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Thin films of the organic superconductor α1-(BEDT-TTF)2I3
St,u IfllTIHI TIIE ELSEVIER
Synthetic Metals 70 (1995) 791-792
Thin Films of the Organic Superconductor (xc-(BEDT-TTF)2I 3 J. Moldenhauer ~, H. Wachtel ~, D. Schweitzer a, B. Gompf b, W. Eisenmenger b, P. Bele c, H. Brunner c and H.J. Keller d ~3. Physikalisches Institut and hl. Physikalisches Insitut, Universit/it Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany CMax-Planck-Institut ftir medizinische Forschung, AG: Molekiilkristalle, Jahnstr. 29, 69120 Heidelberg, Germany aAnorganisch-Chemisches Institut, Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany Abstract ~-(BEDT-TI'F)fl3 is a quasi-two-dimensional organic metal with a metal-insulator phase transition at 135 K. Thermal treatment at about 800 C leads to the metallic system ~,-(BEDT-TTF)fl3, which becomes superconducting below 8 K. Thin films of the c~-phase (between 500 and 3000 A thick) have been evaporated in high vacuum onto several substrates and characterised by means of X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM) and a low field microwave absorption technique. Depending on the temperature of the substrate and the evaporation rate, the films exhibit different degrees of microcristallinity, which under certain conditions can be strongly reduced and a completely covering film can be obtained. X-ray diffraction spectra reveal a high orientation with the c-axis perpendicular to the substrate and as well the successful conversion into the oq- phase by tempering. SEM and AFM investigations prove that the conversion takes place without reducing the mechanical quality of the films. Low-field microwave-absorption experiments show that the eq-films become superconducting with an onset at 9 K. 1. I N T R O D U C T I O N During the last decade organic conductors and superconductors have attracted considerable interest. Apart from the interesting physics of low-dimensional systems in the area of so-called molecular electronics some effort is made to scale down electronic circuits and devices to dimensions of a few unit cells or even molecules. Radical cation salts of the organic donor BEDT-TTF (Bis(ethylenedithiolo)tetrathiafulvalene)exhibit a large variety in their transport properties from semiconducting to metallic and even superconducting behaviour with T c up to 12.4 K. Therefore these salts are excellent candidates for constructing electronic devices on the basis of a thin film technique, because the transport properties can be varied drastically by varying temperature or anions. A most interesting and well characterised system is the orphase of (BEDT-TI'F)fl3. At room temperature it is a metal with a conductivity of 60 - 200 S/cm and exhibits a sharp metal-insulator transition at 135 K [1,2]. This phase can be transformed (at 75°C for 2-3 days) into the metallic ot,-phase, which becomes superconducting below 8 K [3,4]. The fabrication of thin films of the radical cation salt (BEDT-TTF)zI3 was performed for the first time by Kawabata and co-workers [5,6].
pyrolytic graphite (HOPG). When the temperature of the substrate was below 300 K, they were warmed up at a rate of 5 15 K/h after evaporation. After evaporation, the films have been characterised with different methods like X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) in order to improve the quality of these films. Superconductivity of the films has been checked by a low-field microwave absorption (LFMA) technique. 3. R E S U L T S A N D D I S C U S S I O N 3.1. Thin films of ct-(BEDT-TTF)2I 3 First attempts in evaporation thin films onto Si- or glass substrates were performed at substrate temperatures of room temperature (RT) and above. This technique yields results as shown in Fig. 1 (left picture, type I) i.e. a film consisting of many crystals with 1-3 ~m size and a not completely covered substrate surface, Similar results were already obtained in refs. 5 and 6.
2. E X P E R I M E N T A L Concerning the evaporation of radical cation salts, we worked in a vacuum of 10-5 mbar base pressure. The radical cation salts were evaporated from a Knudsen-like source at temperatures between 150 °C and 220 °C at distances between 30 cm and 5 cm away from substrates kept at temperatures between 77 K and 460 K. Rates of deposition have been varied from 0.1 to 4 A/sec. Substrates were silicon, glass, mica and 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved 0379-6779(94)02653-G
SSDI
Fig. l: SEM micrographs of thin films of o~-(BEDT-TFF)fl3 evaporated on substrates at 300 K (left, type I) and 170 K (right, type II) (total area is 16 ~m x 16 ~m each). An extensive investigation of another route - which mainly consists in lowering the substrate temperature with varying
792
J. Moldenhauer et a L I Synthetic Metals 70 (1995) 791-792
source-target distances and evaporation rates - led to much better results, as shown in Fig. 1 (right picture), where the surface is completely covered (type II films). To determinate the crystal phase and the orientation of the unit cell with respect to the substrate plane XRD spectra of type II films were recorded. The spectrum shown in Fig. 2 (upper part) reveals (00n) reflections i.e. the (a,b)-plane of the unit cell is parallel to the substrate plane. . ~ . ',°.
