- Email: [email protected]
Direct observation of a new cationic vacancy ordered defect structure in laser-deposited superconducting YBaCuO thin films
Vofume 10, number t ,Z
September t 990
MATERIALS LETTERS
Direct observation of a new cationic vacancy ordered defect structure in lacer-dc~o~itcd ~u~erco~duct~~g Y-Ba-Cu-0
thin films
X.X. Xi, X.D. Wu, T. Venkatesan Rtctgersuniversity,Pisc~taw~y~ NJ08834, USA
and
Received t QMay I990
The Y2BaJ&& phase is the most common stacking defect observed in laser-deposited Y-Ba-Cu-0 thin Films. fn this Letter, we report for the fist time, the existence of a cationic vacancy ordered Y2Ba4Cus0,, structure in which every alternate pair of Cu atoms in the two chain tayers are missing. This new structure is a potytype of the Yz3a2Cu3c)7phase, i.e. it has the “Y,Ba&t@,,‘* structure but the “Y,Ba.&k&“ composition. Some possible causes and consequences of the formation of this defect structure are discussed.
The existence of different su~erGond~~ing phases Y-Ba-Cu-0 (YBCQ) system is now well established. One of the su~r~ondu~ting phases, with the nominal composition of YaBa&u8Q6 (2-4-g) was originally discovered as stacking defects in samples of nominal composition co~esponding to Y Ba&u& f t -2-3) [ 1f . The rest&s of this high-resolution electron microscopy (HREM) study revealed that the “2-4-8” structure differs from the “ l2-3” structure through the introduction of a CuO chain layer that is glide plane related to the CuO chain layer present in the “ l-2-3” structure. This glide plane is along the [ 1001 direction. Subsequently, the “24-8” phase has been synthesized as thin films f2f and as bulk super~o~du~ta~s [ 3 f . Recently, WREM studies of laser-deposited YBCO of nomind composition of “l-2-3”, revealed the existence of stacking defects of the “2-4-8” phase as well as another in the
0167-5?7x/9~/$
defect phase with the cationic composition of “2-24’” [4]. The “2-4-g” phase is observed as the predominant stacking defect in fiims oriented with their c-axis normal to the substrate surface. In the light of the fact that the target is made up of primarily the “I 2-3” phase, the obse~ation of this Cu-rich phase (compared to the “ 1-2-3’” composition ) and the cationic mass irnb~a~~~ involved in the growth of this structure is puzzling. In this report, we present the first evidence for the existence of an ordered cationdeficient “2-4-8” structure through atomic resolution electron microscopy studies in conjunction with detaited image simulations and processing. The thin films were prepared by pulsed faser deposition, the details of which are reported elsewhere [ 5 1. In the case of the present study, the substrate material was a fOO11 o~entation IsAl&, which is
03.50 0 1990 - Elsevier Science Publishem B.V. ~~o~h-~~o~iaad~
23
Volume 10, number 1,2
MATERIALS LETTERS
closely lattice matched with the “l-2-3” basal plane. Electron transparent samples were prepared from these films using conventional cross-sectioning techniques. The samples were examined in the Berkeley Atomic Resolution Microscope (ARM) at 800 kV, using a double-tilt goniometer stage. Care was taken to minimize electron-beam-induced damage by carrying out all the alignment steps on the substrate material far from the thin film. The epitaxial relationship between the substrate and the thin film ensures that when the substrate is aligned in the [ 1001 zone axis the film is also aligned in the same orientation. The interpretations of the image contrast were verified using the TEMPAS simulation package. Image processing experiments were carried out using the SEMPER program. In YBCO thin films, both the [loo] and [OlO] variants of the “2-4-8” phase have been observed, indicating the random formation of 90” growth twins. In fig. 1 is shown a [ 0 lo] zone axis atomic resolution image of the “2-4-8” structure in the thin film. Only the cationic positions are imaged and oxygen is not directly imaged since it is a very light element.
