- Email: [email protected]
Preparation of a-axis-oriented YBa2Cu3Ox superconducting films on MgO substrates with PrBa2Cu3Ox buffer layers
PHYSlCA ELSEVIER
Physica C 267 (1996)330-336
Preparation of a-axis-oriented YBa 2Cu 3Ox superconducting films on MgO substrates with P r B a 2 C u 3 0 x buffer layers Toshiharu M i n a m i k a w a a,b Mitsutoshi Tazoe a Kazuhito S e g a w a b Yasuto Y o n e z a w a h Akiharu Morimoto a,* Tatsuo S h i m i z u a a Department of Electrical and Computer Engineering, Faculty of Engineering, Kanazawa University, Kanazawa 920, Japan b Industrial Research Institute oflshikawa, Kanazawa 920-02, Japan Received 5 January 1996
Abstract
We prepared high-quality a-axis-oriented superconducting YBa2Cu30 x (YBCO) films on (100)MgO substrates using the c-axis-oriented PrBa2Cu30 x (PBCO) buffer layer and the a-axis-oriented cubic YBCO self-template layer. The YBCO template layers deposited at a low substrate temperature (T~) on MgO substrates without the PBCO buffer layers were found to turn into orientation other than a-axis orientation by annealing at a higher temperature after the deposition. The in-plane aligned c-axis-oriented PBCO layer was inserted between the a-axis-oriented cubic YBCO layer and the MgO substrate to stabilize the template layer. As a result, the YBCO film deposited at higher T~ on these layers was found to be a-axis-oriented with a full width at a half-maximum (FWHM) of the (200) rocking curve of 0.58 °, an FWHM of (200) diffraction of 0.18 °, a critical onset temperature around 90 K and a critical zero temperature of 70 K. Keywords: a-Axis orientation; YBa2Cu30 ~ superconducting films; MgO substrates; PrBa2Cu30 x buffer layer; Pulsed laser ablation
1. Introduction a-axis-oriented superconducting films are expected to be used for sandwich-type Josephson junction devices [1,2] because of the longer coherence length along the a-axis than that along the c-axis. For preparation of a-axis-oriented Y B a 2 C u 3 0 x (YBCO) thin films, a variety of substrates have been used by considering the lattice constant and the thermal expansion coefficient. (110) NdGaO 3, (100) LaA103, (100) SrTiO 3 are known to be substrates for a-axis-oriented growth of YBCO because of the
* Corresponding author. Fax: + 81 762 34 4876.
small lattice mismatch between substrate and YBCO film [3,4]. However, they have not been widely used because of the high cost. Generally, YBCO films deposited directly on the substrates at low substrate temperature (T~) are (n00)-oriented cubic without the superconducting transition [5], and the films deposited at high T~ are c-axis-oriented three-layered perovskite with high Tc [6]. Then, a high-quality a-axis-oriented superconducting film was prepared by the self-template technique using the cubic YBCO layer [7]. MgO is known to be a good substrate for the epitaxial growth of a variety of oxides. It is easy to get high-quality in-plane aligned c-axis-oriented superconducting YBCO films on MgO substrates. It is, however,
0921-4534/96/$15.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved PH S 0 9 2 1 - 4 5 3 4 ( 9 6 ) 0 0 3 6 5 - 6
T. Minamikawa et a l . / Physica C 267 (1996) 330-336
difficult to get a-axis-oriented superconducting YBCO films on MgO substrates by the template technique because of the stability of the template layer as presented in this report. For improving the stability of the template layer, a buffer layer will be required. Lattice constants and crystal structure of PrBa2Cu30 x (PBCO) are similar to those of YBCO [8]. Then, the a-axis length of a PBCO film is closely one third of the c-axis length of an YBCO film. This consideration leads us to the possibility that c-axis-oriented PBCO films can be used as a buffer layer inserted between the template layer and the MgO substrate. Hontsu et al. reported that an a-axis-oriented YBCO layer was grown on a lattice-matched SrTiO 3 substrate using an a-axis-oriented PBCO template layer [9]. The a-axis-oriented YBCO layer is expected to be easily grown on a-axis-oriented PBCO because of the equivalent crystal structure. However, it is interesting to study the growth of an a-axis-oriented YBCO film on the c-axis-oriented PBCO layer. In this study, we prepared a-axis-oriented YBCO films using the self-template technique with YBCO on MgO substrates employing the c-axis-oriented PBCO buffer layer by pulsed laser ablation (PLA). First, the PBCO buffer layer was examined for stabilizing the template layer, and then the preparation of a-axis-oriented films was carried out.
2.
Experimental
Fig. l(a) shows a sample structure for preparing an a-axis-oriented YBCO film on a (100) MgO substrate with a c-axis-oriented PBCO buffer layer and an a-axis-oriented cubic YBCO self-template layer. We designate the c-axis-oriented PBCO buffer layer, the a-axis-oriented cubic YBCO self-template layer and the a-axis-oriented YBCO layer as PBCO layer, a-axis cubic YBCO layer and a-axis YBCO layer, respectively. Here the a-axis-oriented cubic YBCO implies cubic YBCO with the [100] direction perpendicular to the substrate surface. The substrate temperatures of the PBCO layer, the a-axis cubic YBCO layer and the a-axis YBCO layer are denoted by Tp, TVL and TvH, respectively. All the film depositions were carried out at oxygen ambient of 40 Pa by ArF excimer laser ablation (Shibuya SQL
331
a-a~s-odentedYBCOlayer (a-axisYBCOlayer)
TYH
a-axis-orientedcubicYBCOtemplatelayer (a-a~s cubicYBCO layer/ c-axis-orientedPBCO bufferlayer(PBCOlayer)
MgO Substrate (a)Layered structure
I T-~
heatingup/ 10*C/min T~
1 Hz
D..
n
coo,,n ,nO.O m
Dep.
rain 50nm
30
time
(b)Temperature Profile Fig. 1. Sample structure (a) and temperature profile for deposition of a cubic YBCO layer and an a-axis-oriented YBCO layer (b).
