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Fabrication and physical properties of very thin HgBa2CaCu2O6+δ films
Physica C 339 (2000) 253±257
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Fabrication and physical properties of very thin HgBa2CaCu2O6d ®lms L. Fang a,1, S.L. Yan a, A.A. Gapud a, T. Aytug a, B.W. Kang a, Y.Y. Xie a, J.Z. Wu a,*, S.C. Tidrow b, M.H. Ervins b, K.W. Kirchner b a
Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA b US Army Research Laboratory, Adelphi, MD 20783, USA Received 27 July 1999; received in revised form 23 March 2000; accepted 8 May 2000
Abstract Epitaxial c-axis-oriented HgBa2 CaCu2 O6d (Hg-1212) ®lms 50±80 nm thick have been grown on (0 0 1) LaAlO3 substrates using a cation-exchange process. These ®lms show smooth surface morphology and high-quality epitaxy. The superconducting transition temperatures are up to 118 K. In zero applied magnetic ®elds, the critical current densities of these ®lms are typically above 107 A/cm2 at 5 K and remain up to 3:54 105 A/cm2 at 100 K. Ó 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Synthesis; Critical currents; Magnetization; Thin ®lms
Many microelectronic applications require very thin high-Tc superconducting (HTS) ®lms with their thickness less than 100 nm. One such example is HTS bolometric infrared detectors, which oer the promise of matching the sensitivity of semiconductor infrared detectors, but with a detection range extended to wavelengths longer than 15 lm [1±3]. The superconductivity above 130 K discovered on Hg-based high-Tc superconductors (HgHTSs) [4,5] makes their application in infrared detection very promising as a high device operational temperature above 100 K can be realized.
* Corresponding author. Tel.: +1-858-534-0403; fax: +1-858534-2232. E-mail address: [email protected] (J.Z. Wu). 1 Permanent address: Department of Electronics, Nankai University, Tianjin 300071, PeopleÕs Republic of China.
Fabrication of very thin ®lms of these materials, however, has been an extremely dicult task due to the highly volatile nature of the Hg-based compounds. Despite an extensive eort worldwide, most Hg-HTS ®lms obtained so far [6±10] have relatively large thickness of the order of 1 lm and the minimum is still above 250 nm [6]. Two common problems were noticed with these thick ®lms: rough surface morphology and poor-quality epitaxy. These problems are not unexpected as Hg vapor reacts with most metals and oxides. The resulting poor ®lm/substrate interfaces make it nearly impossible to grow Hg-HTS ®lms at much smaller ®lm thicknesses. In addition, the severe airsensitivity of the cuprate precursor (Ba±Ca±Cu±O) used in the conventional thermal-reaction process [6±10] further complicates the processing and results in poor sample reproducibility even for thick Hg-HTS ®lms.
0921-4534/00/$ - see front matter Ó 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 0 3 5 8 - 0
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L. Fang et al. / Physica C 339 (2000) 253±257
These problems can be circumvented in a recently developed cation-exchange process, in which a pre-reacted Tly Ba2 Canÿ1 Cun Ox precursor ®lm (Tl-HTSs: y 1 or 2, n 2 or 3) is employed [11,12]. By annealing these precursor ®lms in Hg vapor, Hg-HTS ®lms can be formed through replacement of Tl by Hg. These Hg-HTS ®lms inherit the crystalline structure and surface morphology from their Tl-HTS precursor ®lms. This allows fabrication of very thin epitaxial HgBa2 CaCu2 O6d (Hg-1212) ®lms since very thin TlBa2 CaCu2 O7ÿd (Tl-1212) ®lms of high-quality epitaxy and smooth surface can be readily fabricated. In addition, Tl-1212 precursor ®lms are stable in air, which substantially simpli®es the sample preparation process and enhances sample reproducibility. In this article, we report superconducting properties of epitaxial Hg-1212 thin ®lms with thickness of 50±80 nm fabricated using the cation-exchange process. Very thin Tl-1212 precursor ®lms were prepared using the facing-magnetron sputtering and postannealing method [13]. The amorphous Tl±Ba± Ca±Cu±O ®lms were sputtered onto (0 0 1) LaAlO3 single crystal substrates from a pair of Tl-1212 targets in a mixture of Ar and O2 gases Ar=O2 4=1 at a total pressure of 