Thinfilm composite mixtures of YBa2Cu3O7−δ and Y2O3P. R. Broussard, V. C. Cestone, and L. H. Allen Citation: Journal of Applied Physics 77, 252 (1995); doi: 10.1063/1.359385 View online: http://dx.doi.org/10.1063/1.359385 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/77/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Incorporation of hyperfine probes into the thinfilm superconductor YBa2Cu3O7−δ during deposition Appl. Phys. Lett. 63, 3224 (1993); 10.1063/1.110205 YBa2Cu3O7−δ thinfilm gradiometers: Fabrication and performance Appl. Phys. Lett. 59, 3473 (1991); 10.1063/1.105657 Performance of a narrow band microwave filter implemented in thinfilm YBa2Cu3O7−δ with ohmiccontacts Appl. Phys. Lett. 58, 537 (1991); 10.1063/1.104579 Scaling behavior of YBa2Cu3O7−δ thinfilm weak links Appl. Phys. Lett. 57, 1155 (1990); 10.1063/1.104223 Magnetic flux noise in thinfilm rings of YBa2Cu3O7−δ Appl. Phys. Lett. 53, 695 (1988); 10.1063/1.100643

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composite mixtures of YBa2Cu307+ and Y,O, P. R. Broussard, V. C. Cestone, and L. H. Allen Naval Research Lab, Washington, DC 20375-5343

(Received i5 June 1994; accepted for publication 12 September 1994)

Thin-film, composite mixtures of YBa,Cu,O,-s (YBCO) and Y203 (yttria) have been grown by off-axis sputtering onto (100) SrTiOs and (100) and (110) MgO. ,Y-ray-diffraction measurements for films grown on (100) substrates show the presence of c-axis-oriented YBCO and (hOO)-ordered yttria. The composites deposited on (110) MgO are insulating, and x-ray analysis shows the presence of Y,Ba,Cu,O, along with c-axis YBCO. Scanning electron microscopy analysis shows the films to be very granular. The superconducting transition temperature and resistivity ratios between 300 and 100 K of the composites grown on the (100) substrates have been measured. A dramatic change is seen in these transport properties around 70% (volume) of YBaZCu,O,-fi, which may be explained by the phase diagram for Y-Ba-Cu-0. Q 1995 American Institute of Physics.

1. INTRODUCTION

In a previous article’ we examined the structural and transport properties of composite films of YBa,CusO,-G (YBCO) and Y,BarCu,O, (211). There we found that the 211 phase was not stable during deposition, decomposing into YBCO plus Y,O, (yttria). On the basis of these results, we decided to examine composite mixtures of YBCO with yttria, even though from the phase diagrams for the Y-Ba-Cu-0 system2 we would not expect these mixtures to be stable. (As is shown later, the composition of these samples falls in the YBCO-211-CuaO region, and under equilibrium conditions one would not expect yttria to be present.) We have now grown these thin-film composite mixtures and characterized their structural and transport properties. Previous studies of yttria grown onto YBC03.4 have shown that yttria grows epitaxially onto c-axis-oriented YBCO, with the following relationships: [OOl] YBC01][001] yttria and [llO] YBC01][100] yttria. There have been previous studies on the effects of yttria precipitates in YBCO films,5@ however, in those studies the compositions were kept near the ideal 123 level with no more than -10% yttria (by volumej present in the films.

II. SAMPLE GROWTH AND RESULTS FOR PURE FILMS

The films were grown by off-axis sputtering using two magnetron sputter guns for the YBCO and yttria targets, as in Ref. 1. The guns were at a relative angle’of 90”, and both were mutually perpendicular to the substrate holder. The sub- strates were heated by radiation from quartz lamps to a tem- perature of approximately 700 “C. The films were deposited in a gas at a pressure of 100 mTorr, composed of 80% Ar and 20% O,, while the YBCO target was dc sputtered at constant current and the yttria target was rf sputtered at constant power. The films were then cooled in 100 Torr of oxygen. The volume percentage of YBCO in the composites (desig- nated in this article as % YBCO) was calibrated from rate data on the two guns. Deposition rates varied from 93 to 258 A/h for the YBCO, while the rate for yttria was typically 40 AllI.