2240). The laser beam was shot at 1 Hz and focused on the target with a fluence of 1.5 J//cm 2. These preparation conditions of layers are the same as the condition of c-axis-oriented YBCO films in our previous paper [6] except for the substrate temperature. Substrates used were (100) MgO single crystals annealed at 1200°C, because reproducible and high-quality c-axis-oriented films were found to be prepared on the MgO substrates by PLA [6]. Prior to the deposition of the two YBCO layers, the crystallinity of all the PBCO films was examined by the X-ray diffraction (XRD) technique. The a-axis cubic YBCO and the a-axis YBCO layers were deposited in accordance with the temperature profile as shown in Fig. l(b). Film thicknesses of the PBCO layer, the a-axis cubic YBCO layer and the a-axis YBCO layer are 50, 50 and 100 nm, respectively. The substrate temperatures for deposition of the PBCO layer (Tp) and the cubic layer (TvL) were
332
T. Minamikawa et a l . / Physica C 267 (1996) 330-336
with PBCO o_ ~" ~"
(d)
0.
v
t-" (D
680°C anneal
(b)
t-
>-
(a) I
20
,
I
3O
°2 ~
J as-dep. --I--
40
I
50
20(deg) Fig. 2. XRD spectra of the films on MgO substrates deposited at 560°C; film without the PBCO layer (a), film annealed at 680°C after the deposition of an a-axis-oriented cubic YBCO layer without the PBCO layer (b), film deposited at 680°C without PBCO layer (c), and film annealed at 700°C after the deposition with the PBCO layer (d).
optimized for preparing high-quality a-axis-oriented superconducting YBCO films, by means of 0 - 2 0 and to scans of XRD measurements and the conventional four-probe measurement.
3. Results and discussion
3.1. Effect of the PBCO buffer layer First, for investigating the effect of the PBCO layer, the a-axis-oriented cubic YBCO films were prepared at a low substrate temperature directly on MgO substrates. XRD spectra of the films deposited at a substrate temperature of 560°C show the largest (200) diffraction, probably coming from the cubic phase [8]. The films deposited at this temperature, however, did not offer superconductivity with a high T~. Fig. 2 shows the XRD spectra of the films on MgO substrates deposited at 560°C; the film without
the PBCO layer (a), the film annealed at 680°C after the deposition of the a-axis-oriented cubic YBCO without the PBCO layer (b), the film deposited at 680°C without PBCO layer (c), and the film annealed at 700°C after the deposition with the PBCO layer (d). When the a-axis-oriented cubic YBCO film without the PBCO buffer layer was annealed at a higher temperature above 560°C, the film does not keep the a-axis orientation. In other words, the a-axis-oriented cubic YBCO layer turned into a random orientation by the high-temperature treatment. Thus, this self-template YBCO layer without the PBCO buffer layer is not suitable for the growth of a-axis-oriented YBCO films on MgO substrates. Then, we tried to insert the c-axis-oriented PBCO buffer layer between the a-axis-oriented cubic YBCO film and the MgO substrate for stabilizing the interface. Similarly to the YBCO film, the PBCO film was grown to be c-axis-orientated at a high substrate temperature on a MgO substrate. The PBCO film shows cube-on-cube growth on the MgO substrate without any rotation in the film plane at 770°C [10]. We carried out the characterization of a low-temperature deposited YBCO film on a MgO substrate with an in-plane aligned c-axis-oriented PBCO buffer layer deposited at 770°C. Fig. 2(d) shows the XRD spectra of the a-axis-oriented cubic YBCO film which was deposited at 560°C on the MgO substrate with the PBCO layer and then annealed at a high temperature of 700°C. The YBCO (200) diffraction is recognized around 20 = 47.6 ° even after 700°C annealing. This suggests that the a-axis-oriented cubic YBCO template layer on the MgO substrate with the PBCO buffer layer keeps the orientation even at high temperature for high-temperature YBCO film deposition. This result clearly indicates that the c-axis-oriented PBCO buffer layer is essential for keeping the cubic YBCO template layer with a favorable orientation at high temperature. This distinguished effect of the PBCO buffer layer is probably attributed to a stabilized interface between the c-axis-oriented PBCO buffer layer and the a-axis cubic YBCO template layer. The interface between the a-axis cubic YBCO template layer and the MgO substrate is not stable enough for high-temperature treatment because of the large lattice mismatch between them and the instability of the a-axis cubic YBCO itself.
T. Minamikawa et a l . / Physica C 267 (1996)330-336
When the c-axis-oriented PBCO film on MgO substrates was prepared at high temperature, the PBCO film is expected to be grown stably with a strong interaction between the substrate and the film by means of the growth mechanism of the step flow [11], and the lattice mismatch between the substrate and the film is expected to be gradually relaxed from the bottom to the top of the PBCO film adjusting the oxygen content in the CuO chain. On the other hand, when the a-axis-oriented cubic YBCO films on the MgO substrate were prepared at a low temperature, the crystallinity of the film appears to be poor and then the interface between the substrate and the film appears to be unstable. Therefore, when the a-axis cubic YBCO film was treated at a higher temperature, the crystal orientation of the interface layer was expected to be turned into the other stable orientation in order to release the stress at the interface.
