 160H), 18 (“0, 160D, 160HH, 170H), 27 (Al), and 63 (Cu). The measured secondary ion current for a given species is, is given by the general expression that Is = Z,S-yCf, where Zp is the primary ion current, S is the sputter yield (i.e. the number of atoms ejected per incident particle), y is the

Q As-impl. 2 M~J~ ________._._._-._._.--.-._.-

3.2. OH species in the as-receiued YBCO film The SIMS depth profile of mass 17 for the as-received sample has been inserted in Fig. la as line 0. For a clearer

b 175°C ,._._._.-_ ._.__._. _._._______..,, +

0.0

0.3

0.6

0.90.0

0.3

DEPTH

0.6

0.90.0

0.3

0.6

0.9

[ pm ]

Fig. 1. SIMS depth profiles from the as-implanted sample (a), the (175” C for 60 min) annealed sample (b), and the (175” C for 60 min and +250° C for 60 min) annealed sample (c). The sumbols 1 M-2 (i.e. D-, line l), 2 M-16 (i.e. 160-, line 2), 3 M-17 [i.e. “O17(160H)-, line 31, 4 M-18 [i.e. 180- and 1s(‘60D)-, line 41, 5 M-27 (i.e. “A-, line 51, and 6 M-6 (i.e. 63Cu-, line 6) were marked in (a). Line 5 in (b) and line 6 in (c) were omitted for clearer view of the others. Line 0 (with marked symbol “0 (M-17) X 0.1”) is the mass 17 signal in the as-received film, with the intensity being multiplied by a factor of 0.1.

Y. Li et al. /Nucl. Instr. and Meth. in Phys. Res. B 99 (1995) 627-631

view the intensity has been multiplied by a factor 0.1. It is interesting to note that the mass 17 signal shows a declining “diffusion-controlled” distribution before it becomes flat (i.e. to the instrumental background level) at a depth of about 150-200 nm. In the top layer of the as-received film, the mass 17 signal with the background subtracted should be contribution of “(160H) signal. It should be noted that for SIMS analysis the absolute quantification of the contribution of 17(160H) could include a large error due to the reasons mentioned earlier. However, we try to discuss its contribution quantively as will be shown later. Before the implantation, the film was left in a desiccator under air for a few months. We think that water (or OH) probably can “diffuse” into the YBCO film through pinholes and twin boundaries (or lattice diffusion) and may react locally with YBCO. The storage conditions for the film and the quality of the film also play a role in determining the level and “penetration” depth of OH in the film. This study shows that the (OH) species shows a “diffusion-controlled” profile in the YBCO film. 3.3. D trapping after implantation Line 1 in Fig. la shows the D depth profile in the as-implanted sample. The concentration calibration shown

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in line 1 of Fig. 2 was achieved by assuming the all implanted D had been trapped in the film. The distribution is essentially Gaussian-like and the maximum D concentration is equal to 4.2 X 10” cme3, i.e. - 0.55 at.%, with the atomic density of YBCO being 7.7 X 10”’ atoms/cm3. Lines 2, 3, and 4 in Fig. 2 show the simulated D, vacancy, and dpa (displacements per atom) depth profiles calculated with TRIM-90 for random media [14]. The depth of maximum D concentration d, (Le. the peak of D concentration), the mean projected range R, and the straggle AR,,, determined from the experimental profiles are clearly larger than the predicted values (see the inserted table in Fig. 2). It is obvious that 1) the target crystallinity and/or 2) an improved model for calculation of stopping powers has to be taken into account for the range data, for the smaller size and lighter D ion implantation. However, the existing simulations provide a useful guide to first order. Some of the implanted D can also be inferred from the mass 18 trace (line 4 in Fig. la), which clearly shows a peak at the position corresponding the peak of the implanted D, and is higher than the signal corresponding to the natural isotopic background for I80 signal in the implanted region. The mass 18 signal combines the contributions from “0 and “(160D) signals. In other words, some of the trapped D must bond with the oxygen. The

,lO'_ Z

c, 0

dr

Fp

420 125

1o18

0.0

ARp

-_

375nm 88

YBCO

nm

1

I

0.2

0.4 DEPTH

1

I

0.6 { ,um

0.8

0.00

1 .O

]

Fig. 2. Line 1: the calibrated D concentration distribution for the as-implanted sample. The simulated ‘H concentration, vacancy, and dpa (displacements per atom) distributions (lines 2, 3 and 4) for YBa,Cu,O, by TRIM-90 are alsodrawn in same scale. For calculating the damage distribution, a threshold displacement energy, Ed of 20 eV for all elements, is assumed. The density of YBCO is chosen to be 6.54 g/cm3. The inserted table shows the measured and calculated rante data for the as-implanted D distribution. The error bar for the range data is mainly from the measured error of the depth by Talystep, which is estimated to be + 5%. For the simulated damage distribution (line 3), the left vertical axis represents the density of vacancies, in units of vacancies/cm3.