20 mTorr. The thickness of the ®lms is dependent on the sputtering time and is typically in the range of 30±80 nm. The ®lm dimension is typically 5 5 mm2 . These ®lms were then annealed in 1 atm O2 using a crucible process at temperatures of 800±850°C [14]. The as-grown Tl-based ®lms were superconducting, and their phases were determined by X-ray diraction (XRD) h±2h scans using CuKa radiation. Fig. 1(a) shows the XRD spectrum for an 80-nm-thick Tl1212 ®lm annealed at 825°C. The dominant (0 0 l) peaks indicate that the ®lm is nearly phase-pure Tl-1212 with a negligible amount of impurity and it grows with the c axis perpendicular to the substrate surface. Using the above-mentioned method, high-quality Tl-HTS thin ®lms were fabricated reproducibly. It is interesting to note that the Tl± Ba±Ca±Cu±O amorphous precursor ®lms were found much less susceptible to humidity. Generally, exposing the Tl±Ba±Ca±Cu±O precursor ®lms to air for several hours or storing them in a dry box for several days would have little eect on the
Fig. 1. XRD h±2h patterns of (a) 80-nm-thick Tl-1212 precursor ®lm and (b) 80-nm-thick Hg-1212 ®lm made using cation-exchange process.
quality of Tl-based superconducting thin ®lms prepared. Although the mechanism is still under investigation, a possible explanation is that the presence of Tl in the Ba±Ca±Cu precursor ®lms may chemically stablize the BaO and CaO that are highly susceptible to moisture and carbonates in air so as to minimize the detrimental eect of air on the precursor ®lms. The Tl-1212 precursor ®lms were then sealed in a pre-cleaned and evacuated quartz tube with bulk pellets of HgBa2 Ca2 Cu3 Ox and Ba2 Ca2 Cu3 Ox , and were annealed at high temperatures, typically 760± 780°C for 3±4 h. The mass ratio between the two bulk pellets was 3:1 in order to maintain an appropriate Hg-vapor pressure. Hg-1212 ®lms were formed via Tl±Hg cation exchange in this annealing process. After the Hg-vapor annealing, the Hg1212 ®lms were further annealed at 300°C in a ¯owing O2 atmosphere for 1 h to optimize the oxygen content of the ®lms. The XRD h±2h pattern for an 80-nm-thick Hg-1212 ®lm is presented in Fig. 1(b). The observation of nearly identical XRD spectra for the Tl-1212 precursor ®lms (Fig. 1(a)) and the Hg-1212 ®lms (Fig. 1(b)) is not dif®cult to understand considering their identical crystalline structures.
L. Fang et al. / Physica C 339 (2000) 253±257
Fig. 2. M(T) curves of 80-nm-thick Hg-1212 ®lm and Tl-1212 precursor ®lm. The inset shows the temperature dependence of the resistivity for the same Hg-1212 ®lm.
The transformation from Tl-1212 to Hg-1212 is con®rmed by a dramatic increase of superconducting transition temperature (Tc ) after the Hgvapor annealing followed by O2 annealing. Fig. 2 shows the temperature (T) dependence of the zero®eld-cooled (ZFC) d.c. magnetization (M) of the same 80-nm-thick Hg-1212 ®lm and its Tl-1212 precursor ®lm in a 5 G magnetic ®eld measured in a superconducting quantum interference device (SQUID) magnetometer. The Tc of the Hg-1212 ®lm is around 118 K, which is slightly lower than that (120±124 K) obtained on thick Hg-1212 ®lms [6,11]. No other transition is visible on the smooth M±T curve of the Hg-1212 ®lm, indicating that the unconverted Tl-1212 phase is minimal. This consists, with the observation, of similar M±T curves on Hg-1212 ®lms processed using longer Hg-vapor annealing times. It should be noted that the Tc value of the Hg-1212 ®lm is about 33 K higher than that of its precursor Tl-1212 ®lm. The transition width for the former is much smaller than that for the latter. The resistivity vs. T curve for the same Hg-1212 ®lm measured by standard fourprobe method is shown in the inset of Fig. 2. Zeroresistance is achieved at 118 K, which is consistent with the magnetic measurement. To estimate the magnitude of critical current density (Jc ), both magnetic and transport measurements were carried out with a magnetic ®eld (H) applied normal to the ®lms. The magnetiza-
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Fig. 3. Jc (T) of a 80-nm-thick Hg-1212 ®lm in zero ®eld; Jc vs. H curves of the same 80-nm-thick Hg-1212 ®lm showed in the inset. The magnetic ®eld was applied parallel to the c axis of the ®lm.