3.2. Preparation condition dependence of PBCO and cubic YBCO layers The substrate temperatures for the PBCO layer and the a-axis cubic YBCO layer were optimized for obtaining high-quality a-axis-oriented YBCO films using the self-template technique on the MgO substrate with the PBCO layer. The substrate temperature for the a-axis YBCO layer TyH was fixed at TVH -- 670°C throughout these experiments, because the temperature was the lowest one for obtaining high-quality c-axis-oriented superconducting YBCO films on MgO substrates [6]. An excessively high substrate temperature is supposed to give rise to a c-axis-oriented YBCO layer. First, the a-axis YBCO films were examined varying the substrate temperature of the PBCO layer. The substrate temperature for the a-axis cubic YBCO layer was fixed at TVL = 560°C, because the a-axis cubic YBCO film on the MgO substrate showed the strongest (200) diffraction at this temperature [6]. Fig. 3 shows the substrate temperature (Tp) dependence of the XRD spectra for the a-axis Y B C O / a axis cubic Y B C O / P B C O sample. The substrate temperatures of the two YBCO layers were fixed at TyL = 560°C and TyH = 670°C, and then only the temperature for the PBCO layer Tp was varied; Tp = 700°C (a), Tp --- 740°C (b), Tp = 770°C (c) and Tp = 800°C (d). All the spectra clearly show a strong
'
333
TYH=670°C
I
(d)
•
I
'
I
TyL=5600C
Tp=800°C
(c) Tp=770°C
lw ~..
• = c-
(b) Tp=740°C o
1-
(a)
~"
o.
o= ,
20
30
40
j
50
20(deg) Fig. 3. Substrate temperature Tp dependence of XRD spectra for a-axis Y B C O / a - a x i s cubic YBCO/PBCO. The substrate temperatures of both YBCO layers are fixed at TyL = 560°C and TyH = 670°C, and then only the temperature of the PBCO layer was varied, Tp = 700°C (a), Tp = 740°C (b), Tp = 770°C (c) and Tp = 800°C (d).
!
I
TyH=6700C TL=560°C • Tp=700°C zx Tp=740°C o TP=770°C
g E: rc
o
o ~ ~~ •Z~b Tc(onset)=901~
V~o
T p=800°C
• Z~ • "
0
O
~ T c ( z e r o l i
,
,
I
50
,
i
,
=70K ,
I
i
100
Temperature(K) Fig. 4. Normalized resistance versus temperature for a-axis Y B C O / a - a x i s cubic Y B C O / P B C O , where the samples used were the same ones as shown in Fig. 3.
334
T. Minamikawa et al./ Physica C 267 (1996)330-336
(200) diffraction from the a-axis YBCO layer and almost no (002) diffraction around 2 0 = 15° coming from the c-axis-oriented YBCO. The volume fraction of c-axis-oriented YBCO grains is estimated to be less than 3%. Only this (002) diffraction of c-axis-oriented YBCO is not obscured by the (002) diffraction of c-axis-oriented PBCO because of the very weak intensity of the PBCO (002) diffraction due to the extinction law of XRD. When the buffer layer is an in-plane aligned c-axis-oriented PBCO film prepared at 770°C, the a-axis YBCO layer with the PBCO layer shows the largest shift of the (200) diffraction toward the high angle. This suggests that the film has the maximum oxygen content. This film showed a full width at half-maximum (FWHM) of the (200) rocking curve of 0.58 °, which was similar to that of the PBCO layer, and showed an FWHM of the (200) diffraction of 0.18 °. Fig. 4 shows the normalized resistance versus t~/nperature for the a # axis YBCO/a-axis cubic-YB)CO/PBCO samples, where samples used were the same ones as shown in Fig. 3. The a-axis YBCO layer with a PBCO layer deposited at 770°C shows a critical onset temperature above 90 K and a critical zero temperature of 70 K. Fig. 5 shows T~zero) and the a-axis length derived from the XRD spectra for the YBCO film as a function of the substrate temperature for the PBCO layer Tp. Tc(zero) strongly depends on the a-axis length of the a-axis YBCO film. This suggests that
. . . . 80 .TH 6~6o d
= 560°C
T
,0
I
..
YL
'
'
'
i
,
./\
....
60 ~
3.84
•
',,
3.83 •
"~
3.82
'o
50
/ •/
40 , , , 680
• .• a-'~is - % ' . . . . . . .
720
760
' '
g ¢ i
o~
3,81
800
T (°C) P
Fig. 5. To(zero) and a-axis length derived from XRD spectra for a-axis YBCO/a-axis cubic YBCO/PBCO as a function of the substrate temperatureTp for the PBCO layer.
I
!
T
(c)
(G)
YH
T =770°C
iTyL=580°C
>,
!
=670°C
P
'g
(b)
F =560°C YL
°
tin 0
0
(a) T
0
=
=540°C
YL
h
20
_
,
30
Jkl
40
J
~.
