CRYSTALGROWTH

ELSEVIER Journalof Crystal Growth 137 (1994)259—267

Epitaxialgrowthof cupratesuperconductorsfrom the gasphase

D.G. Schioma,* D. Anselmetti b J.G. Bednorzc Ch. GerberC J• MannhartThe PennsylvaniaState University, DepartmentofMaterials ScienceandEngineering,UniversityPark, Pennsylvania16802-5005,USA

~ UniversityofBase!, Instituteof Physics, Klingelbergstrasse82, CH-4056Basel,Switzerland

IBM ResearchDivision, Zurich ResearchLaboratory, CH-8803Rüschlikon,Switzerland

Abstract

The growthmechanismof c-axis orientedthin epitaxial films of the mostwidely studiedcupratesuperconductor,YBa2Cu3O7~,formedby a varietyof gasphasecodepositionmethodson commonsubstratematerialsis described.The evolution of the surfacemicrostructure,as revealedby scanningtunneling microscopy(STM), indicatesthatgrowth is dominatedby theaccommodationof depositingspeciesat ledges.Theseledges,which provide energeti-cally favorablepositionsfor this process,lie either alonggrowthspiralsemanatingfrom screwdislocations,or, whenavicinal substrateis used,separatethe low energy(001) planesat the film surface. If thesubstrateis misorientedsufficiently, growth occurs by step propagation.Otherwise,a high densityof screw dislocations (‘.. i0’~cm~)isnucleatedduring the initial stagesof growth, providing a continual supply of ledge incorporationsites for thedepositingspecies.Two likely mechanismsfor thegenerationof thesescrewdislocationsare described.The surfaceevolution reportedappearsto be an intrinsic featureof c-axis oriented YBa2Cu3O7_5films for a wide rangeofgrowth conditions,irrespectiveof thesubstratematerialor vapor phasedepositionmethod.

1. Introduction pendenton the film growth mechanism,which isthe subjectof this paper.

Vapor phase deposition techniques have Efforts to understandthe growth mechanismachievedimmensesuccessin preparing epitaxial of oxide superconductorfilms haveprofited sub-thin films of cupratesuperconductorswith trans- stantially from atomic-scaleimaging techniquesport propertiessuperiorto thosemadeby bulk [1—3],in particularscanningtunnelingmicroscopysynthesis methods. A consequenceof the ex- (STM) andatomic force microscopy(AFM) [4,5].tremely short coherencelengthsin thesesuper- Since experimental data elucidating the growthconductors,typically a few ~ngströms,is that the mechanismof high temperaturesuperconductorselectrical characteristicsof these materials are by gasphasecodepositionmethodsarepredomi-particularly sensitive to their underlying mi- nantly for YBa2Cu3O7~films orientedwith theircrostructure.This microstructure is in turn de- c-axesnormal to the substrate,this materialwill

be the focus of this paper. The applicability ofthe inferred growth mechanism to other high

_______ temperature superconductorsis briefly ad-* Correspondingauthor, dressed,basedon the limited dataavailable.

0022-0248/94/$07.O0© 1994 Elsevier ScienceB.V. All rights reservedSSD!0022-0248(93)E0523-A

260 t).G. Schlomci a!. /Journal of Crystal Growth 137 (1994) 259—267

2. Microstructureof YBa2Cu3O7~films molecular beam epitaxy (MBE) [30,31] (DyBa2

Cu107_5 films) on SrTiO3 (100) [3—7,9,12,14—Despite the obvious differences in the vapor 16,19,23—27,29—31],MgO (100) [3,5,6,8,9,13,15—

