Inplane orientation control of (001)YBa2Cu3O7−δ grown on (001)MgO by pulsedorganometallic beam epitaxyD. B. Buchholz, J. S. Lei, S. Mahajan, P. R. Markworth, R. P. H. Chang, B. Hinds, T. J. Marks, J. L. Schindler, C.R. Kannewurf, Y. Huang, and K. L. Merkle Citation: Applied Physics Letters 68, 3037 (1996); doi: 10.1063/1.115569 View online: http://dx.doi.org/10.1063/1.115569 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/68/21?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Epitaxial YBa2Cu3O7 growth on KTaO3 (001) single crystals Appl. Phys. Lett. 63, 3376 (1993); 10.1063/1.110150 In situ layerbylayer growth of YBa2Cu3O7−x thin films by multitarget sputter deposition Appl. Phys. Lett. 61, 2826 (1992); 10.1063/1.108050 Orientation control of YBa2Cu3O7 thin films on MgO for epitaxial junctions Appl. Phys. Lett. 60, 1516 (1992); 10.1063/1.107262 Epitaxial growth of YBa2Cu3O7−δ thin films on LiNbO3 substrates Appl. Phys. Lett. 55, 1261 (1989); 10.1063/1.102470 High critical currents in epitaxial YBa2Cu3O7−x thin films on silicon with buffer layers Appl. Phys. Lett. 54, 754 (1989); 10.1063/1.101471

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In-plane orientation control of (001)YBa 2Cu3O72d grown on (001)MgOby pulsed organometallic beam epitaxy

D. B. Buchholz, J. S. Lei, S. Mahajan, P. R. Markworth, and R. P. H. ChangDepartment of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208

B. Hinds and T. J. MarksDepartment of Chemistry, Northwestern University, Evanston, Illinois 60208

J. L. Schindler and C. R. KannewurfDepartment of Electrical Engineering, Northwestern University, Evanston, Illinois 60208

Y. Huang and K. L. MerkleMaterials Science Division, Argonne National Laboratory, Argonne, Illinois 60439

~Received 22 January 1996; accepted for publication 19 March 1996!

Thin films of ~001! YBCO are grown on epitaxially polished~001! MgO by pulsed organometallicbeam epitaxy. The in-plane orientation of the film is controlled by the thickness of a BaO layer,grown in situ, prior to the YBCO growth. For thin BaO layers (,'731014 Ba/cm2! the filmsgrown @110#YBCOi@100#MgO. For thick BaO layers (.'1131014 Ba/cm2! the films grow@100#YBCOi@100#MgO. A mechanism that relates the change in YBCO in-plane orientation to achange in the structure of the initial BaO layers with BaO thickness is described. ©1996American Institute of Physics.@S0003-6951~96!04221-0#

It has been well established that high angle grain bound-aries in c-axis normal YBa2Cu3O72d @~001! YBCO# thinfilms exhibit weak link characteristics1 and can degrade thecritical current density.2 Therefore, it is desirable to growsingle-crystallike films where all the grain boundaries arevery nearly 0°~or 90° in the case ofa-b axis twins!. Filmswith only 0° and 90° boundaries are commonly referred to ashaving a single in-plane orientation and are routinely grownon substrates such as LaAlO3 and SrTiO3. By carefully ad-justing the growth parameters~substrate preparation2,3 anddeposition temperature3!, YBCO films with single in-planeorientation have also been grown by laser ablation and sput-tering on the poorer latticed matched substrate, MgO. To ourknowledge, no report of a YBCO film with a single in-planeorientation has been made for films grown on MgO bychemical vapor deposition.

In many microelectronic applications Josephson-likeweak link devices are used. Thus, the depression ofJccaused by high angle grain boundaries can be developed andused in such applications. For example, a weak link can begenerated by a@001# tilt grain boundary. To create such aboundary, two single-crystal films, rotated by some in-planeanglef, must be grown on a single substrate. Bicrystal sub-strates can be used to accomplish this but the tilt boundary ofthe film is limited to the bicrystal grain boundary. A moreflexible technique, sometimes termed biepitaxy, is thegrowth of one or more buffer layers between the substrateand the film to modify the in-plane orientation of the film.4

By patterning the buffer layer~s! prior to the growth of theYBCO film, grain boundaries can be placed at any predeter-mined location on the substrate.5

The key requirement for biepitaxy is that the buffer layerinitiate a rotation relative to the unbuffered substrate. Impu-rity ion segregation to the surface of~100! MgO, which canbe viewed as a buffer layer, has been extensively studiedbecause of the influence it has on the properties of the

material.6 Cotteret al.7 studied the growth of BaO overlayerson ~100! MgO. They concluded that Ba is substitutionallyincorporated into the MgO lattice, at the MgO lattice spac-ing, for BaO coverages less than 2 monolayers, and for cov-erages of 3 monolayers and above, BaO forms an epitaxialoverlayer with the BaO lattice spacing and the^100& in-planedirections rotated 45° with respect to the MgO substrate.BaO has a rock salt structure with a lattice constant of 5.52Å. One monolayer of BaO corresponds to a~100! surfacecation density of'6.631014 Ba/cm2. In a subsequent study,Cotteret al.8 calculated the heat of segregation for both typesof BaO layers as a function of Ba surface concentration. Theformation of a BaO overlayer becomes energetically favor-able over substitutional incorporation as the Ba coverage ex-ceeds'831014 Ba/cm2. The existence of two thermody-namically similar BaO orientations, rotated 45° with respectto each other, poses a possible means by which two in-planeYBCO orientations could be grown on MgO. Here we re-ported the effect of the initial Ba coverage on the in-planeorientation of YBCO films.