I
50
20(deg) Fig. 6. XRD spectra for the a-axis YBCO/a-axis cubic YBCO/PBCO samples prepared at various TyL for the a-axis cubic YBCO layer. The substrate temperatures of the PBCO layer and the a-axis layer are fixed at Tp = 770°C and TvH = 670°C, respectively, and then only the temperature of the cubic layer was v a r i e d , T y L = 5 4 0 ° C (a), T y L = 5 6 0 ° C (b) a n d T y L = 5 8 0 ° C (c).
the superconductivity really comes from a-axis-oriented YBCO grains. The results of the XRD and the resistivity measurements revealed that the PBCO layer deposited at 770°C is the best one for a good a-axis-oriented YBCO layer. The degradation of the superconductivity of the YBCO layer with the PBCO layer prepared at 800°C is attributed to the presence of the 45 ° rotated grains of the PBCO layer confirmed by the XRD 4' measurement Secondly, the a-axis-oriented YBCO films were examined varying the substrate temperature of the a-axis cubic YBCO layers TVL. The substrate temperature of the PBCO layer was fixed at Tp = 770°C, based on the above results. Fig. 6 shows XRD spectra for samples prepared at various TyL for the a-axis cubic YBCO layer. The substrate temperatures of the PBCO layer and the a-axis YBCO layer were fixed at Tp = 770°C and TyH = 670°C, respectively, and then only the substrate temperature for the a-axis cubic YBCO layer was varied, TyL = 540°C (a), TyL = 560°C (b) and TyL = 580°C (c). The a-axis YBCO film with the a-axis cubic YBCO layer prepared at 560°C shows the strongest intensity of the
T. Minamikawa et aL / Physica C 267 (1996) 330-336 I
T
YH
I
=670°C
T =770°C
V
P
T
8 E rr"
O
=540°C
YL
o
T
=560°C
V
TyL=580°C
YL
W v
41, 4b ql,
v
• ° 0° v 00
50
100
Temperature(K) Fig. 7. Normalized resistance versus temperature for a-axis YBCO/a-axis cubic YBCO/PBCO, where the samples used were the same ones as shown in Fig. 6.
335
4. Conclusion YBCO template layers deposited at a low substrate temperature (T~) on MgO substrates without PBCO buffer layers were found to turn into an orientation other than a-axis orientation by annealing at higher temperatures after the deposition. YBCO films with a-axis orientation have been prepared on MgO substrates using an a-axis-oriented cubic YBCO self-template layer and the buffer layer of in-plane aligned c-axis-oriented PBCO. The film showed an FWHM of the rocking curve of 0.58 ° and an FWHM of the (200) diffraction of 0.18 ° and had a critical onset temperature above 90 K and a critical zero temperature 70 K. This successful preparation of an a-axis-oriented YBCO film on MgO with a large lattice mismatch is brought about by employing a c-axis-oriented PBCO buffer layer.
Acknowledgements (200) peak. Fig. 7 shows the normalized resistance versus temperature for a-axis YBCO films on cubic-YBCO/PBCO, where samples used were the same ones as shown in Fig. 6. The film with the cubic YBCO layer prepared at 560°C offers a critical onset temperature above 90 K and a critical zero temperature 70 K. This curve is the same one shown in Fig. 4. Both the degradations of the superconductivity for the YBCO films with TyL = 540 and 580°C are attributed to the degradation of the crystallinity of the a-axis cubic YBCO layer, as we expected based on the results of our previous paper [6]. The crystal structure of YBCO is similar to that of PBCO, The crystallinity of the c-axis-oriented YBCO films on MgO substrates is better than that of the c-axis-oriented PBCO films on MgO substrates. So, when the a-axis-oriented YBCO films are prepared on MgO substrates with a c-axis-oriented YBCO buffer layer and a a-axis-oriented cubic YBCO layer, it must be possible to prepare higher-quality a-axisoriented superconducting YBCO films. Based on the above consideration, a c-axis-oriented YBCO layer, instead of the c-axis-oriented PBCO layer, is another candidate for the buffer layer. The c-axis-oriented YBCO layer, however, causes confusion for characterizing the a-axis-oriented YBCO layer.
We thank M. Kumeda of Kanazawa University for helpful discussions. We also thank H. Yoshida of the Industrial Research Institute of Ishikawa for helping with our experiments. We are grateful to Shibuya Kogyo Co., Ltd. for supplying the excimer laser system. This work was supported in part by the Grant-in-Aid for Developmental Scientific Research No. 07555499 from the Ministry of Education, Science, Sports and Culture of Japan.
References [1] J.B. Barner, C.T. Rogers, A. Inam, R. Ramesh and Bersey, Appl. Phys. Leu. 59 (1991) 742. [2] T. Hashimoto, M. Sagoi, Y. Mizutani, J. Yoshida and K. Mizushima, Appl, Phys. Lett. 60 (1992) 1756. [3] C.B. Eom, A.F. Marshall, S.S. Laderman, R.D. Jacowitz and T.H. Geballe, Science 249 (1990) 1549. [4] K.H. Young and J.Z. Sun, Appl. Phys. Lett. 59 (1991) 2448. [5] J.A. Agostinelli, S. Chen and G. Braunstein, Phys. Rev. B 43 (1991) 11396. [6] T. Minamikawa, T. Suzuki, Y. Yonezawa, K. Segawa, A. Morimoto and T. Shimizu, Jpn. J. Appl. Phys. 34 (1995) 4038. [7] A. lnam, C.T. Rogers, R. Ramesh, K. Remschnig L. Farrow, D. Hart, T. Venkatesan and B. Wilkens, Appl. Phys. Lett. 57 (1990) 2484.
336
T. Minamikawa et aL / Physica C 267 (1996) 330-336
[8] T.C. Poon and J. Gao, Supercond. Sci. Technol. 8 (1995) 415. [9] S. Hontsu, N. Mukai, J. Ishii T. Kawai and S. Kawai, Appl. Phys. Lett. 63 (1993) 1576.
[10] T. Minamikawa, M. Tazoe, A. Morimoto and T. Shimizu, unpublished. [11] M.G. Norton, S.R. Summerfelt and C.B. Carter, Appl. Phys. Lett. 56 (1990) 2246.