September 1990
In this orientation, the glide plane present between the CuO layers is observed as a staggering of the two CuO layers. The following layers are located between two pairs of CuO layers: BaO-CuO,-Y-CuO*-BaO, which we term as the perovskite block. The interpretation of the image contrast is confirmed by the simulated image shown in the inset. When imaged along the [ 1001 orientation, the projection of the glide plane is observed and the two CuO layers are now observed with a mirror symmetry plane between them. It is important to note that due to the symmetry of this structure, cationic defects in the two CuO chain layers can be imaged only in this orientation. The observation of such cationic defects in the two CuO chain layers is the central result of this report. To establish this, supporting evidence is presented from similar HREM experiments using sintered samples that show a large density of the “2-4-8” phase. In fig. 2a, a typical [ 1001 zone axis atomic resolution image of the ordered “2-4-8” structure is illustrated. In this under-focused image the projection of cationic columns corresponds to black dots. The
Fig. 1. [OlO] zone axis atomic image of the “2-4-8” defect in Y-Ba-Cu-0 thin films, illustrating the two CuO layers related by a ( l/2) [ 1001 glide operation. The inset shows a simulated image (foil thickness=20 A; objective defocus= -500 A) of this phase which confirms the interpretation of the image contrast.
24
Volume
10, number
1,2
MATERIALS
Fig. 2. (a) [ IOO] zone axis atomic structure image of the ordered “2-4-8” structure in which every alternate pair of Cu atoms is missing; the inset shows a processed image of the ordered structure, illustrating that the missing atoms are Cu; (b) [ IOO] zone axis image of the “2-4-8” phase obtained from a bulk superconductor, illustrating the perfect structure of the “2-4-8” phase, with the pairs of Cu atoms at a spacing of 3.85 A.
main feature to note is the doubling of the periodicity of the black lines in the two CuO chains. In the perfect structure this periodicity is 3.85 w (i.e. the unit cell spacing along that direction) while in this ordered defect structure the spacing is 7.7 A. The inset to this image shows a portion of the image, enlarged after image processing. This processed image clearly illustrates that the double vacancy occurs in the Cu sites (i.e. directly above the Cu-atom positions in the perovskite block) and not in the oxygen
LETTERS
September
1990
sites. Fig. 2b is a [ 1001 HREM image of the perfect structure of the two CuO chain layers obtained from a sintered bulk sample. In this structure, the image periodicity corresponding to the two Cu-0 chains is half of that shown in fig. 2a for the defective structure. We note that an ordered cationic vacancy structure is less common compared to an ordered arrangement of anionic vacancies, e.g., oxygen vacancies. Several types of ordered structures corresponding to different oxygen vacancy concentrations have been observed in the YBazCusO,_., system [ 61. In order to understand the details of the image contrast we carried out image simulations in the [ 1001 orientation for several possible structural models of the “2-4-8” structure. Specifically, we considered the following: (a) the perfect “2-4-8” structure; (b) an ordered arrangement of Cu vacancies along the [ 0101 direction; (c) an ordered arrangement of oxygen vacancies along the [ 0 lo] direction. The contrast in the simulated images was compared to that in the experimental image. The simulations were carried out for a variety of defocus conditions and foil thicknesses. Here we present calculated images for a foil thickness of 20 A and a defocus of - 500 A. In fig. 3a the simulated image for the perfect “24-8” structure in the [ 1001 orientation is shown. Of particular interest is the periodicity and spacing (3.85 A, as in fig. 2b) of the double CuO chain layers, relative to the Cu-atom positions in the perovskite block. In this zone axis, the Cu atoms in the chain layers are aligned on top of the Cu atoms in the two CuOz planes, as is observed in this image. Fig. 3b shows the image for a structure in which every alternate Cu pair has been removed. Particular attention is drawn to the contrast and periodicity of the intensity distribution in the two CuO chain layers. Every alternate bright band corresponds to a pair of Cu vacancies, which alternates with a pair of Cu atoms. The image contrast for the two chain layers matches well with that experimentally obtained, fig. 2a. The simulated image in fig. 3c, obtained by removing every alternate pair of oxygen in the two CuO chains, exhibits a very weak intensity modulation along the [ 0101 direction. The weak intensity modulation occurs with a bright band (corresponding to a pair of oxygen vacancies) on top of the Ba-atom position, which is different from the experimental 25
Volume IO, number I,2
MATERfALS LETfERS
Fig. 3. Simulated images in the [ 1001 zone axis for three different modeh of the ‘“2-4-8” structure with a foil thickness of 20 A and objective defocus of - 500 A: (a > the perfect “Z-4-8” structure with no vacancies; (b) the “2-4-8” structure in which every alternate pair of Cu atoms has been removed; (c) the “2-4-8” structure in which every alternate pair of oxygen atoms has been removed.
