J . Phys. Chem. 1993,97, 7796-7199
7796
Contact Resistance Measurements Recorded at Conductive Polymer/High-Temperature Superconductor Interfaces Steven G. Haupt, David R. Riley, Jianai Zbao, and John T. McDevitt' Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712-1 167 Received: May 24, 1993
Methods of making electrical contact to high-Tc superconductors with conductive polymers are described. In addition, three- and four-point resistance measurements are acquired on samples of Y BazCu3074, GdBazCu3074, and Pbo.sBil.7Srl.aCa2.4Cu3010 using poly(3-hexylthiophene) contacts in order to determine the temperature dependence oft he conductive pol ymer/superconductor contact resistance. Although the polymer/superconductor contact resistance displays activated behavior above Tc and increases as the temperature is lowered, below Tc there is a precipitous decrease in the resistance at that interface. Similar measurements completed on systems where the superconducting components are substituted by normal metals do not show any signs of such contact resistance decreases. Possible reasons for the decrease in the polymer/superconductor interface resistance at Tc are discussed.
Introduction With the discovery of a number of cuprate superconductor phases which possess transition temperatures above 100 K, the economicand technological difficultiesof cooling superconductor elements for ultrafast electronic devices have been relaxed. Moreover, considerable effort has been devoted to the study of Si and GaAs semiconductor devices for operation at 77 K for high-speedoperation and effective power dissipation. Under such circumstances, it has become important to investigate the prospects for the development of new hybrid semiconductor/ superconductordevices. Unfortunately, the initial workcompleted in the area of high-T, thin-film deposition onto Si and GaAs surfaces has shown that these semiconductors are not chemically compatiblewith the cuprate materials. Although the use of metal oxide buffer layers and low-temperature processing has been shown to inhibit somewhat this adverse reactivity,' the search for new semiconductor materials which do not degrade the cuprate compounds is now warranted. In this regard, we have recently begun to seek procedures that can be utilized to fabricate and study conductive polymer/superconductor structures. Along these lines, we have demonstrated that the superconducting properties of YBazCu3074 films in hybrid conductive polymer/ high-temperature superconductor structures can be modulated in a controllable fashion by changing the polymer's oxidation level.2 Whereas neutral polypyrrole only slightly influences the electrical properties of the YBazCu3074 films as compared to the pristine (uncoated) samples, the oxidized polymer depresses Tc by more than 15 K. In this paper, wedescribe contact resistance experiments based on three- and four-point probe measurements that are completed in order to evaluate the chemical compatibility of oxidatively doped conductive polymers within a number of p-type cuprate phases as well as to explore polymer/superconductor chargetransfer phenomena at temperatures above and below Tc. A number of researchers have previously investigated the contact resistance phenomena which occur between hightemperature superconductors and conventional metals.3-9 Obtaining low contact resistance values is necessary for many practical applications and is diagnostic of a clean interface. In general, it has been found that, in cases where clean metal/ superconductorinterfacescan be achieved,values for the measured contact resistance are small in magnitude and display a metallike temperature dependence. Moreover, precipitous decreases in the interface resistance occur as the temperature is lowered below the transition temperature, T,. Such decreases in the 0022-3654/93/2091-7796%04.00/0
interface resistance have been interpreted previously as being the result of the induction of superconductivityinto the normal metal contacting layer via a process called the proximity effect. Since there are to date no documented examples of conductive organic polymers that by themselves display superconductivity,the study of conductive polymer/superconductor contact resistance phenomena provides a new avenue for research in which conductive polymer proximity effects might be explored.