For pure YBCO films on (loo), (110) MgO, and (100) SrTiO,, we find c-axis growth, with inductive T, values of 86-87 K and inductive critical currents at 77 K of greater than lo6 A/cm”. When we grew yttria directly onto (100) MgO and (100) SrTiO, substrates, we found that the films grew with the close-packed direction (wck) along the normal; however, a layer of yttria grown onto an YBCO base layer showed the expected growth as seen in Refs. 3 and 4, namely (OOlj~~(OOl). We notice that while the (600) reflection is for- bidden in yttria, we do observe it for samples grown by off-axis sputtering.

Ill. COMPOSITES GROWN ON (110) MgO

When we grew YBCO/yttria composites onto the above substrates, we saw a dramatic difference for samples depos- ited on (110) MgO, compared to those on (100) MgO and SrTi03. In particular, the samples on (110) MgO were insu- lating, while those on the (100) substrates were supercon- ducting. In an attempt to understand this we used x-ray dif- fraction to look at the structure of the films. In Fig. 1, we show x-ray-diffraction patterns for YBCO/yttria composites grown on (110) MgO with -88% and 84% YBCO. For the 88% YBCO sample we do see lines from c-axis-oriented YBCO, but we cannot clearly identify lines from yttria. There are three lines that can be indexed to (hOOj-oriented 211 material. To account for this we compared the lattice parameters for (hO0) 211 to (110) MgO. Taking three (110) MgO surface net cells, with dimensions of 4.2X5.94 A,” we can construct a cell with dimensions 12.6X5.94 A.’ The (100) surface net for 211 is 12.18X5.66 Aa? giving a strain of -3.4% along the b direction of “211” and -4.9% along the c direction. We would then expect that the value of the a lattice constant would be smaller than normal, from the di- rection of the strain. We find, however, for the samples on (100) MgO that show the 211 peaks a value of a=7.15 A, which is larger than the nominal value for 211, 7.13 A. The 84% YBCO composite grown on (110) MgO [Fig. l(b)] does not show the 211 (hO0) reflections seen in Fig. l(a), but instead shows lines from c-axis YBCO and (hO0) yttria, which is similar to that for composites on (100) MgO (which

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106

g f IO' 4 5 0 E a, C *1O3 e A

102

lo5

3 c y IO4 -E! d .?? 2 3? E lo3 t.2 k

102

IO 20 30 40 50 60 70 80

ne,r)

10 20 30 40 50 60 70 80 29, (“)

FIG. 1. X-ray diffraction along the sample normal for composites grown on (110) MgO substrates for (a) 88% YBCO and (b) 84% YBCO. The peak labeling is as follows: c=c-axis YBa,Cn,O,-, YO=(hOO) yttria, and G-a =(/ZOO) Y.,BarCu,Os lines: The features labeled s are due to “forbidden” reflectionsin the substrates.

is presented later). Although this T-ray pattern is similar to those for the (100) substrates, this 84% YBCO composite on (110) MgO is still insulating.

The fact that composites of YBCO/yttria on (110) MgO are insulating, whereas pure films of YBCO on the same substrate have exc$lent properties is quite puzzling. Com- paring the c-axis values for all the films does show that the c axis is somewhat larger for films on (110) MgO. Typically, we see values of 11.71-11.72 A for the films on (100) MgO and SrTiOs, while on (110) MgO the value is nearly 11.73 A; however, the c-axis value for a pure YBCO film on (110) MgO is also nearly 11.73 A, so”this does not expIain the phenomena. We do not see a substantial increase in peak widths for the composites on (110) MgO, as all the compos- ites have slightly larger widths compared to pure YBCO, although not as large as seen in the YBCO/211 composites.’ Using the results of Ye and Nakamura,8 we looked for oxy- gen deficiency by comparing the ratio of the c-axis peak intensities. We compared the intensity ratios of the (OOS)-to- (006) c-axis reflections for samples, on (110) and (100) MgO and saw that the ratios for insulating composites on (110) MgO and superconducting composites on (100) MgO were very similar to each other and to our pure YBCO films. Thus,

J. Appl. Phys., Vol. 77, No. 1, 1 January 1995

(4 I6.0pm 04

FIG. 2. Scanning electron microscopy micrographs for YBCO/yttria com- posite films: (a) 91% YBCO on (110) MgO and (b) 87% YBCO on (100) MgO.

we feel that oxygen deficiency is not the cause of the absence of superconductivity in the YBCO/yttria composites on (110) MgO.

