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Rhenium and rhenium oxide as buffer layers for high Tc thin films on metallic substrates
-
ml w
Pergamon
-
Applied Superconductivity Vol. 3, No. 1-3, pp. 55-60, 1995 Copyright 0 1995 Else& Science Ltd 0964-1807(95)00032-l Printed in Great Britain. All rights reserved 0964-1807195 $9.50 + 0.00
RHENIUM AND RHENIUM OXIDE AS BUFFER LAYERS FOR HIGH T, THIN FILMS ON METALLIC SUBSTRATES K. L. JIAO, L. H. CHANG, R. WALLACE and W. A. ANDERSON Department of Electrical and Computer Engineering, State University of New York at Buffalo, 217 C Bonner Hall, Amherst, NY 14260, U.S.A. Abstract-High T, superconductor films on metal wires for high power applications must show longterm stability. Suitable buffer layers are required to prevent interaction between the superconductor thin films and the substrate, and to improve the long-term stability. A high melting-point metal, rhenium, and its oxide, were sputter deposited on the metallic substrate, Hastelloy. The study showed that rhenium thin films were resistant to out-diffusion from the Hastelloy substrates. However, it was found that they are easily oxidized at high temperature and they are, therefore, not suitable to oxygen rich high r, superconductors like YBCO. They might work with other types of high T, superconductors which have a lesser oxygen dependence.
INTRODUCTION
Successful deposition of superconducting YBa2Cu307_x (YBCO) thin films on metallic substrates usually requires a buffer layer. The buffer materials will have the functions to prevent interaction between the superconductor and substrate, and to promote nucleation of a polycrystalline superconductor. They should also have good electrical conductivity, and long-term stability. A variety of materials, such as MgO [I], yttrium stabilized ZQ (YSZ) [2], Ag [3], SrTiOs [4], RuOz [5], BaTi03 (BTO) [6], and YSZ/Ft double layer structures [7, 81, have been used. Rhenium (RE) has been used for electrical contacts and as filaments for electronic devices. Re has a high density of 2 1.04 g/cm3 and high melting point of 3 180°C. It is resistant against some acids and it is not attacked by molten tin, zinc, silver, copper or aluminum. Both Re and its oxide (e.g. ReOz and ReOs) are electrically conductive (resistivity ranging from 1.93 x 10V5 O-cm for Re and 10e3 to 1O-4 R-cm [9] for rhenium oxides). It seemed that this metal and its conductive oxides would be good candidates for the buffer layers between YBCO and metallic substrates. EXPERIMENTAL
As would be expected from its high melting point, Re has a very low vapor pressure, only slightly higher than that of tungsten, and this makes it difficult to evaporate. We attempted direct evaporation using a thin Re wire as the filament. No obvious deposition was observed. Hence, a RF magnetron sputtering method was utilized for Re film deposition and oxygen was introduced for reactive sputtering of rhenium oxide films. A variety of substrates was used including glass slide, Si, SiOJSi, as well as metallic substrates of Hastelloy C-276 (Ni-Cr-MO). The Hastelloy was polished with a 1 pm diamond powder. The sputtering was carried out using a Re target with purity of 99.9%. A distance of 8 cm was used between the target and the substrate holder. Total gas pressure was 10 mTorr which was pure Ar for Re film deposition but was Ar with O2 as the reactive gas for rhenium oxide films. The partial pressure of oxygen was varied from 0.1 to 0.5 mTorr. Two different temperatures were used for Re sputtering; one was high temperature deposition at 5OO”C, the other was room deposition with or without an in-situ, post-deposition, high temperature annealing at 630°C. The deposition rate was loo-150 A/min. The properties of Re and its oxide thin films were studied alone without deposition of YBCO. The crystal structure of the films was analyzed by X-ray diffraction (XRD) from a Nicolet/STOE X-ray diffractometer using Cu-Ka radiation. The surface morphology was examined by a Hitachi S-800 scanning electron microscope (SEM). The composition of the films was investigated by 55
K. L.
56
JIAO et al.
