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Crystalline growth rate and microstructure in YBaCuO thin films
Physica C 351 (2001) 9±12
www.elsevier.nl/locate/physc
Crystalline growth rate and microstructure in YBaCuO thin ®lms F. Pailloux a, R.J. Gaboriaud a,*, C. Champeaux b, A. Catherinot b a
Laboratoire de M etallurgie Physique-UMR 6630 CNRS SP2MI, bd3, T el eport 2, BP30179, 86962 Chasseneuil-Futuroscope Cedex, France b Facult e des Sciences, University of Limoges-SPCTS, UMR 6638 CNRS, 123 av A.Thomas, 87060 Limoges Cedex, France
Abstract The in¯uence of the anisotropy of the crystalline growth rate on the microstructure and morphology of YBaCuO thin ®lm deposited by laser ablation is studied by means of transmission electron microscopy. The results obtained from high resolution lattice imaging and large angle convergent beam diraction, together with the investigations of the thin ®lm surface morphology show the strong in¯uence of the dierent crystalline growth rates on both the internal stress present in the YBaCuO ®lm and the roughness of the outer surface. These results should be of prime importance for the superconducting properties. This is an essential in tailoring these thin ®lms for device applications. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: YBaCuO; Thin ®lms; Laser ablation; HRTEM; LACBED
1. Introduction The most widely synthesized type of oriented YBaCuO ®lms which exhibit a large value of critical current Jc , has its c crystallographic axis orientation normal (c?) to the substrate surface [1]. Further studies clearly show that by varying the substrate holder temperature a progressive change in the orientation of the ®lm from c? oriented to the so-called a-axis oriented (ck) appears which can be due to the lowering of atomic mobilities. Furthermore the growth rate normal to the
*
Corresponding author. Tel.: +33-5-4949-6662; fax: +33-54949-6692. E-mail address: [email protected] (R.J. Gaboriaud).
c-axis is roughly ®ve to ten times higher than the growth rate parallel to the c-axis. This dierence is mainly due to the fact that the c? material requires a greater degree of self-organization than the ck case. The aim of this work is to study what might be the in¯uence of a ck grain distribution in a c? matrix on both the microstructure-induced internal stress distribution and the outer surface morphology of the ®lm. This study is focused on YBaCuO thin ®lm deposited at a temperature and under oxygen partial pressure which allow a distribution of ck grains embedded in a c? matrix.
2. Experimental procedures YBaCuO thin ®lms, 350 nm thick, were grown using in situ pulsed laser deposition technique with
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 6 8 0 - 4
10
F. Pailloux et al. / Physica C 351 (2001) 9±12
a Kr±F excimer laser. The thin ®lms were deposited on [0 0 1] oriented SrTiO3 , MgO and LaAlO3 substrates in a temperature range 725±750°C, under an oxygen pressure of 0.3 mbar. Microstructure of the thin ®lms has been investigated by high resolution transmission electron microscopy (HRTEM) and large angle convergent beam electron diraction (LACBED). 3. Results and discussion Fig. 1a shows an HRTEM image of a plan-view sample of YBaCuO thin ®lm deposited on SrTiO3 . This picture shows grains immersed in a c? axis host matrix and which are elongated in the direction perpendicular to their c-axis. In this direction, the interfaces between the grain and the host matrix are very abrupt indicating a high degree of coherency. Instead, the extremities of the domain exhibit a curved-like shape which corresponds to the faster growth rate direction of the YBaCuO structure. In order to obtain additional insights concerning the atomic structure of the curved sides of the grains, the HRTEM pictures have been digitized and analysed by a phase shift map technique originally described in detail by H ytch et al. [2]. Fig. 1b depicts the curved interface with its Fourier transform in the inset. The corresponding phase shift map is shown in Fig. 1c. Characteristic contrast features clearly appear (white circles) at the boundary between the grain and the host matrix. A thorough examination of the HRTEM
image shows that these defects are interface dislocations with their extra half plane on the host matrix side. The ck axis of the grain is parallel to the a-axis of the c? matrix. Owing to this epitaxial relationship together with the cooling phenomena which follows the deposition, the ck grain should be under compressive stress as depicted in Fig. 2. This stress component can be described as an effective stress rxx acting in the climb plane of the dislocation which experiences a climb force by unit length equal to F rxx b where b is the Burgers vector. In the present case this force acts on the mis®t dislocations toward the host matrix. Instead the surface tension of the curved interface tends to retract this surface in the other direction. These two eects can explain the equilibrium shape of the ck grains embedded in a c? host matrix. Fig. 3 are scanning (SEM), atomic force (AFM) and transmission (TEM) electron microscopy pictures of an YBaCuO thin ®lm deposited on a MgO substrate. The SEM picture shows the distribution of rectangular precipitates. Fig. 2b is atomic force microscopy picture, Fig. 3c and d are cross sectional HRTEM micrographs and Fig. 3e and f are TEM plan views of the same sample. In the plane view sample TEM micrograph, a white contrast appears on two opposite sides of the precipitates depending on the orientation of the grains. Fig. 3d and c clearly indicate the outgrowth aspect of the precipitates. Arrows with a letter L correspond to a lack of matter, in agreement with the AFM picture. The dramatic depletion of matter which is observed is another consequence of the dierence
Fig. 1. HRTEM plan-view sample of YBaCuO on SrTiO3 (a) ck grain in c? matrix (b) detail and FFT (c) phase shift map: dislocations are depicted by white circles.