Experimental Section
The monomer used for the preparation of poly(3-hexylthiophene) was synthesized using the method described by Tamao et al.1° Polymerization of the material was accomplished using FeC13 (Aldrich) as the oxidant accordingto the procedure reported by Sugimoto et al." Thin films of poly(3-hexylthiophene)were prepared via a spray coating deposition technique in which a commercial air brush was utilized with nitrogen gas as the propellant. Typically, -20 mL of a solution of poly(3-hexylthiophene) dissolved in tetrahydrofuran (0.5 g/L) was used to coat an area of 5 mm by 15 mm and yielded a film thickness of -4000 A. Oxidative doping of the poly(3-hexylthiophene)structure was accomplished using both electrochemical and chemical means. Chemical doping was achieved by exposing the poly(3-hexylthiophene) to a 0.06 M solution of FeC13 dissolved in nitromethane. The procedure was completed in an inert atmosphere of argon or nitrogen with exposure time of -20 s. Although the chemical doping was simple to perform, the dopant stoichiometry was difficult to control, and often damage to the underyling superconductor occurred with this method. Electrochemical doping was performed in an inert atmosphere glovebox using a solution of 0.1 M Et4NBF4 in acetonitrile. Oxidative doping of the polymer into the conductive form was accomplished by raising the potential of the superconductor electrode to values more positive than 0.7 V vs SCE. The maximum room temperature conductivityof such doped polymer films was found to be -20 $ 2 1 cm-1, close to values reported previously.11 All the devices discussed in this paper, with the exception of the P ~ o . ~ B ~ ~ , ~ S system, ~ ~ , ~were C ~doped Z,~CU~O~O using electrochemical procedures. Bulk ceramic pellets of YBazCu3074,GdBazCu3G4, and P4.3B i l , ~ S r l , ~ C a z , 4 Calong ~ 3 0 ~with ~ , laser ablated thin films of YBa2CuJ074, were utilized as the superconducting templates. Ceramic pellets of the YBazCu3074 and G d B a ~ C u 3 0 ,super~ conductors were prepared using standard solid-state methods of 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 7797
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Figure 1. Simplified equivalent circuit diagrams for the three- and fourpoint resistance measurements. The idealized circuits shown here do not include line resistance contributions.
heating the metal oxide and carbonate salts to temperatures in excess of 900 OC.12 The Pbo.3Bi1.7Srl.6Ca2.4CU3010 sample was prepared via the oxalate route.13 Thin films of YBa2Cu3074 (- 3000 A) were deposited onto single crystalline MgO (100) substrates using the pulsed laser ablation method.14 Four separate pads (- 5 mm long and 1 mm wide) were created on the surface of such films by using a micromanipulator to scribe the thin-film assembly with a diamond tip. The metal contacting layers that were exploited for the control studies were deposited by thermal evaporation for the cases of Ag and Au and by a sputtering procedure for Cu and Pt.
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Results It is common practice to use linear four-point probe measurements to measure directly sample resistance, thereby eliminating contact resistance contributions to the ascertainedvalues. Accordingly, two outer leads are used to pass current, and the potential is measured across the inner two leads. However, if the current is forced to pass through one of the voltage leads, the measured resistance will include contributions from both the sample resistance (RJ and one contact element ( R J . Figure 1 provides idealized equivalent circuits for both the four-point and the three-point geometries. Contact resistance values are acquired simply by subtractingthe four-point value from the 3-point value. Contact resistivity is calculated by multiplying the value of the contact resistance by the contact area. Figure 2 displays the different geometries that are utilized here to study the high-T, superconductor contact resistance phenomena. In the simplest geometry (Figure 2A), a rectangular segment of a ceramic pellet is coated with metal layers and silver paint is applied to attach the wire electrodes. A slightly different geometry (Figure 2B) is exploited when polymer contacts are made to the ceramic sample. Here the superconductor chip is encapsulated into an insulating epoxy matrix, and the polymer is deposited over both the superconductor and the supporting matrix. Consequently, when electrical contact to the polymer layer is made by using gold paste over the insulating segment, the possibility of direct electrical contact between the gold paste and the superconductor is eliminated. The above two geometriesutilize superconductor samples and normal contacts. In addition, patterned thin film contact
polymer sample
Figure 2. Schematic illustrations showing the electrical attachment geometries that were utilized for the sample and contact resistance measurements. Terminals labeled "I" are common current terminals used in both four- and three-point measurements, leads labeled "lcn are current leads used in three-point measurementsfor the contact resistance measurements, and "Is" are the current terminals exploited in the fourpoint measurementsto acquire the sample resistance. Terminals labeled "V" are used to measure the voltage in both the three- and four-point modes. (A) Geometry used tocollect data for Figure 3A,B. (B) Geometry employed to acquire data for Figure 3C. (C) Geometry used to collect data for Figure 3D,F. (D) Geometry exploited for Figure 3E.
