VOLUME 76, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 25 MARCH 1996

omsa

236

Atomic Processes in Low Temperature Pt-Dendrite Growth on Pt(111)

Michael Hohage, Michael Bott, Markus Morgenstern, Zhenyu Zhang,* Thomas Michely, and George CInstitut für Grenzflächenforschung und Vakuumphysik, Forschungszentrum Jülich, 52425 Jülich, Germany

(Received 24 July 1995)

Shape and branch thickness of dendritic Pt-adatom islands on Pt(111) are compared with the resultsof kinetic Monte Carlo simulations. Atoms attached to just one atom of an island were found to havean asymmetric jump probability towards higher coordinated sites. This asymmetry, which results fromnonequivalent hopping paths, gives rise to preferential growth directions at low temperatures.

PACS numbers: 68.55.Jk, 02.50.Ng, 61.43.Hv, 68.70.+w

do

no

discet

o

o

neae

n

e

eh

Ån

hheh

ere

n-

blest

gecaleityy

tal

anches

Fractal and dendritic two-dimensional adatom islanmay be grown at low temperatures by homoepitaxialheteroepitaxial deposition on fcc (111) or hcp (0001) sufaces [1–6]. Since compactness and step-edge roughof islands are two main factors determining the transpbetween atomic layers, the importance of this patteformation process for thin film growth was readily realize[2,7]. However, considerable discrepancies still exconcerning diffusion processes on the atomic level whidetermine the formation and appearance of the expmentally observed dendritic and fractal islands. FracAu dendrites on Ru(0001) [1] and Pt dendrites oPt(111) [3] were considered as physical realizationsa two-dimensional hit-and-stick model [8], whereas thdifference of step-edge diffusion along the two typesclose-packed steps was suggested to be decisive ingrowth of Ag dendrites on Ag(111) and Pt(111) [4,5].

Here, by an interactive approach of experiments asimulations, a new mechanism is identified, which dtermines the anisotropy of growth of Pt on Pt(111)low temperatures. Experimentally, the analysis is bason the temperature dependence of the island shapesbranch thicknesses of Pt dendrites grown on Pt(111)the range from 150 to 285 K as obtained from scannitunneling microscopy (STM) topographs. The ability okinetic Monte Carlo (MC) simulations to reproduce thbranch thicknesses and the island shapes derived fromexperiments turns out to be a stringent test for assumtions on the growth processes of the Pt dendrites.

The experiments were performed in a UHV chambwith a variable low temperature STM [9]. In eacexperiments9.4 6 0.5d% of a monolayer (ML) Pt wasdeposited at a fixed temperatureTs between 150 and285 K with a deposition rate of6.7 3 1024 ML s21 froma thoroughly cleaned, electron beam heated Pt foil ona Pt(111) surface with a terrace width of 1000–4000After deposition the sample was quenched to 20 K aimaged by STM in order to rule out diffusional changesubsequent to deposition.

Figure 1 exhibits two representative STM topograpobtained after deposition at 180 (a) and 245 K (b). Tislands have a dendritic-branched structure with apparaverage branch thicknesses of 13 (a) and 23 Å (b). Tshape of the islands and the orientation of their branch

6 0031-9007y96y76(13)y2366(4)$10.00

sr

r-essrtrn

thri-alnf

efthe

d-td

anding

f

thep-

r

to.d

s

sente

es

are anisotropic. This is most pronounced at 245 K, whthe branches grow preferentially in the threek1 12l direc-tions of the (111) terrace, giving rise to islands with triagular envelopes.

In order to get a measure of the island structure suitafor comparison with MC simulations, we determined firfrom the average apparent branch thickness,dapp , thetrue one,dtrue, at each growth temperature. The averaapparent branch thickness was determined from grey stopographs like those in Figs. 1(a) or 1(b). This quantis plotted as a function of temperature in Fig. 1(c). Anmeasure valuedapp results from a wideningw of the truebranch thicknessdtrue by the STM tip:dapp ­ dtrue 1 2w[10]. Since the STM-tip-dependent wideningw cannotbe obtained directly, we proceeded as follows: The toapparent coverageQapp was determined for each of the

FIG. 1. STM topographs of600 Å 3 600 Å size obtainedafter deposition of 9.4% ML atTs ­ 180 (a) and245 K (b).(c) Temperature dependence of the average apparent brthicknessdapp of the Pt dendrites and of the corrected valudtrue (see text).

