Neodymium copper oxide Nd/sub 2/CuO/sub 4/ (NCO) has been applied as a buffer material to improve the epitaxy of YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) thin films on reactive substrates and as a potential barrier to construct multi-layer junctions. The microstructures and interfaces in heteroepitaxial Nd/sub 2/CuO/sub 4//YBCO multi-layer have been characterized by using an electron microscopy. Cross-sectional images obtained on a transmission electron microscopy (TEM) revealed an atomically sharp boundary between layers, underlining the excellent compatibility of NCO with YBCO. No chemical reaction occurred between film and substrate. It was found that all layers grow highly epitaxially with their c-axis perpendicular to the substrate surface. On the other hand, various defects such as mis-oriented grains and stacking faults were found near the interfaces.published_or_final_versio

Cheung, YL

Gao, J

So, SM

IEEE Transactions on Applied Superconductivity

HKU Scholars Hub

TitleMicrostructural study in heteroepitaxial YBa/sub 2/Cu/sub3/O/sub 7//Nd/sub 2/CuO/sub 4/ multi-layers by using electronmicroscopyAuthor(s) Gao, J; Cheung, YL; So, SMCitation IEEE Transactions on Applied Superconductivity, 2003, v. 13 n. 2pt 1, p. 885-888Issued Date 2003URL http://hdl.handle.net/10722/43391Rights Creative Commons: Attribution 3.0 Hong Kong LicenseIEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003 885Microstructural Study in HeteroepitaxialYBa2Cu3O7=Nd2CuO4 Multi-Layersby Using Electron MicroscopyJ. Gao, Y. L. Cheung, and S. M. SoAbstract—Neodymium copper oxide Nd2CuO4 (NCO) has beenapplied as a buffer material to improve the epitaxy of YBa2Cu3O7(YBCO) thin films on reactive substrates and as a potential bar-rier to construct multi-layer junctions. The microstructures and in-terfaces in heteroepitaxial Nd2CuO4/YBCO multi-layer have beencharacterized by using an electron microscopy. Cross-sectional im-ages obtained on a transmission electron microscopy (TEM) re-vealed an atomically sharp boundary between layers, underliningthe excellent compatibility of NCO with YBCO. No chemical re-action occurred between film and substrate. It was found that alllayers grow highly epitaxially with their -axis perpendicular to thesubstrate surface. On the other hand, various defects such as mis-oriented grains and stacking faults were found near the interfaces.Index Terms—Nd2CuO4, superconducting films, TEM.I. INTRODUCTIONNEODYMIUM copper oxide Nd CuO (NCO) with a214T’ structure has been introduced as a new buffermaterial to improve the initial epitaxy quality and surfacemorphology of YBCO thin films grown on yttrium stabilizedZrO (YSZ) [1], [2]. Such a NCO material was first reportedin 1973 [3], which has a very stable tetragonal structure withlattice constants and . It behavesas a semiconductor with a high value of resistivity at lowtemperature. For an appropriate buffer layer, it must be inertboth chemically and structurally. Moreover, it should provide alattice strain relief and a high chemical stability [4]. Comparedwith those 123-phase compounds, the 214-phase compoundsshow a higher stability in their crystal structure and physicalproperties. There is no phase transition below the depositiontemperature or when the oxygen content changes. The latticeconstant of NCO matches well with that of YBCO. The thinfilms of NCO always show excellent crystallinity and verysmooth surface. All these features make NCO a good buffermaterial for growing YBCO on reactive substrates. Recently,we also succeeded in applying NCO as a barrier to constructhigh- Josephson junctions with high values [5].The interfaces and microstructures in the heteroepitaxialmulti-layer can greatly affect the initial epitaxy and the crys-tallinity of the grown layers. For instance, it has been knownthat the crystallinity and superconductivity of YBCO filmManuscript received August 05, 2002. This work has been supported by Re-search Grants Council (RGC) of Hong Kong (Project HKU7297/99P) and theUniversity Research Committee of HKU.The authors are with the Department of Physics, The University of HongKong, Pokfulam Road, Hong Kong (e-mail: jugao@hku.hk).Digital Object Identifier 10.1109/TASC.2003.814059deposited on YSZ could be hampered by the intermediate layerformed at the YBCO/YSZ interface. For multi-layer junctions,the boundary between electrode and barrier is a crucial factorto determine the performance of the formed junctions. Apoor interface is usually caused by the structural, thermal andchemical incompatibility between materials. Such interfacialand structural defects can be well identified by transmissionelectron microscopy (TEM). Hence, the study in interfaces andmicrostructures is of significance for many researches basedon multi-layers.In this paper, the detailed structures of heteroepitaxial multi-layer of YBCO/NCO have been investigated by means of TEM.The cross-sectional TEM (XTEM) study gives insight into thenature of epitaxial growth and interface structures. The goal ofour study is to reveal the possible origins which cause the degra-dation in both crystallinity and superconductivity. In particular,the structural compatibility between these two compounds isdiscussed.II. EXPERIMENTALYBCO and NCO thin films were fabricated on a STO sub-strate by using an off-axis rf-magnetron sputtering technique[6]. In brief, sintered stoichiometric discs of YBCO and NCOwith diameter 50 mm and thickness 4 mm were used as sput-tering targets. The chamber was evacuated with the backgroundpressure lower than 5 10 mbar. The sputtering took placein a mixture gas of argon and oxygen with a ration of Ar:O. The sputtering pressure was kept about 1.6 10 mbarduring the deposition. RF power of 80 W with a power den-sity of 4 W/cm was applied to the cathode. The substrate wasmounted on the heater by using silver paste and the substratetemperature was kept at 750 C. After the deposition, the filmwas annealed at 500 C in 1 bar oxygen for 30 minutes.To prepare cross-sectional TEM sample, tri-layer sample wascut and glued together face-by-face using epoxy. The samplewas mechanically ground down to 10 m in thickness by discgrinder (Gatan model 623) after the glue was dry. The samplewas mounted on a single hole Mo grid and was further thinnedby Ar ion milling with two ion-beam blocks to protect the films.During ion milling, the ion-milling holder was rotated and twoion guns were used. The sample was tilted with the beam in-cident angle of 15 . The applied beam energy was about5 keV and beam current was kept as 0.5 mA. When color fringescould be observed, the ion milling could be changed into an ionpolishing procedure. The ion polishing was performed with the1051-8223/03$17.00 © 2003 IEEE886 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003Fig. 1. Low-magnification XTEM image of NCO/YBCO/NCO tri-layergrown on STO substrate.Fig. 2. Interface between the NCO buffer layer and STO substrate.tilting angle of 8 –10 , the beam energy 3–4 keV and thebeam current 0.3 mA.Electron micro-images were observed on a Tecnai 20 trans-mission electron microscope with 200 kV.III. RESULTS AND DISCUSSIONTo be sure the observations made in this study are reliableand reflect the common features of YBCO/NCO heteroepitaxialmulti-layer, five samples made by different processes wereexamined. Fig. 1 gives a low-magnification overview of aNCO/YBCO/NCO tri-layer. The sample was prepared using anoff-axis sputtering technique. The three different layers couldeasily be identified by their different image contrast. All layerswere rather uniform and single crystal like. Strain effects dueto lattice mismatch were unlikely appeared as the TEM imageshows a sharp contrast for each layer. The layer thicknessmeasured from TEM image agreed well with that estimatedfrom the deposition time.Understanding the interfacial microstructures between filmsand substrates is very important because it may precisely con-trol the growth process [7]. In