'? . , ~ .
2000A ( as av'~oota~d )
,~.
T.
35
21000A temperedi~ 3h~,tBS~C I
5
10
15
~ ~t~BEDT-TTF)213
2O
25
3O
3.3. Conductivity and superconductivity Measurements of the dc-conductivity with the standard 4-point method still reveal thermally activated behaviour of the films. This might be due to the fact, that the conductivity is governed by the presence of micro-crystals with grain boundaries and imperfections• This assumption is supported by the following results of the low-field microwave absorption measurements - a method that has been applied successfully to granular samples of YBa2Cu307 [7,8]• For a proof of superconductivity in these films, we measured the microwave absorption of a film in the resonator of an X-band microwave bridge, with a field of -15 G to +15 G perpendicular to the substrate plane. On lowering the temperature, one observes an onset of the absorption at 9 K (Fig. 4, lower left), which does not appear in the films with a-phase (Fig. 4, upper left)• This absorption is ascribed to a microwave loss mechanism in intergranular Josephson junctions.
35
2e
Fig. 2: X-ray diffraction pattern of type II films: as evaporated (above) and afterwards tempered (below).
3.2. Films of t~-(BEDT-TTF)zI 3 During the transformation of crystals of ~-(BEDT-TI'F)213 into ct,-(BEDT-TrF),_I 3 at 75 °C the surface of the crystals usually is corrupted by a loss of iodine, and the whole crystal shows a lot of micro-cracks. Considering films of about 2000 A thickness, best results were obtained at higher temperatures i.e. 85 ° - 90 ° C for 1.5 to 4 hours. Fig. 2 (lower part) shows the XRD spectrum of such a converted film, and - as expected - (0On) reflections of the [3-phase (which is assumed to be equal to the ~,-phase) are observed.
12
10
8
6
4
2
0
temperature ~" 0 03 Same film as above after 2h of tempering. B = 14Gauss
10
/
T=I.7K
-20 -15 -10 -5 c e_. 0
12
10
8
6
4
2
0
5
10 15 20
B [Gauss}
0
temperature [K]
Fig. 4: Temperature dependent microwave absorption of a type II film: as evaporated (upper left) and tempered (lower left). Right: Field dependent absorption amplitude of an ~,- film.
References
Fig. 3: AFM-pictures of type II films: as evaporated (upper left), 1.5 hours (upper right) and 3.5 hours (below) tempered (total area of each picture is 1.5 pm × 1.5 pm). The investigations of the influence of this thermal treatment on the quality of the films with AFM (Fig. 3) show minor changes within the first 2 hours of exposure to elevated temperatures. Longer exposure leads to the formation of microcrystals and uncovered areas on the substrate.
1. K. Bender, K. Dietz, H. Endres, H.W. Helberg, I. Hennig, H.J. Keller, W. Sch~er, D. Schweitzer; Mol. Cryst. Liq. Cryst. 107 (1984), 45. 2. K. Bender, I. Hennig, D. Schweitzer, K. Dietz, H. Endres, H.J. Keller; Mol. Cryst. Liq. Cryst. 108 (1984), 359. 3. G.O. Baram, L.I. Buravov, L.C. Degtariev, M.E. Kozlov, V•N. Laukhin, E.E. Laukhina, V.G. Orischenko, K.I. Pokhodnia, M.K. Scheinkman, R.P. Shibaeva, E.B. Yagubskii; JETP Lett. 44 (1986), 293. 4. D. Schweitzer, P. Bele, H. Brunner, E. Gogu, U. Haeberlen, I. Hennig, T. Klutz, R. Swietlik, H. J. Keller; Z. Phys. B., Condensed Matter 67 (1987), 489. 5. K. Kawabata, K. Tanaka, M. Mizutani; Solid State Comm. 74 (1990), 83. 6. K. Kawabata, K. Tanaka, M. Mizutani; Adv. Mater. 3(1991), 157. 7. A. Dulcic, B. Ravkin, M. Pozek; Europhys. Lett. 10 (1989), 593. 8. M. Pozek, A. Dulcic. B. Ravkin; Physica C 169 (1990), 95. We gratefully acknowledge financial support of this work by the Deutsche Forschungsgemeinschaft (SFB 329).