templatesare utilized to yield the opposite geometry which consists of a polymer sample with superconductor contacts (Figure 2C). For comparison purposes, the behavior of conductive polymer samples with normal metal contacts is also examined (Figure 2D). Prior to studying the organic conductors, we examined the temperature dependence of the contact resistance between a variety of normal metals and a number of cupratesuperconductors. The contact resistivity of noble metal contacting layers, such as Ag, Au, or Pt, is found to have a room temperature value of close to 1 X 10-5 h m 2 , the magnitude of which decreasesas the sample is cooled.15 For such metals, large decreases in the contact resistance are noted near Tc. Typical data for Y B a ~ C u 3 0 ~with 4 Au contacting layers are shown in Figure 3A. On the other hand, when contacting metals such as Cu, Sn, Al, Pb, or In are utilized, the room temperature contact resistivity is significantly higher. Moreover, these systems display activated temperature dependencies, and no decreases in the interface resistance are observed near T,. Metals of this type, which possess stable metal oxides, are known to react chemically with high-T, samples and form a degradation layer at the interface between the two conductors.* Data typical for the reactive metal case are shown in Figure 3B for a YBa2Cu3074 ceramic sample measured with Cu contacts. We infer that good electrical connection is made between the polymer and the superconductor because decreases in the fourpoint sample resistance with onset temperature near 110 K and zero resistance close to 85 K are noted for Pb.3Bil.7Sr1.6Ca2.4Cu3010samples measured with poly(3-hexylthiophene)contacts, as shown in Figure 3C. Almost identical four-point sample resistance results are acquired with the use of silver contacts on the same specimen. These results demonstrate that conductive polymer components can be utilized to prepare superconductor
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Figure 3. Contactresistance (left axis) and sample resistance (right axis) as a function of temperaturerecorded for the following sample/contactelement combinations: (A) YBa2CuoO74 ceramic pellet sample with gold contacts. (B) YBa2CU307d ceramic pellet with copper contacts. (C) Pb.3Bi1.7Srl.6Cal.&u3Olo ceramic pellet with poly(3-hexylthiophene) contacts. (D) Poly(3-hexylthiophene)film sample with YBazCu307-s film contacts. (E) Poly(3-hexylthiophene)film sample with platinum film contacts. (F) Poly(3-hexylthiophene)film samplewith an oxygen-deficientYBa2CusO7-s film contact.
TABLE I: Contact Resistivity Values -.
