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Preparation of YBa2Cu4O8 thin films by MOCVD
Journal of Crystal Growth 115 (1991) 782—787
CRYSTAL GROWTH
J
North-Holland
Preparation of YBa7Cu4O8 thin films by MOCVD H. Hayashi, Y. Yamada, D.J. Baar, T. Sugimoto, K. Sugawara, Y. Shiohara and S. Tanaka .S’uperconducthiiy Research Lahorawrv, ISTEC
,
1—10- 13 .S’hinonome, Aoio—ku, Tokso 135, Japan
We have obtained a film in which the 1 24 phase was dominant. From the results for diflere Ut deposition times, it is dernonstt’ated that the I 24 phase is not formed through solid state reactiorL rather, it grows during deposition. We discuss the growth mechanism of the 124 phase. Films fabricated tinder the same deposition conditions were lound to he different for SrTiO (IOU) and for MgO (11)1)) substrates. The differences originate from the dii terence in ii ueleation and growth of the 124 phase at the early stage of deposition on the two substrates.
1. Introduction The superconducting phase YBa,Cu405 (124) is superior to the YBa~Cu~O~ (123) phase in terms of thermochemical stability of oxygen content. The oxygen content of 124 is not reduced at temperatures up to about 8500 C [1—4].and accordingly bulk samples quenched from high temperature still show superconductivity [51.Although the 1~of 124 is 80 K, which is lower than that of 123, Miyatake et al. have reported that I~. could be increased to 90 K by partial substitution of Ca l’or Y [6]. Therefore, the 124 phase may he useful for practical applications where liquid nitrogen cooling is employed. Previously, the 124 phase was discovered as lattice defects in thin I’ilm samples of the 123 phase [7—101.Thin films of 124 were fabricated by annealing an amorphous film, which was made by PVD (physical vapor deposition), at an appropriate temperature under 1 atm oxygen pressure [11,12]. However, there have been no reports of in-situ growth of 124 films using PVD. As for MOCVD. while several groups have fabricated 123 films [13—16],there have been no reports on 124 film formation. Recently, we reported the fabrication of Y—Ba—Cu—O superconducting films containing the 124 phase by MOCVD without post annealing [17], and that oxygen partial pressure and deposition temperature significantly affeet the appearance of the 124 phase [181. Unfor1)022-1)248/91 /$03.5() s~199!
tunatelv, we have not yet succeeded in the fabrication of single phase 124 films. To confirm the existence of the 124 phase, we tried to use TEM observation. Fig. I shows the X-ray diffraction pattern of the TEM sample which was fabricated according to the previous work: the susceptor temperature is C and the oxygen partial pressure is 7.5 Torr. The c-axis lattice constant of’ the 124 phase evaluated from this pattern is 27.2 A. which is close to the value of bulk 124 samples. Fig. 2a shows the TEM image of the 124 domain which was observed from cross-sectional direction, and the schematic crystal structure of 124 is shown in fig. 2h. The 7750
a .~.
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~.
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10
20
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40
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2 0 (degrees) Fig. I. X-ray diffraction pattern of TEM saniple.
Elsevier Science Publishers B.~’.All rights reserved
H. Havashi ci a!.
/ Preparation
of YBa,Cu
783
405 i/tin films by MOCVD
124 phase is different from the 123 phase in having a double CuO chain, and in the 124 phase 123 units slip off by 1/2 to the direction of the h-axis at double CuO chain. This featuring structure was observed in fig. 2a and the c-axis lattice constant estimated from the image was about 27 A, which was close to that of the 124 phase. In the present work, we have obtained films in which the 124 phase is dominant. We have studied the growth mechanism of the 124 phase in MOCVD film processing, by the result of changing the deposition time under the same conditions.
Table I Deposition conditions of Y—Ba-Cu-O films MO source Temperature (°C) Y(DPM)1 107 Ba(DPM)2 2(15 Cu(DPM)2 10()
Ar flow (SCCM) 50 Sf) 5))
Oxygen flow Substrate , Temperature of suseeptor Total gas pressure Oxygen partial pressure Deposition rate Deposition time
1000 SCCM SrTiO3(lOt)). MgO(lOO) 800 0 C 20 Torr 17.5 Torr 8 A/mm 15, 3)), 60. 120, 240, 480 mm
Cooling rate
100 C/mm
2. Experimental procedure deposition. SrTiO3(100) and MgO(100) 3) were used assingle subcrystals (10>< deposition, 10 x 0.5 mm strates. After the films were cooled to room temperature at a rate of 10°C/mm. The growth rate was about 8 A/mm estimated from SEM and ICP analysis. The crystal structures of the films were analyzed by X-ray diffraction (XRD) using Cu-Kct radiation.
