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Epitaxial growth of superconducting YBa2Cu3O7 thin films on bare R-plane sapphire substrate
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Physica C 235-240 (1994)665~66 North-Holland
Epitaxial
growth
sapphire
substrate
of
superconducting
YBa2Cu307
thin
films
on
bare
R-plane
C. Thivet 1, M. Guilloux-Viry 1, J. Padiou 1, A. Perrin 1, G. Dousselin 2, Y. Pellan 2 and M. Sergent I 1 Laboratoire de Chimie du Solide et Inorganique Moldculaire, LIRA CNRS 1495, Avenue du G6n6ral Leclerc, 35042 Rennes cedex. 2 Laboratoire de Physique des solides, INSA, Avenue des Buttes de CoEsmes, 35042 Rennes cedex. We report epitaxial growth by laser ablation of YBa2Cu307 films on bare sapphire (1]'02) substrate. Owing to suitable substrate treatment, we have succeeded to grow superconducting YBCO films with Tc(R = 0) above 88 K and p(300K)/p(100K) around 2.80" first promising Rs value of 17 mO was measured at 10 GHz and 77 K. The first expectable applications of high temperature cuprates appear to be microwave devices litbrication, which need quite large area, homogeneous and high quality thin films. Thus, the most important parameters are i) the surface resistance Rs which appears strongly correlated to the crystalline quality of the film [1], requiring epitaxial growth, and ii) the intrinsic dielectric constant of the substrate. For that purpose, "R-plane" (1702) oriented sapphire AI203 have been chosen.
to a transition tail in the case of normal substrates. The difference is even more striking when looking at the R(T) curves in the normal state, with p(300K) / p(100K) = 2.88 and 1.55 respectively. Measurements performed on films grown onto normal and chemically etched sapphires, with same deposition conditions, confirm this result, indicating that different types of etching, chemical or ionic, although operating at different ranges (several A to 800 ,~), lead to comparable superconducting properties.
Substrate preparation appears to be an essential step in order to obtain good nucleation of YBCO material on the substrate at the early stages of deposition [2] : when sapphire is used, it is very important to prevent interdiffusion between this kind of reactive substrate and the YBCO film [3]. In this work, three types of R-plane (1T02) sapphire substrates have been used: without special treatment, hereafter called "normal substrate". chemically etched substrate, using a 1/1 HNO3/water solution (ten seconds). - ionic etched substrate, using a low energy Ar ion beam, provided by a 3" Kauffmann source (500 eV, 0.5 mA.cm -2, 2.10 -2 Pa). The deposition process of YBCO films on sapphire is the same as the one we employed with MgO substrate [4], except for the deposition tcmperature which is a slightly higher, in the range 750°-770°C, with an optimum temperature of 761°C.
Figure 1 • Comparison of resistive transitions of YBCO films on normal (N) and ionic etched (I) (1T02 ) sapphires
Figure 1 shows the typical resistive transitions of an YBCO film grown on the same sapphire substrate half divided in a normal part and an ionic etched part. TC(R = 0) = 88.5 K and 81 K were obtained for etched and normal sapphires, respectively : so more than 7 K separate the two types of films, mainly due
Standard 0-20 diffraction shows that the YBCO films, both on normal and etched sapphire, are fully c-oriented. Epitaxy of the YBCO film on the (1102) etched sapphire was first confirmed by in-situ RHEED, as displayed in figure 2 : distances between streaks of substrate and film diffraction, performed at
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0921-4534/94/$07.00 © 1994- ElsevierScienceB.V. All rights reserved. SSDI 0921-4534(94)00893-0
60
90
79
a3
J : a7
91
95
120 i50 180 210 240 270 300 TEMPERATURE K
the same azimut, leed to close paraineter values, thus prooving the alignment of the (100) or/and (010) "fBCO axis with the (22q)1) or/and (02~1) sapphire axis, in the plane of the substratc. Oscillating-crystal mode X-ray diffraction, using a Weissenberg camera, has confinned the epitaxy on etched sapphire.