tion Jc (Jcm ) was estimated using the Bean critical state model [15,16] from hysteresis M±H loops obtained using a 5 T SQUID magnetometer. The Jcm (H) can be calculated from the dierence between the upper and lower magnetization branches of the hysteresis M±H loops at the same ®eld H. In the calculation, the entire area of the ®lm was used. The calculated Jcm (T)Õs at zero ®eld are shown in Fig. 3 (curve (a)). The Jcm of the 80-nmthick Hg-1212 ®lm is around 1:05 107 A/cm2 at 5 K. It drops to 1:12 106 A/cm2 at 77 K, which is close to that previously reported [17] for ultra-thin YBa2 Cu3 O7ÿx ®lms of similar thickness at the same temperature. When the temperature increases to 100 and 110 K, the Jcm Õs are still as high as 3:54 105 and 1 105 A/cm2 , respectively, on the 80-nm-thick Hg-1212 ®lms. It should be noted that these Jcm values are comparable with the best values obtained for thick (250 nm) Hg-1212 ®lms made in the conventional thermal reaction process [18]. The transport Jc (Jct ) measured on an 80-nmthick Hg-1212 ®lm (curve (b) in Fig. 3), which was patterned into a bridge of 150 lm in length and 250 lm in width, is slightly lower than the Jcm value of the same sample. This may be induced by sample degradation during the wet etching process employed for the bridge patterning. A criterion of 1 lV/mm was used to determine the Jct from the I±V curves. Due to instrumental limitations, Jct was
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only measured near the Tc . The inset of Fig. 3 shows the ®eld dependence of the calculated Jcm for the 80-nm-thick ®lm at 5, 77, 100 and 110 K. It should be pointed out that at a ®xed temperature, a nearly identical H-dependence of Jcm was observed on Hg-1212 thick ®lms (thickness larger than 200 nm) made by cation-exchange process, although the absolute value of Jcm for the thick ®lm is a factor of 2 higher at zero ®eld. At low magnetic ®elds, Jcm decreases very slowly as magnetic ®eld increases. At 5 K, the Jcm decreases by a factor of 1.2 when the ®eld increases from 0 to 1000 G,
Fig. 4. (a) X-ray (1 0 2) pole ®gure of epitaxial Tl-1212 precursor ®lm and (b) Hg-1212 ®lm grown with cation-exchange process.
and by a factor of 4.5 to H 1 T. At 77 K, Jcm drops by a factor of 4.7 when the ®eld increases from 0 to 1000 G. The high Jc Õs obtained on the very thin Hg-1212 ®lm can be associated with its high-quality epitaxy inherited from its Tl-1212 precursor ®lm. Fig. 4(b) depicts the XRD pole ®gures of an 80-nm-thick Hg-1212 precursor ®lm. Four sharp (1 0 2) poles, which are 90° apart, were observed on very thin Hg-1212 ®lms. It is not a surprise that the pole ®gure of the Hg-1212 very thin ®lm is nearly identical to that of the corresponding Tl-1212 precursor ®lm (Fig. 4(a)) since the cation exchange occurs on an atomic scale and the crystalline structure of the precursor ®lm remains during the cation-exchange process. An examination of the substrate (1 0 1) poles suggests that the a and b axes of the ®lm align with the (1 0 0) and (0 1 0) axes of the (0 0 1) LaAlO3 substrate, con®rming the epitaxial growth of the Hg-1212 very thin ®lms on the substrate. Fig. 5 shows a typical scanning electron micrograph of an 80-nm-thick Hg-1212 ®lms. Smooth morphology and dense crystal structure can be clearly seen on the surface of the Hg-1212 ®lm. No grain boundary structure or cracks are visible, indicating a well-connected ®lm at the thickness of 80 nm. A small amount of unidenti®ed particulates of irregular shapes was
Fig. 5. Scanning electron microscopy image of a typical 80-nmthick Hg-1212 ®lm.