Synthetic Metals 70 (1995) 791-792
Thin Films of the Organic Superconductor (xc-(BEDT-TTF)2I 3 J. Moldenhauer ~, H. Wachtel ~, D. Schweitzer a, B. Gompf b, W. Eisenmenger b, P. Bele c, H. Brunner c and H.J. Keller d ~3. Physikalisches Institut and hl. Physikalisches Insitut, Universit/it Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany CMax-Planck-Institut ftir medizinische Forschung, AG: Molekiilkristalle, Jahnstr. 29, 69120 Heidelberg, Germany aAnorganisch-Chemisches Institut, Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany Abstract ~-(BEDT-TI'F)fl3 is a quasi-two-dimensional organic metal with a metal-insulator phase transition at 135 K. Thermal treatment at about 800 C leads to the metallic system ~,-(BEDT-TTF)fl3, which becomes superconducting below 8 K. Thin films of the c~-phase (between 500 and 3000 A thick) have been evaporated in high vacuum onto several substrates and characterised by means of X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM) and a low field microwave absorption technique. Depending on the temperature of the substrate and the evaporation rate, the films exhibit different degrees of microcristallinity, which under certain conditions can be strongly reduced and a completely covering film can be obtained. X-ray diffraction spectra reveal a high orientation with the c-axis perpendicular to the substrate and as well the successful conversion into the oq- phase by tempering. SEM and AFM investigations prove that the conversion takes place without reducing the mechanical quality of the films. Low-field microwave-absorption experiments show that the eq-films become superconducting with an onset at 9 K. 1. I N T R O D U C T I O N During the last decade organic conductors and superconductors have attracted considerable interest. Apart from the interesting physics of low-dimensional systems in the area of so-called molecular electronics some effort is made to scale down electronic circuits and devices to dimensions of a few unit cells or even molecules. Radical cation salts of the organic donor BEDT-TTF (Bis(ethylenedithiolo)tetrathiafulvalene)exhibit a large variety in their transport properties from semiconducting to metallic and even superconducting behaviour with T c up to 12.4 K. Therefore these salts are excellent candidates for constructing electronic devices on the basis of a thin film technique, because the transport properties can be varied drastically by varying temperature or anions. A most interesting and well characterised system is the orphase of (BEDT-TI'F)fl3. At room temperature it is a metal with a conductivity of 60 - 200 S/cm and exhibits a sharp metal-insulator transition at 135 K [1,2]. This phase can be transformed (at 75°C for 2-3 days) into the metallic ot,-phase, which becomes superconducting below 8 K [3,4]. The fabrication of thin films of the radical cation salt (BEDT-TTF)zI3 was performed for the first time by Kawabata and co-workers [5,6].
pyrolytic graphite (HOPG). When the temperature of the substrate was below 300 K, they were warmed up at a rate of 5 15 K/h after evaporation. After evaporation, the films have been characterised with different methods like X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) in order to improve the quality of these films. Superconductivity of the films has been checked by a low-field microwave absorption (LFMA) technique. 3. R E S U L T S A N D D I S C U S S I O N 3.1. Thin films of ct-(BEDT-TTF)2I 3 First attempts in evaporation thin films onto Si- or glass substrates were performed at substrate temperatures of room temperature (RT) and above. This technique yields results as shown in Fig. 1 (left picture, type I) i.e. a film consisting of many crystals with 1-3 ~m size and a not completely covered substrate surface, Similar results were already obtained in refs. 5 and 6.
2. E X P E R I M E N T A L Concerning the evaporation of radical cation salts, we worked in a vacuum of 10-5 mbar base pressure. The radical cation salts were evaporated from a Knudsen-like source at temperatures between 150 °C and 220 °C at distances between 30 cm and 5 cm away from substrates kept at temperatures between 77 K and 460 K. Rates of deposition have been varied from 0.1 to 4 A/sec. Substrates were silicon, glass, mica and 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved 0379-6779(94)02653-G
SSDI
Fig. l: SEM micrographs of thin films of o~-(BEDT-TFF)fl3 evaporated on substrates at 300 K (left, type I) and 170 K (right, type II) (total area is 16 ~m x 16 ~m each). An extensive investigation of another route - which mainly consists in lowering the substrate temperature with varying
792
J. Moldenhauer et a L I Synthetic Metals 70 (1995) 791-792
source-target distances and evaporation rates - led to much better results, as shown in Fig. 1 (right picture), where the surface is completely covered (type II films). To determinate the crystal phase and the orientation of the unit cell with respect to the substrate plane XRD spectra of type II films were recorded. The spectrum shown in Fig. 2 (upper part) reveals (00n) reflections i.e. the (a,b)-plane of the unit cell is parallel to the substrate plane. . ~ . ',°.