XII. ION IMPLANTATION/ANALYSIS

Y. Li et al, / Nucl. Instr, and Meth. in Phys. Rex B 99 (1995) 627-631

630

I

1

10

20 -THFr/A

A%

50

2 Fig. 3. XRD patterns for the as-received sample (a) and the as-implanted sample (b).

contribution of “?OD) signal when corrected for the natural 180 abundance is equivalent to about 1.5 X 101’ ‘80/cm2. Therefore, under the assumption that one D bonds with one oxygen and that OD has an identical ionisation probability to 0, about 15% of trapped D within the film must bond with oxygen. The as-implanted D distribution is found to change gradient around the point labelled G (see line 1 in Fig. la). It is interesting to note that there is a step in the mass 17 signal (see line 3 in Fig. la) at same depth. It is clear that during implantation the (OH) species already present from in diffusion is redistributed within the top surface region of the film (see line 0 in Fig. la and line 3 in Fig. lb). The gradient changes, seen in the distribution around point G, is due to a presence (OH) at the same point [12]. Figs. 3a and 3b show XRD patterns from the as-received sample (a) and as-implanted sample (b). They show that after implantation 1) the c-axis lattice parameter increases (from 1.167 to 1.183 nm); 2) the normalised intensity of the (001) (I = 1, 2, 3 . . . ) peaks of 123 phase decreases; 3) the (001) peaks become broader. In addition, after implantation, some new peaks (see, for example, the peak with mark N in the inserted figure of Fig. 3b) located at the old (0031) (3Z= 3, 6, 9 . . > peaks can be seen. There are two possible explanations to the new peaks: 1) they are (hO0) (h = 1,2, 3 . . > peaks of 123 phase, the positions of these peaks, are in good agreement with those accepted for YBCO [15]; it should be noted that these new peaks are located between the shoulders of the broader (0031) peaks and the peaks from the substrate, so the actual intensity must be much smaller than the apparent intensity; or, 2) splitting of (0031) peaks, which has not been reported for the irradiated film, and need further explanation. This result shows that implantation induces some damage and makes the film more granular, with the

mainly expitaxial structure being maintained. Within the irradiated film, the simulated vacancy density (about 43% of which is due to oxygen atoms displacements from the oxygen sublattice) is at least one order of magnitude higher than the implanted D (see lines l-3 in Fig. 2). Thus some of the implanted D is likely to be bound with the displaced oxygen. The extended defects, second phases, and the boundaries are also be thought to be sinks for the displaced oxygen, implanted D, and the formed OD species. In theory, the destruction of superconductivity of the film (i.e. the oxygen sublattice changes from O,,,, to O,,,) needs a dpa level of 0.079, which is slightly lower than the simulated dpa level around the damage peak (see line 4 in Fig. 2). In practice, after implantation, no obvious T, can be detected from the inductive transition curve with the measured temperature down to 18 K (not shown), although with decreasing temperature the film shows some diamagnetic transition (i.e. some regions of the film are transiting to the superconducting state). Non-uniform damage distribution (see, for example, line 4 in Fig. 2) must play a role in determining the above observation. 3.4. D migration and release during annealing Figs. lb and lc show the change of D distributions after the sequential isochronal annealing (175°C for 60 min + 250°C for 60 min), with R, - (103 + lo)% and - (8 f 0.8)%. We define here the relative fraction R, to be the number of retained D atoms after the anneal divided by the number of retained D after implantation. SIMS analysis (not shown) shows that the D signal is at instrumental background (i.e. R, - 0) for the sample after the further sequential annealing at 350°C for 60 min. Clearly the anneal at 175°C for 60 min (see Fig. lb) results in some of the deuterium located around the concentration peak migrating to the wings. The diffusion of deuterium throughout the lower part of the film can also be seen in line 1 of Fig. lb, with a higher deuterium concentration (- 2 X 1019 cmm3, using the as-implanted D concentration as the standard) having been built up in the less damaged bottom of the film. Therefore, it could be thought that at 175°C the apparent “solubility” of 2H within the YBCO is quite high, although the nature of the high “solubility” is under study. The OH species is found to “diffuse” into the deeper film (line 3 in Fig. lb), this could be due to the exchange of positions between hydrogen and deuterium (i.e. hydrogen isotopic exchange) or indiffusion of “( 160H) species, However, the annealing at 175°C results in no obvious release of D from the film, although near the surface a small number of the implanted deuterium may be lost due to exchange with hydrogen or perhaps direct release/desorption. At 250°C (see Fig. lc), most of the implanted D (92 f 0.8%) has been released from the film. Meanwhile, the (OH) continues to “diffuse” into the deeper film. It is interesting to note that the D signal and OH signals give very similar profiles (see lines 1 and 3 in Fig. lc). These observations show that during

