- Email: [email protected]
Control of the in-plane alignment and the critical current of polycrystalline YBa2Cu3O7 − x thin films
Applied Superconductivity Vol. 3, No. 1-3, pp. 105-l 11, 1995 Copyright 0 1995 Elsevier Science Ltd
Pergamon
0964-1807(95)00038-O
Printed in Great Britain.All rights reserved 0964-1807/95$9.50+ 0.00
CONTROL OF THE IN-PLANE ALIGNMENT AND THE CRITICAL CURRENT OF POLYCRYSTALLINE YBazCu307_, THIN FILMS F. YANG, E. NARUMI, S. PATEL
and D. T. SHAW
New York State Institute on Superconductivity, 330 Bonner Hall, State University of New York at Buffalo, Amherst, NY 14260, U.S.A. Abstract-Biaxially aligned yttria-stabilized-zirconia (YSZ) buffer layers are prepared on polycrystalline substrates using ion-beam-assisted laser deposition. The effect of directional ion bombardment on in-plane texturing of YSZ layer is investigated by four-circle X-ray diffractometer. Without the assisting ion beam, uniaxially aligned YSZ layers are obtained at room temperature, exhibiting a preferred orientation of the grains with the [OOI] axis perpendicular to the substrate surface. With the assisting ion beam, the in-plane texture of Y SZ layer is altered as the (110) plane tends to align in the direction of the incident ion beam. The X-ray I#Jscan of (111) poles shows that the in-plane alignment of the YSZ layer improves as the ion beam voltage increases in the range 200-450 \! The improvement of YSZ inplane alignment is also observed as the layer thickness increases. On the biaxially aligned YSZ buffer layers, superconducting YBa&@_, thin films are prepared and their transport critical current densities are found to be directly related to the in-plane alignment of the YBCO thin films. Using a numerical method to model the thin film as a matrix of two dimensional grains, a correlation is established between the line shape of X-ray 4 scan and the measured critical current density of polycrystalline YBa2Cu307_, thin film.
1. INTRODUCTION
Deposition of superconductor thin films on polycrystalline metallic substrates with high transport critical current densities (Jc) at liquid nitrogen temperature (77 K) is very important for largescale applications such as electric power transmission and energy storage, etc.. Although J, values as high as IO6 A/cm2 can be routinely obtained at 77 K and 0 T for YBa&u307_, (YBCO) thin films prepared on suitable single crystal substrates, such as yttria-stabilized-zirconia (YSZ), MgO, SrTi03, and LaA103 by various deposition techniques, the J,s for YBCO thin films prepared on polycrystalline metallic substrates are typically reduced by a factor of 100 or so [l-4]. With the support of YSZ buffer layers, YBCO thin films prepared on polycrystalline metallic substrates have good quality of c-axis alignment, but their a and b axes are randomly oriented in the u-b plane from grain to grain. In these uniaxially aligned YBCO thin films, many high-angle grain boundaries inevitably exist and cause the degradation of critical current density across the sample because of their Josephson weak-link nature [5-71. Top reduce the number of high-angle grain boundaries in YBCO thin film on metallic substrate and improve its critical current density, the in-plane a and b axes need to be aligned in addition to c-axis alignment. Such biaxially aligned YBCO growth was realized on biaxially aligned YSZ buffer layer on the substrate, with the buffer layer being prepared by ion beam-assisted deposition (IBAD) [8-lo]. The [OOl] axes of YSZ grains were oriented to the substrate normal, and [ lOO]/ [OIO] axes were aligned in a common azimuthal direction in the substrate plane. The c-axis oriented YBCO thin film grown subsequently was found to inherit the in-plane texture of the underlying buffer layer in a relation YBCO [l IO]//YSZ [ 1001. In this study, we investigated the evolution of YSZ in-plane alignment with ion energy, chamber pressure, and film thickness, and correlated the crystallographic alignment between YSZ buffer layer and YBCO superconductor layer. Using a numerical method [ 11, 121 to model the biaxially aligned YBCO thin film, the in-plane alignment dependence of transport critical current density was calculated and the result was consistent with the result of four-probe measurements.