The MOCVD system cold-wall used in this conventional horizontal type.study The isex-a perimental setup and deposition process were described in detail elsewhere [17]. Deposition conditions are listed in table 1. The source materials used are the /3-diketonate metal chelates: bis-(2,2,6,6-tetramethyl-3,5-heptanedionate)-yttrium Y(DPM) 3, Ba(DPM)2 and Cu(DPM)2. The temperatures of the vaporizers are fixed at 107, 205 and 100 C, respectively. The susceptor temperature was 800°C.The total gas pressure and oxygen partial pressure in the reactor were maintained at 20 and 17.5 Torr, respectively, during
3. Results and discussion
0
Fig. 3 shows the XRD pattern of a film deposited for 480 mm on a SrTiO3(100) substrate. /~-.-~-.-ç-.-ç. /
/
A
A
_I/
A
1
V
A
~ ~~Ba ~ ~oCu
-~!flT~Tfl° -~-
(a)
(b)
Fig. 2. (a) Cross-sectional TEM image of 124 domain in Y—Ba—Cu—O film. (b) Schematic presentation of the lattice structure of 124.
784
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/ Preparation
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405 thin films by MOCVI)
a
~-~-
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,_
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20 (degrees) Fig.
Fig. 5.
3. X-ray diffraction pattern of the Y—Ba—Cu—O thin grown for 480 mm on a SrTiO5 (IOU) substrate
100
200
300
400
500
Deposition Time(min)
-
Intensities of
the (002) peak of the 124 phase and of
film
the (1)1)1) peak of the 123 phase as a function of deposition time in the case SrTiO3 (lilt)) was used as substrate.
Strong (001) peaks of the 124 phase are detected and peak intensities of the 123 phase are very weak. According to this result, it could be stated that we obtained almost single phase 124 film. To clarify when the 124 phase appears and how the 124 grain grows, we fabricated films for various deposition times keeping the other conditions the same. Composition of films are approximately Y: Ba : Cu = 0.95 : 2.0 :4.38. which is a little richer in Cu than 124 stoichiometry. Fig. 4a. 4b and 4c show the X-ray diffraction patterns of films A, B and C as a function of the deposition time under the same deposition condi-
tions on the SrTiO3 (100) substrate. In film A which was grown for 15 mm, (001) peaks of the 123 phase are detected but the peaks are broad. As for the 124 phase, only a weak and broad (002) peak was detected. In film B, which was grown for 30 mi (00/) peaks of the 123 phase are also detected, and the peak intensities hecome stronger and sharper than those of film A. The (002) peak of 124 is also detected, and the intensity was stronger and sharper than that of film A. However, the intensity is lower than that of the (001) peak of 123. The (001) peaks of 124 other than (002) are also detected. In film C, which was deposited for 60 mm, sharp peaks corresponding to the (00/) peaks of 124 and 123 are clearly identified. The intensity of the (002) peak of 124 became higher than that of the (001) peak of 123. From these results, it is clear that the diffraction intensities of the 124 and the 123 phases become stronger and that the peaks hecome sharper as the growth time increases. Fig. 5 shows the intensities of the (002) peak of
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The for all samples X-ray were done under themeasurements same conditions including
~
20 30 20 (degrees)
40
Fig. 4. X-ray diffraction patterns of Y—Ba—Cu—O thin films grown for (a) 15 mm, (h) 3)) mm. and tel 60 nOn under the same deposition condition,
124 and of 123 deposition timeofinthe the(001) case peak SrTiO3 (100)versus was used as
power, slit width, sensitivity, scan speed, and sampie scale. The peak intensities of both peaks increased as the deposition time increased. However, the intensity of the (002) peak of 124 increased faster than the (001) peak of 123. This
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405 t/o,i film.s by MOCVD
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10
20 30 20 (degrees)
40
Fig. 6. X-ray diffraction patterns of the Y—Ba—Cu—O films: (a) as-grown (growth time = 120 mm), (h) after annealing of the film (a) for 360 mm under the deposition pressure, and (c) after annealing of the film (a) for 1680 mm under the deposition pressure.
result indicates that the volumes of both 124 and 123 also increased as the deposition time became longer, and that the ratio of the amounts of 124 to 123 increased as well, Recently, it has been found that 124 is a stable phase under high oxygen pressure or at low ternperature in the Y—Ba—Cu—O system. There have been some reports that the 124 phase can be fabricated by annealing an amorphous film [11,12] or by annealing a m~tureof 123 and CuO for an extended time period [19], or by the sol—gel method [20]. If, in the present experiment, the 124 phase
10
20 30 40 20 (degrees)
Fig. 7. X-ray diffraction pattern of the Y—Ba—Cu—O thin film grown for 480 mm on a MgO (IOU) substrate.