F. Pailloux et al. / Physica C 351 (2001) 9±12
11
Fig. 2. Plan view of a ck grain embedded in c? crystal ± bold arrows indicate compressive stress. Mis®t dislocation and the related force in the climb plane are also depicted.
(C)
Fig. 3. (a) SEM (b) AFM (c±f) TEM in cross-section and plan view of YBaCuO/MgO.
in the growth rate. When the deposition process is stopped, numerous layers of c? YBaCuO are not achieved. From measurement on Fig. 3d, a huge amount of YBaCuO material, approximately 50 layers of unit cells, are missing beside the ck grains. This depletion of matter at a very particular place, is very likely due to a diusion process which takes place from the c? crystal to the ck grains. YBaCuO thin ®lm is not a perfect single crystal and therefore the HOLZ lines in the LACBED disc obtained on the ®lm are too blurred to be studied in detail. Contrarily, the HOLZ lines obtained with the MgO and LaAlO3 substrates are
very suitable for a thorough investigation of the substrate lattice distortion induced by the stress present in the deposited ®lm. In the LACBED disc performed on LaAlO3 (Fig. 4), ®rst far from the interface (Fig. 4a) with the ®lm and then near the interface (Fig. 4b) underneath the c? oriented crystal of YBaCuO, the HOLZ line (arrow) is ®ne and undistorted showing that this LACBED diffraction pattern corresponds to an area where the LaAlO3 lattice is rather perfect, indicating a lack or a small stress relaxation near the interface. Then, the sample is moved under the LACBED probe, in a direction parallel to the interface, from
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F. Pailloux et al. / Physica C 351 (2001) 9±12
Fig. 4. LACBED experiments on a cross-section of YBaCuO/LaAlO3 ± for details see text.
the c? crystal to a location just underneath the ck grain (Fig. 4c). The salient result is that some HOLZ line strongly broadens and bends when the line is just below the ck grain. This result clearly shows that the top of the substrate experiences strain. This broadening of the HOLZ lines is interpreted as essentially due to the stress relaxation which appears during the TEM sample preparation. These ck grains are surrounded by a c? YBaCuO matrix. During the thinning process, two surfaces of the ck grains are released promoting a substantial lattice strain relaxation and the corresponding reaction in the LaAlO3 substrate, in the thickness direction. The same result is obtained with the MgO substrate [3]. This result shows that the ck oriented grains embedded in a c? oriented thin ®lm of YBaCuO are under stress.