Physica C 267 (1996)330-336
Preparation of a-axis-oriented YBa 2Cu 3Ox superconducting films on MgO substrates with P r B a 2 C u 3 0 x buffer layers Toshiharu M i n a m i k a w a a,b Mitsutoshi Tazoe a Kazuhito S e g a w a b Yasuto Y o n e z a w a h Akiharu Morimoto a,* Tatsuo S h i m i z u a a Department of Electrical and Computer Engineering, Faculty of Engineering, Kanazawa University, Kanazawa 920, Japan b Industrial Research Institute oflshikawa, Kanazawa 920-02, Japan Received 5 January 1996
Abstract
We prepared high-quality a-axis-oriented superconducting YBa2Cu30 x (YBCO) films on (100)MgO substrates using the c-axis-oriented PrBa2Cu30 x (PBCO) buffer layer and the a-axis-oriented cubic YBCO self-template layer. The YBCO template layers deposited at a low substrate temperature (T~) on MgO substrates without the PBCO buffer layers were found to turn into orientation other than a-axis orientation by annealing at a higher temperature after the deposition. The in-plane aligned c-axis-oriented PBCO layer was inserted between the a-axis-oriented cubic YBCO layer and the MgO substrate to stabilize the template layer. As a result, the YBCO film deposited at higher T~ on these layers was found to be a-axis-oriented with a full width at a half-maximum (FWHM) of the (200) rocking curve of 0.58 °, an FWHM of (200) diffraction of 0.18 °, a critical onset temperature around 90 K and a critical zero temperature of 70 K. Keywords: a-Axis orientation; YBa2Cu30 ~ superconducting films; MgO substrates; PrBa2Cu30 x buffer layer; Pulsed laser ablation
1. Introduction a-axis-oriented superconducting films are expected to be used for sandwich-type Josephson junction devices [1,2] because of the longer coherence length along the a-axis than that along the c-axis. For preparation of a-axis-oriented Y B a 2 C u 3 0 x (YBCO) thin films, a variety of substrates have been used by considering the lattice constant and the thermal expansion coefficient. (110) NdGaO 3, (100) LaA103, (100) SrTiO 3 are known to be substrates for a-axis-oriented growth of YBCO because of the
* Corresponding author. Fax: + 81 762 34 4876.
small lattice mismatch between substrate and YBCO film [3,4]. However, they have not been widely used because of the high cost. Generally, YBCO films deposited directly on the substrates at low substrate temperature (T~) are (n00)-oriented cubic without the superconducting transition [5], and the films deposited at high T~ are c-axis-oriented three-layered perovskite with high Tc [6]. Then, a high-quality a-axis-oriented superconducting film was prepared by the self-template technique using the cubic YBCO layer [7]. MgO is known to be a good substrate for the epitaxial growth of a variety of oxides. It is easy to get high-quality in-plane aligned c-axis-oriented superconducting YBCO films on MgO substrates. It is, however,
0921-4534/96/$15.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved PH S 0 9 2 1 - 4 5 3 4 ( 9 6 ) 0 0 3 6 5 - 6
T. Minamikawa et a l . / Physica C 267 (1996) 330-336
difficult to get a-axis-oriented superconducting YBCO films on MgO substrates by the template technique because of the stability of the template layer as presented in this report. For improving the stability of the template layer, a buffer layer will be required. Lattice constants and crystal structure of PrBa2Cu30 x (PBCO) are similar to those of YBCO [8]. Then, the a-axis length of a PBCO film is closely one third of the c-axis length of an YBCO film. This consideration leads us to the possibility that c-axis-oriented PBCO films can be used as a buffer layer inserted between the template layer and the MgO substrate. Hontsu et al. reported that an a-axis-oriented YBCO layer was grown on a lattice-matched SrTiO 3 substrate using an a-axis-oriented PBCO template layer [9]. The a-axis-oriented YBCO layer is expected to be easily grown on a-axis-oriented PBCO because of the equivalent crystal structure. However, it is interesting to study the growth of an a-axis-oriented YBCO film on the c-axis-oriented PBCO layer. In this study, we prepared a-axis-oriented YBCO films using the self-template technique with YBCO on MgO substrates employing the c-axis-oriented PBCO buffer layer by pulsed laser ablation (PLA). First, the PBCO buffer layer was examined for stabilizing the template layer, and then the preparation of a-axis-oriented films was carried out.
2.
Experimental
Fig. l(a) shows a sample structure for preparing an a-axis-oriented YBCO film on a (100) MgO substrate with a c-axis-oriented PBCO buffer layer and an a-axis-oriented cubic YBCO self-template layer. We designate the c-axis-oriented PBCO buffer layer, the a-axis-oriented cubic YBCO self-template layer and the a-axis-oriented YBCO layer as PBCO layer, a-axis cubic YBCO layer and a-axis YBCO layer, respectively. Here the a-axis-oriented cubic YBCO implies cubic YBCO with the [100] direction perpendicular to the substrate surface. The substrate temperatures of the PBCO layer, the a-axis cubic YBCO layer and the a-axis YBCO layer are denoted by Tp, TVL and TvH, respectively. All the film depositions were carried out at oxygen ambient of 40 Pa by ArF excimer laser ablation (Shibuya SQL
331
a-a~s-odentedYBCOlayer (a-axisYBCOlayer)
TYH
a-axis-orientedcubicYBCOtemplatelayer (a-a~s cubicYBCO layer/ c-axis-orientedPBCO bufferlayer(PBCOlayer)
MgO Substrate (a)Layered structure
I T-~
heatingup/ 10*C/min T~
1 Hz
D..
n
coo,,n ,nO.O m
Dep.
rain 50nm
30
time
(b)Temperature Profile Fig. 1. Sample structure (a) and temperature profile for deposition of a cubic YBCO layer and an a-axis-oriented YBCO layer (b).