105
F. YANGet al
106
Excime Laser
I
Pumps
Fig. 1. Schematic diagram of ion beam-assisted deposition system. Two targets are mounted on the holder, one YSZ and one YBCO. During YSZ deposition, the growing film is subject to the simultaneous bombardment of Ar+ ion beam extracted from the ion gun. The ion beam is turned off during the subsequent YBCO deposition.
2. EXPERIMENT
PROCEDURE
The IBAD system is shown in Fig. 1. An ArF excimer laser beam (3, = 193 nm, LPX210, Lambda Physik) was focused at the target. A rotatable target holder was used so that YSZ and YBCO could be deposited alternately. Hastelloy C-276 was used as the substrate, which was painted on the back with silver paste and clamped to the heater block. The substrate was placed at a distance of 10 cm from the target for either YSZ or YBCO deposition. A 3 cm diameter ion gun (model 3.0-1500-100, Ion Tech Inc.) was installed at 7 cm away from the substrate, providing ion bombardment at the substrate surface with the incident angle 8 adjustable from 45” to 75”. The deposition of YSZ buffer layer was carried out at room temperature so that (OO1)instead of (111) oriented Y SZ was formed on polycrystalline substrate. The chamber pressure was controlled in the range from 1.9 to 3.5 mTorr by varying the flow rate of oxygen gas. Ion gun was turned on for YSZ growth and the growing layer was subject to low energy Ar+ bombardment. Due to the simultaneous ion bombardment, in-plane alignment was developed in the YSZ layer. The Ar+ energy was varied from 200 to 450 eV, and beam current was fixed at 10 mA. After formation of a 0.6 ,um thick YSZ buffer layer, the ion gun was turned off and Ar gas supply was stopped. To proceed YBCO growth on the buffer layer, the substrate temperature was set at 650°C and oxygen pressure was increased to 50 mTorr. The YBCO film thickness was 0.4-0.5 pm. A group of c-axis oriented YBCO samples were prepared on Hastelloy substrates covered by (001) YSZ buffer layers deposited at different IBAD conditions. In-plane alignment was determined by X-ray 4 scan measurement for both YSZ and YBCO layers. Full-width at halfmaximum (FWHM) of the 4 scan profile was used to quantitatively describe the degree of inplane alignment. For YBCO layer, (103)/(013) poles were scanned; for YSZ buffer layer, (111) poles were scanned. An 80 pm wide, 500 pm long bridge was patterned on each YBCO sample and the transport critical current density was measured using conventional four-probe method. 3. RESULTS
(A) In-plane
AND
DISCUSSION
texture
All the YSZ buffer layers prepared without IBAD have random in-plane alignment as shown by X-ray 4 scan measurement. With IBAD, YSZ buffer layers show strong enhancement of (001) oriented growth, and the development of in-plane alignment demonstrated by the 4-fold symmetry in the YSZ (111) 4 scan. One of the four peaks in the (111) 4 scan is always found to coincide with the projection of the direction of the ion beam on the (001) plane, indicating that YSZ (110) plane is aligned parallel to the ion beam incidence plane. To study the ion energy effect on in-plane alignment, the incident angle of Ar+ ion beam, 8, was fixed at 55”. The pressure was controlled at 3 mTorr. In Fig. 2, the full-width at half-maximum of
In-plane
alignment
and critical current of polycrystalline
107
YBCO thin films
Ion Energy (eV) Fig. 2. Effect of A? energy on the full-width at half-maximum of YSZ(111) 4 scan (FWHM,). best alignment is shown at an ion energy of 400 eV.
The
the 4 scan (FWHM4) of Y SZ (111) is plotted against ion energy. A minimum is clearly seen at about 400 eV Cross-sectional scanning electron microscopy (SEM) study reveals that YSZ layer thicknesses are very close to one another, about 0.6 pm. Figure 2 clearly show that the in-plane alignment of YSZ layer continues to improve when assisting Ar ion energy is increased up to 400 eV The alignment deterioration at the ion energy above 400 eV was caused by ion bombardment-induced defects. The chamber pressure is an important parameter not only for YSZ deposition, but also for Ar ion beam transport. In Fig. 3, FWHM, of YSZ (111) is plotted against chamber pressure for YSZ made at Ar ion energy of 400 eV In the pressure range investigated, at 3 mTorr YSZ thin film has the narrowest FWHM+. At lower pressure, YSZ layer suffers more severe damage because of excessive ion bombardment. Its crystallographic alignment deteriorates. At higher pressure, the ion beam becomes more turbulent and divergent, which also causes the degradation of YSZ alignment. The thickness dependence of the FWHMb of YSZ (111) is shown in Fig. 4. It shows that YSZ in-plane alignment improves steadily as the layer grows thicker. Based on the assumption that the in-plane alignment was induced by anisotropic ion sputtering yield [ 131, FWHM& at the surface of YSZ layer was estimated to be proportional to t-1’2, where t is the thickness. However,
1
2
3
4
Chamber Pressure (mtorr) Fig. 3. Effect of chamber pressure on the full-width at half-maximum of YSZ (111) 4 scan (FWHM,).