Both the initial Ba layer and the YBCO film are grownby pulsed organometallic beam epitaxy~POMBE!, the de-tails of which have been described elsewhere.9 For the pur-poses of these experiments it is important to note that theprecursors are pulsed onto the substrate via a set of computercontrolled valves and that the pulse sequence and length canbe programmed into the computer. This allows very fine con-trol over the amount of precursor delivered to the substrate.The organometallic source materials used are Y~dpm!3,Ba~hfa!2•~tet!, and Cu~dpm!2. The precursors are pulsed inthe sequence, Ba–~Cu–Y–Cu–Ba–Cu–Ba!–n , wheren isused to indicate the sequence in parentheses is repeatedntimes and Ba–denotes the deposition is started with a vari-able thickness Ba layer prior to the repeat cycle. The filmsare grown on epitaxially polished~001! MgO ~maximum

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peak to valley roughness of 5 Å!, at a substrate temperatureof 670–685 °C.

The in-plane orientation is determined by x-ray diffrac-tion f scans of the~102!~012! YBCO peaks. The criticalcurrent is measured using an offset criteria of 10mV/cm, bythe four probe method on bridges~5 mm wide 3150 mmlong!, patterned with positive photoresist and etched with 1%HNO3.

YBCO films were grown on~100! MgO and the time ofthe first Ba pulse varied. At low initial Ba coverage the filmsare@110#YBCOi@100#MgO ~45° oriented!, see Fig. 1~a!, andat high Ba coverage the films are@100#YBCOi@100#MgO ~0°oriented!, see Fig. 1~b!. A sharp transition from 45° to 0°orientation is observed for an increase in Ba coverage of only50%, between Ba pulse times of 5.7 and 8.6 s,@see Fig.2~a!#. The Ba coverage which results from the initial Ba~hfa!2•tet pulses is believed to be dependent on the substratepreparation and deposition conditions. However, a first orderestimate of the Ba coverage, based on YBCO film growth, is'1.331014 Ba/cm2 s. The transition in YBCO in-plane ori-entation would therefore occur close to the range in whichCotter7 observed a transition from Ba incorporation to 45°rotated BaO overlayer formation. It is our belief that the firstBa ~hfa!2•tet pulses are substitutionally incorporated into theMgO lattice when 45° oriented films are grown, while for 0°oriented films a BaO overlayer is formed. When Ba is sub-stitutionally incorporated into the MgO lattice, a near coin-cidence of four~200! MgO planes with three~110! YBCOplanes defines the epitaxial alignment; a near coincidencelattice ~NCL! mismatch of 2.8%. The100& in-plane direc-tions of the YBCO lattice are rotated 45° with respect to the^100& in-plane directions of the MgO lattice and the overallorientation is @110#YBCOi@100#MgO. Because the Ba issubstitutionally incorporated into the MgO lattice one couldexpect a strong interaction between the substrate and theYBCO film, and therefore a sharp interface with periodic

interface dislocations, similar to that observed under 45° ori-ented grains by Rameshet al.10 For the case where a BaOoverlayer is formed, a near coincidence of one~200! BaOplane with one~110! YBCO plane defines the epitaxial align-ment; a NCL mismatch of 1.1%. The100& in-planedirections of YBCO lattice are rotated 45° with respectto the^100& in-plane directions of the BaO overlayer, whichin turn is rotated 45° with respect to the100& in-planedirection of the MgO substrate. The overallorientation is @100#YBCOi@110#BaOi@100#MgO, i.e.,@100#YBCOi@100#MgO. The 0° orientation, by this mecha-nism, would therefore require an intermediate layer to existbetween the MgO substrate and the YBCO film.

To fabricate devices using a BaO buffer layer, selectiveareas of the substrate must have a thicker initial BaO layer.However, attempts to process the BaO filmex situresulted inYBCO films with mixed in-plane orientation so anin situmeans of selectively depositing more Ba on specific regionsof the substrate must be devised. A~100! MgO surface witha high number of vacancies, ledges, and kink sites is consid-ered to be more reactive than a ‘‘near perfect’’ surface11,12