2240). The laser beam was shot at 1 Hz and focused on the target with a fluence of 1.5 J//cm 2. These preparation conditions of layers are the same as the condition of c-axis-oriented YBCO films in our previous paper [6] except for the substrate temperature. Substrates used were (100) MgO single crystals annealed at 1200°C, because reproducible and high-quality c-axis-oriented films were found to be prepared on the MgO substrates by PLA [6]. Prior to the deposition of the two YBCO layers, the crystallinity of all the PBCO films was examined by the X-ray diffraction (XRD) technique. The a-axis cubic YBCO and the a-axis YBCO layers were deposited in accordance with the temperature profile as shown in Fig. l(b). Film thicknesses of the PBCO layer, the a-axis cubic YBCO layer and the a-axis YBCO layer are 50, 50 and 100 nm, respectively. The substrate temperatures for deposition of the PBCO layer (Tp) and the cubic layer (TvL) were
332
T. Minamikawa et a l . / Physica C 267 (1996) 330-336
with PBCO o_ ~" ~"
(d)
0.
v
t-" (D
680°C anneal
(b)
t-
>-
(a) I
20
,
I
3O
°2 ~
J as-dep. --I--
40
I
50
20(deg) Fig. 2. XRD spectra of the films on MgO substrates deposited at 560°C; film without the PBCO layer (a), film annealed at 680°C after the deposition of an a-axis-oriented cubic YBCO layer without the PBCO layer (b), film deposited at 680°C without PBCO layer (c), and film annealed at 700°C after the deposition with the PBCO layer (d).
optimized for preparing high-quality a-axis-oriented superconducting YBCO films, by means of 0 - 2 0 and to scans of XRD measurements and the conventional four-probe measurement.
3. Results and discussion
3.1. Effect of the PBCO buffer layer First, for investigating the effect of the PBCO layer, the a-axis-oriented cubic YBCO films were prepared at a low substrate temperature directly on MgO substrates. XRD spectra of the films deposited at a substrate temperature of 560°C show the largest (200) diffraction, probably coming from the cubic phase [8]. The films deposited at this temperature, however, did not offer superconductivity with a high T~. Fig. 2 shows the XRD spectra of the films on MgO substrates deposited at 560°C; the film without
the PBCO layer (a), the film annealed at 680°C after the deposition of the a-axis-oriented cubic YBCO without the PBCO layer (b), the film deposited at 680°C without PBCO layer (c), and the film annealed at 700°C after the deposition with the PBCO layer (d). When the a-axis-oriented cubic YBCO film without the PBCO buffer layer was annealed at a higher temperature above 560°C, the film does not keep the a-axis orientation. In other words, the a-axis-oriented cubic YBCO layer turned into a random orientation by the high-temperature treatment. Thus, this self-template YBCO layer without the PBCO buffer layer is not suitable for the growth of a-axis-oriented YBCO films on MgO substrates. Then, we tried to insert the c-axis-oriented PBCO buffer layer between the a-axis-oriented cubic YBCO film and the MgO substrate for stabilizing the interface. Similarly to the YBCO film, the PBCO film was grown to be c-axis-orientated at a high substrate temperature on a MgO substrate. The PBCO film shows cube-on-cube growth on the MgO substrate without any rotation in the film plane at 770°C [10]. We carried out the characterization of a low-temperature deposited YBCO film on a MgO substrate with an in-plane aligned c-axis-oriented PBCO buffer layer deposited at 770°C. Fig. 2(d) shows the XRD spectra of the a-axis-oriented cubic YBCO film which was deposited at 560°C on the MgO substrate with the PBCO layer and then annealed at a high temperature of 700°C. The YBCO (200) diffraction is recognized around 20 = 47.6 ° even after 700°C annealing. This suggests that the a-axis-oriented cubic YBCO template layer on the MgO substrate with the PBCO buffer layer keeps the orientation even at high temperature for high-temperature YBCO film deposition. This result clearly indicates that the c-axis-oriented PBCO buffer layer is essential for keeping the cubic YBCO template layer with a favorable orientation at high temperature. This distinguished effect of the PBCO buffer layer is probably attributed to a stabilized interface between the c-axis-oriented PBCO buffer layer and the a-axis cubic YBCO template layer. The interface between the a-axis cubic YBCO template layer and the MgO substrate is not stable enough for high-temperature treatment because of the large lattice mismatch between them and the instability of the a-axis cubic YBCO itself.
T. Minamikawa et a l . / Physica C 267 (1996)330-336
When the c-axis-oriented PBCO film on MgO substrates was prepared at high temperature, the PBCO film is expected to be grown stably with a strong interaction between the substrate and the film by means of the growth mechanism of the step flow [11], and the lattice mismatch between the substrate and the film is expected to be gradually relaxed from the bottom to the top of the PBCO film adjusting the oxygen content in the CuO chain. On the other hand, when the a-axis-oriented cubic YBCO films on the MgO substrate were prepared at a low temperature, the crystallinity of the film appears to be poor and then the interface between the substrate and the film appears to be unstable. Therefore, when the a-axis cubic YBCO film was treated at a higher temperature, the crystal orientation of the interface layer was expected to be turned into the other stable orientation in order to release the stress at the interface.