phasegrowth techniquesused to produce high 19,27,28,31], LaAIO3 (100) [6,9,20,22], yttria-quality superconductorthin films, the specific stabilized cubic zirconia Y203—Zr02 (l00} [211,conditionsusedby different researchgroups,and oxidized Zr sheets[27], NdGaO3 (001) [10,31],the rangeof lattice match,crystal structure,sub- and Mg7TiO4 {l00) [23].STM and AFM imagesstrateperfection, and surfacepreparationof the of these films reveal a surfacemorphologycon-substratematerialsused, the microstructuresof taming closely spacedsteps [3—29].Thesesteps,the films grown by different methodsexhibit re- which havea height equal to the c-axis length ofmarkable similarity, indicating that the same YBa,Cu3O7_~(11.7A) or an integralmultiple ofgrowth mechanismis operative.The c-axis on- it, separate(001) YBa2Cu7O7_aterraces.Theseented YBa7Cu3O7.~films discussedhere are terraces are typically only a few hundredgrown by sputtering [6,71, off-axis magnetron ~ngströms wide [3,13,15,20,22,26,29],indicatingsputtering [4,5,8—16],pulsed laser deposition the length scaleover which (001) oriented YBa~(PLD) [3,6,15—27],thermal coevaporation[6,28], Cu307_5 films are atomically flat. The highden-chemicalvapordeposition(CVD) [6,1627,29], and sity of steps,which appearsto be a common

_ -

Ic

Fig. L STM image of a c-axis oriented, 1300 A thick YBa Cu 07 film grown on SrTiO1 (100) by sputterdeposition.Growthspiralsemanatingfrom both left- and right-handedscrewdislocationsindicatea screwdislocationdensityof (12.5+ I) l0~cm -

at the film surface.

D.G. Schiomet al/Journal of CrystalGrowth 137 (1994)259—267 261

feature of c-axis oriented YBa2Cu3O7_6 films 20 I

grown by vapor phasemethods, exposesa-axisfaces.Thesetypically amountto 5%—10% of the E 15 ~

surfaceareaof the YBa2Cu3O7_afilms [121. ~ ~

The microstructure of c-axis oriented YBa2 I

Cu3O7....~films is distinctly different for filmsgrown on well-oriented (100) substrates(Fig. 1), ________

comparedto vicinal surfaces(Fig. 2). Irrespective I I

of the substratematerialanddepositionmethod I 4used, on well-oriented (100) substrates,a high adensityof screw dislocations(108_lOb cm

2) is 0 1 2 3 4seenin films morethan a few hundred~ngströms 8 (Degrees)thick, whereasfor growth on substrateswith vici- Fig. 3. Screw dislocation density ~P~crc~ as a function of

nal surfacesthe steppednature of the substrate substratemisorientationangle (0) for sputtered1000—1500A

surfaceis preservedanda much lower densityof thick YBa2Cu307~films grown at ~~uh 750~Canda growth

• 8 —2 ‘ rate of 0.5—1.8 A/s on nominally (1(10) oriented SrTi0~screw dislocations~< 10 cm ) is found. These -

substrates(from Ref. [15]).screw dislocationsnot only may be strongvortexpinning siteswhich enhancethe critical currentdensitiesof the thin films [32], but also revealthefilm growth mechanism [12,14,15]. Films with from (100) andbecomesincreasinglymisoriented,much lower screwdislocationdensitieshavebeen a continual cross-overin dislocationdensityas agrown on vicinal SrTiO3 (100) [15] and LaA1O3 function of misorientation is observed (Fig. 3).(100) [26] surfacesby sputtering [151 and PLD Increased growth temperatureand decreased[261.As the orientationof the substratedeviates growth rate causea similar effect, as discussed

below [4,15].The STM imagesshownare not only represen-

,/‘<lOO) tative of the hundredsof such imagesthat we~ have taken of our own films formed by direct

current (DC) magnetronsputtering [4,12,14,151,but are characteristicof the topological featuresreported by many other researchersusing thevariety of gas phasepreparationtechniquesandsubstrateslisted above[3—31].A lower magnifica-tion STM image of a sputteredfilm grown on a

\<100> well-oriented SrTiO3 (100) substrateis shown inFig. 4. The terrace widths on films grown onwell-oriented substratesare comparableto themaximum terracewidth observedon films grownon vicinal substratesunderthe samegrowth con-ditions at film thicknesseswhere the growth spi-rals emanatingfrom screw dislocationshavebe-come well established.As describedbelow, thegrowth spiralstypically appearafter the growthof

IL~Onm a YBa2Cu3O7 ~ film about 8—10 unit cells thickFig. 2. STM image of a c-axis oriented, 1300 A thick on SrTiO3 (100) [4 6 15] or MgO (100) [3 19]YBa2Cu7O7.5film grown on avicinal SrTiO~(100) substrate .

by sputterdeposition.The surfacemorphology is dominated These are the only two substratematerialsforby stepsbetween(001) YBa2Cu3O7_5terraceswhich accom- which studiesof the developmentof growth spi-modatethesubstratetilt (from Ref. [is]). rals with film thicknesshavebeenpublished.