This paper is focused on two aspects of stress and related relaxation processes in thin ®lms of YBaCuO deposited on dierent substrates. Experimental observations of the ®lm microstructure show the strong in¯uence of the crystallographic oriented growth rate on both the internal defects and the surface roughness. This should be of prime importance for the superconducting properties and for the engineering of the related devices. References [1] E. Garcia-Gonzalez, G. Wagner, M. Reedyk, H.U. Habermeier, J. Appl. Phys. 78 (1995) 353. [2] M.J. Hytch, M. Gandais, Phil. Mag. A 72 (1995) 19. [3] F. Pailloux, R.J. Gaboriaud, Thin Solid Films 368 (2000) 142.
www.elsevier.nl/locate/physc
Crystalline growth rate and microstructure in YBaCuO thin ®lms F. Pailloux a, R.J. Gaboriaud a,*, C. Champeaux b, A. Catherinot b a
Laboratoire de M etallurgie Physique-UMR 6630 CNRS SP2MI, bd3, T el eport 2, BP30179, 86962 Chasseneuil-Futuroscope Cedex, France b Facult e des Sciences, University of Limoges-SPCTS, UMR 6638 CNRS, 123 av A.Thomas, 87060 Limoges Cedex, France
Abstract The in¯uence of the anisotropy of the crystalline growth rate on the microstructure and morphology of YBaCuO thin ®lm deposited by laser ablation is studied by means of transmission electron microscopy. The results obtained from high resolution lattice imaging and large angle convergent beam diraction, together with the investigations of the thin ®lm surface morphology show the strong in¯uence of the dierent crystalline growth rates on both the internal stress present in the YBaCuO ®lm and the roughness of the outer surface. These results should be of prime importance for the superconducting properties. This is an essential in tailoring these thin ®lms for device applications. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: YBaCuO; Thin ®lms; Laser ablation; HRTEM; LACBED
1. Introduction The most widely synthesized type of oriented YBaCuO ®lms which exhibit a large value of critical current Jc , has its c crystallographic axis orientation normal (c?) to the substrate surface [1]. Further studies clearly show that by varying the substrate holder temperature a progressive change in the orientation of the ®lm from c? oriented to the so-called a-axis oriented (ck) appears which can be due to the lowering of atomic mobilities. Furthermore the growth rate normal to the
*
Corresponding author. Tel.: +33-5-4949-6662; fax: +33-54949-6692. E-mail address: [email protected] (R.J. Gaboriaud).
c-axis is roughly ®ve to ten times higher than the growth rate parallel to the c-axis. This dierence is mainly due to the fact that the c? material requires a greater degree of self-organization than the ck case. The aim of this work is to study what might be the in¯uence of a ck grain distribution in a c? matrix on both the microstructure-induced internal stress distribution and the outer surface morphology of the ®lm. This study is focused on YBaCuO thin ®lm deposited at a temperature and under oxygen partial pressure which allow a distribution of ck grains embedded in a c? matrix.
2. Experimental procedures YBaCuO thin ®lms, 350 nm thick, were grown using in situ pulsed laser deposition technique with
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 6 8 0 - 4
10
F. Pailloux et al. / Physica C 351 (2001) 9±12
a Kr±F excimer laser. The thin ®lms were deposited on [0 0 1] oriented SrTiO3 , MgO and LaAlO3 substrates in a temperature range 725±750°C, under an oxygen pressure of 0.3 mbar. Microstructure of the thin ®lms has been investigated by high resolution transmission electron microscopy (HRTEM) and large angle convergent beam electron diraction (LACBED). 3. Results and discussion Fig. 1a shows an HRTEM image of a plan-view sample of YBaCuO thin ®lm deposited on SrTiO3 . This picture shows grains immersed in a c? axis host matrix and which are elongated in the direction perpendicular to their c-axis. In this direction, the interfaces between the grain and the host matrix are very abrupt indicating a high degree of coherency. Instead, the extremities of the domain exhibit a curved-like shape which corresponds to the faster growth rate direction of the YBaCuO structure. In order to obtain additional insights concerning the atomic structure of the curved sides of the grains, the HRTEM pictures have been digitized and analysed by a phase shift map technique originally described in detail by H ytch et al. [2]. Fig. 1b depicts the curved interface with its Fourier transform in the inset. The corresponding phase shift map is shown in Fig. 1c. Characteristic contrast features clearly appear (white circles) at the boundary between the grain and the host matrix. A thorough examination of the HRTEM
image shows that these defects are interface dislocations with their extra half plane on the host matrix side. The ck axis of the grain is parallel to the a-axis of the c? matrix. Owing to this epitaxial relationship together with the cooling phenomena which follows the deposition, the ck grain should be under compressive stress as depicted in Fig. 2. This stress component can be described as an effective stress rxx acting in the climb plane of the dislocation which experiences a climb force by unit length equal to F rxx b where b is the Burgers vector. In the present case this force acts on the mis®t dislocations toward the host matrix. Instead the surface tension of the curved interface tends to retract this surface in the other direction. These two eects can explain the equilibrium shape of the ck grains embedded in a c? host matrix. Fig. 3 are scanning (SEM), atomic force (AFM) and transmission (TEM) electron microscopy pictures of an YBaCuO thin ®lm deposited on a MgO substrate. The SEM picture shows the distribution of rectangular precipitates. Fig. 2b is atomic force microscopy picture, Fig. 3c and d are cross sectional HRTEM micrographs and Fig. 3e and f are TEM plan views of the same sample. In the plane view sample TEM micrograph, a white contrast appears on two opposite sides of the precipitates depending on the orientation of the grains. Fig. 3d and c clearly indicate the outgrowth aspect of the precipitates. Arrows with a letter L correspond to a lack of matter, in agreement with the AFM picture. The dramatic depletion of matter which is observed is another consequence of the dierence
Fig. 1. HRTEM plan-view sample of YBaCuO on SrTiO3 (a) ck grain in c? matrix (b) detail and FFT (c) phase shift map: dislocations are depicted by white circles.
