Channeling analyses of epitaxially grown YB2C3O7−x thin film

cm

Nuclear Instruments and Methods in Physics Research B 118( 1996) 684-687

B

__

NONI

__

Beam Interactions with Materials&Atoms

@ ELSEVIER

Channeling analyses of epitaxially grown YB,C,O,_, J.Z. Qu, J.R. Liu Texus

thin film

*,

Centerftir Superconductivity

Q.Y. Chen, X.T. Cui, W.K. Chu

at Uniuersity of’Houston. Houston TX772045932,

USA

Abstract The structure features of YBCO (I IO&films on (110) SrTiO, substrate were studied by 3.05 MeV He’+ and 6 MeV Li3+ ion channeling measurement. Li3+ Ion was used to enhance the mass resolution of Ba, Y and Cu. Angular scan along the [I IO] directions reflected a misorientation between the film and the substrate. A sample consisting of three distinctive layers deposited under three different conditions were used to study the relationships between deposition conditions and crystallographic orientation. The angle of misorientation is consistent with a tilting in compensating for the lattice parameter difference between the substrate and the YBCO film.

1. Introduction Cuprate high temperature superconductors are layer compounds of orthorhombic crystal structure. In YBa,Cu,O,_, (YBCO), two CuO planes, being parallel to the a- and b-axes and thus also called the ab-planes, exist in a unit cell with c-axis perpendicular to them. Many believe that the coupling of CuO planes carries important clue to the mechanism of high temperature superconductivity and that understanding the anisotropic behaviors of these layer compounds would help shed some light [I]. Bulk crystals of these layer compounds, however, are thin in the c-direction (- 0.1 mm) and thicker (2-4 mm) in the ab directions. This makes the measurements of out-of-plane behaviours, i.e., the properties along c-direction, much more difficult. As a means to solve this problem, thin film deposition has been successfully used to fabricate c- and u-oriented materials with c- or a-axis, respectively, perpendicular to the substrate (or film) surface [2-41. While c-films are now almost the state-of-the-art, u-films are more difficult to produce in a reliable fashion. Both c- and a-films are usually deposited on a {IOO} surface of a substrate with cubic structure. During thin film growth, the film c-axis has almost equal access to the substrate (100) axis as c _ 3b and the difference in lattice constants a (m 3.82 A) and b (- 3.88 A) is small. Twin structures are thus a common problem in achieving high quality in-plane calignment for an a-oriented film [S]. To tackle this problem, {I IO}-cut substrates, instead of the {OOl}-cut used in c- or a-films, have been found to be able to limit the access of the in-plane c-axis to the

* Corresponding author. Tel. 713 743 8255, fax 713 743 8201, e-mail: [email protected]. 0168-583X/96/$15.00

.SSD/ 0168-583X(95)01

substrate [OOI] direction only, provided that the u-film deposition conditions are largely contained [6]. Although { l03)-phase, with film c-axis tilting _ 45” may also develop under certain growth conditions [7], there is a temperature and pressure range in which the in-plane aligned {I IO}-phase would be more favorable. We have been able to successfully fabricate pure {I IO&oriented YBCO thin films with no discernible (103) phase, as judged by the X-ray analysis based on the symmetry of the YBCO-IO8 pole figure. This work is part of an effort aiming at understanding the underlying kinetics and thermodynamics that govern the film structure in relation to deposition conditions. By varying deposition temperature and pressure, we have observed consistent crystallographic transition. Ion beam channeling techniques have been used to characterize crystal orientation and film quality in general. We have found better lattice match to the substrate for low temperature deposition as films tend to exist in tetragonal form (a = b) despite the oxygen deficiency and related poor superconducting properties. As temperature rises (accompanied by increasing pressure) in order to achieve higher oxygen content, orthorhombic structure develops accordingly in the film. Lattice mismatch with the substrate, which would have been abrupt, is now accommodated by the template zone through the gradually varying lattice constants. This article reports the observation of such gradual change via channeling analysis.

2. Sample preparations

and experimental

setups

The YBCO samples used in this work were deposited on heated SrTiO, substrate in Ar and 0, mixture (_ 2: 1) using an inverted cylindrical magnetron sputtering gun [8]. The YBCO targets were powder compressed and sintered

Copyright 0 1996 Elsevier Science B.V. All rights reserved 175-7

J.Z. QU et ul./l?ucl.

