Clarke, Roy

Tsui, F.

Uher, C.

Wellman, J.

Carolina Digital Repository

VOLUME 76, NUMBER 17 P H Y S I C A L R E V I E W L E T T E R S 22 APRIL 1996111).eaturesleationTheease inonentcked3164Morphology Transition and Layer-by-Layer Growth of Rh(111)F. Tsui,* J. Wellman, C. Uher, and Roy ClarkeDepartment of Physics, University of Michigan, Ann Arbor, Michigan 48109(Received 2 August 1995)We have observed a morphological transition in the nucleation and growth of epitaxial Rh(The transition occurs near 600 K and is signaled by a change in the shape of the surface ffrom fingered to compact. The transition appears to be related to a change in the critical nucsize. On both sides of the transition, there lies a regime of persistent layer-by-layer growth.general surface features exhibit well-defined length scales and as growth proceeds they incrsize following a power-law dependence on film thickness with a morphology-independent expof 0.33 6 0.03. The results suggest a general pathway to the layer-by-layer growth of close-pametals. [S0031-9007(96)00058-0]PACS numbers: 68.55.Jk, 61.16.Ch, 68.35.Bshisfee[5olffrv-luagrieoesthtiaoeesullykyezthlaTora.stiaccin-edatcetedf 1s.d aten-innd-gyceMntea-resiththectheuresndan-he00ns.ea-fi-rna-s.atntsRecent epitaxial growth experiments [1–4] show tpresence of surface features that exhibit characterlength scales. The observed three-dimensional (3D)tures arise from instabilities due to the competition btween various diffusion barriers near step edgesThese findings suggest that the non-self-affine morphgies may be a common occurrence when surface dision is sufficiently large with profound implications fothe growth of thin films in this regime. In particular, seeral models [6,7] have focused on the power-law evotion of surface feature size with respect to the averfilm thickness, leading to predictions which our expemental results can address.Quantitative experimental data on the thickness depdence are still very sketchy and many important questiare yet to be addressed: How is the evolution of th3D features related to layer-by-layer growth, what areeffects of “kinks” at step edges, and what role does ininucleation play? In this Letter we report on the evolutiof surface morphology during epitaxial growth of a modsystem, Rh(111), as a function of temperature, thicknand growth rate. We show that surface features incling those resulting from layer-by-layer growth generaexhibit characteristic dimensions which scale with thicness and temperature, and that aglobal transformationof the surface morphology accompanied by layer-by-lagrowth arises from a change in the critical nucleation siThe results have important general implications forgrowth of close-packed metal films.Our studies were performed in a VG 80M molecubeam epitaxy (MBE) chamber equipped with anin situscanning tunneling electron microscope (STM) andreal-time reflection high energy electron diffraction (RRHEED) imaging system [8]. The growth template fthe study consists of a synthetic fluorine mica substr[9] and a buffer layer of 250 monolayers (ML) RhWe chose this system for two main reasons. First, aclassic close-packed system, Rh is much less suscepto contamination and chemical reaction than most met0031-9007y96y76(17)y3164(4)$10.00etica--].o-u--e-n-nseelnls,d--re.era-teablels;and second, it is known [9] that Rh grows well on the f(111) face on synthetic mica.The mica substrates were freshly cleaved beforetroduction into the MBE chamber, and then degassat 1200 K. The Rh buffer was subsequently grown600 K and then annealed at 1100 K, giving typical terralengths of several thousand Å. The Rh was evaporafrom an electron-beam hearth with respective rates oand2 MLymin for the nucleation and the growth studieThe base pressure of the MBE chamber was maintainebelow10210 torr. The samples were cooled at50 Kyminafter growth was terminated, and STM images, the frozin configurations, were then taken at room temperatureconstant current mode with a typical current of 1 nA aa bias of20.1 V . Continuous RHEED observations confirm that there is no change in the surface morpholoduring cooling.The strong influence of growth temperature on surfamorphology and Rh(111) is demonstrated by the STimages of a series of 10.1 ML films grown at differetemperatures, as shown in Fig. 1. Several important ftures are immediately apparent: (1) the surface featushow a characteristic length scale typified by islands wwell-defined size and separation; (2) the shape offeatures exhibits a transition near 600 K from compaat high growth temperatures [Figs. 1(d)–1(f)] where tstep edges are smooth, to fingered at low temperat[Figs. 1(a)–1(c)] where the step edges have “kinks”; a(3) the height of the features