The appearance‘of 211 in these films seems to contradict our work on YBCO/211 composites’ where we saw yttria instead of 211. In that study, we assumed that the 211 was decomposing in the presence of YBCO. An alternative view is that there were limitations on the kinetics for the phase formation of 211 from the constituent species. Composite growth on (110) MgO may be a situation where the kinetics for formation of 211 are not limited, perhaps due to the lat- tice match between 211 and (110) MgO described earlier. Applying the lever rule to the location of our composites in the Y-Ba-Cu-0 phase diagram (shown later), we find that the formation of 211 would result in films with substantially lower % of YBCO and thus~poorer properties; however, the film with a lower YBCO Percentage that is also insulating [shown in Fig. l(b)] did not show 211 lines, but did show yttria lines. A comparison of the intensity ratio of YBCO c-axis peaks to yttria peaks (discussed in more detail later) for this sample shows that the YBCO/yttria ratio is even higher than its companion on (100) MgO, so an argument based on depletion of YBCO will not work here. The surface morphology of a 91% YBCO composite on (110) MgO (that also shows the presence of 211) is shown in Fig. 2(a), and displays a lack of connectedness that might explain the insu- lating nature of the samples. There are, however, clearly structures underneath these whose composition could not be determined by x-ray anaIysis (EDAX) due to their small size. A complete explanation for the behavior of the samples on (110) MgO will require further investigation.

IV. COMPOSITES ON (100) MgO AND SrTiO,

In Fig. 2(b) we show the surface morphology of a 87% YBCO composite on (100) MgO. We see that the composite has a very granular and nonuniform surface. [The films on (100) SrTiO, are similar.] As the amount of YBCO varies in the samples there is little variation in the surface morphol- ogy. We believe that the light-colored grains are yttria and the surrounding darker grains are YBCO, but, due to their small size, we were unable to verify this with EDAX. The x-ray analysis on this film, presented in Fig. 3(a), shows

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?rl”“l”““““““~ I (a)

F .Z

5 e

IO4

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s = P 103

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IO2

10 20 30 40 50 60 70 80 2Q,C)

FIG. 3. X-ray diffraction along the sample normal for composites on (100) MgO with (a) 88% YBCO and (b) 69% YBCO. The peak labeling is as in Fig. 1, with G-b=(OkO) Y2BaICu,0S lines.

similar growth to that seen for an yttria film grown on c-axis YBCO, with c-axis YBCO (IzOO) yttria reflections. [For all our composites, we obtain from the yttria (hO0) lines a value of ao=10.60+0.01 A, which agrees with the literature.‘] This pattern is also identical to that seen for our YBC0/211 composites.’ For samples on (100) MgO with lower percent- ages of YBCO, we see slightly different behavior, as seen in Fig. 3(b) for an -70% YBCO composite grown on (100) MgO. We see identical lines to those in Fig. 3(a), but also a new series of lines, which can be indexed to 211 (OkO) lines, with a b-axis lattice constant of 12.21 A, compared to 12.18 A for bulk 211. The linewidth of these 211 lines is unusually narrow (roughly one-half as wide) compared to the YBCO and yttria lines. We were unable to find a simple lattice match with (100) MgO that would lead us to expect this orientation of 211 material in the film. The appearance of 211 in the composites on (100) MgO is again unexpected based on our earlier work.

In Fig., 4 we show x-ray-diffraction patterns for YBCO/ yttria composites with ~88% and 71% YBCO grown on (100) SrTiOs . As the amount of YBCO is reduced for samples on (100) SrTiO, we see that, in contrast to the samples on (100) MgO, the x-ray pattern remains the same, with only c-axis YBCO and (IzOO) yttria lines. It may be expected then that the low YBCO % composites on (100)

10 20 30 40 50 60 70 80

ne,o

Id !lhmk.LL I , I I I t 1 ! I , I I , I I , 1 r I I , dLLuL~.J

10 20 30 40 50 60 70 80 28,(o)

FIG. 4. X-ray diffraction along the sample normal for composites on (100) SffiO, with (a) 88% YEXO and (b) 71% YBCO. The peak labeling is as in Fig. 1.