image shown in fig. 2a. We thus conclude that the ordered structure is caused by alternate pairs of Cu vacancies, resulting in a cationic stoichiometry of ” I2-3” with structure of the “2-4-8” phase. This defect structure, which by definition is a polytype of the “ l2-3” phase, is part of a variety of polytypoidic and polytypic structural defects observed in the c-axis oriented thin films. Such an ordered structure was consistently observed in all the in-situ YBCO thin films examined 26
September 1990
to date although the density was variable. There appears to be a propensity for a higher density of this ordered structure in films deposited at substrate temperatures slightly lower than the optimum required for the growth of the “l-2-3” structure. In fig. 4 we show an instance of a sample in which the density of the ordered structure was appreciable. There are two important questions regarding these structural defects. The first aspect is the origin of such defects. It has been suggested recently that polytypoidic stacking defects in the c-axis oriented films arise due to the ledge growth mechanism [ 7 ‘j. The Cu-vacancy ordered structure presumably forms because: (i) the growth temperature is close to the stability regime of the “2-4-8” structure and hence fluetuations in the instantaneous substrate temperature are likely to stabilize the “2-4-8” type structure. However, the film is provided with the composition of “l-2-3” from the target, thus leading to the formation of cationic vacancy structures. The high quenching rates involved in the laser-deposition process may be another factor, although only a minor one, since such ordered structures have been observed in sputtered films also. The second aspect is their effect on the transport properties, viz. r,, J,.,,and microwave surface resistance of the c-axis oriented films. Since these defects are all aberrations in the crystalline purity of the “l2-3” structure, they could potentially contribute to an increase in the hip-frequency surface resistance of these films. On the other hand, a beneficial aspect of such defects could be in providing good fluxon pinning sites, enabling the films to carry a large J,_ The plytypic defect discussed in this paper does not have a precedent in these cuprates and its properties cannot be inferred by examining systematic series, as in the case of the Bi cuprates or the “l-2-3” and “24-8” phases, Further studies are required to understand the role of such defects on the transport properties. The support and encouragement of P.L. Key, J-Ii. Wernick, M.J. Bowden, V.G. Keramidas and P.F. Liao is greatly appreciated. We also wish to acknowledge the continued support of Professor G. Thomas and the staff of the National Center for Electron Microscopy.
Volume
10, number
I ,2
MATERIALS
LETTERS
Fig. 4. A low-magnification lattice image from a sample deposited at a substrate temperature lower than the optimum 3” phase, showing the existence of the ordered “2-4-8” structure over a large area of the film.
1990
value for the “ 1-2-
[ 31 J. Karpinski,
References [ 1 ] H.W. Zandbergen, R. Gronsky, Nature 33 1 ( 1988) 496.
September
K. Wang and G. Thomas,
[2] A. Kapitulnik, Physica C 153-155 (1988) 520; A.F. Marshall, R.W. Barton, K. Char, A. Kapitulnik, B. Oh, R.H. Hammond and S.S. Laderman, Phys. Rev. B 37 ( 1988) 9353; K. Char, M. Lee, R.W. Barton, A.F. Marshall, I. Bozovic, R.H. Hammond, M.R. Beasley, T.H. Geballe, A. Kapitulnik and S.S. Laderman, Phys. Rev. B 38 (1988) 834.
E. Kaldis, E. Jilek, S. Rusiecki and B. Bucher, Nature 336 (1988) 660. [ 4 ] R. Ramesh, D.M. Hwang T. Venkatesan, T.S. Ravi, L. Nazar, A. Inam, X.D. Wu, B. Dutta, G. Thomas, A.F. Marshall and T.H. Geballe, Science 247 ( 1990) 57. [ 51 A. Inam, M.S. Hegde, X.D. Wu, T. Venkatesan, P. England, P.F. Miceli, E. W. Chase, CC. Chang, J.M. Tarascon and J.B. Wachtman, Appl. Phys. Letters 53 ( 1988) 908. [ 61 R. Beyers and T.M. Shaw, Solid State Phys. 42 ( 1989) 135. (71 R. Ramesh et al., Appl. Phys. Letters, submitted for publication.
