Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.cplire.ru/html/oxide233/paper8.pdf Äàòà èçìåíåíèÿ: Thu Sep 1 13:25:15 2005 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 00:54:38 2012 Êîäèðîâêà:

Physica C 334 Z2000. 168 ­ 174 www.elsevier.nlrlocaterphysc

Orientation relations and twinning in heterostructures YBa 2 Cu 3 O x ´NdGaO3 and YBa 2 Cu 3 O x ´CeO 2 ´Al 2 O 3
I.K. Bdikin
a

a, )

, A.D. Mashtakov b, P.B. Mozhaev b, G.A. Ovsyannikov

b

Institute of Solid State Physics, Russian Academy of Sciences, Chernogoloóka, Moscow District, 142432, Russia b Institute of Radio Engineering and Electronics, Moscow 103907, Russia Received 11 November 2000; accepted 29 February 2000

Abstract Epitaxial YBa 2 Cu 3 O 7y x ZYBCO. thin films on Z110. NdGaO 3 and Z100. CeO 2 ´Z1102.Al 2 O 3 substrates were studied with X-ray diffraction methods, orientation features of film twinning were determined. Films on both substrates were mainly ° c-oriented with c s 11.67 A. Orthorhombic structure of NdGaO 3 results in increase of the angle between Z110. and Z110. twinning planes in YBCO films to 90.208 and in difference in volume of two twin domain systems. About 60% of YBCO film on CeO 2 ´Al 2 O 3 substrates show no twinning. The lattice direction ²1102: of the Al 2 O 3 substrate and ²100: of the CeO 2 film were inclined by 0.158; this inclination can result from high lattice mismatch between CeO 2 film and Al 2 O 3 substrate. The ²001: direction of the YBCO film was inclined to ²1102: Al 2 O 3 direction only by 0.068. The spread of in-substrate-plane misorientation in CeO 2 film of 1.658 was also higher than that of the YBCO film Z1.228.. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Thin films; Multilayes; Twinning; Domains; X-ray diffraction

1. Introduction Modern technology of deposition of the superconductive YBa 2 Cu 3 O 7y x ZYBCO. thin films provides films of crystal structure close to the structure of single crystals. Most results on high-Tc superconducting thin films were obtained with YBCO grown with the c-axis normal to the surface of different substrates. Similar to single crystals, twinning in c-oriented films occurs on the ä 1104 r²110: scheme with an angle of twinning about 18 w1,2x. The type of twinning structure of YBCO films is correlated with the structure of the substrate. In order to interpret the
Corresponding author. Tel.: q 7-95-913-224; fax: q 7-96-5764111.
)

transport property such as the critical current density, the twin structure in the a ­ b plane is very important. We present results of comparative studies of twinning structure in two c-oriented YBCO films on Z110. NdGaO 3 and Z100.CeO 2 ´Z1102.Al 2 O 3 substrates with similar property. 2. Experimental

° YBCO films Z1000 ­ 1500 A thick. were deposited with the DC-sputtering of a stoichiometric YBCO target at high oxygen pressure w3x on Z110. NdGaO 3 and r-cut sapphire substrates. Sapphire substrates ° were covered with a thin Z300 A. cerium oxide ZCeO 2 . buffer layer to prevent aluminium diffusion from the substrate into the film. The deposition

0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Z 0 0 . 0 0 2 4 4 - 6


I.K. Bdikin et al. r Physica C 334 (2000) 168 ­ 174

169

conditions of CeO 2 provided formation of the buffer layer in Z001. orientation. The film deposition parameters Zsubstrate temperature, oxygen pressure, and discharge current density. were optimised to obtain best superconducting properties of the resulting film. X-ray studies were performed using Siemens D500 and DRON-3M diffractometer systems. Both symmetric and asymmetric diffraction geometries were used. To study twinning in c-oriented YBCO films, it is necessary to observe reflections from crystallographic planes, tilted to Z001. planes. The chosen Z103. and Z113. reflections are of the most intensive reflections in the YBCO structure. Relative positions of diffraction peaks from corresponding planes provide the twinning angle value

and the value of the angle between substrate and YBCO crystallographic planes. These angles were calculated using

d s acos Z g . q bsin Z g . ,

Z 1.

where d -- measured misorientation angle between film grains Ztwin domains., a and b -- angles of misorientation between these grains in selected mutually perpendicular planes, g -- angle between planes corresponding to d and a . a and b axes were chosen in substrate plane and normal to substrate surface. Grain misorientation spread is present in two perpendicular directions: normal to the substrate sur-

Fig. 1. X-ray diffraction u ­2 u scans of c-oriented YBCO films on Z110. NdGaO 3 Za. and on Z1102. Al 2 O 3 with Z100. CeO 2 buffer layer Zb..