these conditions and decomposes slightly. Comparing the XRD patterns of fig. 6c to fig. 3, the total time for which the film at high temperature is about 1800was mm maintained and is about four times as long as the case of the sample in fig. ~ However, the intensity of the 124 from the sample in fig. 3 is much stronger than that of the sample in fig. 6c. Accordingly, it is apparent that the 124 phase in fig. 3 was not produced through a solid state reaction, but grew during deposition. We have discussed the films fabricated on the SrTiO3 (100) substrates, we also set MgO (100) substrates together with SrTiO3 (100) in the same
10’ ‘124(002) 0 123(001)
was formed from 123 and CuO through solid state reactions during the long time deposition, by annealing we would expect the as-deposited to obtain single filmphase for a long 124 films time under the deposition pressure. Figs. 6a, 6b and 6c show the X-ray diffraction patterns of the film (a) as grown for 120 mi (b) after annealing of the film (a) for 360 mm under the deposition pressure, and (c) after annealing of the film (a) for 1680 mm under the deposition pressure. The peak intensity of 124 was constant and the intensity of 123 decreased slightly as the annealing time was increased. From this result, it seemed that the 124 phase is stable under the deposition conditions, while the 123 phase is unstable under
50
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10
0~0
~ 10 i7 0
C
~
—.
I
10
100
.
0
100
200
300
400
500
Time(min) Fig. 8. Intensities of the (002) peak of the 124 phase and of the (001) peak of the 123 phase as a function of deposition time in the case MgO (100) was used as substrate.
78(i
1/. llavashi ci a!.
/ Preparation
run. Fig. 7 shows the XRD pattern of a film deposited for 480 mm on MgO (100) substrate. Fig. 8 shows the intensity of the (002) peak of 124 and of the (001) peak of 123 versus deposition time in the case MgO (100) was used as substrate. In the case of MgO (100), the same tendencies were seen as described before in the case of SrTiO3 (100). As the deposition time increased, the peaks of 123 and 124 became sharp, and its intensities became stronger and the ratio of the heights of the (002) peak of 124 to the (001) peak of 123 increased. From the result that the 124 phase grew during deposition, there should be differences in the volume ratio of 124 to 123 between the part near the film surface and the part near the substrate surface. This result could he explained by assuming that the growth of the 124 grains perpendicular to the substrate surface is superior to that of the 123 grains. This occurs in both cases of using MgO (100) and SrTiO5 (100). However, there is a quite pronounced difference between fig. 3 and fig. 7, in which the deposition conditions were the same except for the substrates used. From this result, it was again confirmed that the 124 phase formation did not occur as a result of solid state reaction, hut instead grew during the process of deposition. Provided that the 124 phase grew by such solid state reactions, we would not expect any difference between SrTiO5 (100) and MgO (100) substrates. However, a clear difference was observed between the results shown in fig. 3 and fig. 7. This difference could he caused by the ratios of 123 and 124 at the early stage of the deposition on SrTiO3 (100) and on MgO (1(0). On the hypothesis that both 124 and 123 could nucleate on the substrate during the early stage of deposition before the substrate surface is covered with the film, the phenomenon is explained tentatively as follows: (i) As the lattice constant mismatch for 124 to SrTiO3 (100) is smaller than that to MgO (100), 124 nucleates more easily on SrTiO3 (100) than on MgO (100). When the substrate surface was covered with the mixture of 124 and 123, the area ratio, as well as the number of grains, of 124 to 123 in the case of the SrTiO3 (100) is higher than
of
Yiia~(’u
4O5. thin tthn.s
lot’ MO(k[)
in the case of MgO (100). Once the substrate surface is covered with film, such that the growth of the 124 grains perpendicular to the substrate surface become superior to that of the 123 grains, and the volume of 124 becomes higher than that of 123 as the growth time increases. (ii) The 123 grains on MgO (100) are in disorder because of the lattice constant mismatch between 123 and the substrate. The grtwth rate perpendicular to the substrate surface of the disordered 123 grain would he higher than that of the ordered 123 grain and not so different from that of the 124 grain. Appropriate thickness is needed to overcome the disorder. After the disorder is overcome, the growth rate of the 124 grain perpendicular to the substrate surface becomes higher than that of the 123 grain. On the contrary, in the case of SrTiO~ (100). the lattice constant mismatch between 123 and the substrate is small, and there is not such a strong discrder in the 123 grains. Therefore in the case of the MgO (100) substrate. the 123 phase is more dominant than in the case of the SrTiO5 (100) substrate at the early stage of deposition. We could not definitely conclude by only XRD data that the actual behavior is really the case presented in (i) or (ii). Other explanations might exist. Cross-sectional TEM observation close to the substrate surface would explain the actual nucleation of 123 and 124 on the substrate, for example, which phase exists on the substrate or how dislocations exist in the grains. However, we have not succeeded in obtaining a good 1’EM image to explain the nucleation. In this discussion. we made the assumption that both 124 and 123 on the substrate could nucleate during the early stage of deposition before the substrate surface is covered: however, we did not discuss the nucleation kinetics of 124 an 123. To clarify these points, further studies will he needed.