27
September t 990
MATERIALS LETTERS
Direct observation of a new cationic vacancy ordered defect structure in lacer-dc~o~itcd ~u~erco~duct~~g Y-Ba-Cu-0
thin films
X.X. Xi, X.D. Wu, T. Venkatesan Rtctgersuniversity,Pisc~taw~y~ NJ08834, USA
and
Received t QMay I990
The Y2BaJ&& phase is the most common stacking defect observed in laser-deposited Y-Ba-Cu-0 thin Films. fn this Letter, we report for the fist time, the existence of a cationic vacancy ordered Y2Ba4Cus0,, structure in which every alternate pair of Cu atoms in the two chain tayers are missing. This new structure is a potytype of the Yz3a2Cu3c)7phase, i.e. it has the “Y,Ba&t@,,‘* structure but the “Y,Ba.&k&“ composition. Some possible causes and consequences of the formation of this defect structure are discussed.
The existence of different su~erGond~~ing phases Y-Ba-Cu-0 (YBCQ) system is now well established. One of the su~r~ondu~ting phases, with the nominal composition of YaBa&u8Q6 (2-4-g) was originally discovered as stacking defects in samples of nominal composition co~esponding to Y Ba&u& f t -2-3) [ 1f . The rest&s of this high-resolution electron microscopy (HREM) study revealed that the “2-4-8” structure differs from the “ l2-3” structure through the introduction of a CuO chain layer that is glide plane related to the CuO chain layer present in the “ l-2-3” structure. This glide plane is along the [ 1001 direction. Subsequently, the “24-8” phase has been synthesized as thin films f2f and as bulk super~o~du~ta~s [ 3 f . Recently, WREM studies of laser-deposited YBCO of nomind composition of “l-2-3”, revealed the existence of stacking defects of the “2-4-8” phase as well as another in the
0167-5?7x/9~/$
defect phase with the cationic composition of “2-24’” [4]. The “2-4-g” phase is observed as the predominant stacking defect in fiims oriented with their c-axis normal to the substrate surface. In the light of the fact that the target is made up of primarily the “I 2-3” phase, the obse~ation of this Cu-rich phase (compared to the “ 1-2-3’” composition ) and the cationic mass irnb~a~~~ involved in the growth of this structure is puzzling. In this report, we present the first evidence for the existence of an ordered cationdeficient “2-4-8” structure through atomic resolution electron microscopy studies in conjunction with detaited image simulations and processing. The thin films were prepared by pulsed faser deposition, the details of which are reported elsewhere [ 5 1. In the case of the present study, the substrate material was a fOO11 o~entation IsAl&, which is
03.50 0 1990 - Elsevier Science Publishem B.V. ~~o~h-~~o~iaad~
23
Volume 10, number 1,2
MATERIALS LETTERS
closely lattice matched with the “l-2-3” basal plane. Electron transparent samples were prepared from these films using conventional cross-sectioning techniques. The samples were examined in the Berkeley Atomic Resolution Microscope (ARM) at 800 kV, using a double-tilt goniometer stage. Care was taken to minimize electron-beam-induced damage by carrying out all the alignment steps on the substrate material far from the thin film. The epitaxial relationship between the substrate and the thin film ensures that when the substrate is aligned in the [ 1001 zone axis the film is also aligned in the same orientation. The interpretations of the image contrast were verified using the TEMPAS simulation package. Image processing experiments were carried out using the SEMPER program. In YBCO thin films, both the [loo] and [OlO] variants of the “2-4-8” phase have been observed, indicating the random formation of 90” growth twins. In fig. 1 is shown a [ 0 lo] zone axis atomic resolution image of the “2-4-8” structure in the thin film. Only the cationic positions are imaged and oxygen is not directly imaged since it is a very light element.