170

I.K. Bdikin et al. r Physica C 334 (2000) 168 ­ 174

face and in the substrate plane. Values of this decomposition can be found as follows D d 2 s D a 2cos 2 Z x . q D b 2 sin2 Z x . , where D d from the d tion spread axes, x --

° ° 3.880Z2. A, c s 11.670Z2. A. Low c parameter level supposes high oxygen contents in both films w4,5x.
3.1. YBCO film twinning on (110) NdGaO3

Z 2.

-- angle of grain misorientation spread axis, D a and D b -- grain misorientaangles from the mutually perpendicular angle between axes d and a .

3. Results and discussion X-ray diffraction ur2 u-scans of the YBCO films on Z110. NdGaO 3 and on Z001. CeO 2 ´Z1102. Al 2 O 3 are presented on Fig. 1. The obtained films were epitaxial with the c-axis oriented normal to the substrate Z c-oriented film.. Film lattice parameters were determined using Z0 0 13., Z3 0 10., and Z0 3 10. reflections. Values ° ° ° a s 3.827Z1. A, b s 3.889Z1. A, c s 11.674Z2. A were found for films on Z110. NdGaO 3 , while on ° CeO 2 ´Al 2 O 3 , these were a s 3.830Z2. A, b s

u-scan diffraction pattern in vicinity of Z113. YBCO film reflection is shown on Fig. 2 and presents a distinctive picture of YBCO twinning. The obtained pattern can be arithmetically decomposed into four diffraction curves. A and AX curves of this decomposition correspond to different twinning systems of Z110. plane, B curve results from other two twinning orientation on Z110. plane, showing no splitting, and C curve corresponds to Z020. NdGaO 3 planes. Relative peak positions reflect misorientation of corresponding planes. Diffraction patterns Fig. 2 allow determination of mutual orientation of NdGaO 3 and YBCO atomic planes ZFig. 3a. as well as mutual orientations of twinning parts. Twinning angle was calculated using Eq. Z1., where g is the angle between Z113. and Z110. planes in the YBCO lattice, g s 358. For the investigated YBCO film on NdGaO 3 we obtain b s 08 and d s 1.398 Zangle between A

Fig. 2. X-ray diffraction u-scans axis of Z113. reflection of c-oriented YBCO films on Z110. NdGaO 3 . On the insert: scheme of the experiment.


I.K. Bdikin et al. r Physica C 334 (2000) 168 ­ 174

171

equal to 90.208. Such discrepancy from 908 was also observed in Ref. w2x in YBCO film on NdGaO 3 . The increase of the angle between twinning planes can be explained by symmetry of NdGaO 3 lattice. Lattice in Z110. plane of NdGaO 3 is close to tetragonal with angle between Z111. and Z001. planes of 44.948 Zmeasured NdGaO 3 lattice parameters were a s ° ° ° 5.428Z2. A, b s 5.499Z2. A, c s 7.711Z3. A.. Knowledge of NdGaO 3 and YBCO lattice parameters allows determination of mutual atom positions in the Z001. plane of the film and Z110. plane of the substrate ZFig. 3b.. Distance between the M atom of the A film twin orientation and N substrate atom corresponds to distance between the K atom of the AX film twin orientation and the same N substrate atom. These distances can be calculated and are ° ° R 1 s 0.047 A and R 2 s 0.063 A, correspondingly. With increase of the twin domain size, the discrepancy between film and substrate atom positions will increase also. This strain will increase faster for the AX twin domain, resulting in smaller size of the twin domain. This effect can be seen in Fig. 2: the ratio of A and AX peak amplitudes, corresponding to the volume of the different twin domain systems, is about 1.7. The evaluation of the volume ratio can be given as Z R 2rR1 . 2 s 1.8, that is close to the experimentally observed value. 3.2. YBCO film twinning on (001) CeO2 ´ (1102) Al 2 O3

Fig. 3. Za. Scheme of atomic planes of YBCO film on Z110. NdGaO 3 substrate. Zb. Scheme of mutual location of atoms in Z001. YBCO plane and in Z110. NdGaO 3 planes.