F. YANG et al.
108
l
$ scan data
-
least square
---
calculated
fit
(cct-1'3 )
(c&l12
)
lo0.1
1
YSZ Thickness, t (pm) Fig. 4. The full-width at half-maximum of YSZ (111) 4 scan (FWI-IM+) on buffer layers prepared by IBAD as a function of thickness in a log-log plot. The data points follow a t-*” dependence. The FWHM+ of YSZ surface layer is predicted to be proportional to t-l’* (see Ref. 12). t is the thickness of the YSZ buffer layer.
experimental data of FWHM+ show a t-“3 dependence as shown in Fig. 4. The increase in deviation for thicker YSZ layer is due to the fact that in X-ray 4 scan measurement, the whole layer of YSZ contributes to the diffraction signal and the corresponding FWHM+ The difference increases between the peak breadth of the 4 scan for the surface and that for the whole layer as it grows thicker. The texture of the YBCO top layer is determined by the YSZ buffer layer. Figure 5a is a typical X-ray diffraction pattern for a YBCO thin film prepared on Hastelloy substrate with a biaxially aligned YSZ buffer layer, indicating good c-axis alignment. The alignment in the YBCO u-b plane is displayed by an X-ray 4 scan profile, shown in Fig. 5b. The FWHM is 34” in YSZ (111) and 27” in YBCO (103)/(013) 4 scan. The in-plane texture of YBCO thin film has a relation YBCO[l lO]/iYSZ[lOO] with respect to YSZ buffer layer, similar to YBCO thin films grown on single crystal YSZ substrates. Figure 6 shows the correlation between the in-plane texture of YSZ buffer layers and the YBCO top layers. (B) Critical current density
For the sample shown in Fig. 5, J, was found to be 2 x lo5 A/cm2 at 77 K by four-probe measurement. In comparison, the J, value for a 0.4 pm thick YBCO thin film with random in-
YBCO
a
50
28 (“1
70
-180
(103)/(013)
-90
b
0
90
$0
Fig. 5. The crystallographic structure of YBCO thin film deposited on the biaxially aligned YSZ buffer layer: (a) X-ray 20 diffraction pattern, showing YBCO (001) peaks and YSZ (200) peak; (b) X-ray I#J scan patterns for YBCO (103)/(013) and YSZ (11 l), respectively.
1
In-plane
alignment
YBCO thin films
and critical current of polycrystalline
60-
109
. .
60.
.
40.
30-
.
40
20
0
60
FWHM of YBCO (103)1(013) $ ktn (“1 Fig. 6. In-plane texture correlation between YEXO top layers and YSZ buffer layers.
plane alignment was only 2.5 x lo4 A/cm’. An order of magnitude improvement was achieved in biaxially aligned YBCO thin film with FWHMg of 27”. In order to investigate the correlation between YBCO in-plane texture and J,, a numerical model [l l] was used to calculate the transport critical current density of polycrystalline YBCO thin film. c-axis oriented YBCO thin films grown on polycrystalline substrates contain a single grain along the substrate normal in most area, and each grain usually has five to six neighbors in u-b plane [7]. To simplify the problem, the bridge area for J, measurement is treated as a twodimensional hexagon matrix (sides of the hexagons are chosen to be unity) with m rows of grains, n grains for each odd row and n - 1 grains for each even row. Each grain is c-axis oriented. Its a (or b) axis forms an angle $ with respect to an arbitrary but fixed direction. The intergrain critical current across a grain boundary (indexed by k), j,, is solely determined by the misorientation angle 0 = 1c#+& 1 between the Q (or b) axes of two adjacent grains [5, 6, 141, i.e. j,-k = j=(e). Angle Cpfor each grain in the matrix was determined using a distribution function f&($), bp(4)
= A exp{ -4 ln[A/(A - 0.5)]42/A2} + (1 - A), with 0.5 < A < 1.0
where A is the FWHM of YBCO (103)/(013) X-ray 4 scan and A an adjustable constant. Figure 7 shows the result of the numerical fit of two samples with different FWHM+s. To calculate J,, a
---
numerical M wi+AA-0.97.
