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YBa2Cu3O7−x thin film deposition by MOCVD for microwave applications
Physica C 235-240 (1994) 653-654 North-Holland
YBa2Cu307_ x
PHI$1CA
thin film deposition by MOCVD for microwave applications
C. D u b o u r d i e u a, B. C h e n e v i e r a, J.P. S 6 n a t e u r a, O. T h o m a s b aLaboratoire des Mat6riaux et G6nie Physique, INPG, CNRS URA 1109,BP46, 38402 S t Martin d'H~res, France bLaboratoire MATOP, Facult6 des Sciences et Techniques de St J6r6me, 13397 Marseille Cedex 20, France Y B a 2 C u 3 0 7 _ x thin films have been deposited by Metal Organic Chemical Vapor Deposition (MOCVD) simultaneously on (001) MgO, (001) SrTiO3 and (012) LaAtO3 substrates. A comparison of the properties of two sets of samples has been done. The microstructures induced by the different substrates have been studied by SEM and X ray diffraction. The morphology is very sensitive to the nature of the substrate and to the overall composition of the films. The films are strongly textured, with the c axis perpendicular to the substrate but "a axis grains" are observed when the ratio Ba/Y is above 2. The highest critical temperatures and lowest resistivities are obtained on SrTiO3 and LaA103, with T c _ 89 K and p(100 K) _ 90 ~f2.
1. E X P E R I M E N T A L
2. R E S U L T S
YBCO thin films have been deposited on (001) MgO, (001) SrTiO 3 (STO) and (012) LaA10 3 (LAO) substrates by Metal Organic Chemical Vapour Deposition (MOCVD). The three different substrates are covered in a same run ; they are clamped to the substrate block heater whose diameter is 12 cm. Therefore the growth conditions of the three films are the same (same vapour phase), which allows direct comparison of their properties. The starting materials are the ~-diketonates 2,2,6,6 tetramethyl 3,5 heptanedionates of Y, Ba and Cu. The Argon flow is 40 sccm in each source. The total pressure in the reactor is fixed to 666 Pa and the oxygen partial pressure to 266 Pa. The gas phase composition is determined by the evaporation temperature in the sources. The depositions were carried out at 875 °C and 900 °C (substrate block heater temperature). After deposition, the films are cooled down to room temperature in 1 atm of oxygen. For more details, see [1]. The critical temperature is deduced from resistivity measurements (4 point probe method with I = 100 ~A)) in the range 70 K - 300 K or from shielding measurements (f = 800 Hz, H = 20 mOe). The morphology is studied by Scanning Electron Microscopy and the overall composition is calculated from Energy Dispersive X ray spectrometry measurements. The structure is determined by X ray diffraction (0/20 spectrum). The in plane orientation is analysed by ~ scans performed on the (102)/(012) poles with a D5000 Siemens diffractometer.
Two sets of three samples have been prepared, one at 875 °C (#1) and the other one at 900 °C (#2). In set #1, as deduced from the EDX analysis, the three films are copper rich, the ratio Ba/Y is equal to 1.50 and the thickness is 2500 ~,. The SEM micrographs show lumps on the surface of the three films, with a larger size on MgO substrate. The EDX analysis performed on these lumps shows that they are copper rich secondary phases. Holes are also observed on the matrix of the three films. The films are strongly textured, with the c axis perpendicular to the surface of the substrate. As evidenced by the ~) scans, there are very few "a axis grains" in the films, i.e. grains with a or b axis perpendicular to the substrate. The critical temperature deduced from the resistivity measurements (TCR) is about 91.5 K for STO and LAO substrates, and from shielding measurement TcS - 88.5 K, AT c _ 0.5 K. For the sample on MgO, TcR = 88.5 K, TcS = 77.0 K, with ATc = 3.3 K. The ratio of the resistivities at 300 K and 100 K, p(300 K)/p(100 K), is higher than 3 for films on STO and LAO (with p(100 K) ~ 150 [.t~) and equal to 1.9 for the film on MgO (p(100K) = 330 ~Q). Unlike the two films on STO and LAO, the film on MgO presents a broad transition width and a low ratio p(300 K)/p(100 K). It may be due to chemical interactions between MgO and YBCO. In set #2, the ratio Ba/Y is equal to 2.50, the films are all copper rich and the thickness is 3500/k. Here again, the critical temperatures are higher on STO a n d L A O : T c R - 8 9 K , T c s - 8 4 K a n d A T c - 1 K,
0921-4534/94/$07.00 © 1994 -Elsevicr Science B.V. All rights rescrvcd. SSDI 0921-4534(94)00887-6
AND
DISCUSSION
o54
( '.