energy dispersive X-ray spectroscopy (EDX) using a PGT IMIX X-ray analyzer. The interface properties of the films and the substrates were revealed by Auger electron spectroscopy (AES) from a Perkin-Elmer PHI-660 scanning Auger microprobe. RESULTS
AND
DISCUSSION
The high temperature as-deposite Re and its oxide thin films had smooth and shiny surfaces. Structure analysis using XRD indicated the Re films to be crystalline although the preference of crystal orientation was observed depending on different substrates, as shown in Figs 1 and 2 for Re films on Hastelloy and Si substrates, respectively. It can be seen that Re films on Hastelloy
Re/Hastelloy
(101)
I (100)
(002)
)t I
10
I
30
,\
/
/
40
20 Fig. 1. XRD results for a Re film on Hastelloy. :lol)
RelSi Si:(lll)
(002)
ReSi: (100)
10
I
I
I
40
2: Fig. 2. XRD results for a Re film on Si substrate.
Re in buffer layers for high T, films
Fig. 3. SEM picture to show grain structure of a Re film.
0.0
5.0
10.0
15.0
KeV Fig. 4. EDX result for a Re film on Hastelloy.
20.0
K. L. JIAO et al.
58
RelHastelloy
(annealed
in 0,)
60.0
0.0
10.0
20.0
30.0
40.0
Sputter Time (min) Fig. 5. AES profile for a Re film after annealing at 600°C in oxygen.
Fig. 6. SEM picture for a Re film deposited at room temperature without post-annealing.
Re in buffer layers for high TC films
Fig. 7. SEM picture for a Re film deposited at room temperature with post-anne :aling.
75.0
60.0
Post-annealed
30.0
15.0
0.0
Sputter Time (min) Fig. 8. AES profile
for the sample
pictured
in Fig. 7
59
60
K. L. JIAOetnl.
exhibited three features (IOO), (002), and (101) in addition to the identity peak of Hastelloy, while for Re films on Si, the orientation (100) was not observed. Instead, two peaks at the small angles appeared and they were due to rhenium silicide formation [9]. An SEM picture, as shown in Fig. 3, also exhibited some grain structures for Re thin films. However, for the rhenium oxide thin films, no XRD peaks were revealed except for the one corresponding to Hastelloy, indicating the oxide films to be in the amorphous state. The composition of Re thin films was examined by EDX; the peaks of Re was clear, as shown in Fig. 4. But, there were no Re signals for the oxide films. Different partial pressures of oxygen at 0.12, 0.2 and 0.5 mTorr were used to sputter oxide films on different substrates, including Si, SiOz, Hastelloy, and SiOZ coated Hastelloy, but the results were the same. This disappointing result was then verified by AES measurements and explained by the instability of the oxide films at high temperature (even during cooling down from high temperature). They might continue oxidization to the heptoxide form (Re207) which is volatile [lo]. This caused the later experiments to be focused on pure Re films. The superconductor YBCO thin films were then sputter deposited on the Re buffer layers. Unfortunately, the films were found to have poor morphology. It was then noticed that the Re layer was cracked and/or peeled off even under the pre-sputtering condition while the oxygen was already introduced. To further investigate this phenomenon, a Re film deposited on Hastelloy was heated in vacuum with a partial pressure of oxygen of 0.5 mTorr for 1 h. The AES profile measurement, as illustrated in Fig. 5, revealed a diffusion of oxygen into the sample and an out diffusion of Ni. Also, the very small signal of Re indicated a possible loss of Re during the annealing in oxygen (compare with Fig. 8). This suggested that Re may not be a good buffer layer, at least, for sputtering at high temperature. A room temperature deposition was used with and without in-situ post-annealing in vacuum. The Re films without post-annealing were cracked due perhaps to compressive stress, but the annealed ones were smooth, as shown in Figs 6 and 7, respectively. An AES profile showed a clear Re signal and a quite sharp interface at the Hastelloy surface, although there was a small oxygen peak at the interface due perhaps to chromium oxide. However, the YBCO thin films deposited on such as-annealed samples were also of poor quality. The T, was not measurable. An EDX analysis revealed that the peak of Re was very small, which was due very likely to the reaction of Re with oxygen, forming a heptoxide (Rez07) which was then vaporized. CONCLUSION
In conclusion, Re and its oxide films were explored as intermediate buffer layers between YBCO and metallic substrates. However, it was found that both are not suitable for this purpose since Re tends to be oxidized at an elevated temperature in oxygen and the finalized heptoxide form Rez07 is volatile. This detrimental factor make them unfit for the processing of YBCO films in which oxygen is necessary to be introduced at high temperatures. Their use for other types of superconductors has not been explored but could be interesting since Re prevents penetration of the metal substrate. Acknowledgemenrs-We wish to thank P Bush for the EDX and AES measurements. State Institute for Superconductivity.