sample highly doped PHT film [p(RT) = 7.5 Q-cm] highly doped PHT film [p(RT) = 51 Qam]
contactb YBa2Cu307.8 film, Tc(onset) = 73 K, AT=3K YBa~Cu307.6film, Tc(onset) = 72 K, AT=45K
290 K 250
90K 21000
45K 1200
notesC geometryC
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lightly doped PHT film (p(RT) = 1500 Qan] YBa2Cu307.8 film nonsuperconducting 14000 down to 20 K 16 30 7.2 P ~ O . ~ B ~ ~ , . I S ~ ceramic ~C~~C pellet U ~ O I O highly doped PHT film Pt film 30 660 3600 highly doped PHT film [p(RT) = 12 Qtm] 4.9 X lv 1 X gold film 3.1 X 1 k 5 YBazCuaO7.s ceramic pellet copper film 0.002 0.36 YBazCu307.6 ceramic pellet indium layer 112.16 (300 K) 84 (77 K) Yh2Cu307.6 ceramic pellet a PHT = Poly(3-hexylthiophene). AT = 10%90% transition width. Geometries as specified in Figure 2. circuits which operate at temperatures both above and below T,. Moreover, the normal state values of the contact resistance acquired for the poly( 3-hexylthiophene)/Y Ba2Cu307~structure are comparable to values acquired for systems in which the superconductor component is replaced with a noble metal material such as platinum (see Table I). Such results indicate that the oxidized polymer and the cuprate superconductor are chemically compatible. Although the values obtained for the noble metal/ superconductor contact resistance are far superior to those obtained for polymer/superconductor contact resistance, the organic material's performance is comparable to that achieved with indium metal and is much better than that achieved with conventional semiconductors, such as silicon. The temperature dependence of the poly(3-hexylthiophene)/ P b o . ~ B i l , ~ S r ~ , 6 C a 2 .contact ~ c u ~ o lresistance ~ above Tc displays an activated behavior, with the contact resistance increasing as
geometry B geometryD geometry A geometry A ref 13
the temperature is lowered from room temperature to 110 K (Figure 3C). However, as T,(onset) is approached, the contact resistance decreases dramatically.16 Similar decreases in the contact resistance are noted also for theoppositegeometry (Figure 2C) where a poly(3-hexylthiophene) thin film sample is measured with YBa2Cu3074 contacts (Figure 3D). In each case, the temperature at which the contact resistance decrease occurs correlates well with the transition temperature for the underlying superconductor. As an important control, contact resistance values for a number of poly(3-hexylthiophene)/normal metal interfaces were investigated also (Figure 3E). Unlike the measurements made with the superconducting templates, no decreases in the contact resistance values are observed near 90 K. When an oxygen-deficient YBa2Cu3074 film with a very broad metal-superconductor transition region (AT, = 45 K) is used to make contact to the poly(3-hexylthiophene), only a slight
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The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 7799
decrease in contact resistance is noted near Tc (onset). Further cooling of the structure resulted in continued increases in the interface resistance (Figure 3F).
Mscussion From the magnitudes of the polymer/superconductor contact resistivities, it appears unlikely that conductivepolymers such as poly( 3-hexylthiophene) will be used as contacting materials for high current superconductor applications. The fact that the conductive polymers display low carrier densities and low mobilities below 100K is likely to be responsible for this behavior. In spite of these limitations, conductive polymers may find utility for the fabrication of hybrid polymer/superconductor devices. Important in this regard is the fact that the conductivity of conductive polymers, such as poly(3-hexylthiophene), can be controlled by varying the polymer oxidation state (i.e,, doping level). Moreover, unlike silicon and other conventional semiconductors,the doping of these polymers is highly reversible, and these organic materials appear to be chemically compatible with the cuprate compounds. The decrease of the poly(3-hexylthiophene)/high-T0 superconductor contact resistance can be attributed possibly to the induction of superconductivity at that interface caused by a proximity effect. Similar decreases in contact resistance values have been noted by other authors in the study ofnormal metal/ superconductor ~tructures.~ The proximity effect occurs when normal metals and superconductors are brought into intimate contact with each other and a leakage of the Cooper pairs (superconductingelectron pairs) from the superconductorto the normal metal occurs along with quasiparticle (normal isolated electrons) penetration from the metal to the s~perconductor.1~J~ The phenomenon is most pronounced directly at the interface between the two conductors. If the proximity effect is responsible for the decrease in poly(3-hexylthiophene)/superconductor contact resistance, this data would provide the initial evidence for the induction of superconductivity into a conductive polymer system.
Conclusions Oxidatively doped conductive polymer samples appear to be chemically compatiblewith the p-doped cuprate compounds, and these polymers can be utilized to make electrical contacts to highTc superconductors. Although the polymeric contacts will not likely be utilized for high current applications, they may find utility for the fabrication of novel polymer/superconductor devices and circuits. Moreover, the intriguing decreasesin the polymer/ superconductor interface resistance that are observed near Tc suggest that the proximal location of a superconductor can influence the electrical transport propertiesin conductive polymer structures. Accordingly, the data presented in this paper may provide the first evidence for the induction of superconductivity into a doped organic polymer. Modulation of transition tem-
perature and critical current in conductive polymer/superconductor sandwich structures reported recently is also consistent with this notion.2 Further experiments are now in progress to search for additional evidence for such an unprecedented effect.