262 D.G. Schlometa!./Journal of Crystal Growth 137(1994) 259—267

3. Growth mechanism the growth surfaceis increased,the supersatura-tion at positionsfar from the ledgesbecomeshigh

The presenceof a high density of growth spi- enoughfor two-dimensional nucleation to occur

rals emanatingfrom screw dislocations in c-axis on terraces.oriented YBa2Cu3O7~films [3—29] indicates At the initiation of growth, the only availablethat film growth takes place by accommodating ledge sites on a vicinal, but otherwise perfect,speciesalongthe spiral-shapedgrowthsteps.The surfaceare surface steps. Indeed transmissiongrowth processof c-axis orientedYBa2Cu3O7~ electronmicroscopy(TEM) studiesshowthat thefilms may thus be described as ledge growth, initial nucleationoccurs predominantlythere[2].emphasizingthe energeticpreferenceof the de- lf the substrateis sufficiently well oriented andpositing speciesto attach to existing ledges(step the substratetemperaturelow enoughthat theedges),which propagatelaterally acrossthe sub- supersaturationon the terracecan reachthe level

stratesurfaceas they accommodatespecies.This required for two-dimensionalnucleation,growthprocessand the various typesof ledgesites avail- will take place by the nucleationof new layersable for attachmentare schematicallyshown in (two-dimensionalnucleation)in regions far fromFig. 5. As the spacingbetweenexisting ledgeson ledges.However, if a sufficient density of screw

I 1pm IFig. 4. STM imageof a c-axis oriented,1200 A thick YBa2Cu7O7_5film grown on SrTiO3 (100) by sputterdeposition.Growthspiralsemanatingfrom both left- and right-handedscrewdislocations indicatea screwdislocationdensityof (13.5±2) x i0~cm 2

at the film surface.

D.G. Schiometa!./Journal of CrystalGrowth 137 (1994)259—267 263

dislocationswereto begenerated,possiblemech- which appearto impedethe nucleationof screwanismsfor which aredescribedbelow, an endless dislocations and lead to continued two-dimen-supply of attachmentsiteswould be availableto sionalnucleationfor the growth of thick films areaccommodatethe depositingspecies.Directmor- as yet unclear.phologicalevidencefrom STM andAFM studies RHEED intensityoscillationsare not expected[3,4,6,15,191as well as indirect evidence from for growthoccurringby the additionof speciestoreflection high energy electron diffraction establishedgrowth spirals or to a propagating(RHEED) intensity oscillations [33,34] indicate sequenceof surfacesteps,since the surfacestepthat two-dimensionalnucleationis only dominant density is then essentially time independent.In-for the first 100 to 200 A of film growth. By this deed,the observationof RHEED intensityoscil-thicknesshigh densitiesof screw dislocationsare lations has only been reported[33,34] to occurpresent in YBa2Cu3O7_~films grown on well during the initial stagesof growth (for thicknessoriented substrates,dominatingthe growth pro- ~ 200 A for YBa2Cu3O7~films) on bare sub-cessfrom thenon. Only in a few caseshasgrowth strates, indicative of the time necessaryfor adominated by two-dimensionalnucleation been steady-statesurfacestepdensityto becomeestab-documentedfor thicker films [8,35,36].The sur- lished. On thicker films, no RHEED intensityface morphology of thesefilms, which has been oscillationsareseen,even if the growth is brieflydescribedas “Tower of Hanoi” [351or “layer- interrupted[33,34], nor are RHEED oscillationscake-like” [361exhibits narrow terraces with observedfor growth on sufficiently vicinal sur-widths comparable to other c-axis oriented faces[37].YBa2Cu3O7_~films, with the exception that When slightly misoriented from (100), a Se-thesefilms appearto be devoid of screwdisloca- quenceof stepsseparating(100) terracesis pre-tions at the tops of the stacks of concentric sent on the surfaceof the substrate.Although theYBa2Cu3O7_8 layers. The growth parameters substratestep height does not match the layer