Y. Li et al. /Nucl. Instr. and Meth. in Phys. Res. B 99 (1995) 627-631

implantation and lower temperature annealing hydroxide species are formed. If we assume that no retrapping process happens for the implanted deuterium during the sequential annealing at 250°C for 60 min and that the diffusion process can be described by Fick’s II diffusion law [16], then we can estimate the diffusivity of the deuterium from the deuterium outdiffusion process. Based on line 1 in Fig. lb and line 1 in Fig. lc, we can consider that during the outdiffusion, on average, the released deuterium needs to move a distance of 420 nm (R,). So using AX2 = 20 * t (i.e. via Brownian movement [16], where AX” is the mean square displacement of diffusing deuterium, and t is the diffusion time), the apparent diffusion coefficient in the irradiated YBCO film, D *, is roughly 2.5 X lo-l3 cm2/s at 250°C. It is clear that the implanted D is a fast diffuser in the YBCO. This diffusion coefficient is many orders of magnitude larger than that expected for oxygen in the c-direction and half an order of magnitude larger than that for oxygen in the a-b plane at the same temperature.

Acknowledgement This work is funded by the Engineering Science Research Council, UK.

and Physical

References [l] 0. Meyer, T. Kroener, J. Remmel, J. Geerk, G. Linker, B. Strehlau and Th. Wolf, Nucl. Instr. and Meth. B 64 (1992) 539.

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[2] L.E. Rehn, Nucl. Instr. and Meth. B 64 (19921 161. [3] Y.J. Zhao, W.K. Chu, J.R. Liu, J. Kulik, H. Zandbergen and Y.K. Tao, Appl. Phys. L&t. 61 (1992) 1968. [4] Takahiko Kato, Katsuzo Aihara, Jiro Kuniya, Tomoichi Kamo and Shin Pei Matsuda, Jpn J. Appl. Phys. 27 (19881 L564. [5] F.M. Saba, J.A. Kilner, T.J. Tate, M.J. Lee, J.W. Radcliffe, L.F. Cohen, P. Quincey, R.E. Somekh and P. Przyslupski. J. Alloys and Compounds 195 (1993) 141. [6] J.W. Radcliffe, L.F. Cohen, G.K. Perkins, A.D. Caplin, T.J. Tate, M.J. Lee, F.M. Saba, P. Quincey, R.E. Somekh and P. Przslupski, J. Alloys and Compounds 195 (1993) 467. [7] M.F. Yan, R.L. Barns, H.M. O’Bryan, Jr., P.K. Gallagher, R.C. Scherwood and S. Jin, Appl. Phys. Lett. 51 (1987) 532. [8] A. Barkatt, H. Hojaji, R.W. Amarakoon and J.G. Fagan, MRS Bulletin 18(9) (1993) 45. [9] X.X. Xi, G. Linker, 0. Meyer, E. Nold, B. Obst, F. Ratzel, R. Smithey, B. Strehlau, F. Weschenfelder and J. Geerk, 2. Phys. B 74 (1989) 13. [lo] F.J. Muller, J.C. Gallop and A.D. Caplin, Supercond. Sci. Tech. 4 (19911 616. [ll] A. Kulpa, A.C.D. Chaklader, G. Roemer, D.L.I. Williams and W.N. Hardy, Supercond. Sci. Technol. 3 (1990) 483. [12] Y. Li, J.k Kilner, T.J. Tate, M.J. Lee, F.M. Saba, L.F. Cohen, A.D. Caplin and P.G. Quincey, J. Appl. Phys. 7.5 (1994) 4081. [13] Y. Li, J.A. Kilner, T.J. Tate, M.J. Lee, F.M. Saba, L.F. Cohen, A.D. Caplin and P.G. Quincey. Nucl. Instr. and Meth. B 85 (19941 281. [14] J.F. Ziegler, J.P. Biersack and U. Littmark, in: The Stopping and Ranges of Ions in Solids (Pergamon, New York, 1985). [15] Powder Diffration File, International Centre for Diffraction Data, PA, USA 1993. [16] See, for example, W. Jost, Diffusion in Solids, Liquids, and Gases (Academic Press, New York, 1960).

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