l:tg,~rc 2 " RIIEED pattern of an YBCO film (m io~m: elched (1702) sapplzin:
ones we usually observed c)n MgO and Srl~()t subtrates 151. In the case of a normal sapphire, RHEED pattcr~'<, consist of spots and rings, showing the absence c,q epitaxy and the rough character of the surfacc. Both FWHM values and ECP patterns confirm this poor crystalline quality, well explained by lhe SEM observations showing granular surface. Properties in the microwave range were studied b> nieasuring the surface resistance Rs by the dielectric resonator methode, at 10 GHz and 77 K 161 : the values are higher than those of a standard conductox like Cu, but a value of 17 m-Q obtained on a YBC() fihn on ionic etched sapphire is very proniising. Thi~ value is lower than those obtained on chemical etched substrales (70-100 m(2), showing that the crystalline arrangement on the first type of sapphire is far better l : r o n l this point o f view. the illoderate crystalline characteristics arc probably the reason el the difference of surface resistance between fihns o~ A I 2 0 3 and MgO (in the later case, typicall> R S = 1 I n ~ ) . In fact, it seems that ~er,, good supcrconducting properties, in particuhir crilic'~d lemperalure Tc, are not sufficient for the oblenlion el very low values of R S on sapphire. We thank J.C. Mage and B. Marcilhac (l]lonisoT~ I.CR Corbcville, Orsay) for the Rs measurcuienl~. This work was supported by the Centre NatitmaJ d'Etude des Tdlecommunications (conlracl 92 8B045 ). REFERENCES
l'ig~re 3 " Wide. angle ct~nneling paltenl )'17('0 ~hn on ionic: etched (1 l()2) sapphire
of
a#i
Crystalline quality of the fihns is refle,cted by the [TWit M values of the rocking-curves recorded around the (005) YBCO peak : the YBCO values, higher than 1.5 °, are not ,,'cry good, pointing out that the mesait spread along the c direction is significantly larger than in the case of YBCO films on MgO and SrTiO3 substrates. This is confirmed by wkte angle channeling pattern photographs, figure 3 : although the typical YBCO pattern is cleary observed in the case of etched substrate, it is not so sharp than the
1. K.H. Young and J.Z. Sun, Appl. Phys. [,cu 70(7) (1991) 37. 2. B.H. Moeckly, 1).[t. Lathrop, S.E.Russel, P,...\ Butmnan, M.G. Norton and C.B. Carter, IEEE :fran, on Magnetics, 27(2) ( 1991 I 1017 ?. K. CMr, D.K. Fork, ]'.t4. GcMIIc. S.5 l_aderman, R.(7. Taber, R.I). Jacowitz, V. P,ridgc,. G.A.N. Counel and J.B. Boyce, Appl. Phys. lcu. 56(8) (1990) 785. 4. M. Guilloux-Viry, ('. Yhivet, M.(}. Karkut. i Padiou, O. Pefia, A. Perrin, M. Sergent and NI. Gauneau, Mater. Sc. ~md Eng. B, 18 (.1993) 115 5. A. Perrin, M. Guilloux-Viry, C. T h i v e l . . I . ( Jcgaden, M. Sergent and J. [,e t, annic, Jeol News. " (1992) 26. 6. J.C. Mage, D. Dieumegardc, AGARD Conlercncc Proceedings, chap 10 (1090) 481.
Physica C 235-240 (1994)665~66 North-Holland
Epitaxial
growth
sapphire
substrate
of
superconducting
YBa2Cu307
thin
films
on
bare
R-plane
C. Thivet 1, M. Guilloux-Viry 1, J. Padiou 1, A. Perrin 1, G. Dousselin 2, Y. Pellan 2 and M. Sergent I 1 Laboratoire de Chimie du Solide et Inorganique Moldculaire, LIRA CNRS 1495, Avenue du G6n6ral Leclerc, 35042 Rennes cedex. 2 Laboratoire de Physique des solides, INSA, Avenue des Buttes de CoEsmes, 35042 Rennes cedex. We report epitaxial growth by laser ablation of YBa2Cu307 films on bare sapphire (1]'02) substrate. Owing to suitable substrate treatment, we have succeeded to grow superconducting YBCO films with Tc(R = 0) above 88 K and p(300K)/p(100K) around 2.80" first promising Rs value of 17 mO was measured at 10 GHz and 77 K. The first expectable applications of high temperature cuprates appear to be microwave devices litbrication, which need quite large area, homogeneous and high quality thin films. Thus, the most important parameters are i) the surface resistance Rs which appears strongly correlated to the crystalline quality of the film [1], requiring epitaxial growth, and ii) the intrinsic dielectric constant of the substrate. For that purpose, "R-plane" (1702) oriented sapphire AI203 have been chosen.
to a transition tail in the case of normal substrates. The difference is even more striking when looking at the R(T) curves in the normal state, with p(300K) / p(100K) = 2.88 and 1.55 respectively. Measurements performed on films grown onto normal and chemically etched sapphires, with same deposition conditions, confirm this result, indicating that different types of etching, chemical or ionic, although operating at different ranges (several A to 800 ,~), lead to comparable superconducting properties.
Substrate preparation appears to be an essential step in order to obtain good nucleation of YBCO material on the substrate at the early stages of deposition [2] : when sapphire is used, it is very important to prevent interdiffusion between this kind of reactive substrate and the YBCO film [3]. In this work, three types of R-plane (1T02) sapphire substrates have been used: without special treatment, hereafter called "normal substrate". chemically etched substrate, using a 1/1 HNO3/water solution (ten seconds). - ionic etched substrate, using a low energy Ar ion beam, provided by a 3" Kauffmann source (500 eV, 0.5 mA.cm -2, 2.10 -2 Pa). The deposition process of YBCO films on sapphire is the same as the one we employed with MgO substrate [4], except for the deposition tcmperature which is a slightly higher, in the range 750°-770°C, with an optimum temperature of 761°C.