C

~2-D

Z Nucleation2-D~ Nucleation ________________________________

HScrewDislocation

MediatedGrowth

Nucleation 1—a”~ePsH_ _H

I c ~ Mediated4 GrowthI £terrace ____________

~TimeNucleation ContinuedGrowthon Substrate

Fig. 5. Schematicrepresentationof the ledgegrowthprocessinferredfrom the growthof c-axis orientedYBa2Cu3O7_5films. Thehigh nucleationbarrier to forming a new layer by two-dimensionalnucleationrequiresa high supersaturationto be reachedforsuchnucleationto takeplace.Thepresenceof moreenergeticallyfavorable ledgeattachmentsites in thevicinity of the depositingspecies(e.g., emanatingfrom screwdislocationsor thosepresenton vicinal substrates)accommodatesthe depositingspeciesandreducesthe supersaturationsuch that after the initial stagesof growth, two-dimensionalnucleationis rare.

264 D.G. Schiomci a!. /Journalof CrystalGrowth 137(1994) 259—267

height of YBa2Cu1O7~,stepsdevelop,separat- lO~~cm~2)reportedalso for growth on annealed

ing (001) YBa2Cu3O7~terracesand accommo- or cleaved substrates.For example, YBa2Cu3

dating the tilt of the substrate.The amount of O7~films grown on MgO (100) substratesthatsubstratetilt necessaryto achievegrowth mainly wereannealedat 1100°Cfor 12 h in oxygenpriorby this step propagationgrowth mechanism is to growth [8], conditionsunderwhich reorderingpresumably that which causesthe steps to be of the substratesurfaceoccurs[2,40,41],exhibitedspaced sufficiently close together that the de- screw dislocationdensities in the 108_lOb cm

2positing speciesdiffuse to andareaccommodated range, despite this significant difference in sub-by the ledgeswithout reachinga supersaturation strate surfacepreparation.The sameis true foron the terracesufficient for two-dimensionalnu- YBa

7Cu3O7~films grown on cleaved surfacescleation [38]. Experimentally, a cross-overfrom of MgO, which underwent heating to 1250°C,screwdislocationmediatedgrowth to steppropa- followed by a topotactic solid state reactionwithgation, shown in Fig. 3, has been observed on Ti02 at 1100°Cto form Mg,Ti04 substrates,avicinal SrTiO1 (100) surfaces[15]. Under these processknown to bury any defectswhich mightgrowth conditions,the transition occurs at about have been presenton the initial MgO surface

2°,correspondingto an averageterracespac- [23,42].Another proposedmechanismfor the in-ing of about 30 nm. Growth on less misoriented troduction of screw dislocationsinvolves the nu-substratesunder the samegrowth conditions re- cleation of dislocation half-loops at the film sur-suIts in a comparableterrace spacing(~20 nm face, which then glide to the substrateinterface[15]) betweenturns of the growth spirals,as is to to relieve the lattice mismatch between thebe expected for this analogous ledge-mediated YBa7Cu3O7~film and the substrate[3,19,22].growth process.The influenceof substrateorien- Although the Burgers vector of the proposedtation on the film growth mechanismexplainsthe dislocation half-loops was not explicitly stated,apparentlydisparateresultsof severalstudies(in note that if b = c[OOI] (i.e., the endsof the dislo-which substrate misorientations were not re- cation loop intersectingthe film surfaceare pureported)where nominally identical growth condi- screw), the pure edgeinterfacial segmentof thistions yielded drastically different growth mor- loop does not relieve any of the mismatch he-phologies[7,8]. tween the film and the substrate.Choosing a

Burgers vector which would relieve interfacialmisfit (i.e., one that containsan a[100] or h[0l0]

4. Nucleation of screwdislocations component), results in an even longer Burgersvector; energeticallysuch a dislocationwould be

