Dopant incorporation during epitaxial growth of a multicomponent oxide thin film fromvapor phase: A case study of Fe/YBa2Cu3O7−δ systemS. B. Ogale, I. Takeuchi, M. Rajeswari, R. L. Greene, T. Venkatesan, and D. D. Choughule Citation: Applied Physics Letters 66, 1674 (1995); doi: 10.1063/1.113889 View online: http://dx.doi.org/10.1063/1.113889 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/66/13?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Novel solidphase epitaxial growth of YBa2Cu3O7−δ films from precursor oxides J. Appl. Phys. 76, 2807 (1994); 10.1063/1.357515 Incorporation of hyperfine probes into the thinfilm superconductor YBa2Cu3O7−δ during deposition Appl. Phys. Lett. 63, 3224 (1993); 10.1063/1.110205 Epitaxial growth of YBa2Cu3O7−δ films on oxidized silicon with yttria and zirconiabased buffer layers J. Appl. Phys. 74, 3614 (1993); 10.1063/1.354500 Gasphase oxidation chemistry during pulsed laser deposition of YBa2Cu3O7−δ films J. Appl. Phys. 73, 7877 (1993); 10.1063/1.353939 Gasphase oxidation of copper during laser ablation of YBa2Cu3O7−δ in different oxidizing ambients Appl. Phys. Lett. 62, 102 (1993); 10.1063/1.108824

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Dopant incorporation during epitaxial growth of a multicomponent oxidethin film from vapor phase: A case study of Fe/YBa 2Cu3O72d system

S. B. Ogale,a) I. Takeuchi, M. Rajeswari, R. L. Greene, and T. Venkatesanb)Center for Superconductivity Research, Department of Physics, University of Maryland, College Park,Maryland 20742

D. D. ChoughuleCenter for Advanced Studies in Materials Science and Solid State Physics, Department of Physics,University of Poona, India

~Received 27 June 1994; accepted for publication 30 January 1995!

Incorporation of Fe in YBa2Cu3O72d matrix during its epitaxial growth on~001! LaAlO3 by pulsedexcimer laser ablation is examined. It is shown that oxygen pressure during deposition plays acritical role in the incorporation process and that a low-temperature postsynthesis annealing isessential for complete substitution. Clustering of iron atoms and its control via the anion controlledreactions are identified as the key features in this context. It is also shown by using atomic forcemicroscopy that Fe incorporation influences the surface morphology. These issues are of importancein the formation of cationic defects in multicomponent thin films. ©1995 American Institute ofPhysics.

High quality epitaxial crystalline films of multicompo-nent materials are synthesized routinely using techniquessuch as sputtering, evaporation, and pulsed laser deposition.1

In the last six years, there has been a proliferation of researchin the area of multicomponent oxide films of importance to avariety of technologically important fields such as supercon-ductors, ferroelectrics, piezoelectrics, electro-optics, andmagneto-optics.2 In all these cases, stoichiometric films havebeen prepared under suitable conditions of substrate tem-perature and background oxygen pressure with the implicitassumption that under optimal conditions the cation ratio inthe film is determined by the ratio of the cations arriving atthe surface during the deposition process. However, the pres-ence of a background gas such as oxygen tends to enhancethe probability for the formation of compounds other thanthe desired phase and this issue is particularly of importancewhen one wants to dope the films with the desired impurityatom. The incorporation of dopants into the crystalline filmsis particularly of relevance to the emerging junction basedtechnologies.

In this work, we have examined the case of Fe incorpo-ration in YBa2Cu3O72d ~YBCO! thin film during its epitaxialgrowth on ~001! LaAlO3 by pulsed excimer laser ablation.We demonstrate that Fe incorporation is very sensitive to thegrowth conditions and that the growth parameter window forcomplete incorporation of Fe is narrower than that forachieving high quality films of undoped YBCO. We believethe issues addressed here are generic since the various thinfilm growth techniques employed for multicomponent oxidesall involve common physical processes such as chemisorp-tion, surface diffusion, surface and interface reactions. TheYBa2Cu32xFexO72d system used in this work has beenwidely studied in the bulk in the context of the influence ofmagnetic impurities on the properties of highTcsuperconductors.3–9

In our experiments, the Fe doped YBCO bulk pellets

used as target materials for pulsed laser deposition~PLD!were prepared by standard ceramic techniques.5 The resultsreported here are for thex50.2 case. The details regardingthe PLD process are described elsewhere.1 The substrate~100 LaAlO3! was at a nominal temperature of 800 °C. Theoxygen ambient pressure during deposition was varied in therange 130 to 350 mTorr. After deposition, the films werecooled in 200 Torr oxygen. The deposited films~2000 Åthick! were characterized by ac susceptibility, x-ray diffrac-tion and atomic force microscopy. Structural analysis byfour-circle x-ray diffraction revealed that the films werec-axis oriented @rocking full width at half-maximum~FWHM!,0.4# and had good in-plane alignment with thesubstrate. In specific cases, the deposited films were annealedin an oxygen ambient~380 Torr! at temperatures in the rangeof 550 and 650 °C for 1 h.