© 1996 The American Physical Society

VOLUME 76, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 25 MARCH 1996

teg

ine

h5m

ns

cdtay

ew

asfgoths

meb

rve

tedof

ra-cy

e-ononsedasr-l-tly

Ptnmat

d3).I

dlye

heI”of

aynlyld

hee.

ldns,

images. While the true coverage was in all caseQtrue ­s9.4 6 0.5d% ML [11], Qapp ranged typically from 13% to17% of a ML. Since the branch thickness of the dendriinvestigated here is much smaller than the branch lendtrue ­ dappQtrueyQapp. The wideningw ­

12 sdapp 2

dtrued ­12 dapps1 2 QtrueyQappd caused by the STM tip

ranges in the present experiments from 2 to 5 Å, beconsistent with Ref. [10]. Using the procedure describabove, dtrue is plotted in Fig. 1(c). The true brancthickness appears to stay nearly constant around 6.between 150 and 180 K, then increases, reaching a splateau of about 12 Å between 225 and 265 K beforefinal rapid increase sets in.

The averagein-planecoordination numberC of atomsin the aggregate is a direct output of the MC simulatioAccordingly, the corrected branch thicknessesdtrue havebeen transformed into experimental average in-planeordination numbersCexp. One obtains an upper bounfor Cexp [curve “exp-up” in Fig. 2(a)] by assuming thathe edges of the branches are atomically flat. A betterproximation forCexp is obtained by assuming that everfifth edge atom contributes to branch roughness (i.e., toonly twofold coordinated), which gives rise to the curvexp in Fig. 2(a). The task of the MC simulation is no

FIG. 2. (a) Temperature dependence of average in-plcoordination numberC of atoms in Pt dendrites. Full symbolresult from transformation ofdtrue under the assumption oatomically smooth branches (exp-up) and of every fifth edatom contributing to branch roughness (exp). Open symbresult from MC simulations assuming hit-and-stick grow(type I), immobile bonding only in twofold coordinated site(type II), and temperature dependent asymmetry in juprobability from onefold to twofold coordinated sites (asy) (salso text). Simulated island shapes at 180 K of type I (type II (c), and with the asymmetry assumption (d).

sth,

gd

Åalla

.

o-

p-

be

ne

els

pe),

to reproduce the island shapes qualitatively and the cuexp quantitatively.

In the kinetic MC code used, Pt atoms are deposiat random in space and time with the deposition rate6.7 3 1024 ML s21 onto a hexagonal grid with periodicboundary conditions containing3 3 104 fcc sites until acoverage of 10% ML is reached. For Pt-adatom migtion an activation energy 0.26 eV and an attempt frequen5 3 1012 Hz are used, which were determined in indpendent experiments [12]. The value of the activatienergy agrees with a previous determination by field imicroscopy (FIM) [13]. Other processes and energies uin the simulation will be introduced below, chosen to besimple as possible while still consistent with FIM obsevations. In the two-dimensional MC simulation, funneing [14] is included, and the few atoms deposited direconto aggregatess,0.3% ML d are incorporated into the stepedge at the nearest available sites.

The simplest assumption for the growth of thedendrites is that the only thermally activated diffusiomechanism allowed is adatom migration. An adatoarriving at an aggregate sticks and stays immobilethe impact site, even if it is only onefold coordinateto the aggregate (the lightly shaded atoms in Fig.This gives rise to the growth of islands of fractal type(in the terminology of Ref. [15]). The simulated islanshape at 180 K shown in Fig. 2(b), which is strongbranched and isotropic, is obviously different from thexperimentally observed one [(Fig. 1(a)]. Moreover, tresulting coordination number plotted as curve “typein Fig. 2(a) is by far too low and almost independenttemperature.

In order to increase the coordination number one massume that each atom attaching to an island in an oonefold coordinated site is mobile and jumps to a twofo

FIG. 3. Ball model of small agglomerates on Pt(111). Tlightly shaded atoms are only onefold coordinated in planDashed and full arrows indicate diffusional jumps into twofocoordinated sites via atop positions ad via bridge positiorespectively.

2367

VOLUME 76, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 25 MARCH 1996

dwp

llt

atoo

fp

thn

ue

om

aeu

ioljau

pthgtms3

eoetm-

enere

tva

tethe

ob-f ano-d.ed

ncegk-

thepeheive.m-n ofonsp-ed

er-bym-

ges de-

ro-at

rede-e

-ep-e of

hisci-

s isld

coordinated site before its initial position is stabilized banother arriving atom (fractal type II [15]). The islanshape simulated with this assumption at 180 K, shoin Fig. 2(c), is almost compact, and the overall shais also isotropic, as can be seen from the analysismany islands. This differs again from the experimentaobserved shape [Fig. 1(a)]. The corresponding simulacoordination number plotted as curve “type II” in Fig. 2(is almost temperature independent in contrast toexperimental curve. The failure of curve type II tdescribe the experiment adequately is also obvious frthe fact that below 200 K it exhibits a higher coordinationumber than curve exp-up, which is an upper boundthe experimental coordination number. Including steedge diffusion along close-packed steps would shifttype-II curve to even higher coordination numbers athus can definitely by ruled out below 200 K.