and AX peaks and a -- double twinning angle.. This gives a twinning angle of 0.858, corresponding to ° a ­ b s 0.057 A. The a ­ b value calculated from lat° tice parameters gives 0.061 A in good agreement with the previous evaluation. Application of Eq. Z1. in a similar way allows evaluation of the angle between twinning YBCO planes from mutual positions of A, AX , and B peaks,

ur2 u-scans of YBCO films on Al 2 O 3 with CeO 2 buffer layer showed only Z00 l . peak family, supposing absence of a-oriented grains ZFig. 1.. Presence of b-orientation can be checked by comparison of relative intensities of the Z00 l . reflections w6x. The Z010. and Z020. reflections will increase the Z003. and Z006. peak intensities from the theoretically calculated values for totally c-oriented film. This will result in change of the intensity ratios I Z005.rI Z003. and I Z005.rI Z006., compared with the theoretical values. In our case, the difference between observed intensity ratios and theoretical for x s 0.1 w7x is negligible, so probably b-orientation is also absent in the studied film. Twinning in YBCO films on Al 2 O 3 with CeO 2 buffer layer does not reveal in the distinctive splitting ZFig. 4b.. Diffraction curves are more narrow


172

I.K. Bdikin et al. r Physica C 334 (2000) 168 ­ 174

Fig. 4. Za. X-ray diffraction u-scans of Z103. reflection of YBCO c-oriented films on Z110. NdGaO 3 . A and B correspond to two twin orientations for different Z110. and Z110. twinning planes. C peak corresponds to diffraction from Z112. NdGaO 3 planes. Zb. X-ray diffraction u-scans of Z103. reflection of c-oriented YBCO films on Z1102. Al 2 O 3 with Z100. CeO 2 buffer layer.

Fig. 5. X-ray diffraction u-scans of YBCO c-oriented film on Z1102. Al 2 O 3 with Z100. CeO 2 buffer layer, Z103. reflection. Solid line -- theoretical calculation in supposition of 40% twinned film and 1.18 grain misorientation; dotted line -- 100% and 1.18; dashed line -- 100% and 0.88.

than for YBCO films on NdGaO 3 , if we suppose merging of twin peaks into one broad peak. The nZ hkl . reflection width does not change with the increase of n, supposing broadening of the curve mainly due to grain misorientation, rather then size of domains or film defects w8x. The obtained X-ray diffraction curve was analysed in supposition of presence of twinned and not twinned YBCO phases in the studied film. Ratio of twinned and not twinned phase volumes, film grain misorientation angle, and parameter of orthorombicity have been varied. The best approximation was ° obtained with lattice parameters a s 3.830 A, b s ° c s 11.670 A, 1.18 misorientation angle ° 3.880 A, and 40% part of the film twinned ZFig. 5.. Similar calculation of twinned phase concentration was performed for films containing a-oriented inclusions. The a-oriented inclusions gave contribution into the not twinned phase reflections. Presence of the not twinned phase in 100% c-oriented film can be explained by following possible reasons. Za. The interface between twins can be broadened, and transformation from one twin orientation into another can form a phase with smaller twinning angle w9x.

Zb. The size of CeO 2 grain is smaller than size of ° a twin domain in the YBCO film Zabout 500 A.. In this case, the size of YBCO grains will be determined by the CeO 2 grain size and twinning in such small grains will be hardly probable. X-ray diffraction u-scans of Z005. YBCO and Z200. CeO 2 reflections are shown on Fig. 6. Deposi-

Fig. 6. X-ray diffraction u-scans of Z200. CeO 2 reflection Za,c. and Z005. YBCO Zb,d. in symmetric diffraction geometry. Za,b. and Zc,d. scans were obtained after rotation of the sample 908 in substrate plane. CeO 2 deposition temperature 7708C. Dashed lines -- position of normal to substrate Z0.08. and positions of Z1102. Al 2 O 3 .


I.K. Bdikin et al. r Physica C 334 (2000) 168 ­ 174

173

4. Conclusions Twin structure and domain orientations of the Y BCO film s on Z110. N dG aO 3 and Z100. CeO 2 ´Z1102. Al 2 O 3 were been investigated. Comparison of twinning structures of YBCO films with close crystallographic parameters, but on these different substrates shows particularities resulting from the nature of substrates. Twinning orientation features of the YBCO film on Z110. NdGaO 3 correlated with symmetry substrate. Presence of the not twinned phase in c-oriented Z100. CeO 2 ´Z1102. Al 2 O 3 film can be helpful to possibility of reducing twinning degrees and quantity of twinning borders in large part of film. Study of mutual orientations of layers in YBCO´CeO2 ´Al 2 O 3 heterostructures reveals inclinations of crystallographic planes between neighbour layers. Grain misorientation spread from the substrate normal is equal for all layers, but in substrate plane spread of grain misorientation is greater for the buffer CeO 2 layer. This effect proves significant influence of the sapphire substrate on the YBCO film. Further studies are necessary to find out the nature of this substrate ­ film interaction.