F. Pailloux et al. / Physica C 351 (2001) 9±12
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Fig. 2. Plan view of a ck grain embedded in c? crystal ± bold arrows indicate compressive stress. Mis®t dislocation and the related force in the climb plane are also depicted.
(C)
Fig. 3. (a) SEM (b) AFM (c±f) TEM in cross-section and plan view of YBaCuO/MgO.
in the growth rate. When the deposition process is stopped, numerous layers of c? YBaCuO are not achieved. From measurement on Fig. 3d, a huge amount of YBaCuO material, approximately 50 layers of unit cells, are missing beside the ck grains. This depletion of matter at a very particular place, is very likely due to a diusion process which takes place from the c? crystal to the ck grains. YBaCuO thin ®lm is not a perfect single crystal and therefore the HOLZ lines in the LACBED disc obtained on the ®lm are too blurred to be studied in detail. Contrarily, the HOLZ lines obtained with the MgO and LaAlO3 substrates are
very suitable for a thorough investigation of the substrate lattice distortion induced by the stress present in the deposited ®lm. In the LACBED disc performed on LaAlO3 (Fig. 4), ®rst far from the interface (Fig. 4a) with the ®lm and then near the interface (Fig. 4b) underneath the c? oriented crystal of YBaCuO, the HOLZ line (arrow) is ®ne and undistorted showing that this LACBED diffraction pattern corresponds to an area where the LaAlO3 lattice is rather perfect, indicating a lack or a small stress relaxation near the interface. Then, the sample is moved under the LACBED probe, in a direction parallel to the interface, from
12
F. Pailloux et al. / Physica C 351 (2001) 9±12
Fig. 4. LACBED experiments on a cross-section of YBaCuO/LaAlO3 ± for details see text.
the c? crystal to a location just underneath the ck grain (Fig. 4c). The salient result is that some HOLZ line strongly broadens and bends when the line is just below the ck grain. This result clearly shows that the top of the substrate experiences strain. This broadening of the HOLZ lines is interpreted as essentially due to the stress relaxation which appears during the TEM sample preparation. These ck grains are surrounded by a c? YBaCuO matrix. During the thinning process, two surfaces of the ck grains are released promoting a substantial lattice strain relaxation and the corresponding reaction in the LaAlO3 substrate, in the thickness direction. The same result is obtained with the MgO substrate [3]. This result shows that the ck oriented grains embedded in a c? oriented thin ®lm of YBaCuO are under stress.
This paper is focused on two aspects of stress and related relaxation processes in thin ®lms of YBaCuO deposited on dierent substrates. Experimental observations of the ®lm microstructure show the strong in¯uence of the crystallographic oriented growth rate on both the internal defects and the surface roughness. This should be of prime importance for the superconducting properties and for the engineering of the related devices. References [1] E. Garcia-Gonzalez, G. Wagner, M. Reedyk, H.U. Habermeier, J. Appl. Phys. 78 (1995) 353. [2] M.J. Hytch, M. Gandais, Phil. Mag. A 72 (1995) 19. [3] F. Pailloux, R.J. Gaboriaud, Thin Solid Films 368 (2000) 142.