A-26.6’
b
a
0 XRDdatr
4
0.0 n -45
7 0 $0
45
-45
0 $(“)
Fig. 7. Numerical fit of the peak profile of YBCO (103)/(013) 4 scan using the function&(d) (see text) (a) for a sample with full-width at half-maximum of 27”, and (b) for a sample with full-width at half-maximum of 58”.
45
110
F. YANGet al.
. ._
II
j o=
3.5x
106Afcm2
0 l
0
(see
ref.
14)
our expefiemental data data from lifima al al. (see ref. 9)
30
60
90
FWHM of YBCO (103)/(013) Q Scan (“) Fig. 8. J, dependence on YBCO in-plane texture. The solid line is the result of the numerical calculation. Filled circles are our experimental data. Two open circles are from Iijima et al. [9].
matrix of 24 x 24 grains was used and 4 for each grain was determined using a random number generator with the distribution functionf&(4). To eliminate the influence from the limited matrix size, for each sample, ten different sets of 4s were generated using the same f&(4) and corresponding J, was calculated. The macroscopic J, was taken as the average value of the ten runs of calculation. Figure 8 shows the calculated result along with the data by four-probe measurements on YBCO thin films with different in-plane alignment. The result calculated from the model fits very well to the experimental data, confirming that the improvement of J, found in biaxially aligned YBCO thin films is directly related to the reduction of the number of high-angle grain boundaries. These grain boundaries have the same current limiting characteristics as those formed on bicrystal substrates.
4. CONCLUSION
We have demonstrated that controlling the in-plane alignment of a and b axes of grains is vital to achieving high critical current density in polycrystalline YBCO thin films. The J, enhancement becomes noticeable as the FWHM of YBCO (103)/(013) 4 scan is reduced to smaller than 55”. The u-b plane texture of YBCO is determined by the in-plane order at the surface of biaxially aligned YSZ prepared by ion beam-assisted pulsed laser deposition. The in-plane order of YSZ surface can be controlled by adjusting ion energy, chamber pressure, and the layer thickness. The numerical calculation reveals that misoriented grain boundaries in polycrystalline YBCO thin films have similar J,-limiting characteristics of the grain boundary in YBCO thin film grown on bicrystal YSZ. It shows that J, of polycrystalline YBCO thin film increases gradually with FWHM of YBCO (103)/(013) 4 scan. For the FWHM N 27”, J, was found to be an order of magnitude higher than the value in uniaxially aligned YBCO thin film. Further improvement in the a (or b) axis alignment will result in more rapid increase in the critical current density of YBCO thin film. AcRnowZedgements-This work was supported in part by New York State Institute on Superconductivity.
REFERENCES 1. E. Narumi, L. W. Song, F. Yang, S. Patel, Y. H. Kao and D. T. Shaw, Appl. Phys. Letf. 56, 2684 (1990). 2. A. Kumar, L. Gananathi, S. M. Kanetkar and J. Narayan. Appt. 3. E. N-i, L. W. Song, P. Yang, S. Patel, Y. H. Kao-and D-T. 4. R. P Reade, X. L. Mao and R. E. Russo, Appl. Phys. Lett. 59, 5. D. Dimos, P Chaudhari, J. Mannhart and F. K. LeGoues, Phys. 6. D. Dimos, l? Chaudhari and J. Mannhart, Phys. Rev. B 41,4038
Phys. Letf. 57, 2594 (1990).
Shaw, Appl. Phys. Lek 58,‘1202 (1991). 739 (1991). Rev. Left. 61, 219 (1988). (1990).