3.2. Preparation condition dependence of PBCO and cubic YBCO layers The substrate temperatures for the PBCO layer and the a-axis cubic YBCO layer were optimized for obtaining high-quality a-axis-oriented YBCO films using the self-template technique on the MgO substrate with the PBCO layer. The substrate temperature for the a-axis YBCO layer TyH was fixed at TVH -- 670°C throughout these experiments, because the temperature was the lowest one for obtaining high-quality c-axis-oriented superconducting YBCO films on MgO substrates [6]. An excessively high substrate temperature is supposed to give rise to a c-axis-oriented YBCO layer. First, the a-axis YBCO films were examined varying the substrate temperature of the PBCO layer. The substrate temperature for the a-axis cubic YBCO layer was fixed at TVL = 560°C, because the a-axis cubic YBCO film on the MgO substrate showed the strongest (200) diffraction at this temperature [6]. Fig. 3 shows the substrate temperature (Tp) dependence of the XRD spectra for the a-axis Y B C O / a axis cubic Y B C O / P B C O sample. The substrate temperatures of the two YBCO layers were fixed at TyL = 560°C and TyH = 670°C, and then only the temperature for the PBCO layer Tp was varied; Tp = 700°C (a), Tp --- 740°C (b), Tp = 770°C (c) and Tp = 800°C (d). All the spectra clearly show a strong
'
333
TYH=670°C
I
(d)
•
I
'
I
TyL=5600C
Tp=800°C
(c) Tp=770°C
lw ~..
• = c-
(b) Tp=740°C o
1-
(a)
~"
o.
o= ,
20
30
40
j
50
20(deg) Fig. 3. Substrate temperature Tp dependence of XRD spectra for a-axis Y B C O / a - a x i s cubic YBCO/PBCO. The substrate temperatures of both YBCO layers are fixed at TyL = 560°C and TyH = 670°C, and then only the temperature of the PBCO layer was varied, Tp = 700°C (a), Tp = 740°C (b), Tp = 770°C (c) and Tp = 800°C (d).
!
I
TyH=6700C TL=560°C • Tp=700°C zx Tp=740°C o TP=770°C
g E: rc
o
o ~ ~~ •Z~b Tc(onset)=901~
V~o
T p=800°C
• Z~ • "
0
O
~ T c ( z e r o l i
,
,
I
50
,
i
,
=70K ,
I
i
100
Temperature(K) Fig. 4. Normalized resistance versus temperature for a-axis Y B C O / a - a x i s cubic Y B C O / P B C O , where the samples used were the same ones as shown in Fig. 3.
334
T. Minamikawa et al./ Physica C 267 (1996)330-336
(200) diffraction from the a-axis YBCO layer and almost no (002) diffraction around 2 0 = 15° coming from the c-axis-oriented YBCO. The volume fraction of c-axis-oriented YBCO grains is estimated to be less than 3%. Only this (002) diffraction of c-axis-oriented YBCO is not obscured by the (002) diffraction of c-axis-oriented PBCO because of the very weak intensity of the PBCO (002) diffraction due to the extinction law of XRD. When the buffer layer is an in-plane aligned c-axis-oriented PBCO film prepared at 770°C, the a-axis YBCO layer with the PBCO layer shows the largest shift of the (200) diffraction toward the high angle. This suggests that the film has the maximum oxygen content. This film showed a full width at half-maximum (FWHM) of the (200) rocking curve of 0.58 °, which was similar to that of the PBCO layer, and showed an FWHM of the (200) diffraction of 0.18 °. Fig. 4 shows the normalized resistance versus t~/nperature for the a # axis YBCO/a-axis cubic-YB)CO/PBCO samples, where samples used were the same ones as shown in Fig. 3. The a-axis YBCO layer with a PBCO layer deposited at 770°C shows a critical onset temperature above 90 K and a critical zero temperature of 70 K. Fig. 5 shows T~zero) and the a-axis length derived from the XRD spectra for the YBCO film as a function of the substrate temperature for the PBCO layer Tp. Tc(zero) strongly depends on the a-axis length of the a-axis YBCO film. This suggests that
. . . . 80 .TH 6~6o d
= 560°C
T
,0
I
..
YL
'
'
'
i
,
./\
....
60 ~
3.84
•
',,
3.83 •
"~
3.82
'o
50
/ •/
40 , , , 680
• .• a-'~is - % ' . . . . . . .
720
760
' '
g ¢ i
o~
3,81
800
T (°C) P
Fig. 5. To(zero) and a-axis length derived from XRD spectra for a-axis YBCO/a-axis cubic YBCO/PBCO as a function of the substrate temperatureTp for the PBCO layer.
I
!
T
(c)
(G)
YH
T =770°C
iTyL=580°C
>,
!
=670°C
P
'g
(b)
F =560°C YL
°
tin 0
0
(a) T
0
=
=540°C
YL
h
20
_
,
30
Jkl
40
J
~.