I)ld,oln'Ui~'u <'I al
Fi~. I a. : YBCO fihn on MgO (set #2)
Fi U. I t~ : YBCO film on STO (set #2)
t>hv.<,i~a ( 23 ~ 240 t / 9!74, 6<, 7 (}'>4
whereas TcR = 86 K, Tog = 80.5 K and ATcs : I 7 K tor the film on MgO. The ratio p(3()() K)/p(10OK) is higher lhan 3 for ST() arid I~AO (Ot 10(1 K) --~)() bl£]) and about 2.5 for MgO (t)(10() K) = 12() bi_£2! As in set # l , the films are lexlured with the c axis perpendicular to the substrate, but there arc also ' a axis grains", as inferred by the 0 scans. The SEM micrographs of those three fihns are shown ( F i g 1~ Needle like grains arc clearl\ xisible on ST() ami I~AO. The much higher cl)nlenl (>1 it axis .urains in lhe sc~ # 2 - although the deposition temperature was itlc highest ( 9 0 0 "C) - Call he attributed to the pln)r Yttrium content (Ba/Y > 2 ) i n these f i l m s It ~a,, shown that when Ba/Y is above 2, needle like ", grains" are visible on the surfiice of the fihns {2). A TEM sitidy of these grains has been performed and ~ill be presented a ! this conference 131. When compared with the /Jrsl sd. we also obser\ e. it1 sc'l #2, a decrease of the critical temperatures as ~ell :l~ a decrease o f tile resisti\,ities This b e h a \ i o u r i~ n~l in line w i l h 'what is g e n e r a l l y o b s e r \ e d , !<)! o x y g e n a t i o n processes o i cationic substitulion {-l.]. where Ihe crilical lenlperalure increases when lhc i-esi sii \:it.'+ ctecreases The @ scans perfornled i)ll ihe l w o fihns on M U ( ) reveal in both cases il poor in plane orientalion will~ misoriented grains nlostiy tit 45 '. Such orientations tire often observed on MgO. However fihns with a ~ood in plane epitaxy have already been obtained on M g O , with critical current densities abo~,e I() f' A/cm 2 and surface resistances less than 2()ln~} (S7 GHz, ().$5 To), close to the best values ine,:lstlrcd so far on YBCO layers I I ]. The cridcal temperatures and crilical current densities :.ire h i u h e r on l_,\() substr;.ites than on M g O The lnierowave properties ~)t the films prepared on I,A() will bc nn~t investigated.