In-plane
alignment
and critical current of polycrystalline
YBCO thin films
111
7. F. Yang, E. Nanuni, S. Pate1 and D. T. Shaw, Appl. Phys. Lett. 60, 249 (1992). 8. 9. 10. 11. 12. 13. 14.
Y. Iijima, N. Tanabe, 0. Kohno and Y. Ikeno, Appl. Phys. Lett. 60, 769 (1992). Y. Iijima, K. Onabe, N. Futaki, N. Tanabe, N. Sadakata, 0. Kohno and Y. Ikeno, J Appl. Phys. 74, 1905 (1993). R. I? Reade, P Berdahl, R. E. Russo and S. M. Garrison, Appl. Phys. Lett. 61, 223 1 (1992). J. Rhyner and G. Blatter, Phys. Rev. B 40, 829 (1989). F. Yang, E. Narumi, S. Pate1 and D. T. Shaw, unpublished. R. M. Bradley, J. M. E. Harper and D. A. Smith, L Appl. Phys. 60, 4160 (1986). Z. G. Ivanov, P. A. Nilsson, D. Winkler, J. A. Alarco, T. Claeson, E. A. Stepantsov and A. Ya. Tzalenchuk, Appl. Phys. Lett. 59, 3030 (1991).
Pergamon
0964-1807(95)00038-O
Printed in Great Britain.All rights reserved 0964-1807/95$9.50+ 0.00
CONTROL OF THE IN-PLANE ALIGNMENT AND THE CRITICAL CURRENT OF POLYCRYSTALLINE YBazCu307_, THIN FILMS F. YANG, E. NARUMI, S. PATEL
and D. T. SHAW
New York State Institute on Superconductivity, 330 Bonner Hall, State University of New York at Buffalo, Amherst, NY 14260, U.S.A. Abstract-Biaxially aligned yttria-stabilized-zirconia (YSZ) buffer layers are prepared on polycrystalline substrates using ion-beam-assisted laser deposition. The effect of directional ion bombardment on in-plane texturing of YSZ layer is investigated by four-circle X-ray diffractometer. Without the assisting ion beam, uniaxially aligned YSZ layers are obtained at room temperature, exhibiting a preferred orientation of the grains with the [OOI] axis perpendicular to the substrate surface. With the assisting ion beam, the in-plane texture of Y SZ layer is altered as the (110) plane tends to align in the direction of the incident ion beam. The X-ray I#Jscan of (111) poles shows that the in-plane alignment of the YSZ layer improves as the ion beam voltage increases in the range 200-450 \! The improvement of YSZ inplane alignment is also observed as the layer thickness increases. On the biaxially aligned YSZ buffer layers, superconducting YBa&@_, thin films are prepared and their transport critical current densities are found to be directly related to the in-plane alignment of the YBCO thin films. Using a numerical method to model the thin film as a matrix of two dimensional grains, a correlation is established between the line shape of X-ray 4 scan and the measured critical current density of polycrystalline YBa2Cu307_, thin film.
1. INTRODUCTION
Deposition of superconductor thin films on polycrystalline metallic substrates with high transport critical current densities (Jc) at liquid nitrogen temperature (77 K) is very important for largescale applications such as electric power transmission and energy storage, etc.. Although J, values as high as IO6 A/cm2 can be routinely obtained at 77 K and 0 T for YBa&u307_, (YBCO) thin films prepared on suitable single crystal substrates, such as yttria-stabilized-zirconia (YSZ), MgO, SrTi03, and LaA103 by various deposition techniques, the J,s for YBCO thin films prepared on polycrystalline metallic substrates are typically reduced by a factor of 100 or so [l-4]. With the support of YSZ buffer layers, YBCO thin films prepared on polycrystalline metallic substrates have good quality of c-axis alignment, but their a and b axes are randomly oriented in the u-b plane from grain to grain. In these uniaxially aligned YBCO thin films, many high-angle grain boundaries inevitably exist and cause the degradation of critical current density across the sample because of their Josephson weak-link nature [5-71. Top reduce the number of high-angle grain boundaries in YBCO thin film on metallic substrate and improve its critical current density, the in-plane a and b axes need to be aligned in addition to c-axis alignment. Such biaxially aligned YBCO growth was realized on biaxially aligned YSZ buffer layer on the substrate, with the buffer layer being prepared by ion beam-assisted deposition (IBAD) [8-lo]. The [OOl] axes of YSZ grains were oriented to the substrate normal, and [ lOO]/ [OIO] axes were aligned in a common azimuthal direction in the substrate plane. The c-axis oriented YBCO thin film grown subsequently was found to inherit the in-plane texture of the underlying buffer layer in a relation YBCO [l IO]//YSZ [ 1001. In this study, we investigated the evolution of YSZ in-plane alignment with ion energy, chamber pressure, and film thickness, and correlated the crystallographic alignment between YSZ buffer layer and YBCO superconductor layer. Using a numerical method [ 11, 121 to model the biaxially aligned YBCO thin film, the in-plane alignment dependence of transport critical current density was calculated and the result was consistent with the result of four-probe measurements.