4. Summary We have obtained films in which YBa2Cu4O5 is the main phase. From the results for different deposition times, it is clear that the 124 phase is not the result of solid state reaction, hut grows
H. Hayas/o ci a!.
/ Preparation
during deposition. The upper part and the lower part of the film differ in the volume ratio of 124 to 123, and in the part near the film surface, the 124 phase is dominant. Also, films fabricated under the same conditions are different for SrTiO3 (100) and MgO (100). These differences originate from the difference of volume of 124 and 123 at the early stage of deposition; the growth of 124 is favored under the deposition conditions.
of YBa,Cu
405 thin films by MOCVD
787
19]
J. Kwo. M. Hong, R.M. Fleming. A.F. Hehard. ML. Mandich, AM. Desantolo, BA. Davidson, P. Marsh and W.D. Hobbins. AppI. Phys. Letters 52 (1988) 1625. [10] K. Char, Mark Lee, R.W. Barton, A F Marshall, 1. Bozovic. RH. Hammond, MR. Beasley. TN. Gehalle and A. Kapitulnik, Phys. Rev. B 38 (1988) 834. [11] A.F. Marshall, R.W. Barton. K. Car, A. Kapitulnik, B. Oh. RH. Hammond and S.S. Laderman. Phys. Rev. B 37 (1988) 9353. 112] K Char, M. Lee. R.W. Barton. A.F Marshall. I Bozovic, RH. Hammond, MR. Beasley. T.H. Gehalle and A. Kapitulnik. Phys. Rev. B 38 (1988) 834. 113] H. Abe, T. Turuoka and T. Nakamori, Japan. J. AppI. Phys. 27(1988) Ll473.
1141 K. Watanahe, T. Matsushita, N. Kohayashi, H. Kawabe,
References II] J. Karpinski, E. Kaldis. E. JOck, S. Rusiecki and B. Bueher. Nature 336 (1988) 660. 12] J. Karpinski, S. Rusiecki, F. Kaldis, B. Bucher and F. Jilek, Physica C 160 (1989) 449. [3] T. Wada. N. Suzuki, A. lehinose, Y. Yaegashi, H. Yamauehi and S. Tanaka, Appl. Phys. Letters 57 (1990) 81. [4] H. Murakami, S. Yaegashi, J. Nishino, Y. Shiohara and S. Tanaka, Japan J. AppI. Phys. 29 (1990) 2715. [5] T. Wada. N. Suzuki, K. Yamaguchi. A. lehinose, ‘~‘ Yaegashi, H. Yamauchi, N. Koshizuka and S. Tanaka, -~ Mater. Res. 6 (1991) 18. [6] T. Miyatake. K. Yamaguchi. T. Takaka, S. Gotoh, N. Koshizuka and S. Tanaka, Physica C 160 (1988) 541. [7] A.F. Marshall, R.W. Barton, K. Char, A. Kapitulnik, B. Oh, RH. Hammond and S.S. Laderman, Phys. Rev. B 37 (1988) 9353, 181 ML. Mandich, AM. DeSantolo, R.M. Fleming, P. Marsh, S. Nakahara, S. Sunshine. J. Kwo, M. Hong, T. Boone and T.Y. Kometani, Phys. Rev. B 38 (1988) 5031.
115] 116]
117]
118] 119]
[20]
F. Aoyagi. K. Hiraga. H. Yamane, H. Kurosawa. T. Hirai and Y. Muto, AppI. Phys. Letters 56(1990)1490. K. Shinohara. F. Munakata and M. Yamanaka. Japan. J. AppI. Phys. 27 (1988) L1683. H. Ohnishi. Y. Kusakahe, M. Kohayashi. S. Hoshinouchi, H. Harima and K. Tachibana, Japan. J. AppI. Phys. 29 (1990) 1070. H. Hayashi, T. Sugimoto, K. Kikuchi, S. Yuhya. Y. Yamada. M. Yoshida, K. Sugawara and Y. Shiohara, in: Proc. 2nd Intern. Symp. on Superconductivity. Tsukuba. 1989 (Springer, Tokyo, 1990) p. 431. H. Hayashi, Y. Yamada, K. Sugawara. Y. Shiohara and S. Tanaka. Japan. J. AppI. Phys. 30 (1991) L352. S. Jin, ElM. O’Bryan. P.K. Gallagher, T.H. Tiefel. R.J. Cava. R.A. Fastanaeht and G.W. Kammlott, Physica C 165 (1990) 415. H. Murakami, S. Yaegashi, J. Nishino. Y. Shiohara and 5. Tanaka. Japan. J. AppI. Phys. 29 (1990) 2715.