L. Fang et al. / Physica C 339 (2000) 253±257
also visible on the ®lm surface after the Tl±Hg cation exchange. It has been shown, however, that these particulates can be wiped out from the surface. It should be mentioned that both Tc and Jc are considerably lower for these very thin (<100 nm thick) Hg-1212 ®lms than for those thick cationexchange processed Hg-1212 ®lms [11], and that they reduce more signi®cantly as the ®lm thickness is further decreased. For a 50-nm-thick Hg-1212 ®lm, the Tc and Jc deteriorates rapidly, showing values of 95 K and 105 A/cm2 at 5 K, respectively. This reduction of Jc becomes more signi®cant at higher temperatures. At 5 K, the Jc of the 80-nmthick Hg-1212 ®lm is a factor of 2 less than that of 200-nm-thick Hg-1212 ®lm. At 110 K, Jc is reduced by a factor of 5±8. Further optimization of processing conditions is necessary to improve the quality of the very thin Hg-1212 ®lms. On the other hand, it should be realized that such reduction in Tc and Jc is not uncommon and has been reported for other high-Tc superconducting ultrathin ®lms [17,19]. It was argued [5] that oxygen depletion near the ®lm surface may be responsible for the degraded superconducting properties of very thin superconducting ®lms. A comparative investigation of sample quality with and without a protective layer would provide insights into this issue. In summary, c-axis-oriented epitaxial Hg-1212 ®lms of thickness 50±80 nm have been fabricated on (1 0 0) LaAlO3 substrates using the cationexchange process. Tc Õs up to 118 K were obtained with 80-nm-thick Hg-1212 ®lms. In zero magnetic ®eld, Jc is up to 1:05 107 A/cm2 at 5 K, 1:12 106 A/cm2 at 77 K, and close to 1 105 A/ cm2 at 110 K. These very thin Hg-1212 superconducting ®lms are promising for various microelectronic applications.
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Acknowledgements This work was supported in part by AFOSR, NSF, NSF EPSCoR, and DEPSCoR.
References [1] S.J. Berkowitz, A.S. Hirahara, K. Char, E.N. Grossman, Appl. Phys. Lett. 69 (1996) 2125. [2] J.P. Rice, E.N. Grossman, D.A. Rudman, Appl. Phys. Lett. 65 (1994) 773. [3] R.L. Richards, J. Appl. Phys. 76 (1994) 1. [4] S.N. Putlin, E.V. Antipov, O. Chmassem, M. Marzio, Nature (London) 362 (1993) 226. [5] A. Schilling, M. Cantoni, J.D. Go, H.R. Ott, Nature 363 (1993) 56. [6] C.C. Tsuei, A. Gupta, G. Trafas, D. Mitzi, Science 263 (1994) 1259. [7] S.H. Yun, J.Z. Wu, Appl. Phys. Lett. 68 (1996) 862. [8] S.H. Yun, J.Z. Wu, S.C. Tidrow, D.W. Eckart, Appl. Phys. Lett. 68 (1996) 2565. [9] F. Foong, B. Bedard, Q.L. Xu, S.H. Liou, Appl. Phys. Lett. 68 (1996) 1153. [10] W.N. Kang, R.L. Meng, C.W. Chu, Appl. Phys. Lett. 73 (1998) 381. [11] J.Z. Wu, S.L. Yan, Y.Y. Xie, submitted for publication. [12] S.L. Yan, Y.Y. Xie, J.Z. Wu, T. Aytug, A.A. Gapud, B.W. Kang, L. Fang, M. He, S.C. Tidrow, K.W. Kirchner, J.R. Liu, W.K. Chu, Appl. Phys. Lett. 73 (1998) 2989. [13] S.L. Yan, L. Fang, Q.X. Song, J. Yan, Y.P. Zhu, J.H. Chen, S.B. Zhang, Appl. Phys. Lett. 63 (1993) 1845. [14] M.P. Siegal, E.L. Venturini, D.L. Overmyer, P.P. Newcomer, J. Supercond. X (Y) (1997). [15] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [16] E.M. Gyorgy, R.B. van Dover, K.A. Jackson, L.F. Schneemeyer, J.V. Waszczak, Appl. Phys. Lett. 55 (1989) 283. [17] T. Venkatesan, X.D. Wu, B. Dutta, A. Inam, M.S. Hegde, D.M. Hwang, C.C. Chang, L. Nazar, B. Wilkens, Appl. Phys. Lett. 54 (1998). [18] L. Krusin-Elbaum, C.C. Tsuei, A. Gupta, Nature 373 (1995) 679. [19] X.X. Xi, J. Geerk, G. Linker, Q. Li, O. Meyer, Appl. Phys. Lett. 54 (1989) 23.