general, the features of the inter-Fig. 3. Interface between the YBCO and NCO. A few tiny anti-domainsappeared near the interface, as indicated by an arrow.face could be mainly controlled by three factors including thechemistry of the join compounds, the growth kinetics and thepresence of defects [4]. Since both chemical and structural com-patibility between NCO and STO were very good, the resultedNCO/STO interface was well defined. Fig. 2 shows NCO grownon STO substrate epitaxially with the -axis perpendicular to theSTO surface. The NCO layer could be easily distinguished fromthe adjacent STO substrate. From Fig. 1, the surface of STO wasquite smooth without any considerable steps. It has been recog-nized by in-situ resistance measurement that the initial growthof La Sr CuO , also of the 214T’ compound, on YSZ sur-face could be layer-by-layer [8]. The early formation of NCO onSTO has not been investigated, but it had to be similar to that forLa Sr CuO . On the other hand, although the structure ofNCO matches well with STO (both are perovskite oxides andtheir lattice mismatch is 1 ), some defects were still visiblenear the interface, such as mis-oriented grains and misfit dis-locations. They might be due to the growth kinetics at the ini-tial growing stage. Defects on the substrate surface propagate asdefects in the c-axis oriented films. Therefore, good quality ofsubstrate is also a main factor to grow defect-free films.To obtain an excellent epitaxy and crystallinity in the subse-quently grown YBCO film, the surface of the base material mustbe extremely smooth. Observed under a scanning electron mi-croscopy (SEM) and atomic force microscopy (AFM), the sur-face of NCO layer is very flat and smooth. It ensures the qualityof grown YBCO and the formed interface. Fig. 3 demonstrateshigh resolution TEM (HREM) image of the NCO/YBCO in-terface. It was sharp and atomically flat. YBCO thin film wasgrown well oriented and crystallized on NCO. Except for a fewtiny anti-domains (about 2 nm in size) in the NCO layer closeto the NCO/YBCO boundary, no considerable defects could befound. These anti-domains might act as nucleation centers withan unwanted orientation if they were further developed. How-ever, as the growth continued, such tiny anti-domains becameGAO et al.: MICROSTRUCTURAL STUDY IN HETEROEPITAXIAL YBa Cu O Nd CuO MULTI-LAYERS 887Fig. 4. Interface of YBCO layer and up NCO layer. Mis-oriented grains couldbe found in NCO layer near the NCO/YBCO boundary.Fig. 5. HREM image of YBCO layer, stacking faults were quite commonpresent near the NCO buffer/YBCO layer interface.extinct and were covered by the YBCO film with the desiredorientation. This was similar to what we observed on coherentlytilted YBCO films with a (105) orientation [9].A high-resolution TEM (HREM) image of the NCO/YBCOinterface is shown in Fig. 4. In comparing with the YBCO/NCOinterface, the interface formed by growing NCO on YBCO sur-face was relatively poor, although the interface was still clearand sharp with an atomic scale. Stacking faults could be seenat the boundary. Mis-oriented grains in the local area of NCOlayer were also formed. It was not clear that such grains werecaused by film growth or by the ion-milling process.The microstructure and defects formed in the YBCO filmgrown on NCO surface were studied. The epitaxy and crys-tallinity in the YBCO film are of high quality (see Fig. 5).The YBCO layer was well epitaxially grown with the -axisorientation. No big defects like grains or dislocations couldFig. 