CRYSTAL GROWTH
J
North-Holland
Preparation of YBa7Cu4O8 thin films by MOCVD H. Hayashi, Y. Yamada, D.J. Baar, T. Sugimoto, K. Sugawara, Y. Shiohara and S. Tanaka .S’uperconducthiiy Research Lahorawrv, ISTEC
,
1—10- 13 .S’hinonome, Aoio—ku, Tokso 135, Japan
We have obtained a film in which the 1 24 phase was dominant. From the results for diflere Ut deposition times, it is dernonstt’ated that the I 24 phase is not formed through solid state reactiorL rather, it grows during deposition. We discuss the growth mechanism of the 124 phase. Films fabricated tinder the same deposition conditions were lound to he different for SrTiO (IOU) and for MgO (11)1)) substrates. The differences originate from the dii terence in ii ueleation and growth of the 124 phase at the early stage of deposition on the two substrates.
1. Introduction The superconducting phase YBa,Cu405 (124) is superior to the YBa~Cu~O~ (123) phase in terms of thermochemical stability of oxygen content. The oxygen content of 124 is not reduced at temperatures up to about 8500 C [1—4].and accordingly bulk samples quenched from high temperature still show superconductivity [51.Although the 1~of 124 is 80 K, which is lower than that of 123, Miyatake et al. have reported that I~. could be increased to 90 K by partial substitution of Ca l’or Y [6]. Therefore, the 124 phase may he useful for practical applications where liquid nitrogen cooling is employed. Previously, the 124 phase was discovered as lattice defects in thin I’ilm samples of the 123 phase [7—101.Thin films of 124 were fabricated by annealing an amorphous film, which was made by PVD (physical vapor deposition), at an appropriate temperature under 1 atm oxygen pressure [11,12]. However, there have been no reports of in-situ growth of 124 films using PVD. As for MOCVD. while several groups have fabricated 123 films [13—16],there have been no reports on 124 film formation. Recently, we reported the fabrication of Y—Ba—Cu—O superconducting films containing the 124 phase by MOCVD without post annealing [17], and that oxygen partial pressure and deposition temperature significantly affeet the appearance of the 124 phase [181. Unfor1)022-1)248/91 /$03.5() s~199!
tunatelv, we have not yet succeeded in the fabrication of single phase 124 films. To confirm the existence of the 124 phase, we tried to use TEM observation. Fig. I shows the X-ray diffraction pattern of the TEM sample which was fabricated according to the previous work: the susceptor temperature is C and the oxygen partial pressure is 7.5 Torr. The c-axis lattice constant of’ the 124 phase evaluated from this pattern is 27.2 A. which is close to the value of bulk 124 samples. Fig. 2a shows the TEM image of the 124 domain which was observed from cross-sectional direction, and the schematic crystal structure of 124 is shown in fig. 2h. The 7750
a .~.
I ‘~-
~
-~
>,
0
a a
~
a
~.
:~
I —
10
20
30
40
50
2 0 (degrees) Fig. I. X-ray diffraction pattern of TEM saniple.
Elsevier Science Publishers B.~’.All rights reserved
H. Havashi ci a!.
/ Preparation
of YBa,Cu
783
405 i/tin films by MOCVD
124 phase is different from the 123 phase in having a double CuO chain, and in the 124 phase 123 units slip off by 1/2 to the direction of the h-axis at double CuO chain. This featuring structure was observed in fig. 2a and the c-axis lattice constant estimated from the image was about 27 A, which was close to that of the 124 phase. In the present work, we have obtained films in which the 124 phase is dominant. We have studied the growth mechanism of the 124 phase in MOCVD film processing, by the result of changing the deposition time under the same conditions.