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Fabrication and physical properties of very thin HgBa2CaCu2O6d ®lms L. Fang a,1, S.L. Yan a, A.A. Gapud a, T. Aytug a, B.W. Kang a, Y.Y. Xie a, J.Z. Wu a,*, S.C. Tidrow b, M.H. Ervins b, K.W. Kirchner b a
Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA b US Army Research Laboratory, Adelphi, MD 20783, USA Received 27 July 1999; received in revised form 23 March 2000; accepted 8 May 2000
Abstract Epitaxial c-axis-oriented HgBa2 CaCu2 O6d (Hg-1212) ®lms 50±80 nm thick have been grown on (0 0 1) LaAlO3 substrates using a cation-exchange process. These ®lms show smooth surface morphology and high-quality epitaxy. The superconducting transition temperatures are up to 118 K. In zero applied magnetic ®elds, the critical current densities of these ®lms are typically above 107 A/cm2 at 5 K and remain up to 3:54 105 A/cm2 at 100 K. Ó 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Synthesis; Critical currents; Magnetization; Thin ®lms
Many microelectronic applications require very thin high-Tc superconducting (HTS) ®lms with their thickness less than 100 nm. One such example is HTS bolometric infrared detectors, which oer the promise of matching the sensitivity of semiconductor infrared detectors, but with a detection range extended to wavelengths longer than 15 lm [1±3]. The superconductivity above 130 K discovered on Hg-based high-Tc superconductors (HgHTSs) [4,5] makes their application in infrared detection very promising as a high device operational temperature above 100 K can be realized.
* Corresponding author. Tel.: +1-858-534-0403; fax: +1-858534-2232. E-mail address: [email protected] (J.Z. Wu). 1 Permanent address: Department of Electronics, Nankai University, Tianjin 300071, PeopleÕs Republic of China.
Fabrication of very thin ®lms of these materials, however, has been an extremely dicult task due to the highly volatile nature of the Hg-based compounds. Despite an extensive eort worldwide, most Hg-HTS ®lms obtained so far [6±10] have relatively large thickness of the order of 1 lm and the minimum is still above 250 nm [6]. Two common problems were noticed with these thick ®lms: rough surface morphology and poor-quality epitaxy. These problems are not unexpected as Hg vapor reacts with most metals and oxides. The resulting poor ®lm/substrate interfaces make it nearly impossible to grow Hg-HTS ®lms at much smaller ®lm thicknesses. In addition, the severe airsensitivity of the cuprate precursor (Ba±Ca±Cu±O) used in the conventional thermal-reaction process [6±10] further complicates the processing and results in poor sample reproducibility even for thick Hg-HTS ®lms.