In conclusion, neither island shape nor the temperatdependence of the coordination number of the experimtal observed Pt dendrites on Pt(111) above 150 K canexplained by a hit-and-stick mechanism (fractals typeas suggested previously by four of the present auth[3] or by other assumptions suggested for similar syste[1,4,5,15]. In particular, there exists a new anisotropgrowth regime between 150 and 180 K which has anerage coordination number lying between those of typand type-II fractal growth regimes as defined in a previotheoretical study [15].

To get a physical basis for more complicated assumtions to be used in the MC simulation a closer inspectof Fig. 3 is helpful. For each of the lightly shaded onefocoordinated atoms two different diffusion paths to adcent sites are indicated: An atom moving along the frow is able to remain in a higher coordination with respeto the substrate (moving via hcp and bridge positions) than atom moving along the dashed arrow (via an atopsition [16]). Thus it appears reasonable to assumejumps from onefold to higher coordinated sites via bridpositions have a lower onset temperature than via aones [17]. Defining the onset temperature as the teperature at which the jump frequency is 1 Hz, we chofor the onset temperatures of the two types of jumps 1and 200 K, respectively. The temperature dependencthe coordination number simulated with these assumpti[curve “asy” in Fig. 2(a)] reproduces quantitatively thexperimental data. Also the branched structure andtendency towards anisotropy at 180 K are similar in siulation [Fig. 2(d)] and experiment [Fig. 1(a)]. This similarity is strikingly confirmed also at higher temperaturas seen in Fig. 4, where the shapes of islands grow245 K in experiment (a) and simulation (b) are comparIn both cases islands with triangular envelopes and perential branch growth in thek1 12l directions are visible,as well as less regular shaped islands. The agreementween simulation and experiment is remarkable and gistrong evidence for the correctness of the asymmetry

2368

y

neofyed)he

mnor-ed

ren-beI)rss

icv--Is

p-n

d-llctano-ateop

-e0of

ns

he-

sat

d.f-

be-ess-

FIG. 4. (a) Experimentals500 Å 3 500 Åd and (b) simulateds482 Å 3 556 Åd islands shapes at 245 K. In order to facilitathe comparison a widening of 2 Å has been added tobranches of the simulated islands.

sumption. The further increase in branch thicknessserved above 245 K has to be attributed to the onset oadditional diffusion mechanism which involves the mtion of atoms which are more than onefold coordinateThis is probably the motion of atoms along close-packsteps [18].

We note that, in view of the above results, the preseof step-edge diffusion is improbable for the growth of Adendrites on Pt(111) at 110 K [4,5], where a branch thicness of 2.5 atomss,6.5 Åd [6] similar to that measuredhere below 180 K has been observed. Instead, sinceAg dendrites on Pt(111) exhibit also a triangular envelooriented identically to that of Pt dendrites on Pt(111) tsame mechanism as found here is likely to be operatWe also note that in the MC results in Ref. [15] this asymetry is absent, because there only in-plane interactiothe adatoms were considered. In recent MC simulatifor Pt on Pt(111) growth at 255 K, Pt islands with the oposite orientation of the triangular envelope were obtain[19] (triangle corners pointing into thek112l directions).Such an orientation, which is at variance with the expimental one, can be obtained in our simulations onlyreversing the asymmetry, i.e., by using a lower onset teperature for the jump via atop positions than via bridones, and thus can be traced back to the asymmetry arived from EMT calculations of diffusion barriers.

Strong support for the asymmetry assumption is pvided by the nature of the aggregation of Pt adatomsthe two types of close-packedk110l steps preexistent onPt(111). The two types of steps with different structuas obvious from the ball model in Fig. 5(a) have beenfined asA- andB-type steps in Ref. [20]. The STM imagin Fig. 5(b) shows that while theA-type step grows veryrough with strong asperities, theB-type one remains relatively smooth. The MC simulation performed at the samtemperature of 180 K with the correct asymmetry assumtion shows the same difference between the appearancthe two types of steps upon growth. The reason for tbehavior can be easily recognized from Fig. 5(a). Desive for the degree of roughness of the growing stepthe fate of the atoms (lightly shaded), which are onefo

VOLUME 76, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 25 MARCH 1996

geh,s

nth

o

teeth

pe

tyoroaa

ekels

s

bee

thE

e

m,

.

e

l.

.

,

.