Table I Deposition conditions of Y—Ba-Cu-O films MO source Temperature (°C) Y(DPM)1 107 Ba(DPM)2 2(15 Cu(DPM)2 10()
Ar flow (SCCM) 50 Sf) 5))
Oxygen flow Substrate , Temperature of suseeptor Total gas pressure Oxygen partial pressure Deposition rate Deposition time
1000 SCCM SrTiO3(lOt)). MgO(lOO) 800 0 C 20 Torr 17.5 Torr 8 A/mm 15, 3)), 60. 120, 240, 480 mm
Cooling rate
100 C/mm
2. Experimental procedure deposition. SrTiO3(100) and MgO(100) 3) were used assingle subcrystals (10>< deposition, 10 x 0.5 mm strates. After the films were cooled to room temperature at a rate of 10°C/mm. The growth rate was about 8 A/mm estimated from SEM and ICP analysis. The crystal structures of the films were analyzed by X-ray diffraction (XRD) using Cu-Kct radiation.
The MOCVD system cold-wall used in this conventional horizontal type.study The isex-a perimental setup and deposition process were described in detail elsewhere [17]. Deposition conditions are listed in table 1. The source materials used are the /3-diketonate metal chelates: bis-(2,2,6,6-tetramethyl-3,5-heptanedionate)-yttrium Y(DPM) 3, Ba(DPM)2 and Cu(DPM)2. The temperatures of the vaporizers are fixed at 107, 205 and 100 C, respectively. The susceptor temperature was 800°C.The total gas pressure and oxygen partial pressure in the reactor were maintained at 20 and 17.5 Torr, respectively, during
3. Results and discussion
0
Fig. 3 shows the XRD pattern of a film deposited for 480 mm on a SrTiO3(100) substrate. /~-.-~-.-ç-.-ç. /
/
A
A
_I/
A
1
V
A
~ ~~Ba ~ ~oCu
-~!flT~Tfl° -~-
(a)
(b)
Fig. 2. (a) Cross-sectional TEM image of 124 domain in Y—Ba—Cu—O film. (b) Schematic presentation of the lattice structure of 124.
784
H. Ilavashi ci a!.
~.
a
~-
~
/ Preparation
of YBa,Co
405 thin films by MOCVI)
a
~-~-
•124(002)
,_
0123(001)
I
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.6
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—
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,
~
0
.-
10
20
30
40
50
20 (degrees) Fig.
Fig. 5.
3. X-ray diffraction pattern of the Y—Ba—Cu—O thin grown for 480 mm on a SrTiO5 (IOU) substrate
100
200
300
400
500
Deposition Time(min)
-
Intensities of
the (002) peak of the 124 phase and of
film
the (1)1)1) peak of the 123 phase as a function of deposition time in the case SrTiO3 (lilt)) was used as substrate.
Strong (001) peaks of the 124 phase are detected and peak intensities of the 123 phase are very weak. According to this result, it could be stated that we obtained almost single phase 124 film. To clarify when the 124 phase appears and how the 124 grain grows, we fabricated films for various deposition times keeping the other conditions the same. Composition of films are approximately Y: Ba : Cu = 0.95 : 2.0 :4.38. which is a little richer in Cu than 124 stoichiometry. Fig. 4a. 4b and 4c show the X-ray diffraction patterns of films A, B and C as a function of the deposition time under the same deposition condi-
tions on the SrTiO3 (100) substrate. In film A which was grown for 15 mm, (001) peaks of the 123 phase are detected but the peaks are broad. As for the 124 phase, only a weak and broad (002) peak was detected. In film B, which was grown for 30 mi (00/) peaks of the 123 phase are also detected, and the peak intensities hecome stronger and sharper than those of film A. The (002) peak of 124 is also detected, and the intensity was stronger and sharper than that of film A. However, the intensity is lower than that of the (001) peak of 123. The (001) peaks of 124 other than (002) are also detected. In film C, which was deposited for 60 mm, sharp peaks corresponding to the (00/) peaks of 124 and 123 are clearly identified. The intensity of the (002) peak of 124 became higher than that of the (001) peak of 123. From these results, it is clear that the diffraction intensities of the 124 and the 123 phases become stronger and that the peaks hecome sharper as the growth time increases. Fig. 5 shows the intensities of the (002) peak of
_________________________________ ~ ~
if
a
a
g ~
I
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U)
—
-Hi -~,
(c)
‘
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-~
~.
substrate.