In Fig. 1 we show the dependence ofTc of the undopedand Fe doped films on the oxygen partial pressure duringdeposition.~All other deposition parameters are held con-stant.! The bulkTc value is also indicated which is expectedto result from complete and homogeneous incorporation ofFe at Cu sites. With an oxygen pressure of 130 mTorr, the Fedoped films show aTc;80 K. ~This is the optimum pres-sure value for high quality undoped YBCO films.! Clearly,much of the Fe is not incorporated in the lattice substitution-ally under this growth condition. Deposition at higher pres-sures improves the incorporation up to a pressure of about325 mTorr bringing down theTc to about 5863 K. At stillhigher pressures the incorporation seems to worsen again assuggested by the increase inTc . It is noteworthy that there isno significant change in theTc of the undoped films over thepressure range up to 325 mTorr; however, at higher pressureseven the undoped filmTc is seen to be lowered.

X-ray diffraction data for films deposited under differentconditions indicate that all the films arec-axis oriented. No-table differences were observed in the spectra only over cer-tain spectral regions and the corresponding results are shown

a!On study leave from University of Poona.b!Also with the Electrical Engineering Department.

1674 Appl. Phys. Lett. 66 (13), 27 March 1995 0003-6951/95/66(13)/1674/3/$6.00 © 1995 American Institute of Physics

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in Fig. 2. It may be noted that even over these selected re-gions the cases of undoped and doped film deposited at 325mTorr ~which corresponds to the maximum incorporation!show mostly similar features. However, the doped films de-posited at 130 mTorr exhibit two distinctly different peakswhich correspond to the~220! and ~440! planes of Fe3O4.The presence of these peaks suggests that Fe not incorpo-rated in the YBCO matrix under this deposition conditionforms ~x-ray discernible! clusters of Fe3O4. The x-ray peaksalso suggest that the growth of this phase is predominantlyoriented along the~110! family of planes.10 This is under-standable because thed values for the~110! family of planesof Fe3O4 has a minimum lattice mismatch~;6.25%! with theLaAlO3 ~100!. Further, from the top box of Fig. 2 it can beseen that the undoped case shows two peaks while the dopedcase~325 mTorr! shows merging of these peaks. This canoccur if the height of the intermediate peak attributed toFe3O4 ~220! reduces and broadens due to the decrease in thenumber and size of the Fe3O4 clusters for deposition at 325mTorr. Second, the position of the main peak@which is the~004! peak# is seen to be shifted to lower 2u value or higherd value representing change in the ‘‘c’’ parameter from 11.67to 11.684 A. This is consistent with the shift reported in ironsubstituted bulk YBCO case for equivalent doping.3,9

It may be noted from the results of Fig. 1 that eventhough the incorporation is maximum at an oxygen pressureof about 325 mTorr it is still not complete as indicated by theTc . We attempted deposition at this pressure at lower sub-strate temperatures down to 740 °C but it did not lead to animprovement in incorporation. Thus we attempted annealingof the deposited films. It was seen that an annealing at550 °C in 380 Torr O2 for 2

12 h completes the incorporation

process in the 2000 Å film deposited at 325 mTorr as indi-cated by the desiredTc value. We would like to mention herethat we have also undertaken Rutherford backscattering

~RBS! of these films and the results indicate that the targetstoichiometry is maintained in the doped films. This suggeststhat it is reasonable to expect that the observed differencesare related to the tendency of Fe to form Fe3O4 which com-petes with the Fe incorporation in the copper sites of YBCOlattice.

Our AFM studies showed significant changes in thequality of the surface over the pressure window examined. InFigs. 3~a!–3~c! we show the morphology of the surface forthe Fe doped films deposited at oxygen pressures of 130,325, and 390 mTorr, respectively. It can be clearly seen thatthe morphology is smoothest for the film deposited at 325mTorr. The increased roughness at 130 and 390 mTorr isqualitatively different; several small humps for 130 mTorrcase and smaller number of large humps for 325 mTorr case.