Instr. and Meth. in Phys. Rex B II8

(19961684-687

685

in order to increase the window between the Ba-Y surface edge. We used dilute HCI to etch out part of our film samples to reduce dechanneling effect while channeling the substrate. Substrate [1101

3. Results and discussions

Fig. I. Two possible orientations, (1 lo)- and (I 03)-orientation, of YBCO film on I IO SrTiO, substrate.

hollow cylinder (50 mm X 40 mm X 25 mm). The substrate temperatures for deposition employed in this study were 575, 745, and 785°C at which a total pressure of 80, 280, and 320 mTorr, respectively, was applied. Typical purely {l IO&oriented films produced with this system shows Tc, of 80-86 K with transition width (lo-90%) of 1-3 K. Meanwhile, typical A(28) of the {1lo)-peak in X-ray 20 scan was less than 0.085”. As mentioned earlier, the (110) and {103) phases have very similar inter-planar lattice spacing and hence difficult to resolve, pole figure was used to further confu-rn the phase purity. RBS channeling measurements were performed at the 1.7 MeV Pelletron Tandem accelerator at Texas Center For Superconductivity, University of Houston. The ion beam passes through two 0.5 mm apertures, which are 1 m apart. The samples were mounted on a three-axis goniometer with 0.01” accuracy. 3.05 MeV o-particles and 6 MeV Li3+ ions were used for different measurements. 3.05 MeV o-particles were used for thinner films with enhanced sensitivity for oxygen due to the 3.05 MeV resonance of 160. The 6 MeV Li3+ ion beam was used for thicker films

a

Fig. 1 depicts the crystal orientations of the {110) and {103) phases. Fig. 2 shows the distinct differences between a (1 lo} and a {103) pattern, where peaks l-4 represent the symmetry of {110) phase and peaks 5-12 represent the symmetry of (103) phase. A typical channeling measurement for an (1 IO&oriented film with He*+ is shown in Fig. 3. The minimum channeling yield of Ba-RBS was about 20% for the _ 4000 A thick film. The lowest minimum yield measured on the best {I 10) film prepared in our laboratory was about 13-15%, as compared to < 6% for a typical c-film. The stoichiometry of Y, Ba and Cu was close to 1: 2 : 3 with a maximum deviation of - 10%. The reduced backscattering yields in the surface region are different in spectrum 2 and 3, representing, respectively, the channeling along the film and substrate [l lo] directions. The difference between film- and substrate-aligned spectra suggests a misalignment between the [ 1lo] channels of the film and the substrate. The degree of this misalignment depends on the film growth conditions. To analyze the Ba peak throughout the film thickness in an angular scan, we used a 6 MeV Li3+ beam with which the Y- and Ba-peaks can be clearly separated. In making direct comparisons among various angular scans, we etched out part of the film so as to reduce dechanneling effect while channeling the substrate. After aligning the samples with substrate [l lo] direction, we

b

Fig. 2. (a) (108) pole symmetries of (1 IO) and (103) phases mixture. Peaks 1-4 represent {I IO) phase symmetry, peaks 5- 12 represent (103) phase symmetry. (b) Pole figure of high purity {l IO)-oriented YBCO film (purity > 99%).

IX. NEW MATERIALS

J.Z. Qu er al./Nucl.

Instr. and Meth. in Phys. Res. B 118 (1996) 684487

Substrate Align&

220

200

240

260

280

300

320

.

340

Channel Fig. 3. RBS spectraof {I IO&oriented YBCO thin film: (1) random spectrum, (2) film aligned channeling

moved the ion beam to the film region and repeated the angular scan. The angular scans for samples prepared under different growth parameters are given in Fig. 4. T$e thickness of each layer in this analysis is around 700 A. Curve (a) is the angular scan of the substrate, while curves (b)-(d) are taken from films grown under various deposition conditions (temperature/total pressure): 575”C/80 mTorr for curve (b), 785”C/320 mTorr for curve (c) and 74X/280 mTorr for curve (d). The angular half-width

m .-....

d

-2

2? e

3 0

0

-I

I

2

t#YIIlL TIV ..