'? . , ~ .
2000A ( as av'~oota~d )
,~.
T.
35
21000A temperedi~ 3h~,tBS~C I
5
10
15
~ ~t~BEDT-TTF)213
2O
25
3O
3.3. Conductivity and superconductivity Measurements of the dc-conductivity with the standard 4-point method still reveal thermally activated behaviour of the films. This might be due to the fact, that the conductivity is governed by the presence of micro-crystals with grain boundaries and imperfections• This assumption is supported by the following results of the low-field microwave absorption measurements - a method that has been applied successfully to granular samples of YBa2Cu307 [7,8]• For a proof of superconductivity in these films, we measured the microwave absorption of a film in the resonator of an X-band microwave bridge, with a field of -15 G to +15 G perpendicular to the substrate plane. On lowering the temperature, one observes an onset of the absorption at 9 K (Fig. 4, lower left), which does not appear in the films with a-phase (Fig. 4, upper left)• This absorption is ascribed to a microwave loss mechanism in intergranular Josephson junctions.
35
2e
Fig. 2: X-ray diffraction pattern of type II films: as evaporated (above) and afterwards tempered (below).
3.2. Films of t~-(BEDT-TTF)zI 3 During the transformation of crystals of ~-(BEDT-TI'F)213 into ct,-(BEDT-TrF),_I 3 at 75 °C the surface of the crystals usually is corrupted by a loss of iodine, and the whole crystal shows a lot of micro-cracks. Considering films of about 2000 A thickness, best results were obtained at higher temperatures i.e. 85 ° - 90 ° C for 1.5 to 4 hours. Fig. 2 (lower part) shows the XRD spectrum of such a converted film, and - as expected - (0On) reflections of the [3-phase (which is assumed to be equal to the ~,-phase) are observed.
12
10
8
6
4
2
0
temperature ~" 0 03 Same film as above after 2h of tempering. B = 14Gauss
10
/
T=I.7K
-20 -15 -10 -5 c e_. 0
12
10
8
6
4
2
0
5
10 15 20
B [Gauss}
0
temperature [K]
Fig. 4: Temperature dependent microwave absorption of a type II film: as evaporated (upper left) and tempered (lower left). Right: Field dependent absorption amplitude of an ~,- film.
References
Fig. 3: AFM-pictures of type II films: as evaporated (upper left), 1.5 hours (upper right) and 3.5 hours (below) tempered (total area of each picture is 1.5 pm × 1.5 pm). The investigations of the influence of this thermal treatment on the quality of the films with AFM (Fig. 3) show minor changes within the first 2 hours of exposure to elevated temperatures. Longer exposure leads to the formation of microcrystals and uncovered areas on the substrate.
1. K. Bender, K. Dietz, H. Endres, H.W. Helberg, I. Hennig, H.J. Keller, W. Sch~er, D. Schweitzer; Mol. Cryst. Liq. Cryst. 107 (1984), 45. 2. K. Bender, I. Hennig, D. Schweitzer, K. Dietz, H. Endres, H.J. Keller; Mol. Cryst. Liq. Cryst. 108 (1984), 359. 3. G.O. Baram, L.I. Buravov, L.C. Degtariev, M.E. Kozlov, V•N. Laukhin, E.E. Laukhina, V.G. Orischenko, K.I. Pokhodnia, M.K. Scheinkman, R.P. Shibaeva, E.B. Yagubskii; JETP Lett. 44 (1986), 293. 4. D. Schweitzer, P. Bele, H. Brunner, E. Gogu, U. Haeberlen, I. Hennig, T. Klutz, R. Swietlik, H. J. Keller; Z. Phys. B., Condensed Matter 67 (1987), 489. 5. K. Kawabata, K. Tanaka, M. Mizutani; Solid State Comm. 74 (1990), 83. 6. K. Kawabata, K. Tanaka, M. Mizutani; Adv. Mater. 3(1991), 157. 7. A. Dulcic, B. Ravkin, M. Pozek; Europhys. Lett. 10 (1989), 593. 8. M. Pozek, A. Dulcic. B. Ravkin; Physica C 169 (1990), 95. We gratefully acknowledge financial support of this work by the Deutsche Forschungsgemeinschaft (SFB 329).