In Fig. 4 we show the quantitative estimates of rough-ness in terms of the difference in surface area over an area of25 mm2 for the doped as well as undoped films. Both thedoped and the undoped films have the lowest degree of sur-face roughness at about the same pressure value at which thehighest Fe incorporation was observed. This minimum inroughness is flat over a small pressure window but is quitedeep in the doped case. In the undoped case the roughnessdecreases up to a pressure of about 300 mTorr and then risesrather weakly. The observation of higher roughness at lowerdeposition pressure for the doped case should correspond tothe clustering of Fe3O4 inferred from the x-ray diffractiondata. If the Fe atoms undergo a fast surface diffusion at thispressure, they can nucleate the iron oxide phase at different

FIG. 2. Two regions ofu-2u x-ray diffraction spectra forc-axis orientedYBa2Cu3O72d films and YBa2Cu32xFexO72 films at 130 and 325 mTorroxygen pressure. The arrows denote the~220! and ~440! peaks of Fe3O4.

FIG. 1. Tc vs oxygen pressure during deposition for undoped YBa2Cu3O72dand YBa2Cu2.8Fe0.2O72d films by PLD.

1675Appl. Phys. Lett., Vol. 66, No. 13, 27 March 1995 Ogale et al.

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positions and a faster growth of YBCO can ensue at thesenuclei leading to a rough morphology. At higher and higheroxygen pressure the Fe surface diffusion is expected to besuppressed by reactive incorporation of oxygen, and both themorphology as well as incorporation can improve. At pres-sures higher than 325 mTorr the surface diffusion of all cat-ions may decrease creating an unfavorable situation for asmooth layer by layer growth process and the roughnesswould rise. In this case the grains are expected to be increas-ingly smaller and the relatively faster moving Fe atoms mayincorporate more and more at the grain boundary regions

rather than in the lattice, leading to a small increase intheTc.

In conclusion, it is shown that~a! oxygen pressure dur-ing deposition plays a critical role in the process of incorpo-ration of Fe in YBaCuO films during their epitaxial growthby laser ablation;~b! a postsynthesis annealing is essentialfor complete substitution;~c! clustering of iron atoms byrapid surface diffusion and its control via the anion con-trolled reactions are the key features;~d! dopant incorpora-tion influences the surface morphology. While Fe incorpora-tion into the lattice may be an extreme case, this study pointsout the role of the preparation conditions in reducing cationdefects in the multicomponent films.

The work at Poona is supported by the Department ofScience and Technology and the Council for Scientific andIndustrial Research, India.

1T. Venkatesan, S. Bhattacharya, C. Doughty, A. Findikoglu, C. Kwon, QiLi, S. N. Mao, A. Walkenhost, and X. X. Xi,MultiComponent and Mul-tilayered Thin Films for Advanced Microtechnologies: Fundamentals,Techniques and Devices, edited by O. Auciello and J. Engemann, Proceed-ings of NATO Advanced Study Institute, Bad Windsheim, Germany, 1992~Kluwer, The Netherlands, 1993!, p. 209.

2S. L. Swartz, IEEE Trans. Insulators25, 937 ~1990!.3K. Char, L. Antagonazza, and T. H. Geballe, Appl. Phys. Lett.63, 2420~1993!.

4J. M. Tarascon, P. Barboux, P. F. Miceli, L. H. Greene, G. W. Hull, M.Eibschutz, and S. A. Sunshine, Phys. Rev. B37, 7458~1988!.

5S. B. Ogale, D. D. Choughule, S. M. Kanetkar, P. Guptasharma, and M.Multani, Solid State Commun.89, 1 ~1994!.

6S. Katsuyama, Y. Ueda, and K. Kosuge, Physica C165, 404 ~1990!.7Y. Xu, R. L. Sabatini, A. R. Moodenbaugh, Y. Zhu, S. G. Shyu, M.Suenaga, K. W. Dennis, and R. W. McCallum, Physica C169, 205~1990!.

8L. Nunez, R. D. Rogers, G. W. Crabtree, U. Welp, K. Vandervoort, A.Umezawa, and Y. Fang, Phys. Rev. B4, 4526~1991!.

9G. Kallias, I. Panagiotopoulos, D. Niarchos, and A. Kostikas, Phys. Rev. B48, 15 992~1993!.

10M. S. Islam and C. Ananthamohan, Phys. Rev. B44, 9492 ~1991!, andreferences therin.

FIG. 3. AFM images of YBa2Cu2.8Fe0.2O72d films prepared with differentoxygen pressures during deposition~a! 130 mTorr,~b! 325 mTorr, and~c!390 mTorr.

FIG. 4. Surface area difference~a measure of roughness! measured by AFMfor doped and undoped YBCO films prepared under different oxygen pres-sures during deposition. The uncertainty in these measurements is;0.2%.

1676 Appl. Phys. Lett., Vol. 66, No. 13, 27 March 1995 Ogale et al.

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