1(a)
10
The for all samples X-ray were done under themeasurements same conditions including
~
20 30 20 (degrees)
40
Fig. 4. X-ray diffraction patterns of Y—Ba—Cu—O thin films grown for (a) 15 mm, (h) 3)) mm. and tel 60 nOn under the same deposition condition,
124 and of 123 deposition timeofinthe the(001) case peak SrTiO3 (100)versus was used as
power, slit width, sensitivity, scan speed, and sampie scale. The peak intensities of both peaks increased as the deposition time increased. However, the intensity of the (002) peak of 124 increased faster than the (001) peak of 123. This
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U)
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Fig. 6. X-ray diffraction patterns of the Y—Ba—Cu—O films: (a) as-grown (growth time = 120 mm), (h) after annealing of the film (a) for 360 mm under the deposition pressure, and (c) after annealing of the film (a) for 1680 mm under the deposition pressure.
result indicates that the volumes of both 124 and 123 also increased as the deposition time became longer, and that the ratio of the amounts of 124 to 123 increased as well, Recently, it has been found that 124 is a stable phase under high oxygen pressure or at low ternperature in the Y—Ba—Cu—O system. There have been some reports that the 124 phase can be fabricated by annealing an amorphous film [11,12] or by annealing a m~tureof 123 and CuO for an extended time period [19], or by the sol—gel method [20]. If, in the present experiment, the 124 phase
10
20 30 40 20 (degrees)
Fig. 7. X-ray diffraction pattern of the Y—Ba—Cu—O thin film grown for 480 mm on a MgO (IOU) substrate.
these conditions and decomposes slightly. Comparing the XRD patterns of fig. 6c to fig. 3, the total time for which the film at high temperature is about 1800was mm maintained and is about four times as long as the case of the sample in fig. ~ However, the intensity of the 124 from the sample in fig. 3 is much stronger than that of the sample in fig. 6c. Accordingly, it is apparent that the 124 phase in fig. 3 was not produced through a solid state reaction, but grew during deposition. We have discussed the films fabricated on the SrTiO3 (100) substrates, we also set MgO (100) substrates together with SrTiO3 (100) in the same
10’ ‘124(002) 0 123(001)
was formed from 123 and CuO through solid state reactions during the long time deposition, by annealing we would expect the as-deposited to obtain single filmphase for a long 124 films time under the deposition pressure. Figs. 6a, 6b and 6c show the X-ray diffraction patterns of the film (a) as grown for 120 mi (b) after annealing of the film (a) for 360 mm under the deposition pressure, and (c) after annealing of the film (a) for 1680 mm under the deposition pressure. The peak intensity of 124 was constant and the intensity of 123 decreased slightly as the annealing time was increased. From this result, it seemed that the 124 phase is stable under the deposition conditions, while the 123 phase is unstable under
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100
.
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100
200
300
400
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Time(min) Fig. 8. Intensities of the (002) peak of the 124 phase and of the (001) peak of the 123 phase as a function of deposition time in the case MgO (100) was used as substrate.
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/ Preparation
run. Fig. 7 shows the XRD pattern of a film deposited for 480 mm on MgO (100) substrate. Fig. 8 shows the intensity of the (002) peak of 124 and of the (001) peak of 123 versus deposition time in the case MgO (100) was used as substrate. In the case of MgO (100), the same tendencies were seen as described before in the case of SrTiO3 (100). As the deposition time increased, the peaks of 123 and 124 became sharp, and its intensities became stronger and the ratio of the heights of the (002) peak of 124 to the (001) peak of 123 increased. From the result that the 124 phase grew during deposition, there should be differences in the volume ratio of 124 to 123 between the part near the film surface and the part near the substrate surface. This result could he explained by assuming that the growth of the 124 grains perpendicular to the substrate surface is superior to that of the 123 grains. This occurs in both cases of using MgO (100) and SrTiO5 (100). However, there is a quite pronounced difference between fig. 3 and fig. 7, in which the deposition conditions were the same except for the substrates used. From this result, it was again confirmed that the 124 phase formation did not occur as a result of solid state reaction, hut instead grew during the process of deposition. Provided that the 124 phase grew by such solid state reactions, we would not expect any difference between SrTiO5 (100) and MgO (100) substrates. However, a clear difference was observed between the results shown in fig. 3 and fig. 7. This difference could he caused by the ratios of 123 and 124 at the early stage of the deposition on SrTiO3 (100) and on MgO (1(0). On the hypothesis that both 124 and 123 could nucleate on the substrate during the early stage of deposition before the substrate surface is covered with the film, the phenomenon is explained tentatively as follows: (i) As the lattice constant mismatch for 124 to SrTiO3 (100) is smaller than that to MgO (100), 124 nucleates more easily on SrTiO3 (100) than on MgO (100). When the substrate surface was covered with the mixture of 124 and 123, the area ratio, as well as the number of grains, of 124 to 123 in the case of the SrTiO3 (100) is higher than
of
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4O5. thin tthn.s
lot’ MO(k[)
in the case of MgO (100). Once the substrate surface is covered with film, such that the growth of the 124 grains perpendicular to the substrate surface become superior to that of the 123 grains, and the volume of 124 becomes higher than that of 123 as the growth time increases. (ii) The 123 grains on MgO (100) are in disorder because of the lattice constant mismatch between 123 and the substrate. The grtwth rate perpendicular to the substrate surface of the disordered 123 grain would he higher than that of the ordered 123 grain and not so different from that of the 124 grain. Appropriate thickness is needed to overcome the disorder. After the disorder is overcome, the growth rate of the 124 grain perpendicular to the substrate surface becomes higher than that of the 123 grain. On the contrary, in the case of SrTiO~ (100). the lattice constant mismatch between 123 and the substrate is small, and there is not such a strong discrder in the 123 grains. Therefore in the case of the MgO (100) substrate. the 123 phase is more dominant than in the case of the SrTiO5 (100) substrate at the early stage of deposition. We could not definitely conclude by only XRD data that the actual behavior is really the case presented in (i) or (ii). Other explanations might exist. Cross-sectional TEM observation close to the substrate surface would explain the actual nucleation of 123 and 124 on the substrate, for example, which phase exists on the substrate or how dislocations exist in the grains. However, we have not succeeded in obtaining a good 1’EM image to explain the nucleation. In this discussion. we made the assumption that both 124 and 123 on the substrate could nucleate during the early stage of deposition before the substrate surface is covered: however, we did not discuss the nucleation kinetics of 124 an 123. To clarify these points, further studies will he needed.