105
F. YANGet al
106
Excime Laser
I
Pumps
Fig. 1. Schematic diagram of ion beam-assisted deposition system. Two targets are mounted on the holder, one YSZ and one YBCO. During YSZ deposition, the growing film is subject to the simultaneous bombardment of Ar+ ion beam extracted from the ion gun. The ion beam is turned off during the subsequent YBCO deposition.
2. EXPERIMENT
PROCEDURE
The IBAD system is shown in Fig. 1. An ArF excimer laser beam (3, = 193 nm, LPX210, Lambda Physik) was focused at the target. A rotatable target holder was used so that YSZ and YBCO could be deposited alternately. Hastelloy C-276 was used as the substrate, which was painted on the back with silver paste and clamped to the heater block. The substrate was placed at a distance of 10 cm from the target for either YSZ or YBCO deposition. A 3 cm diameter ion gun (model 3.0-1500-100, Ion Tech Inc.) was installed at 7 cm away from the substrate, providing ion bombardment at the substrate surface with the incident angle 8 adjustable from 45” to 75”. The deposition of YSZ buffer layer was carried out at room temperature so that (OO1)instead of (111) oriented Y SZ was formed on polycrystalline substrate. The chamber pressure was controlled in the range from 1.9 to 3.5 mTorr by varying the flow rate of oxygen gas. Ion gun was turned on for YSZ growth and the growing layer was subject to low energy Ar+ bombardment. Due to the simultaneous ion bombardment, in-plane alignment was developed in the YSZ layer. The Ar+ energy was varied from 200 to 450 eV, and beam current was fixed at 10 mA. After formation of a 0.6 ,um thick YSZ buffer layer, the ion gun was turned off and Ar gas supply was stopped. To proceed YBCO growth on the buffer layer, the substrate temperature was set at 650°C and oxygen pressure was increased to 50 mTorr. The YBCO film thickness was 0.4-0.5 pm. A group of c-axis oriented YBCO samples were prepared on Hastelloy substrates covered by (001) YSZ buffer layers deposited at different IBAD conditions. In-plane alignment was determined by X-ray 4 scan measurement for both YSZ and YBCO layers. Full-width at halfmaximum (FWHM) of the 4 scan profile was used to quantitatively describe the degree of inplane alignment. For YBCO layer, (103)/(013) poles were scanned; for YSZ buffer layer, (111) poles were scanned. An 80 pm wide, 500 pm long bridge was patterned on each YBCO sample and the transport critical current density was measured using conventional four-probe method. 3. RESULTS
(A) In-plane
AND
DISCUSSION
texture
All the YSZ buffer layers prepared without IBAD have random in-plane alignment as shown by X-ray 4 scan measurement. With IBAD, YSZ buffer layers show strong enhancement of (001) oriented growth, and the development of in-plane alignment demonstrated by the 4-fold symmetry in the YSZ (111) 4 scan. One of the four peaks in the (111) 4 scan is always found to coincide with the projection of the direction of the ion beam on the (001) plane, indicating that YSZ (110) plane is aligned parallel to the ion beam incidence plane. To study the ion energy effect on in-plane alignment, the incident angle of Ar+ ion beam, 8, was fixed at 55”. The pressure was controlled at 3 mTorr. In Fig. 2, the full-width at half-maximum of
In-plane
alignment
and critical current of polycrystalline
107
YBCO thin films
Ion Energy (eV) Fig. 2. Effect of A? energy on the full-width at half-maximum of YSZ(111) 4 scan (FWHM,). best alignment is shown at an ion energy of 400 eV.