Fig. 7. X-ray diffraction u-scans of YBCO´CeO2 ´Al 2 O 3 film. Za. Z1120. Al 2 O 3 reflection; Zb. Z111. CeO 2 reflection, x s 558; Zc. Z103. YBCO reflection, x s 458.

tion temperature is 7708C for CeO 2 and 6958C for YBCO. Two diffraction scans in perpendicular directions are necessary to determine an unambiguous orientation and misorientation spread parameters of the film ZFig. 6a ­ d.. Analysis of these diffraction patterns shows that Z1102. Al 2 O 3 direction is tilted from the substrate normal by 0.98. The main Z001. direction of YBCO is inclined by 0.068 and Z100. CeO 2 direction is inclined by 0.158 to the Z1102. Al 2 O 3 direction. Position of normal to substrate corresponds to the dotted line on Fig. 6. Misorientation of grains can be evaluated as peak width on X-ray diffraction patterns Fig. 6a ­ d, and was equal both for YBCO film and for CeO 2 buffer layer in all scan directions Z0.82 ­ 0.838.. Applying Eq. Z2. for diffraction patterns Fig. 6a ­ c and Fig. 7 grain misorientation in substrate plane can be calculated, being 1.658 for CeO 2 buffer layer and 1.18 ­ 1.298 for YBCO film. Incomplete Z40%. twinning of YBCO films results in uncertainty in the grain orientation spread calculation w10x. The YBCO grain orientation differs for different twin domain systems, increasing observed reflection width by 0.18. Actual misorientation is smaller than calculated value of 1.188 ­ 1.298. The misorientation of YBCO film is sufficiently smaller than that of CeO 2 buffer layer; this effect can be explained if seeding of YBCO occurs only on CeO 2 grains of some certain orientation.

Acknowledgements The authors would like to thank P.V. Komissinskii for the help in the films deposition and I.M. Kotelaynskii for the useful discussion. Work was partially financed by the Russian Foundation for Basic Research Z95-02-06184. and State Program of Russia `` Modern Problems of the Solid State Physics'', division ``Superconductivity''.

References
w1x T. Scheme, P. Marienhoff, R. Herwig, M. Neuhaus, W. Jutzi, Physica C 197 Z1992. 79. w2x T. Steinborn, G. Miehe, J. Wiesner, E. Brecht, H. Fuess, G. Wirth, B. Schulte, M. Speckmann, H. Adrian, M. Maul, K. Petersen, W. Blau, M. McConnel, Physica C 220 Z1994. 219. w3x P.B. Mozhaev, G.A. Ovsyannikov, S.N. Polyakov, E.K. Kov'ev, N.P. Kukhta, Supercond. Phys. Chem. Technol. 9 Z1997. 304, Zin Russian..


174

I.K. Bdikin et al. r Physica C 334 (2000) 168 ­ 174 w7x J. Ye, K. Nakamura, Phys. Rev. B 48 Z1993. 7554. w8x J.P. Gong, M. Kawasaki, K. Fujito, R. Tsuchiya, M. Yoshimoto, H. Koinuma, Phys. Rev. B 50 Z1994. 3280. w9x Y.A. Osip'yan, A.N. Afonikova, T.S. Parsamyan, V.S. Shehkman, I.M. Shmyt'ko, Pis'ma Zh. Eksp. Teor. Fiz. 47 Z1988. 501, Zin Russian.. w10x I.K. Bdikin, A.D. Mashtakov, P.B. Mozhaev, G.A. Ovsyannikov, Fiz. Tverd. Tela 40 Z1998. 609, Zin Russian..

w4x M.S. Osofsky, J.L. Cohn, E.F. Skelton, M.M. Miller, R.J. Soulen Jr., S.A. Wolf, T.O. Vanderah, Phys. Rev. B 45 Z1992. 4916. w5x J.D. Jorgensen, B.W. Veal, A.P. Paulikas, L.J. Nowicki, G.W. Crabtree, H. Claus, W.K. Kwok, Phys. Rev. B 41 Z1990. 1863. w6x C. Dubourdieu, J.P. Senateur, O. Thomas, F. Weiss, B.P. Thane, M. Brunel, Appl. Phys. Lett. 69 Z1996. 1942.