,

s,the

s.

to,

t.

tohsd

topehene

er.

nheg,

ov,

FIG. 5. (a) Diffusional jump towards step edge via bridpositions is possible for the onefold coordinated atoms attacto step adatoms at theB-type step (via bridge positions)whereas no such easy path exists for those attached toadatoms at theA-type step (only via atop positions). Iexperiment (b) and simulation (c) at 180 K asperity growis observed atA-type step edges, while theB-type step edgesremain relatively smooth.

coordinated at step adatoms: while those near theB-typestep can reach the step by an easy jump via bridge ptions, the onefold coordinated atoms near theA-type stepcan reach this step only via the unfavorable atop siAccordingly, the latter will have a correspondingly highchance to be stabilized by an arriving atom. ThusB-type step stays relatively smooth, while on theA-typestep strong asperities are present as obvious from exment [Fig. 5(b)] and simulation [Fig. 5(c)].

In conclusion, the asymmetry in the jump probabilifor atoms attached onefold to Pt islands towards twofcoordinated sites at the island edge plays the decisivefor the temperature dependence of branch thicknessfor Pt-dendrite shape on Pt(111) from 150 up to 250 Kwell as for the shape of the aggregation at the differtypes of close-packed step edges. This process is lito introduce anisotropy in the low temperature growth aon other surfaces with hexagonal symmetry.

The authors acknowledge Max Lagally for discusions, which initiated this work, as well as the comments and the critical reading of the manuscriptTed Einstein and Stefanie Esch. M. H. acknowledgsupport by the Konrad-Adenauer-Stiftung, M. M. by thStudienstiftung des deutschen Volkes, and Z. Z. byU.S. Department of Energy under Contract No. DAC05-84OR21400 with Martin Marietta Energy SystemInc. and by the Forschungszentrum Jülich GmbH.

ed

tep

si-

s.re

ri-

ldlends

ntlyo

--ys

e-,

*Permanent address: Solid State Division, Oak RidgNational Laboratory, Oak Ridge, TN 37831.

[1] R. Q. Hwang, J. Schröder, C. Günther, and R. J. BehPhys. Rev. Lett.67, 3279 (1991).

[2] M. Bott, Th. Michely, and G. Comsa, Surf. Sci.272, 161(1992).

[3] Th. Michely, M. Hohage, M. Bott, and G. Comsa, PhysRev. Lett.70, 3943 (1993).

[4] H. Brune, C. Romainszyk, H. Röder, and K. Kern, Natur(London)369, 469 (1994).

[5] H. Brune, C. Romainszyk, H. Röder, and K. Kern, AppPhys. A60, 167 (1995).

[6] H. Röder, K. Bromann, H. Brune, and K. Kern, Phys. RevLett. 74, 3217 (1995).

[7] R. Kunkel, B. Poelsema, L. K. Verheij, and G. ComsaPhys. Rev. Lett.65, 733 (1990).

[8] T. A. Witten and L. M. Sander, Phys. Rev. Lett.47, 1400(1981); P. Meakin, Phys. Rev. A27, 1495 (1983).

[9] M. Bott, Th. Michely, and G. Comsa, Rev. Sci. Instrum66, 4135 (1995).

[10] J. Wintterlin, J. Wiechers, H. Brune, T. Gritsch, H. Höferand R. J. Behm, Phys. Rev. Lett.68, 59 (1989).

[11] Qtrue was calibrated by deposition at higher temperaturewhere large compact islands resulted and the effect ofwideningw is negligible.

[12] M. Bott, M. Hohage, M. Morgenstern, Th. Michely, andG. Comsa (to be published)

[13] P. J. Feibelman, J. S. Nelson, and G. L. Kellogg, PhyRev. B 49, 10 548 (1994).

[14] J. W. Evans, D. E. Sanders, P. A. Thiel, and A. E. DePrisPhys. Rev. B41, 5410 (1990).

[15] Z. Zhang, X. Chen, and M. G. Lagally, Phys. Rev. Let65, 733 (1990).

[16] The actual diffusion path of onefold coordinated atomstwofold coordinated sites is not necessarily along the patindicated by the arrows. For instance, the jump indicateby the dashed arrow does not have to proceed via the aposition but may also proceed via two or more bridgand hcp positions. However, such a path implies that tatom moves further away from the step and loses in placoordination.

[17] This assumption is consistent with the observation of thdifferent frequencies of intracell and intercell jumps for Idimers on Ir(111) [cf. S. C. Wang and G. Ehrlich, SurfSci. 239, 301 (1990)].

[18] An onset temperature of 245 K for step-edge diffusiocorresponds to the onset of Pt-adatom diffusion along tchannels on Pt(113) and Pt(331) surfaces [cf. G. KellogSurf. Sci. Rep.21, 1 (1994)], which have a structureidentical to the close-packed steps on Pt(111).

[19] J. Jacobsen, K. W. Jacobsen, P. Stoltze, and J. K. NorskPhys. Rev. Lett.74, 2295 (1995).

[20] S. C. Wang and G. Ehrlich, Phys. Rev. Lett.67, 2509(1991).

2369