The
the 4 scan (FWHM4) of Y SZ (111) is plotted against ion energy. A minimum is clearly seen at about 400 eV Cross-sectional scanning electron microscopy (SEM) study reveals that YSZ layer thicknesses are very close to one another, about 0.6 pm. Figure 2 clearly show that the in-plane alignment of YSZ layer continues to improve when assisting Ar ion energy is increased up to 400 eV The alignment deterioration at the ion energy above 400 eV was caused by ion bombardment-induced defects. The chamber pressure is an important parameter not only for YSZ deposition, but also for Ar ion beam transport. In Fig. 3, FWHM, of YSZ (111) is plotted against chamber pressure for YSZ made at Ar ion energy of 400 eV In the pressure range investigated, at 3 mTorr YSZ thin film has the narrowest FWHM+. At lower pressure, YSZ layer suffers more severe damage because of excessive ion bombardment. Its crystallographic alignment deteriorates. At higher pressure, the ion beam becomes more turbulent and divergent, which also causes the degradation of YSZ alignment. The thickness dependence of the FWHMb of YSZ (111) is shown in Fig. 4. It shows that YSZ in-plane alignment improves steadily as the layer grows thicker. Based on the assumption that the in-plane alignment was induced by anisotropic ion sputtering yield [ 131, FWHM& at the surface of YSZ layer was estimated to be proportional to t-1’2, where t is the thickness. However,
1
2
3
4
Chamber Pressure (mtorr) Fig. 3. Effect of chamber pressure on the full-width at half-maximum of YSZ (111) 4 scan (FWHM,).
F. YANG et al.
108
l
$ scan data
-
least square
---
calculated
fit
(cct-1'3 )
(c&l12
)
lo0.1
1
YSZ Thickness, t (pm) Fig. 4. The full-width at half-maximum of YSZ (111) 4 scan (FWI-IM+) on buffer layers prepared by IBAD as a function of thickness in a log-log plot. The data points follow a t-*” dependence. The FWHM+ of YSZ surface layer is predicted to be proportional to t-l’* (see Ref. 12). t is the thickness of the YSZ buffer layer.
experimental data of FWHM+ show a t-“3 dependence as shown in Fig. 4. The increase in deviation for thicker YSZ layer is due to the fact that in X-ray 4 scan measurement, the whole layer of YSZ contributes to the diffraction signal and the corresponding FWHM+ The difference increases between the peak breadth of the 4 scan for the surface and that for the whole layer as it grows thicker. The texture of the YBCO top layer is determined by the YSZ buffer layer. Figure 5a is a typical X-ray diffraction pattern for a YBCO thin film prepared on Hastelloy substrate with a biaxially aligned YSZ buffer layer, indicating good c-axis alignment. The alignment in the YBCO u-b plane is displayed by an X-ray 4 scan profile, shown in Fig. 5b. The FWHM is 34” in YSZ (111) and 27” in YBCO (103)/(013) 4 scan. The in-plane texture of YBCO thin film has a relation YBCO[l lO]/iYSZ[lOO] with respect to YSZ buffer layer, similar to YBCO thin films grown on single crystal YSZ substrates. Figure 6 shows the correlation between the in-plane texture of YSZ buffer layers and the YBCO top layers. (B) Critical current density
For the sample shown in Fig. 5, J, was found to be 2 x lo5 A/cm2 at 77 K by four-probe measurement. In comparison, the J, value for a 0.4 pm thick YBCO thin film with random in-
YBCO
a
50
28 (“1
70
-180
(103)/(013)
-90
b
0
90
$0
Fig. 5. The crystallographic structure of YBCO thin film deposited on the biaxially aligned YSZ buffer layer: (a) X-ray 20 diffraction pattern, showing YBCO (001) peaks and YSZ (200) peak; (b) X-ray I#J scan patterns for YBCO (103)/(013) and YSZ (11 l), respectively.
1
In-plane
alignment
YBCO thin films
and critical current of polycrystalline
60-
109
. .
60.
.
40.
30-
.