Acknowledgment. This research was supportedby the National Science Foundation, Grant DMR-9221589, the Texas Advanced Research Program, Grant 003658-453, and by the Welch Foundation, Grant F-1193. The authors thank Raymond Cole for providing the Pbo.sBil,,Sr,.6Ca2.4Cu30~~ samples, Sukkeun Kim for completing the metal/superconductor measurements, Brett Yamazi for software development, and Christopher Jones and David Jurbergs for technical support. Royce W. Murray, James P. Collman, Matthew J. Holcomb, and William A. Little are thanked for useful discussion and insightful comments. Princeton Applied Research and Exxon Corporationsare thanked for generous gifts. References and Notes (1) Mogro-Campero, A. Supercond. Sci. Technol. 1990,3, 155-158. (2) Haupt, S.G.; Riley, D. R.; Jones, C. T.; Zhao, J.; McDevitt, J. T. J. Am. Chem. Soc. 1993,115,1196-1198. (3) Caton, R.; Selim, R.; Buoncristiani, A. M.; Byvik, C. E. Appl. Phys. Lett. 1988,52, 1014-1016. (4) Wieck, A. D.Appl. Phys. Lert. 1988,52,1017-1019. (5) Eldn,J.W.;Larson,T.M.;Bergren,N.F.;Ne~n,A.J.;Swartzlander, A. B.; Kazmerski, L. L.; Panson, A. J.; Blankenship, B. A. Appl. Phys. Lett. 1988,52,1819-1821. (6) Sugimoto, I.; Tajima, Y.; Hikita, M. Jpn. J. Appl. Phys. 1988,27, L864L866. (7) Wiek, A. D. Appl. Phys. Lett. 1988,53,12161218, (8) Suzuki, Y.; Kusaka, T.; Aoki, A.; Aoyama, T.; Yotsuya, T.; Ogawa, S . Jpn. J. A&. Phys. 1989,28,2463-2467. (9) Chen, Y.C.; Chong, K. K.; Meen, T. H. Jpn. J. Appl. Phys. 1991, 30,33-37. (10) Tamao, K.; Kodama, S.;Nakajima, I.; Kumada, M. Tetrahedron 1982,38,3347-3354. (1 1) Sugimoto, R.; Takeda, S.;Gu, H.; Yoshino, K.Chem. Express 1986, I, 635-638. (12) Riley, D.R.;McDevitt, J. T. J. Electroanul.Chem. 1990,295,373384. (13) Gritzner, G.; Bemhard, K. Physica C 1991,181,201-205. (14) Dijkkamp, D.; Venkatesan, T.; Wu, X.D.; Shaheen, S.A.; Jisrawi, N.; Min-Lee, Y. H.; McLean, W.L.; Croft, M. Appl. Phys. Lett. 1987,51, 619-621. (15) Values reported here for the contact resistivities are for samples in which no post metal deposition anneal was conducted. Further decreases of the contact resistivity by factors of at least 103 were noted when the metal/ superconductor specimenswere heated to temperaturesof 50&950 OC. Since the polymer structures arenot compatiblewith such heat treatments,allcontact resistivity values reported in this paper are for sampleswhich were not thermally annealed. (16) Unlike data acquired with normal metal/high-T, structures where the values of the contact resistance become vanishingly small below To,& for the polymer/superconductor possesses a small, but finite, value at low temperatures. The presence of a short segment of polymer which extends onto the insulating matrix and is not in direct contact with the superconductor material contributes line resistance to the circuit and may be responsible for the majority of the residual resistance associated with geometry C. (17) Meissner, H. Phys. Reo. 1960,117,672-680. (18) Clarke, J. Proc. R. Soc. London, A 1969,308, 447-471.