4. Summary We have obtained films in which YBa2Cu4O5 is the main phase. From the results for different deposition times, it is clear that the 124 phase is not the result of solid state reaction, hut grows
H. Hayas/o ci a!.
/ Preparation
during deposition. The upper part and the lower part of the film differ in the volume ratio of 124 to 123, and in the part near the film surface, the 124 phase is dominant. Also, films fabricated under the same conditions are different for SrTiO3 (100) and MgO (100). These differences originate from the difference of volume of 124 and 123 at the early stage of deposition; the growth of 124 is favored under the deposition conditions.
of YBa,Cu
405 thin films by MOCVD
787
19]
J. Kwo. M. Hong, R.M. Fleming. A.F. Hehard. ML. Mandich, AM. Desantolo, BA. Davidson, P. Marsh and W.D. Hobbins. AppI. Phys. Letters 52 (1988) 1625. [10] K. Char, Mark Lee, R.W. Barton, A F Marshall, 1. Bozovic. RH. Hammond, MR. Beasley. TN. Gehalle and A. Kapitulnik, Phys. Rev. B 38 (1988) 834. [11] A.F. Marshall, R.W. Barton. K. Car, A. Kapitulnik, B. Oh. RH. Hammond and S.S. Laderman. Phys. Rev. B 37 (1988) 9353. 112] K Char, M. Lee. R.W. Barton. A.F Marshall. I Bozovic, RH. Hammond, MR. Beasley. T.H. Gehalle and A. Kapitulnik. Phys. Rev. B 38 (1988) 834. 113] H. Abe, T. Turuoka and T. Nakamori, Japan. J. AppI. Phys. 27(1988) Ll473.
1141 K. Watanahe, T. Matsushita, N. Kohayashi, H. Kawabe,
References II] J. Karpinski, E. Kaldis. E. JOck, S. Rusiecki and B. Bueher. Nature 336 (1988) 660. 12] J. Karpinski, S. Rusiecki, F. Kaldis, B. Bucher and F. Jilek, Physica C 160 (1989) 449. [3] T. Wada. N. Suzuki, A. lehinose, Y. Yaegashi, H. Yamauehi and S. Tanaka, Appl. Phys. Letters 57 (1990) 81. [4] H. Murakami, S. Yaegashi, J. Nishino, Y. Shiohara and S. Tanaka, Japan J. AppI. Phys. 29 (1990) 2715. [5] T. Wada. N. Suzuki, K. Yamaguchi. A. lehinose, ‘~‘ Yaegashi, H. Yamauchi, N. Koshizuka and S. Tanaka, -~ Mater. Res. 6 (1991) 18. [6] T. Miyatake. K. Yamaguchi. T. Takaka, S. Gotoh, N. Koshizuka and S. Tanaka, Physica C 160 (1988) 541. [7] A.F. Marshall, R.W. Barton, K. Char, A. Kapitulnik, B. Oh, RH. Hammond and S.S. Laderman, Phys. Rev. B 37 (1988) 9353, 181 ML. Mandich, AM. DeSantolo, R.M. Fleming, P. Marsh, S. Nakahara, S. Sunshine. J. Kwo, M. Hong, T. Boone and T.Y. Kometani, Phys. Rev. B 38 (1988) 5031.
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[20]
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