An important, but so far unresolvedissue is favored to dissociate(i.e., into dislocationswiththe mechanismof nucleationof the screwdisloca- b = c[00l] and b = a[lOO] or b[OlO]).tions. One possibility is that they are simply in- Two likely mechanismsfor the generationofherited from the substrate.Although the bulk screwdislocations,first proposedby Fisheret al.dislocationdensitiesof the substratesusedvary [43] and observedin the growth of layeredmica-significantly and are in generalorders of magni- ceous materials [44—47],are (1) the coalescencetude lower than the screw dislocation densities of two separategrowth fronts having inclinedobservedin c-axis oriented YBa2Cu3O7~films surfaceswith respect to eachother, and (2) the[4,15,20],AFM studieshaverevealedthat polish- recombinationof two branchesof a singlegrowthing damagecan result in screw dislocationdensi- front offset in layer height. This secondmecha-ties at the substratesurfacecomparable(~l0~ nism is shown schematicallyin Fig. 6. The abilitycm

2) to whatis observedin YBa2Cu3O7 ,~ films of YBa2Cu7O75 to flow over irregularities [1]

[39]. Althoughscrewdislocation inheritancefrom (creatingoffsets in layer height) is facilitated bythe substrateis likely to occur to some extent, it the ledge growth mechanism.The first mecha-appearsincapableof explainingthe rather consis- nism of screw dislocationnucleation is likely totently observedscrew dislocation densities(i0~— occur during the coalescenceof nuclei that form

D.G. Seldomci al. /Journalof CrystalGrowth 137(1994) 259—267 265

(a) ~ alignmentof the coalescingnuclei is necessaryfor

the first nucleationmechanismto occur, andthis/ ~ is madepossibleby the epitaxialalignmentof the

individual nuclei to the underlying substrate.

~ 5. Other cuprate superconductors

Although the nucleationand growth of other

~bi / ~ cuprate materials have not been as intensively

/ “~ studiedas YBa2Cu3O7...,~,the presenceof screw/ . dislocations and their associatedgrowth spirals

have been reported for (001) oriented films ofother ReBa2Cu3O7_6materialsgrown by vapor

~ phasedepositionmethods,where Re = Sm [6,52],

~ Dy [30,31], and Ho [35]; screw dislocationswithc-axisBurgersvectorshavealso beenreportedforbulk Bi2Sr2CaCu2O8[53,54] and Pb2Sr2(Y,Ca)Cu5O8±acrystals[55]. Due to the analogouslay-ered nature of all the known cupratesupercon-

,‘ ,, ,.~ ductors, it is anticipatedthat a ledge growth/ ~ mechanismis applicableto c-axis orientedfilms

ft ~ of all layeredcupratesuperconductors.The mini-

C ~ mum substratetilt necessaryfor growth to take/ place by step propagation(the cross-overshownJ ~ ~ in Fig. 3) dependson surfacediffusion, which is a

~ ~ function of thegrowth conditionsandthe particu-_______ lar layeredcupratesuperconductormaterial.

6. Acknowledgements

We gratefullyacknowledgeinformativediscus-Fig. 6. Schematicillustration of screw dislocation nucleation sionswith A. Catana,N. Chandrasekhar,T. Frey,by the incoherentmeetingof growth fronts (from Ref. [14]). H.P. Lang, I. Maggio-Aprile,A.F. Marshall, and

S.K. Streiffer. The work at Penn State is sup-ported by ONR through contact N00014-93-1-

on the substratesurface.Obstaclesleadingto the 0512.second mechanismof screw dislocation nucle-ation may include impurity particlesor evenstepson the substratesurface.For example,nanorne- 7. Referencester-size Y203 precipitates with concentrationsover two orders of magnitudehigher than the 11] See,for example:5K. Streiffer,B.M. Lairson. C.B. Eom,

surfacescrew dislocationdensityhavebeeniden- B.M. Clemens, J.C. Bravman and T.H. Geballe, Phys.tified in YBa2Cu3O7~films [14,48,49].In-plane Rev. B 43 (1991) 13007.

266 D.G. Schlomci a!. /Journalof CrystalGrowth 137 (1994)259—267

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