SrTiOs would have better superconducting properties. We see, however, that this is not completely the case. For all the composites on (100) substrates, however, as the amount of yttria is increased, we see that the c-axis constant does not vary, but remains around 11.71-11.72 A, and the width of the c-axis peaks does not increase substantially.

For the composites on the (100) substrates, we were able to compare the YBCO/yttria ratio obtained from x-ray data to that expected from the deposition rates. We made an empiri- cal comparison of the intensity ratios for the YBCO (005) and yttria (400) reflections on several bilayers and trilayers of YBCO and yttria, where the volume ratio of YBCO was calculated. from known layer thicknesses. We found an ex- cellent scaling equation between the known volume ratio and the measured intensity ratio for the layered samples, namely

YBCO VOl %=[ 0.998+0.48( $$$)I-‘. (1)

Using this equation on the composites has a built-in error due to the different absorption coefficient terms from varying YBCO percentages, but these will be small. In Fig. 5 we compare the results of Eq. (1) to the predicted YBCO volume percentages. We see overall that the agreement between the two is fair until we go below -70% YBCO, where we see

254 J. Appl. Phys., Vol. 77, No. 1, 1 January 1995 Broussard, Cestone, and Allen [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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0

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t I 11 ~aLiLt..t.>i~L-~u.a LLIL.IJ I : I I 1 X.LLL~

60 65 70 75 80 85 90 95

YBCO Volume % from measurements of deposit ion rates

F3G. 5. Plot of YBCO volume percentage determined by x-ray diffraction vs the expected percentage from rate measurements. The line descr ibes a unity relationship.

that the measured percentage of YBCO is much lower than that predicted by rate measurements. We come back to this point later.

The critical temperatures for the composites on (100) substrates were measured using both inductive and resistance methods. Curves of resistance as a function of temperature R(T) were obtained eon unpatterned films using a dc method with current reversal. In Fig. 6 we present both the inductive and resistive transition temperature measurements, along with an indication of the transition width in the resistive method (onset to complete). We found that the samples on SrTi03 do have higher T, values than those on MgO, and typically that the onset of the inductive transition and the completion of the resistive transitions agree to within a de- gree, until the YBCO percentage reaches ~70%. Here the samples, even those on (100) SrTi03, had no inductive T, above 4.2 K. We see, however, from the resistive measure- ments that a complete transition was observed, but the tran- sition widths have broadened considerably. The difference between the inductive and resistive methods in these films is likely due to the more stringent requirements for an inductive signal compared to a resistive transition. For the lowest YBCO percentage film (61%), there was no evidence for superconductivity in the R(T) curve, as the resistance from room temperature down to 4.2 K followed a l/T dependence.

We found for all the higher ,YBCO percentage samples linear R(T) curves (above 150 K) with resistivity ratios (RR, defined between 300 and 100 K)>l for all samples mea- sured, and typically 2-3 for samples with the highest YBCO percentages. We always find that the values of RR are higher for composites on SrTi03 compared to MgO. As seen in the T, results, for samples with -70% YBCO there is a dramatic change in the transport properties. In Fig. 7 we show R(T) curves for composites on (100) SrTiO, with 71%, 75%, and 78% YBCO. We see that for films with >72% YBCO that the RR decreases gradually as the amount of YBCO de- creases. For the 71% sample, however, we see a sudden drop in the RR (1.25) and a very broad transition width (~54 K).

We attempted to explain the’sudden change in transport properties by the presence of an oxygen deficiency. Using the

90

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1” ‘Ir z @ Inductive T, MgO X T, midpoint MgO

w inductive T, SiTiO, VT, midpoint SrTiO,

-I 0 ~I=, * m ’ r ’ aaIms ’ s ’ s ’ m ) ’ ’ * ’ ’ ’ ’ * ’ - ’ ’ ’ ’ * ’

60 65 70 75 80 85 90 95 % YBCO (volume) from rates

FIG. 6. Inductive and resistive superconduct ing transition temperatures vs the volume percentage of YBCO in the samples (from rate measurements) for the YEZO/yttria composites. Here T, inductive refers to the onset of the inductive transition, while for the resistive transition T, midpoint refers to the midpoint and the “error” bars indicate the onset to complet ion points. Notice for samples below 72% YBCO that no inductive transition was seen above 4.2 K.