0921-4534/00/$ - see front matter Ó 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 0 3 5 8 - 0
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L. Fang et al. / Physica C 339 (2000) 253±257
These problems can be circumvented in a recently developed cation-exchange process, in which a pre-reacted Tly Ba2 Canÿ1 Cun Ox precursor ®lm (Tl-HTSs: y 1 or 2, n 2 or 3) is employed [11,12]. By annealing these precursor ®lms in Hg vapor, Hg-HTS ®lms can be formed through replacement of Tl by Hg. These Hg-HTS ®lms inherit the crystalline structure and surface morphology from their Tl-HTS precursor ®lms. This allows fabrication of very thin epitaxial HgBa2 CaCu2 O6d (Hg-1212) ®lms since very thin TlBa2 CaCu2 O7ÿd (Tl-1212) ®lms of high-quality epitaxy and smooth surface can be readily fabricated. In addition, Tl-1212 precursor ®lms are stable in air, which substantially simpli®es the sample preparation process and enhances sample reproducibility. In this article, we report superconducting properties of epitaxial Hg-1212 thin ®lms with thickness of 50±80 nm fabricated using the cation-exchange process. Very thin Tl-1212 precursor ®lms were prepared using the facing-magnetron sputtering and postannealing method [13]. The amorphous Tl±Ba± Ca±Cu±O ®lms were sputtered onto (0 0 1) LaAlO3 single crystal substrates from a pair of Tl-1212 targets in a mixture of Ar and O2 gases Ar=O2 4=1 at a total pressure of 20 mTorr. The thickness of the ®lms is dependent on the sputtering time and is typically in the range of 30±80 nm. The ®lm dimension is typically 5 5 mm2 . These ®lms were then annealed in 1 atm O2 using a crucible process at temperatures of 800±850°C [14]. The as-grown Tl-based ®lms were superconducting, and their phases were determined by X-ray diraction (XRD) h±2h scans using CuKa radiation. Fig. 1(a) shows the XRD spectrum for an 80-nm-thick Tl1212 ®lm annealed at 825°C. The dominant (0 0 l) peaks indicate that the ®lm is nearly phase-pure Tl-1212 with a negligible amount of impurity and it grows with the c axis perpendicular to the substrate surface. Using the above-mentioned method, high-quality Tl-HTS thin ®lms were fabricated reproducibly. It is interesting to note that the Tl± Ba±Ca±Cu±O amorphous precursor ®lms were found much less susceptible to humidity. Generally, exposing the Tl±Ba±Ca±Cu±O precursor ®lms to air for several hours or storing them in a dry box for several days would have little eect on the
Fig. 1. XRD h±2h patterns of (a) 80-nm-thick Tl-1212 precursor ®lm and (b) 80-nm-thick Hg-1212 ®lm made using cation-exchange process.
quality of Tl-based superconducting thin ®lms prepared. Although the mechanism is still under investigation, a possible explanation is that the presence of Tl in the Ba±Ca±Cu precursor ®lms may chemically stablize the BaO and CaO that are highly susceptible to moisture and carbonates in air so as to minimize the detrimental eect of air on the precursor ®lms. The Tl-1212 precursor ®lms were then sealed in a pre-cleaned and evacuated quartz tube with bulk pellets of HgBa2 Ca2 Cu3 Ox and Ba2 Ca2 Cu3 Ox , and were annealed at high temperatures, typically 760± 780°C for 3±4 h. The mass ratio between the two bulk pellets was 3:1 in order to maintain an appropriate Hg-vapor pressure. Hg-1212 ®lms were formed via Tl±Hg cation exchange in this annealing process. After the Hg-vapor annealing, the Hg1212 ®lms were further annealed at 300°C in a ¯owing O2 atmosphere for 1 h to optimize the oxygen content of the ®lms. The XRD h±2h pattern for an 80-nm-thick Hg-1212 ®lm is presented in Fig. 1(b). The observation of nearly identical XRD spectra for the Tl-1212 precursor ®lms (Fig. 1(a)) and the Hg-1212 ®lms (Fig. 1(b)) is not dif®cult to understand considering their identical crystalline structures.