40
20
0
60
FWHM of YBCO (103)1(013) $ ktn (“1 Fig. 6. In-plane texture correlation between YEXO top layers and YSZ buffer layers.
plane alignment was only 2.5 x lo4 A/cm’. An order of magnitude improvement was achieved in biaxially aligned YBCO thin film with FWHMg of 27”. In order to investigate the correlation between YBCO in-plane texture and J,, a numerical model [l l] was used to calculate the transport critical current density of polycrystalline YBCO thin film. c-axis oriented YBCO thin films grown on polycrystalline substrates contain a single grain along the substrate normal in most area, and each grain usually has five to six neighbors in u-b plane [7]. To simplify the problem, the bridge area for J, measurement is treated as a twodimensional hexagon matrix (sides of the hexagons are chosen to be unity) with m rows of grains, n grains for each odd row and n - 1 grains for each even row. Each grain is c-axis oriented. Its a (or b) axis forms an angle $ with respect to an arbitrary but fixed direction. The intergrain critical current across a grain boundary (indexed by k), j,, is solely determined by the misorientation angle 0 = 1c#+& 1 between the Q (or b) axes of two adjacent grains [5, 6, 141, i.e. j,-k = j=(e). Angle Cpfor each grain in the matrix was determined using a distribution function f&($), bp(4)
= A exp{ -4 ln[A/(A - 0.5)]42/A2} + (1 - A), with 0.5 < A < 1.0
where A is the FWHM of YBCO (103)/(013) X-ray 4 scan and A an adjustable constant. Figure 7 shows the result of the numerical fit of two samples with different FWHM+s. To calculate J,, a
---
numerical M wi+AA-0.97.
A-26.6’
b
a
0 XRDdatr
4
0.0 n -45
7 0 $0
45
-45
0 $(“)
Fig. 7. Numerical fit of the peak profile of YBCO (103)/(013) 4 scan using the function&(d) (see text) (a) for a sample with full-width at half-maximum of 27”, and (b) for a sample with full-width at half-maximum of 58”.
45
110
F. YANGet al.
. ._
II
j o=
3.5x
106Afcm2
0 l
0
(see
ref.
14)
our expefiemental data data from lifima al al. (see ref. 9)
30
60
90
FWHM of YBCO (103)/(013) Q Scan (“) Fig. 8. J, dependence on YBCO in-plane texture. The solid line is the result of the numerical calculation. Filled circles are our experimental data. Two open circles are from Iijima et al. [9].
matrix of 24 x 24 grains was used and 4 for each grain was determined using a random number generator with the distribution functionf&(4). To eliminate the influence from the limited matrix size, for each sample, ten different sets of 4s were generated using the same f&(4) and corresponding J, was calculated. The macroscopic J, was taken as the average value of the ten runs of calculation. Figure 8 shows the calculated result along with the data by four-probe measurements on YBCO thin films with different in-plane alignment. The result calculated from the model fits very well to the experimental data, confirming that the improvement of J, found in biaxially aligned YBCO thin films is directly related to the reduction of the number of high-angle grain boundaries. These grain boundaries have the same current limiting characteristics as those formed on bicrystal substrates.
4. CONCLUSION
We have demonstrated that controlling the in-plane alignment of a and b axes of grains is vital to achieving high critical current density in polycrystalline YBCO thin films. The J, enhancement becomes noticeable as the FWHM of YBCO (103)/(013) 4 scan is reduced to smaller than 55”. The u-b plane texture of YBCO is determined by the in-plane order at the surface of biaxially aligned YSZ prepared by ion beam-assisted pulsed laser deposition. The in-plane order of YSZ surface can be controlled by adjusting ion energy, chamber pressure, and the layer thickness. The numerical calculation reveals that misoriented grain boundaries in polycrystalline YBCO thin films have similar J,-limiting characteristics of the grain boundary in YBCO thin film grown on bicrystal YSZ. It shows that J, of polycrystalline YBCO thin film increases gradually with FWHM of YBCO (103)/(013) 4 scan. For the FWHM N 27”, J, was found to be an order of magnitude higher than the value in uniaxially aligned YBCO thin film. Further improvement in the a (or b) axis alignment will result in more rapid increase in the critical current density of YBCO thin film. AcRnowZedgements-This work was supported in part by New York State Institute on Superconductivity.
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