analysis (as mentioned earlier) of the c-axis peak intensity ratios’ [in the case of (100) SrTiO, the (005)-to-(007) ratio], we did not see an indication of a sudden oxygen deficiency around 70% YBCO. In fact, the values of the ratios varied significantly even for samples with high-T, values, which is in contrast to a similar analysis performed on oxygen- deficient thin films prepared for photoemission studies,”

-7 7 71% YBCO

RR=1 25 \ In’ % 1 . R;;=2.31b..: .

.- = .- .I

.’ . l . 8 .* .c,

A #p -.._ 78% YBCO

P

RR=2.64 A

0.0 0 50 100 150 200 250 300

Temperature (K)

RG. 7. R(T,/R(SOO K) vs temperature for composi tes with 78%, 75%, and 71% YBCO on (100) SrTi03. Here RR refers to the resistivity ratio between 300 and 100 K.

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Tie line crossing is at = 66 % YE3CO

/ “211” jQ/

/ \k ~~~~~~ples

FIG. 8. A portion of the Y-Ba-Cu-Q phase diagram near YBa,Cu,O,-, at an oxygen pressure of -1-1.5X1O-3 atm and 850 “C. Here 123,211, and 132 refer to YBa&usO,-, YaBarCurO,, and Y,Ba,CuzOe+d, respectiveiy. The dark line represents the range of compositions covered by the samples in this study. Figure adapted from Ref. 2.

which clearly showed a dependence as that seen in Ye and Nakamura’s work.8 We also did not see an increase in YBCO peak widths for samples with 70% or less YBCO, which might indicate disordering, which would affect the transport properties.

Collectively, the rather sudden changes in sample prop- erties for the composites on (100) substrates near 70% YBCO including the drop in measured YBCO percentage from x-ray data, the increase in transition width, and the decrease in resistivity ratio, are quite puzzling when taken together. We found, however, that a comparison of where our samples are on the Y-Ba-Cu-0 phase diagram reveals some interesting points. From Ref. 2 we can infer on which oxy- gen pressure plateau our deposition conditions would be lo- cated (assuming we can extrapolate the pressure-temperature regions to lower oxygen pressure and temperature), and from that which pseudoternary section would be most appropriate, which is shown in Fig. 8. We have also shown on this figure the location of the film compositions. Now from our previ- ous work on YBC0/211 composites,’ we know that the bulk phase diagram cannot give us information about phase for- mation for our deposition conditions; however, we calculate that our range of compositions crosses the tie line between the 211 phase and CuaO at a YBCO volume percentage of =66%, which is exactly in the region where we see the sud- den changes in sample properties. This would seem to imply

that although the phase formation is not completely deter- mined by the phase diagram (since samples deposited ‘on the wrong side of the tie line should not contain the YBCO phase, which we still see), the relative amount of phases is influenced by the proximity of the phase boundary. If this is the case, then it would be difficult to fabricate suitable com- posite samples of YBCO/yttria, as the usable range of com- positions would be very small and limited to near the phase boundary. In addition, this -would imply that although the bulk phase diagrams cannot predict what will be produced in thin-film processes, neither can they be completely disre- garded.

V. CONCLUSIONS

We have successfully prepared thin-film composite mix- tures of YBCO/yttria by off-axis sputtering. Samples grown on (110) MgO are insulating and show the presence of 211 (/ZOO) lines. The composites grown on (100) substrates show that the YBCO is oriented c axis, and the yttria is oriented (hO0). For composites on (100) MgO with low YBCO per- centages we also see evidence of 211 (OkO) lines, while on (100) SrTiO, we see only the YBCO c-axis and yttria (/ZOO) lines. We see a sudden change in transport properties around 70% YBCO, with a drop in the resistivity ratio and broaden- ing in the resistive T,. We have interpreted the sudden changes in sample properties as being due to the influence of the bulk phase diagram, which may affect the growth, if not the presence, of phases in thin-film processes.

ACKNOWLEDGMENTS

We would like to acknowledge Terrell Vanderah for helpful discussions and Robert Soulen for careful reading of the manuscript.

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