The project was funded by the New York
REFERENCES 1. J. Saitoh, M. Fukutomi, 2. 3. 4. 5. 6. 7. 8. 9. 10.
Y. Tanaka, T. Asano, H. Maeda and H. Takahara, Jpn J Appl. Phys. 29, L1117 (1990). E. Narumi, L. W. Song, F. Yang, S. Patel, Y. H. Kao and D. T. Shaw, Appl. Pkys. Left. 56, 2684 (1990). R. E. Russo, R. P. Reade, J. M. McMillan and B. L. Olsen, J Appl. Phys. 68, 1354 (1990). T. Yamaguchi, S. Aoki, N. Sadakata, 0. Kohno and H. Osanai, Appl. Phys. Lett. 55, 1581 (1989). Q. X. Jia and W. A. Anderson, High temperature superconducting thin films on metallic substrates using metal or metallic oxide as a buffer layer. Presented at the 5th Annual Conf: on Superconductivity and its Applications, Buffalo, N. Y. (1991). Q. X. Jia and W. A. Anderson, Appl. Phys. Lett. 60, 2689 (1992). E. Narumi, L. W. Song, F. Yang, S. Patel, Y. H. Kao and D. T. Shaw, Appl. Phys. Lett. 58, 1202 (1990). J. Saitoh, M. Fukutomi, K. Komari, Y. Tanaka, T. Asano, H. Maeda and H. Takahara, Jpn 1 Appl. Phys. 30, L898 (1991). G. V Samsonov, The Oxide Handbook, p. 277 IFhPlenum, New York (1973). E D. Snell and L. S. Ettre, Encyclopedia of Industrial Chemical Analysis, p. S 18. Interscience, New York (1973).
ml w
Pergamon
-
Applied Superconductivity Vol. 3, No. 1-3, pp. 55-60, 1995 Copyright 0 1995 Else& Science Ltd 0964-1807(95)00032-l Printed in Great Britain. All rights reserved 0964-1807195 $9.50 + 0.00
RHENIUM AND RHENIUM OXIDE AS BUFFER LAYERS FOR HIGH T, THIN FILMS ON METALLIC SUBSTRATES K. L. JIAO, L. H. CHANG, R. WALLACE and W. A. ANDERSON Department of Electrical and Computer Engineering, State University of New York at Buffalo, 217 C Bonner Hall, Amherst, NY 14260, U.S.A. Abstract-High T, superconductor films on metal wires for high power applications must show longterm stability. Suitable buffer layers are required to prevent interaction between the superconductor thin films and the substrate, and to improve the long-term stability. A high melting-point metal, rhenium, and its oxide, were sputter deposited on the metallic substrate, Hastelloy. The study showed that rhenium thin films were resistant to out-diffusion from the Hastelloy substrates. However, it was found that they are easily oxidized at high temperature and they are, therefore, not suitable to oxygen rich high r, superconductors like YBCO. They might work with other types of high T, superconductors which have a lesser oxygen dependence.