REFERENCES
Fig. 1 c : YBCO fihn on I~AO {set #2) 1.2 cm on the photos correspond to 10 him
1. C. Dubourdieu, G. l)ehlbouglise, O. Thomas, J.P. Sdnateur, D. Chateigner, P. Germi, M. Perncl, S. Hcnsen, S. Orbach, G. Mfiller, in .\pl~tied Superconductivity, edited I v FreyhardI, l)eutsche Gesellschaft for Materialkunde I 1993), p. I()81 2. J.A. Edwards, N . G Chew, S.W. Goodyeal. I S Satchell, S.E. Blenkinsop and R.G. Humphreys..i of t,ess Common Metal, 164-165 (1990) 411. 3. B. Chenevier, ('. Dubourdieu, ,I.P. Stinateur, 1). Thomas, to be presented at this conference 4. Z. B u k o w a k i , K. R o g a c k i , R. Horyn, proceedings of the ICMAS conference, Paris 1992
YBa2Cu307_ x
PHI$1CA
thin film deposition by MOCVD for microwave applications
C. D u b o u r d i e u a, B. C h e n e v i e r a, J.P. S 6 n a t e u r a, O. T h o m a s b aLaboratoire des Mat6riaux et G6nie Physique, INPG, CNRS URA 1109,BP46, 38402 S t Martin d'H~res, France bLaboratoire MATOP, Facult6 des Sciences et Techniques de St J6r6me, 13397 Marseille Cedex 20, France Y B a 2 C u 3 0 7 _ x thin films have been deposited by Metal Organic Chemical Vapor Deposition (MOCVD) simultaneously on (001) MgO, (001) SrTiO3 and (012) LaAtO3 substrates. A comparison of the properties of two sets of samples has been done. The microstructures induced by the different substrates have been studied by SEM and X ray diffraction. The morphology is very sensitive to the nature of the substrate and to the overall composition of the films. The films are strongly textured, with the c axis perpendicular to the substrate but "a axis grains" are observed when the ratio Ba/Y is above 2. The highest critical temperatures and lowest resistivities are obtained on SrTiO3 and LaA103, with T c _ 89 K and p(100 K) _ 90 ~f2.
1. E X P E R I M E N T A L
2. R E S U L T S
YBCO thin films have been deposited on (001) MgO, (001) SrTiO 3 (STO) and (012) LaA10 3 (LAO) substrates by Metal Organic Chemical Vapour Deposition (MOCVD). The three different substrates are covered in a same run ; they are clamped to the substrate block heater whose diameter is 12 cm. Therefore the growth conditions of the three films are the same (same vapour phase), which allows direct comparison of their properties. The starting materials are the ~-diketonates 2,2,6,6 tetramethyl 3,5 heptanedionates of Y, Ba and Cu. The Argon flow is 40 sccm in each source. The total pressure in the reactor is fixed to 666 Pa and the oxygen partial pressure to 266 Pa. The gas phase composition is determined by the evaporation temperature in the sources. The depositions were carried out at 875 °C and 900 °C (substrate block heater temperature). After deposition, the films are cooled down to room temperature in 1 atm of oxygen. For more details, see [1]. The critical temperature is deduced from resistivity measurements (4 point probe method with I = 100 ~A)) in the range 70 K - 300 K or from shielding measurements (f = 800 Hz, H = 20 mOe). The morphology is studied by Scanning Electron Microscopy and the overall composition is calculated from Energy Dispersive X ray spectrometry measurements. The structure is determined by X ray diffraction (0/20 spectrum). The in plane orientation is analysed by ~ scans performed on the (102)/(012) poles with a D5000 Siemens diffractometer.
Two sets of three samples have been prepared, one at 875 °C (#1) and the other one at 900 °C (#2). In set #1, as deduced from the EDX analysis, the three films are copper rich, the ratio Ba/Y is equal to 1.50 and the thickness is 2500 ~,. The SEM micrographs show lumps on the surface of the three films, with a larger size on MgO substrate. The EDX analysis performed on these lumps shows that they are copper rich secondary phases. Holes are also observed on the matrix of the three films. The films are strongly textured, with the c axis perpendicular to the surface of the substrate. As evidenced by the ~) scans, there are very few "a axis grains" in the films, i.e. grains with a or b axis perpendicular to the substrate. The critical temperature deduced from the resistivity measurements (TCR) is about 91.5 K for STO and LAO substrates, and from shielding measurement TcS - 88.5 K, AT c _ 0.5 K. For the sample on MgO, TcR = 88.5 K, TcS = 77.0 K, with ATc = 3.3 K. The ratio of the resistivities at 300 K and 100 K, p(300 K)/p(100 K), is higher than 3 for films on STO and LAO (with p(100 K) ~ 150 [.t~) and equal to 1.9 for the film on MgO (p(100K) = 330 ~Q). Unlike the two films on STO and LAO, the film on MgO presents a broad transition width and a low ratio p(300 K)/p(100 K). It may be due to chemical interactions between MgO and YBCO. In set #2, the ratio Ba/Y is equal to 2.50, the films are all copper rich and the thickness is 3500/k. Here again, the critical temperatures are higher on STO a n d L A O : T c R - 8 9 K , T c s - 8 4 K a n d A T c - 1 K,
0921-4534/94/$07.00 © 1994 -Elsevicr Science B.V. All rights rescrvcd. SSDI 0921-4534(94)00887-6
AND
DISCUSSION
o54
( '.