-2

-1

*-

0

I

0

1

c 2

--

spectrum,

(3) substrate

aligned

values !lf,,* of the film are comparable with the one of the substrate. However, the minimum yield of the film angular scan always shifts from the substrate minimum yield, which suggests that the film [ 1lo] channel direction has a small misalignment with the substrate [ 1lo] channel direction. From Fig. 4, it can be seen clearly that the shifts depend on the growth conditions. At 575”C/80 mTorr, the angle of shift is hardly identifiable, while at 745”C/280 mTorr, the shift reaches a maximum value of _ 1”. This phenomenon can be qualitatively explained by the orthorhombic structure of the YBCO crystal. The angle of the shift reflects the oxygen content in the film. As in YBCO crystal, an increase of oxygen content leads to a decrease in lattice constant in c- and u-directions and an increase in b-direction. The more oxygen, the larger the difference between the a- and b-axis. Fig. 5 shows the [l lo] direction of the SrTiO, substrate, sketched as a simple cubic lattice, is perpendicular to its surface. When the YBCO film, which has an orthorhombic structure, grows on the SrTiO,, the angle between the film [I lo] direction and the substrate [ 1 lo] direction can be easily

b

-1

‘i\/v 2

t

-2

-1 0 Angle (de&)

2

’ 2

Fig. 4. Angular scan of (1 IO)-oriented YBCO films prepared by various deposition parameters (temperature/total pressure of Ar, 0,). (a) substrate, (b) film grown under 575”C/80 mTorr. (c) film grown under 785’C/320 mTorr, (d) film grown under 745”C/280 mTorr.

Fig. 5. {110) direction misorientation between SrTiO, and YBCO films grown on it.

J.Z. Qu et (11./ Nucl. Instr. and Meth. in Phys. Res. B I18 (I 996) 684-687

681

calculated as (Y= 90” - 2(tan- ’ a/b). For a fully oxidized YBa,Cu,O,, this angle could be as large as 0.9”. which is consistent with our experimental results. In the film grown under 575”C/80 mTorr, the low oxygen content YBCO film, it has a tetragonal structure (a = b) so that the shift angle (a) is zero. Grown under the optimal condition (745”C/280 mTorr). the high oxygen content film, which has an orthorhombic structure, has its [I IO] direction 1” off the substrate [llO] direction. In other intermediate cases, since the oxygen content 6 (= 7 - x) is between 6 and 7, the angle of shift falls between 0” and I”.

substrate [l lo] direction. It has been interpreted as due to the lattice mismatch between YBCO film and the SrTiO, substrate. We speculated that the value of the angle of shift reflected the oxygen content in the YBCO film.

4. Conclusion

[4] G. Gieres, H. Schmidt, K. Hradil, W. Hasler and R. Seebiick, Phys. C (1991) 185. [5] B. Roas and L. Schultz, Phys. Rev. Lett. 64 (1990) 479. [6] J.Z. Wu and W.K. Chu, Philos. Mag. B 67 (1993) 587. [7] B. Elkin, H.-U. Habermeier, B. Leibold and D. Shen, private communication. [81 X.X. Xi, G. Linker, 0. Meyer, E. Nold, B. Obst, F. Ratzel, R. Smithey, B. Strehlau, F. Weschenfelder and J. Geerk, Z. Phys. B Condensed Matter 74 (1989) 13.

We have investigated (1 IO]-oriented YBaCuO thin film on SrTiO, substrate. The X-ray results indicated high purity {l IO]-phase. Besides the x,,,~,, the comparison of of the substrate and the film shows good film the q,,, quality. In the RBS channeling study, we observed a misorientation between the film [ 1lo] direction and the

References

[I] M. Mukaida and S. Miyazawa, Appl. Phys. Lett. 63 (1993) 999. [2] H.C. Li, G. Linker, F. Ratzel, R. Smithey and J. Geerk,

Appl. Phys. Lett. 52 (1988) 1098. [3] Y.Z. Zhang, L. Li, Y.Y. Zhao, B.R. Zhao, J.W. Li, J.R. Sun, Q.X. Su and P. Xu, Appl. Phys. Lett. 61 (1992) 348.

IX. NEW MATERIALS