INTRODUCTION
Successful deposition of superconducting YBa2Cu307_x (YBCO) thin films on metallic substrates usually requires a buffer layer. The buffer materials will have the functions to prevent interaction between the superconductor and substrate, and to promote nucleation of a polycrystalline superconductor. They should also have good electrical conductivity, and long-term stability. A variety of materials, such as MgO [I], yttrium stabilized ZQ (YSZ) [2], Ag [3], SrTiOs [4], RuOz [5], BaTi03 (BTO) [6], and YSZ/Ft double layer structures [7, 81, have been used. Rhenium (RE) has been used for electrical contacts and as filaments for electronic devices. Re has a high density of 2 1.04 g/cm3 and high melting point of 3 180°C. It is resistant against some acids and it is not attacked by molten tin, zinc, silver, copper or aluminum. Both Re and its oxide (e.g. ReOz and ReOs) are electrically conductive (resistivity ranging from 1.93 x 10V5 O-cm for Re and 10e3 to 1O-4 R-cm [9] for rhenium oxides). It seemed that this metal and its conductive oxides would be good candidates for the buffer layers between YBCO and metallic substrates. EXPERIMENTAL
As would be expected from its high melting point, Re has a very low vapor pressure, only slightly higher than that of tungsten, and this makes it difficult to evaporate. We attempted direct evaporation using a thin Re wire as the filament. No obvious deposition was observed. Hence, a RF magnetron sputtering method was utilized for Re film deposition and oxygen was introduced for reactive sputtering of rhenium oxide films. A variety of substrates was used including glass slide, Si, SiOJSi, as well as metallic substrates of Hastelloy C-276 (Ni-Cr-MO). The Hastelloy was polished with a 1 pm diamond powder. The sputtering was carried out using a Re target with purity of 99.9%. A distance of 8 cm was used between the target and the substrate holder. Total gas pressure was 10 mTorr which was pure Ar for Re film deposition but was Ar with O2 as the reactive gas for rhenium oxide films. The partial pressure of oxygen was varied from 0.1 to 0.5 mTorr. Two different temperatures were used for Re sputtering; one was high temperature deposition at 5OO”C, the other was room deposition with or without an in-situ, post-deposition, high temperature annealing at 630°C. The deposition rate was loo-150 A/min. The properties of Re and its oxide thin films were studied alone without deposition of YBCO. The crystal structure of the films was analyzed by X-ray diffraction (XRD) from a Nicolet/STOE X-ray diffractometer using Cu-Ka radiation. The surface morphology was examined by a Hitachi S-800 scanning electron microscope (SEM). The composition of the films was investigated by 55
K. L.
56
JIAO et al.
energy dispersive X-ray spectroscopy (EDX) using a PGT IMIX X-ray analyzer. The interface properties of the films and the substrates were revealed by Auger electron spectroscopy (AES) from a Perkin-Elmer PHI-660 scanning Auger microprobe. RESULTS
AND
DISCUSSION
The high temperature as-deposite Re and its oxide thin films had smooth and shiny surfaces. Structure analysis using XRD indicated the Re films to be crystalline although the preference of crystal orientation was observed depending on different substrates, as shown in Figs 1 and 2 for Re films on Hastelloy and Si substrates, respectively. It can be seen that Re films on Hastelloy
Re/Hastelloy
(101)
I (100)
(002)
)t I
10
I
30
,\
/
/
40
20 Fig. 1. XRD results for a Re film on Hastelloy. :lol)
RelSi Si:(lll)
(002)
ReSi: (100)
10
I
I
I
40
2: Fig. 2. XRD results for a Re film on Si substrate.
Re in buffer layers for high T, films
Fig. 3. SEM picture to show grain structure of a Re film.
0.0
5.0
10.0
15.0
KeV Fig. 4. EDX result for a Re film on Hastelloy.
20.0
K. L. JIAO et al.
58
RelHastelloy
(annealed
in 0,)
60.0
0.0
10.0
20.0
30.0
40.0
Sputter Time (min) Fig. 5. AES profile for a Re film after annealing at 600°C in oxygen.
Fig. 6. SEM picture for a Re film deposited at room temperature without post-annealing.
Re in buffer layers for high TC films
Fig. 7. SEM picture for a Re film deposited at room temperature with post-anne :aling.