I)ld,oln'Ui~'u <'I al
Fi~. I a. : YBCO fihn on MgO (set #2)
Fi U. I t~ : YBCO film on STO (set #2)
t>hv.<,i~a ( 23 ~ 240 t / 9!74, 6<, 7 (}'>4
whereas TcR = 86 K, Tog = 80.5 K and ATcs : I 7 K tor the film on MgO. The ratio p(3()() K)/p(10OK) is higher lhan 3 for ST() arid I~AO (Ot 10(1 K) --~)() bl£]) and about 2.5 for MgO (t)(10() K) = 12() bi_£2! As in set # l , the films are lexlured with the c axis perpendicular to the substrate, but there arc also ' a axis grains", as inferred by the 0 scans. The SEM micrographs of those three fihns are shown ( F i g 1~ Needle like grains arc clearl\ xisible on ST() ami I~AO. The much higher cl)nlenl (>1 it axis .urains in lhe sc~ # 2 - although the deposition temperature was itlc highest ( 9 0 0 "C) - Call he attributed to the pln)r Yttrium content (Ba/Y > 2 ) i n these f i l m s It ~a,, shown that when Ba/Y is above 2, needle like ", grains" are visible on the surfiice of the fihns {2). A TEM sitidy of these grains has been performed and ~ill be presented a ! this conference 131. When compared with the /Jrsl sd. we also obser\ e. it1 sc'l #2, a decrease of the critical temperatures as ~ell :l~ a decrease o f tile resisti\,ities This b e h a \ i o u r i~ n~l in line w i l h 'what is g e n e r a l l y o b s e r \ e d , !<)! o x y g e n a t i o n processes o i cationic substitulion {-l.]. where Ihe crilical lenlperalure increases when lhc i-esi sii \:it.'+ ctecreases The @ scans perfornled i)ll ihe l w o fihns on M U ( ) reveal in both cases il poor in plane orientalion will~ misoriented grains nlostiy tit 45 '. Such orientations tire often observed on MgO. However fihns with a ~ood in plane epitaxy have already been obtained on M g O , with critical current densities abo~,e I() f' A/cm 2 and surface resistances less than 2()ln~} (S7 GHz, ().$5 To), close to the best values ine,:lstlrcd so far on YBCO layers I I ]. The cridcal temperatures and crilical current densities :.ire h i u h e r on l_,\() substr;.ites than on M g O The lnierowave properties ~)t the films prepared on I,A() will bc nn~t investigated.
REFERENCES
Fig. 1 c : YBCO fihn on I~AO {set #2) 1.2 cm on the photos correspond to 10 him
1. C. Dubourdieu, G. l)ehlbouglise, O. Thomas, J.P. Sdnateur, D. Chateigner, P. Germi, M. Perncl, S. Hcnsen, S. Orbach, G. Mfiller, in .\pl~tied Superconductivity, edited I v FreyhardI, l)eutsche Gesellschaft for Materialkunde I 1993), p. I()81 2. J.A. Edwards, N . G Chew, S.W. Goodyeal. I S Satchell, S.E. Blenkinsop and R.G. Humphreys..i of t,ess Common Metal, 164-165 (1990) 411. 3. B. Chenevier, ('. Dubourdieu, ,I.P. Stinateur, 1). Thomas, to be presented at this conference 4. Z. B u k o w a k i , K. R o g a c k i , R. Horyn, proceedings of the ICMAS conference, Paris 1992