I
50
20(deg) Fig. 6. XRD spectra for the a-axis YBCO/a-axis cubic YBCO/PBCO samples prepared at various TyL for the a-axis cubic YBCO layer. The substrate temperatures of the PBCO layer and the a-axis layer are fixed at Tp = 770°C and TvH = 670°C, respectively, and then only the temperature of the cubic layer was v a r i e d , T y L = 5 4 0 ° C (a), T y L = 5 6 0 ° C (b) a n d T y L = 5 8 0 ° C (c).
the superconductivity really comes from a-axis-oriented YBCO grains. The results of the XRD and the resistivity measurements revealed that the PBCO layer deposited at 770°C is the best one for a good a-axis-oriented YBCO layer. The degradation of the superconductivity of the YBCO layer with the PBCO layer prepared at 800°C is attributed to the presence of the 45 ° rotated grains of the PBCO layer confirmed by the XRD 4' measurement Secondly, the a-axis-oriented YBCO films were examined varying the substrate temperature of the a-axis cubic YBCO layers TVL. The substrate temperature of the PBCO layer was fixed at Tp = 770°C, based on the above results. Fig. 6 shows XRD spectra for samples prepared at various TyL for the a-axis cubic YBCO layer. The substrate temperatures of the PBCO layer and the a-axis YBCO layer were fixed at Tp = 770°C and TyH = 670°C, respectively, and then only the substrate temperature for the a-axis cubic YBCO layer was varied, TyL = 540°C (a), TyL = 560°C (b) and TyL = 580°C (c). The a-axis YBCO film with the a-axis cubic YBCO layer prepared at 560°C shows the strongest intensity of the
T. Minamikawa et aL / Physica C 267 (1996) 330-336 I
T
YH
I
=670°C
T =770°C
V
P
T
8 E rr"
O
=540°C
YL
o
T
=560°C
V
TyL=580°C
YL
W v
41, 4b ql,
v
• ° 0° v 00
50
100
Temperature(K) Fig. 7. Normalized resistance versus temperature for a-axis YBCO/a-axis cubic YBCO/PBCO, where the samples used were the same ones as shown in Fig. 6.
335
4. Conclusion YBCO template layers deposited at a low substrate temperature (T~) on MgO substrates without PBCO buffer layers were found to turn into an orientation other than a-axis orientation by annealing at higher temperatures after the deposition. YBCO films with a-axis orientation have been prepared on MgO substrates using an a-axis-oriented cubic YBCO self-template layer and the buffer layer of in-plane aligned c-axis-oriented PBCO. The film showed an FWHM of the rocking curve of 0.58 ° and an FWHM of the (200) diffraction of 0.18 ° and had a critical onset temperature above 90 K and a critical zero temperature 70 K. This successful preparation of an a-axis-oriented YBCO film on MgO with a large lattice mismatch is brought about by employing a c-axis-oriented PBCO buffer layer.
Acknowledgements (200) peak. Fig. 7 shows the normalized resistance versus temperature for a-axis YBCO films on cubic-YBCO/PBCO, where samples used were the same ones as shown in Fig. 6. The film with the cubic YBCO layer prepared at 560°C offers a critical onset temperature above 90 K and a critical zero temperature 70 K. This curve is the same one shown in Fig. 4. Both the degradations of the superconductivity for the YBCO films with TyL = 540 and 580°C are attributed to the degradation of the crystallinity of the a-axis cubic YBCO layer, as we expected based on the results of our previous paper [6]. The crystal structure of YBCO is similar to that of PBCO, The crystallinity of the c-axis-oriented YBCO films on MgO substrates is better than that of the c-axis-oriented PBCO films on MgO substrates. So, when the a-axis-oriented YBCO films are prepared on MgO substrates with a c-axis-oriented YBCO buffer layer and a a-axis-oriented cubic YBCO layer, it must be possible to prepare higher-quality a-axisoriented superconducting YBCO films. Based on the above consideration, a c-axis-oriented YBCO layer, instead of the c-axis-oriented PBCO layer, is another candidate for the buffer layer. The c-axis-oriented YBCO layer, however, causes confusion for characterizing the a-axis-oriented YBCO layer.
We thank M. Kumeda of Kanazawa University for helpful discussions. We also thank H. Yoshida of the Industrial Research Institute of Ishikawa for helping with our experiments. We are grateful to Shibuya Kogyo Co., Ltd. for supplying the excimer laser system. This work was supported in part by the Grant-in-Aid for Developmental Scientific Research No. 07555499 from the Ministry of Education, Science, Sports and Culture of Japan.
References [1] J.B. Barner, C.T. Rogers, A. Inam, R. Ramesh and Bersey, Appl. Phys. Leu. 59 (1991) 742. [2] T. Hashimoto, M. Sagoi, Y. Mizutani, J. Yoshida and K. Mizushima, Appl, Phys. Lett. 60 (1992) 1756. [3] C.B. Eom, A.F. Marshall, S.S. Laderman, R.D. Jacowitz and T.H. Geballe, Science 249 (1990) 1549. [4] K.H. Young and J.Z. Sun, Appl. Phys. Lett. 59 (1991) 2448. [5] J.A. Agostinelli, S. Chen and G. Braunstein, Phys. Rev. B 43 (1991) 11396. [6] T. Minamikawa, T. Suzuki, Y. Yonezawa, K. Segawa, A. Morimoto and T. Shimizu, Jpn. J. Appl. Phys. 34 (1995) 4038. [7] A. lnam, C.T. Rogers, R. Ramesh, K. Remschnig L. Farrow, D. Hart, T. Venkatesan and B. Wilkens, Appl. Phys. Lett. 57 (1990) 2484.
336
T. Minamikawa et aL / Physica C 267 (1996) 330-336
[8] T.C. Poon and J. Gao, Supercond. Sci. Technol. 8 (1995) 415. [9] S. Hontsu, N. Mukai, J. Ishii T. Kawai and S. Kawai, Appl. Phys. Lett. 63 (1993) 1576.
[10] T. Minamikawa, M. Tazoe, A. Morimoto and T. Shimizu, unpublished. [11] M.G. Norton, S.R. Summerfelt and C.B. Carter, Appl. Phys. Lett. 56 (1990) 2246.