September 1990
In this orientation, the glide plane present between the CuO layers is observed as a staggering of the two CuO layers. The following layers are located between two pairs of CuO layers: BaO-CuO,-Y-CuO*-BaO, which we term as the perovskite block. The interpretation of the image contrast is confirmed by the simulated image shown in the inset. When imaged along the [ 1001 orientation, the projection of the glide plane is observed and the two CuO layers are now observed with a mirror symmetry plane between them. It is important to note that due to the symmetry of this structure, cationic defects in the two CuO chain layers can be imaged only in this orientation. The observation of such cationic defects in the two CuO chain layers is the central result of this report. To establish this, supporting evidence is presented from similar HREM experiments using sintered samples that show a large density of the “2-4-8” phase. In fig. 2a, a typical [ 1001 zone axis atomic resolution image of the ordered “2-4-8” structure is illustrated. In this under-focused image the projection of cationic columns corresponds to black dots. The
Fig. 1. [OlO] zone axis atomic image of the “2-4-8” defect in Y-Ba-Cu-0 thin films, illustrating the two CuO layers related by a ( l/2) [ 1001 glide operation. The inset shows a simulated image (foil thickness=20 A; objective defocus= -500 A) of this phase which confirms the interpretation of the image contrast.
24
Volume
10, number
1,2
MATERIALS
Fig. 2. (a) [ IOO] zone axis atomic structure image of the ordered “2-4-8” structure in which every alternate pair of Cu atoms is missing; the inset shows a processed image of the ordered structure, illustrating that the missing atoms are Cu; (b) [ IOO] zone axis image of the “2-4-8” phase obtained from a bulk superconductor, illustrating the perfect structure of the “2-4-8” phase, with the pairs of Cu atoms at a spacing of 3.85 A.
main feature to note is the doubling of the periodicity of the black lines in the two CuO chains. In the perfect structure this periodicity is 3.85 w (i.e. the unit cell spacing along that direction) while in this ordered defect structure the spacing is 7.7 A. The inset to this image shows a portion of the image, enlarged after image processing. This processed image clearly illustrates that the double vacancy occurs in the Cu sites (i.e. directly above the Cu-atom positions in the perovskite block) and not in the oxygen
LETTERS
September
1990
sites. Fig. 2b is a [ 1001 HREM image of the perfect structure of the two CuO chain layers obtained from a sintered bulk sample. In this structure, the image periodicity corresponding to the two Cu-0 chains is half of that shown in fig. 2a for the defective structure. We note that an ordered cationic vacancy structure is less common compared to an ordered arrangement of anionic vacancies, e.g., oxygen vacancies. Several types of ordered structures corresponding to different oxygen vacancy concentrations have been observed in the YBazCusO,_., system [ 61. In order to understand the details of the image contrast we carried out image simulations in the [ 1001 orientation for several possible structural models of the “2-4-8” structure. Specifically, we considered the following: (a) the perfect “2-4-8” structure; (b) an ordered arrangement of Cu vacancies along the [ 0101 direction; (c) an ordered arrangement of oxygen vacancies along the [ 0 lo] direction. The contrast in the simulated images was compared to that in the experimental image. The simulations were carried out for a variety of defocus conditions and foil thicknesses. Here we present calculated images for a foil thickness of 20 A and a defocus of - 500 A. In fig. 3a the simulated image for the perfect “24-8” structure in the [ 1001 orientation is shown. Of particular interest is the periodicity and spacing (3.85 A, as in fig. 2b) of the double CuO chain layers, relative to the Cu-atom positions in the perovskite block. In this zone axis, the Cu atoms in the chain layers are aligned on top of the Cu atoms in the two CuOz planes, as is observed in this image. Fig. 3b shows the image for a structure in which every alternate Cu pair has been removed. Particular attention is drawn to the contrast and periodicity of the intensity distribution in the two CuO chain layers. Every alternate bright band corresponds to a pair of Cu vacancies, which alternates with a pair of Cu atoms. The image contrast for the two chain layers matches well with that experimentally obtained, fig. 2a. The simulated image in fig. 3c, obtained by removing every alternate pair of oxygen in the two CuO chains, exhibits a very weak intensity modulation along the [ 0101 direction. The weak intensity modulation occurs with a bright band (corresponding to a pair of oxygen vacancies) on top of the Ba-atom position, which is different from the experimental 25
Volume IO, number I,2
MATERfALS LETfERS
Fig. 3. Simulated images in the [ 1001 zone axis for three different modeh of the ‘“2-4-8” structure with a foil thickness of 20 A and objective defocus of - 500 A: (a > the perfect “Z-4-8” structure with no vacancies; (b) the “2-4-8” structure in which every alternate pair of Cu atoms has been removed; (c) the “2-4-8” structure in which every alternate pair of oxygen atoms has been removed.