75.0
60.0
Post-annealed
30.0
15.0
0.0
Sputter Time (min) Fig. 8. AES profile
for the sample
pictured
in Fig. 7
59
60
K. L. JIAOetnl.
exhibited three features (IOO), (002), and (101) in addition to the identity peak of Hastelloy, while for Re films on Si, the orientation (100) was not observed. Instead, two peaks at the small angles appeared and they were due to rhenium silicide formation [9]. An SEM picture, as shown in Fig. 3, also exhibited some grain structures for Re thin films. However, for the rhenium oxide thin films, no XRD peaks were revealed except for the one corresponding to Hastelloy, indicating the oxide films to be in the amorphous state. The composition of Re thin films was examined by EDX; the peaks of Re was clear, as shown in Fig. 4. But, there were no Re signals for the oxide films. Different partial pressures of oxygen at 0.12, 0.2 and 0.5 mTorr were used to sputter oxide films on different substrates, including Si, SiOz, Hastelloy, and SiOZ coated Hastelloy, but the results were the same. This disappointing result was then verified by AES measurements and explained by the instability of the oxide films at high temperature (even during cooling down from high temperature). They might continue oxidization to the heptoxide form (Re207) which is volatile [lo]. This caused the later experiments to be focused on pure Re films. The superconductor YBCO thin films were then sputter deposited on the Re buffer layers. Unfortunately, the films were found to have poor morphology. It was then noticed that the Re layer was cracked and/or peeled off even under the pre-sputtering condition while the oxygen was already introduced. To further investigate this phenomenon, a Re film deposited on Hastelloy was heated in vacuum with a partial pressure of oxygen of 0.5 mTorr for 1 h. The AES profile measurement, as illustrated in Fig. 5, revealed a diffusion of oxygen into the sample and an out diffusion of Ni. Also, the very small signal of Re indicated a possible loss of Re during the annealing in oxygen (compare with Fig. 8). This suggested that Re may not be a good buffer layer, at least, for sputtering at high temperature. A room temperature deposition was used with and without in-situ post-annealing in vacuum. The Re films without post-annealing were cracked due perhaps to compressive stress, but the annealed ones were smooth, as shown in Figs 6 and 7, respectively. An AES profile showed a clear Re signal and a quite sharp interface at the Hastelloy surface, although there was a small oxygen peak at the interface due perhaps to chromium oxide. However, the YBCO thin films deposited on such as-annealed samples were also of poor quality. The T, was not measurable. An EDX analysis revealed that the peak of Re was very small, which was due very likely to the reaction of Re with oxygen, forming a heptoxide (Rez07) which was then vaporized. CONCLUSION
In conclusion, Re and its oxide films were explored as intermediate buffer layers between YBCO and metallic substrates. However, it was found that both are not suitable for this purpose since Re tends to be oxidized at an elevated temperature in oxygen and the finalized heptoxide form Rez07 is volatile. This detrimental factor make them unfit for the processing of YBCO films in which oxygen is necessary to be introduced at high temperatures. Their use for other types of superconductors has not been explored but could be interesting since Re prevents penetration of the metal substrate. Acknowledgemenrs-We wish to thank P Bush for the EDX and AES measurements. State Institute for Superconductivity.
The project was funded by the New York
REFERENCES 1. J. Saitoh, M. Fukutomi, 2. 3. 4. 5. 6. 7. 8. 9. 10.
Y. Tanaka, T. Asano, H. Maeda and H. Takahara, Jpn J Appl. Phys. 29, L1117 (1990). E. Narumi, L. W. Song, F. Yang, S. Patel, Y. H. Kao and D. T. Shaw, Appl. Pkys. Left. 56, 2684 (1990). R. E. Russo, R. P. Reade, J. M. McMillan and B. L. Olsen, J Appl. Phys. 68, 1354 (1990). T. Yamaguchi, S. Aoki, N. Sadakata, 0. Kohno and H. Osanai, Appl. Phys. Lett. 55, 1581 (1989). Q. X. Jia and W. A. Anderson, High temperature superconducting thin films on metallic substrates using metal or metallic oxide as a buffer layer. Presented at the 5th Annual Conf: on Superconductivity and its Applications, Buffalo, N. Y. (1991). Q. X. Jia and W. A. Anderson, Appl. Phys. Lett. 60, 2689 (1992). E. Narumi, L. W. Song, F. Yang, S. Patel, Y. H. Kao and D. T. Shaw, Appl. Phys. Lett. 58, 1202 (1990). J. Saitoh, M. Fukutomi, K. Komari, Y. Tanaka, T. Asano, H. Maeda and H. Takahara, Jpn 1 Appl. Phys. 30, L898 (1991). G. V Samsonov, The Oxide Handbook, p. 277 IFhPlenum, New York (1973). E D. Snell and L. S. Ettre, Encyclopedia of Industrial Chemical Analysis, p. S 18. Interscience, New York (1973).