Figure 1 • Comparison of resistive transitions of YBCO films on normal (N) and ionic etched (I) (1T02 ) sapphires
Figure 1 shows the typical resistive transitions of an YBCO film grown on the same sapphire substrate half divided in a normal part and an ionic etched part. TC(R = 0) = 88.5 K and 81 K were obtained for etched and normal sapphires, respectively : so more than 7 K separate the two types of films, mainly due
Standard 0-20 diffraction shows that the YBCO films, both on normal and etched sapphire, are fully c-oriented. Epitaxy of the YBCO film on the (1102) etched sapphire was first confirmed by in-situ RHEED, as displayed in figure 2 : distances between streaks of substrate and film diffraction, performed at
•
°
-go
• N°
o
o
.8
O3 Od
.6
.i
."
rr
-.
°
.4
N
r~ :..." z
-.-. i
r
15
30
0921-4534/94/$07.00 © 1994- ElsevierScienceB.V. All rights reserved. SSDI 0921-4534(94)00893-0
60
90
79
a3
J : a7
91
95
120 i50 180 210 240 270 300 TEMPERATURE K
the same azimut, leed to close paraineter values, thus prooving the alignment of the (100) or/and (010) "fBCO axis with the (22q)1) or/and (02~1) sapphire axis, in the plane of the substratc. Oscillating-crystal mode X-ray diffraction, using a Weissenberg camera, has confinned the epitaxy on etched sapphire.
l:tg,~rc 2 " RIIEED pattern of an YBCO film (m io~m: elched (1702) sapplzin:
ones we usually observed c)n MgO and Srl~()t subtrates 151. In the case of a normal sapphire, RHEED pattcr~'<, consist of spots and rings, showing the absence c,q epitaxy and the rough character of the surfacc. Both FWHM values and ECP patterns confirm this poor crystalline quality, well explained by lhe SEM observations showing granular surface. Properties in the microwave range were studied b> nieasuring the surface resistance Rs by the dielectric resonator methode, at 10 GHz and 77 K 161 : the values are higher than those of a standard conductox like Cu, but a value of 17 m-Q obtained on a YBC() fihn on ionic etched sapphire is very proniising. Thi~ value is lower than those obtained on chemical etched substrales (70-100 m(2), showing that the crystalline arrangement on the first type of sapphire is far better l : r o n l this point o f view. the illoderate crystalline characteristics arc probably the reason el the difference of surface resistance between fihns o~ A I 2 0 3 and MgO (in the later case, typicall> R S = 1 I n ~ ) . In fact, it seems that ~er,, good supcrconducting properties, in particuhir crilic'~d lemperalure Tc, are not sufficient for the oblenlion el very low values of R S on sapphire. We thank J.C. Mage and B. Marcilhac (l]lonisoT~ I.CR Corbcville, Orsay) for the Rs measurcuienl~. This work was supported by the Centre NatitmaJ d'Etude des Tdlecommunications (conlracl 92 8B045 ). REFERENCES
l'ig~re 3 " Wide. angle ct~nneling paltenl )'17('0 ~hn on ionic: etched (1 l()2) sapphire
of
a#i
Crystalline quality of the fihns is refle,cted by the [TWit M values of the rocking-curves recorded around the (005) YBCO peak : the YBCO values, higher than 1.5 °, are not ,,'cry good, pointing out that the mesait spread along the c direction is significantly larger than in the case of YBCO films on MgO and SrTiO3 substrates. This is confirmed by wkte angle channeling pattern photographs, figure 3 : although the typical YBCO pattern is cleary observed in the case of etched substrate, it is not so sharp than the
1. K.H. Young and J.Z. Sun, Appl. Phys. [,cu 70(7) (1991) 37. 2. B.H. Moeckly, 1).[t. Lathrop, S.E.Russel, P,...\ Butmnan, M.G. Norton and C.B. Carter, IEEE :fran, on Magnetics, 27(2) ( 1991 I 1017 ?. K. CMr, D.K. Fork, ]'.t4. GcMIIc. S.5 l_aderman, R.(7. Taber, R.I). Jacowitz, V. P,ridgc,. G.A.N. Counel and J.B. Boyce, Appl. Phys. lcu. 56(8) (1990) 785. 4. M. Guilloux-Viry, ('. Yhivet, M.(}. Karkut. i Padiou, O. Pefia, A. Perrin, M. Sergent and NI. Gauneau, Mater. Sc. ~md Eng. B, 18 (.1993) 115 5. A. Perrin, M. Guilloux-Viry, C. T h i v e l . . I . ( Jcgaden, M. Sergent and J. [,e t, annic, Jeol News. " (1992) 26. 6. J.C. Mage, D. Dieumegardc, AGARD Conlercncc Proceedings, chap 10 (1090) 481.