image shown in fig. 2a. We thus conclude that the ordered structure is caused by alternate pairs of Cu vacancies, resulting in a cationic stoichiometry of ” I2-3” with structure of the “2-4-8” phase. This defect structure, which by definition is a polytype of the “ l2-3” phase, is part of a variety of polytypoidic and polytypic structural defects observed in the c-axis oriented thin films. Such an ordered structure was consistently observed in all the in-situ YBCO thin films examined 26
September 1990
to date although the density was variable. There appears to be a propensity for a higher density of this ordered structure in films deposited at substrate temperatures slightly lower than the optimum required for the growth of the “l-2-3” structure. In fig. 4 we show an instance of a sample in which the density of the ordered structure was appreciable. There are two important questions regarding these structural defects. The first aspect is the origin of such defects. It has been suggested recently that polytypoidic stacking defects in the c-axis oriented films arise due to the ledge growth mechanism [ 7 ‘j. The Cu-vacancy ordered structure presumably forms because: (i) the growth temperature is close to the stability regime of the “2-4-8” structure and hence fluetuations in the instantaneous substrate temperature are likely to stabilize the “2-4-8” type structure. However, the film is provided with the composition of “l-2-3” from the target, thus leading to the formation of cationic vacancy structures. The high quenching rates involved in the laser-deposition process may be another factor, although only a minor one, since such ordered structures have been observed in sputtered films also. The second aspect is their effect on the transport properties, viz. r,, J,.,,and microwave surface resistance of the c-axis oriented films. Since these defects are all aberrations in the crystalline purity of the “l2-3” structure, they could potentially contribute to an increase in the hip-frequency surface resistance of these films. On the other hand, a beneficial aspect of such defects could be in providing good fluxon pinning sites, enabling the films to carry a large J,_ The plytypic defect discussed in this paper does not have a precedent in these cuprates and its properties cannot be inferred by examining systematic series, as in the case of the Bi cuprates or the “l-2-3” and “24-8” phases, Further studies are required to understand the role of such defects on the transport properties. The support and encouragement of P.L. Key, J-Ii. Wernick, M.J. Bowden, V.G. Keramidas and P.F. Liao is greatly appreciated. We also wish to acknowledge the continued support of Professor G. Thomas and the staff of the National Center for Electron Microscopy.
Volume
10, number
I ,2
MATERIALS
LETTERS
Fig. 4. A low-magnification lattice image from a sample deposited at a substrate temperature lower than the optimum 3” phase, showing the existence of the ordered “2-4-8” structure over a large area of the film.
1990
value for the “ 1-2-
[ 31 J. Karpinski,
References [ 1 ] H.W. Zandbergen, R. Gronsky, Nature 33 1 ( 1988) 496.
September
K. Wang and G. Thomas,
[2] A. Kapitulnik, Physica C 153-155 (1988) 520; A.F. Marshall, R.W. Barton, K. Char, A. Kapitulnik, B. Oh, R.H. Hammond and S.S. Laderman, Phys. Rev. B 37 ( 1988) 9353; K. Char, M. Lee, R.W. Barton, A.F. Marshall, I. Bozovic, R.H. Hammond, M.R. Beasley, T.H. Geballe, A. Kapitulnik and S.S. Laderman, Phys. Rev. B 38 (1988) 834.
E. Kaldis, E. Jilek, S. Rusiecki and B. Bucher, Nature 336 (1988) 660. [ 4 ] R. Ramesh, D.M. Hwang T. Venkatesan, T.S. Ravi, L. Nazar, A. Inam, X.D. Wu, B. Dutta, G. Thomas, A.F. Marshall and T.H. Geballe, Science 247 ( 1990) 57. [ 51 A. Inam, M.S. Hegde, X.D. Wu, T. Venkatesan, P. England, P.F. Miceli, E. W. Chase, CC. Chang, J.M. Tarascon and J.B. Wachtman, Appl. Phys. Letters 53 ( 1988) 908. [ 61 R. Beyers and T.M. Shaw, Solid State Phys. 42 ( 1989) 135. (71 R. Ramesh et al., Appl. Phys. Letters, submitted for publication.
27