L. Fang et al. / Physica C 339 (2000) 253±257
Fig. 2. M(T) curves of 80-nm-thick Hg-1212 ®lm and Tl-1212 precursor ®lm. The inset shows the temperature dependence of the resistivity for the same Hg-1212 ®lm.
The transformation from Tl-1212 to Hg-1212 is con®rmed by a dramatic increase of superconducting transition temperature (Tc ) after the Hgvapor annealing followed by O2 annealing. Fig. 2 shows the temperature (T) dependence of the zero®eld-cooled (ZFC) d.c. magnetization (M) of the same 80-nm-thick Hg-1212 ®lm and its Tl-1212 precursor ®lm in a 5 G magnetic ®eld measured in a superconducting quantum interference device (SQUID) magnetometer. The Tc of the Hg-1212 ®lm is around 118 K, which is slightly lower than that (120±124 K) obtained on thick Hg-1212 ®lms [6,11]. No other transition is visible on the smooth M±T curve of the Hg-1212 ®lm, indicating that the unconverted Tl-1212 phase is minimal. This consists, with the observation, of similar M±T curves on Hg-1212 ®lms processed using longer Hg-vapor annealing times. It should be noted that the Tc value of the Hg-1212 ®lm is about 33 K higher than that of its precursor Tl-1212 ®lm. The transition width for the former is much smaller than that for the latter. The resistivity vs. T curve for the same Hg-1212 ®lm measured by standard fourprobe method is shown in the inset of Fig. 2. Zeroresistance is achieved at 118 K, which is consistent with the magnetic measurement. To estimate the magnitude of critical current density (Jc ), both magnetic and transport measurements were carried out with a magnetic ®eld (H) applied normal to the ®lms. The magnetiza-
255
Fig. 3. Jc (T) of a 80-nm-thick Hg-1212 ®lm in zero ®eld; Jc vs. H curves of the same 80-nm-thick Hg-1212 ®lm showed in the inset. The magnetic ®eld was applied parallel to the c axis of the ®lm.
tion Jc (Jcm ) was estimated using the Bean critical state model [15,16] from hysteresis M±H loops obtained using a 5 T SQUID magnetometer. The Jcm (H) can be calculated from the dierence between the upper and lower magnetization branches of the hysteresis M±H loops at the same ®eld H. In the calculation, the entire area of the ®lm was used. The calculated Jcm (T)Õs at zero ®eld are shown in Fig. 3 (curve (a)). The Jcm of the 80-nmthick Hg-1212 ®lm is around 1:05 107 A/cm2 at 5 K. It drops to 1:12 106 A/cm2 at 77 K, which is close to that previously reported [17] for ultra-thin YBa2 Cu3 O7ÿx ®lms of similar thickness at the same temperature. When the temperature increases to 100 and 110 K, the Jcm Õs are still as high as 3:54 105 and 1 105 A/cm2 , respectively, on the 80-nm-thick Hg-1212 ®lms. It should be noted that these Jcm values are comparable with the best values obtained for thick (250 nm) Hg-1212 ®lms made in the conventional thermal reaction process [18]. The transport Jc (Jct ) measured on an 80-nmthick Hg-1212 ®lm (curve (b) in Fig. 3), which was patterned into a bridge of 150 lm in length and 250 lm in width, is slightly lower than the Jcm value of the same sample. This may be induced by sample degradation during the wet etching process employed for the bridge patterning. A criterion of 1 lV/mm was used to determine the Jct from the I±V curves. Due to instrumental limitations, Jct was
256
L. Fang et al. / Physica C 339 (2000) 253±257
only measured near the Tc . The inset of Fig. 3 shows the ®eld dependence of the calculated Jcm for the 80-nm-thick ®lm at 5, 77, 100 and 110 K. It should be pointed out that at a ®xed temperature, a nearly identical H-dependence of Jcm was observed on Hg-1212 thick ®lms (thickness larger than 200 nm) made by cation-exchange process, although the absolute value of Jcm for the thick ®lm is a factor of 2 higher at zero ®eld. At low magnetic ®elds, Jcm decreases very slowly as magnetic ®eld increases. At 5 K, the Jcm decreases by a factor of 1.2 when the ®eld increases from 0 to 1000 G,
Fig. 4. (a) X-ray (1 0 2) pole ®gure of epitaxial Tl-1212 precursor ®lm and (b) Hg-1212 ®lm grown with cation-exchange process.