and the possibility that a greater fraction of Ba~hfa!2•tetcould be converted to Ba on such a surface is reasonable.Low energy Ar1 sputtering of MgO is known to create va-cancies, ledges, and king sites in the MgO surface13 and hasbeen observed to modify the in-plane orientation of YBCOfilms.14,15YBCO films were grown on~100! MgO substratesthat had been sputtered with a 500 eV,'1 mA/cm2 Ar1 ionbeam for 4 min, at an incidence angle of 90°. The time of thefirst Ba pulse was varied. For all Ba coverages, the YBCOfilms on the sputtered substrates are predominantly 0° ori-ented, although a small fraction of 45° oriented material ex-ists at the lowest Ba coverage, see Fig. 2~b!. When comparedto Fig. 2~a!, a window for the deposition of 100% 45° ori-ented YBCO on nonsputtered MgO and 100% 0° orientedYBCO on sputtered MgO exists around an initial Ba pulsetimes of 5.0–5.7 s. YBCO films with an initial Ba pulse timeof 5.0–5.7 s have been simultaneously grown on sputteredand nonsputtered MgO. Films on sputtered MgO are typi-cally 100% 0° oriented@see Fig. 1~c!# but occasionally films

FIG. 1. f-scans of the~102!~012! planes of~001! YBCO grown with:~a! aninitial Ba pulse time of 5.3 s on polished MgO,~b! an initial Ba pulse timeof 8.6 s on polished MgO,~c! an initial Ba pulse time of 5.3 s on Ar1

sputtered MgO.

FIG. 2. ~a! YBCO in-plane orientation on epitaxially polished MgO as afunction of first Ba pulse time.~b! YBCO in-plane orientation on Ar1 sput-tered MgO as a function of first Ba pulse time.

3038 Appl. Phys. Lett., Vol. 68, No. 21, 20 May 1996 Buchholz et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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with 90%–100% 0° orientation are observed. Films on non-sputtered MgO are typically 100% 45° oriented@see Fig.1~a!#, but occasionally films with 98%–100% 45° orientationare observed. The cause of the occasional small fraction ofsecond orientation is under investigation and believed to berelated to the oxygen activity of the plasma enhanced reac-tant gas. The critical current, measured at 78 K, for a set ofsimultaneously grown films is 2.03106 A/cm2 on LaAlO3,1.13106 A/cm2 on sputtered MgO, and 1.73106 A/cm2 onnonsputtered MgO. Although slightly depressed from thevalue obtained on LaAlO3, both the 0° and 45° oriented filmshave aJc.106 A/cm2. Figure 3~a! is a cross-sectional TEMof a nonsputtered~001! MgO substrate upon which YBCOhas been deposited. The surface of the MgO substrate iseasily discernible and can be seen to be atomically flat. Thein-plane orientation of the film, as determined by selectivearea electron diffraction~SAD!, is @110#YBCOi@100#MgO.The remarkably distinct periodic contrast features in thisHREM image indicate the presence of misfit dislocations andof coherent lattice planes in spite of the extremely large mis-fit which results in an'8.4 Å repeat unit, the NCL spacingof '4 3d(200) MgO'33d(110) YBCO. Figure 3~b! is aTEM of a sputtered~001! MgO substrate upon which YBCOhas been deposited. The film is deposited under the sameconditions as the film in Fig. 3~a!. The in-plane orientation ofthe film, as determined by SAD, is@100#YBCOi@100#MgO.Although the interface between the substrate and the film isindistinct, it is still possible to observe that the MgO surfaceis rough relative to the nonsputtered substrate and has manyvertical surface steps. No interfacial dislocations are observ-

able, however, there is a crystalline intermediate layer,20–35 Å thick, between the MgO substrate and the YBCOfilm.

In conclusion, the in-plane orientation of~001! YBCOthin films grown on epitaxially polished~001! MgO byPOMBE can be controlled by the thickness of a BaO layer,grown in situ, prior to the YBCO growth. For thin BaOlayers the film grows@110#YBCOi@100#MgO. For thick BaOlayers the films grows@100#YBCOi@100#MgO. The changein YBCO in-plane orientation is believed to result from anorientation change in the initial BaO layer from the substitu-tional incorporation of Ba in the MgO lattice at low Baocoverage to the formation of a 45° rotated BaO overlayer athigh BaO coverage. Low energy Ar1 sputtering of the MgOsurface prior to YBCO growth is also used to induce the@100#YBCOi@100#MgO orientation. The change of in-planeorientation is proposed to result from a greater conversion oforganometallic to BaO on the sputtered MgO surface than onthe epitaxially polished surface. The greater conversion re-sults in a thicker BaO layer and hence the@100#YBCOi@100#MgO orientation. There may be other fac-tors, such as ion implantation during Ar1 sputtering, thatalso influence the transition from Ba incorporation to BaOoverlayer formation. These are still under study.

We acknowledge the generous support of the office ofNaval Research and the National Science Foundation GrantCHE 94-21910, the Science and Technology Center for Su-perconductivity under DRM 91-20000, the U.S. Departmentof Energy under W31-109-ENG-38, and the Central Facili-ties of the Materials Research Center of Northwestern Uni-versity under the Grant DMR 91-20521.

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FIG. 3. ~a! TEM of YBCO grown on epitaxially polished MgO,~b! TEM ofYBCO grown on Ar1 sputtered MgO.

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