6. HREM image of the NCO layer grown on YBCO. In comparing withthe YBCO layer deposited on NCO, the growth of NCO is relatively poor andmore defects could be seen.be found under the electron microscope. Most of the defectsappeared near the NCO buffer layer/YBCO layer interface.Stacking faults were the most common defects in YBCO thinfilm. It is well known that stacking faults may be produced byexcess Cu-O layers. After the initial growing stage, the YBCOfilm was almost free of defect. Thus, the defects in YBCOmight be caused by the interface only.The quality of the NCO film grown on the YBCO surface wasfound relatively poor in comparing with the NCO layer directlygrown STO, which might be caused by the relatively poor sur-face of the YBCO. Our experiments proved that the NCO filmsalways show an extremely smooth surface, which ensures thehigh quality growth of YBCO [1]. In the NCO layer depositedon YBCO, more defects such as mis-oriented grains and dislo-cations, as shown in Fig. 6. Similar to the case for YBCO grownon NCO, most of defects were near the NCO/YBCO interface.The microstructure study demonstrates that the epitaxialgrowth of YBCO can be significantly improved by such a NCObuffer layer. The NCO material with a 214T’ structure is wellcompatible with YBCO material. It can serve as a good bufferlayer or barrier material for various applications.IV. CONCLUSIONMicrostructures and interfaces in the heteroepitaxialYBCO/NCO multi-layer have been characterized by means oftransmission electron microscopy (TEM). The HREM resultsshow that both YBCO and NCO layers are grown epitaxiallywith -axis orientation. The YBCO/NCO interface is very clearand atomically sharp. A few tiny mis-oriented structures canbe seen, but the further growth of such grains is effectivelydepressed by the film growth along the -axis. The interfaceformed by grown NCO on YBCO surface is relatively defec-tive. For all the samples that we studied, both interfaces andbulk films are free of amorphous layers and secondary phaseparticles. The results demonstrate the excellent compatibilitybetween NCO and YBCO.888 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003ACKNOWLEDGMENTThe authors would like to thank the Electron Microscope Unitof The University of Hong Kong for their assistance in operatingthe transmission electron microscopy.REFERENCES[1] J. Gao, W. H. Tang, and T. C. Chui, “Enhanced initial epitaxy ofYBa Cu O ultrathin films grown on YSZ substrates by using a newbuffer layer of Nd CuO ,” Physica C, vol. 330, pp. 33–38, March 2000.[2] J. Gao, T. C. Chui, and W. H. Tang, “Ultrathin films of Y-Ba-Cu-Ogrown on YSZ substrates with a new buffer layer Nd-Cu-O,” IEEETrans. on Appl. Supercon., vol. 9, pp. 1661–1664, June 1999.[3] P. Ganguly and C. N. R. Rao, Mat. Res. Bull., vol. 8, p. 405, 1973.[4] J. Y. Dai, F. H. Kaatz, P. R. Markworth, D. B. Bucholz, X. Liu, W. A.Chiou, and R. P. H. Chang, “Electron microscopy study of interfacialstructure and reaction of YBa Cu O =Y-ZrO films on LaAlO sub-strates,” J. Mater. Res., vol. 13, pp. 1485–1491, June 1998.[5] J. Gao, S. M. So, and Z. H. Wang, “Characterization of highT ramp-type Josephson junctions constructed with an Nd CuObarrier,” Supercon. Sci. & Tech., vol. 15, pp. 1685–1688, December2002.[6] J. Gao, B. Häuser, and H. Rogalla, “High critical current densityultrathin YBa Cu O films made by a modified rf-magnetron sput-tering technique,” J. Appl. Phys., vol. 67, pp. 2512–2515, March1990.[7] V. C. Matijasevic, B. Ilge, B. Strauble-Pumpin, G. Rietveld, F. Tuin-stra, and J. E. Mooij, “Nucleation of a complex oxide during epitaxialfilm growth: SmBa Cu O on SrTiO ,” Phys. Rev. Lett., vol. 76, pp.4765–4768, June 1996.[8] J. Gao, G. J. Lian, and G. C. Xiong, “Improved initial epitaxial growthof superconducting YBa Cu O thin on Y-ZrO substrates with aLa Sr CuO buffer layer,” Physica C, vol. 330, pp. 160–164,March 2000.[9] Y. Yang, J. Gao, T. C. Chui, and L. Li, “Microstructural study of titledepitaxial thin films of YBa Cu O with a (105) orientation,” PhysicaC, vol. 290, pp. 23–30, October 1997.

Microstructural study in heteroepitaxial YBa/sub 2/Cu/sub 3/O/sub 7//Nd/sub 2/CuO/sub 4/ multi-layers by using electron microscopy

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Microstructural study in heteroepitaxial YBa/sub 2/Cu/sub 3/O/sub 7//Nd/sub 2/CuO/sub 4/ multi-layers by using electron microscopy

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