and by a factor of 4.5 to H 1 T. At 77 K, Jcm drops by a factor of 4.7 when the ®eld increases from 0 to 1000 G. The high Jc Õs obtained on the very thin Hg-1212 ®lm can be associated with its high-quality epitaxy inherited from its Tl-1212 precursor ®lm. Fig. 4(b) depicts the XRD pole ®gures of an 80-nm-thick Hg-1212 precursor ®lm. Four sharp (1 0 2) poles, which are 90° apart, were observed on very thin Hg-1212 ®lms. It is not a surprise that the pole ®gure of the Hg-1212 very thin ®lm is nearly identical to that of the corresponding Tl-1212 precursor ®lm (Fig. 4(a)) since the cation exchange occurs on an atomic scale and the crystalline structure of the precursor ®lm remains during the cation-exchange process. An examination of the substrate (1 0 1) poles suggests that the a and b axes of the ®lm align with the (1 0 0) and (0 1 0) axes of the (0 0 1) LaAlO3 substrate, con®rming the epitaxial growth of the Hg-1212 very thin ®lms on the substrate. Fig. 5 shows a typical scanning electron micrograph of an 80-nm-thick Hg-1212 ®lms. Smooth morphology and dense crystal structure can be clearly seen on the surface of the Hg-1212 ®lm. No grain boundary structure or cracks are visible, indicating a well-connected ®lm at the thickness of 80 nm. A small amount of unidenti®ed particulates of irregular shapes was
Fig. 5. Scanning electron microscopy image of a typical 80-nmthick Hg-1212 ®lm.
L. Fang et al. / Physica C 339 (2000) 253±257
also visible on the ®lm surface after the Tl±Hg cation exchange. It has been shown, however, that these particulates can be wiped out from the surface. It should be mentioned that both Tc and Jc are considerably lower for these very thin (<100 nm thick) Hg-1212 ®lms than for those thick cationexchange processed Hg-1212 ®lms [11], and that they reduce more signi®cantly as the ®lm thickness is further decreased. For a 50-nm-thick Hg-1212 ®lm, the Tc and Jc deteriorates rapidly, showing values of 95 K and 105 A/cm2 at 5 K, respectively. This reduction of Jc becomes more signi®cant at higher temperatures. At 5 K, the Jc of the 80-nmthick Hg-1212 ®lm is a factor of 2 less than that of 200-nm-thick Hg-1212 ®lm. At 110 K, Jc is reduced by a factor of 5±8. Further optimization of processing conditions is necessary to improve the quality of the very thin Hg-1212 ®lms. On the other hand, it should be realized that such reduction in Tc and Jc is not uncommon and has been reported for other high-Tc superconducting ultrathin ®lms [17,19]. It was argued [5] that oxygen depletion near the ®lm surface may be responsible for the degraded superconducting properties of very thin superconducting ®lms. A comparative investigation of sample quality with and without a protective layer would provide insights into this issue. In summary, c-axis-oriented epitaxial Hg-1212 ®lms of thickness 50±80 nm have been fabricated on (1 0 0) LaAlO3 substrates using the cationexchange process. Tc Õs up to 118 K were obtained with 80-nm-thick Hg-1212 ®lms. In zero magnetic ®eld, Jc is up to 1:05 107 A/cm2 at 5 K, 1:12 106 A/cm2 at 77 K, and close to 1 105 A/ cm2 at 110 K. These very thin Hg-1212 superconducting ®lms are promising for various microelectronic applications.
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Acknowledgements This work was supported in part by AFOSR, NSF, NSF EPSCoR, and DEPSCoR.
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