has a minimum near the trsition, where we also observe layer-by-layer growth. Tpresence of a layer-by-layer growth regime between 5and 700 K is supported by persistent RHEED oscillatioThese observations reinforce the view that surface ftures are generally not self-affine when diffusion is sufciently high [1] and that the morphology of layer-by-layegrowth is a special case simply because the coherentture of the process predicates well defined length scaleQuantitative analysis of the STM images reveals ththe surfaces exhibit sharp ringlike Fourier compone© 1996 The American Physical SocietyVOLUME 76, NUMBER 17 P H Y S I C A L R E V I E W L E T T E R S 22 APRIL 1996adrshmpeaa)]nueueitmMuneinceeenho-yureureepofondgesr-ksarestesrsr-wceeserachd,ateofs.eges,ges,essofheaityonge5],Inntnd,ter)FIG. 1. STM images of 10.1 ML Rh(111) surfaces grownvarious temperatures, (a) 375 K, (b) 450 K, (c) 530 K, (680 K, (e) 725 K, and (f) 810 K. The horizontal white bacorrespond to 400 Å for images (a)–(e), and 1000 Å for (f).in the growth plane and long-ranged oscillations in theight-height correlation functions as well, thus confiring the presence of non-self-affine features. The temature dependences of the average heightW and separationR of the surface features are shown in Fig. 2. The brominimum in mean feature heightW [Fig. 2(a)] is accom-panied by a break in the otherwise monotonically decreing aspect ratioWyR as temperature increases [Fig. 2(ba very distinct signature of the morphological transitioThe general decreasing trend ofWyR indicates the im-portant role played by capillary-induced smoothing dto diffusion [5], but the fact that the surface area donot decrease monotonically with increasing temperatand that mounds do form both indicate the importanceasymmetric diffusion barriers at step edges [5]. The brof WyR, on the other hand, is completely consistent wcapillary effects alone at a morphological transition frofingered to compact.FIG. 2. Size and shape of the surface features on 10.1Rh(111) as a function of temperature. (a) Feature heightW ,the rms height from average film thickness in Å, and (b) feataspect ratioWyR, average heightW versus average separatioR. Points above and below the morphological transition n600 K are indicated by squares and circles, respectively. Lare to guide the eye.t)e-r-ds-,.esreofakhLrearesOne of the key issues in the formation of surfafeatures is the nature of diffusion at step edges. As sin Fig. 1, the compact mounds formed above the morplogical transition exhibit some reflection symmetr(mounds and craters) not shown by the low temperatfingered features. The asymmetry at low temperatis thought to be associated with the kinetics of stpropagation for terraces at the top and at the bottomthe mounds [4], and, moreover, the morphology transitiobserved here suggests that the shape of the step eplays an important role in the kinetics.It is interesting to point out that the observed layeby-layer growth occurs onboth sides of the morphologytransition. This indicates that the presence of kinat the step edges [10,11] and island spacing [12]two important factors in producing smooth growth. Jubelow the transition the presence of kinks modifithe diffusion barriers by introducing wells and barriealong the step edges [10], thus facilitating the layeby-layer growth. In this regime our experiments shothat increasing the growth rate will increase the chanfor nucleation and induce rougher growth. Above thtransition the compact mounds become smaller and cloas temperature decreases [Fig. 2(b)], and when they rea critical size [12] (in this case,100 Å separation) layer-by-layer growth occurs. In this regime, on the other hanour experiments reveal that increasing the growth rreduces island spacing and promotes the likelihoodproducing layer-by-layer growth at higher temperatureHowever, the origin of the observed large critical sizis unclear.What ultimately triggers the transition? The strondependence of diffusion barriers on the local geometriparticularly those along as well as across the step edsuggests that the answer lies in the nucleation proc[11,13]. We have carried out a detailed investigationthe initial nucleation at the submonolayer level, and tresults reveal that the critical cluster size also exhibitstransition near 600 K. Figure 3 shows the island densas a function of temperature and island size distributifor a Rh submonolayer coverage of 0.1 ML. The chanof slope in the island densityN shown in Fig. 3(a)indicates a change in the activation energy [14,1separating the diffusion process into two branches.addition, the island size distribution exhibits two differescaling functions [15–19], as shown in Figs. 3(b) a3(c). In the low coverage limit with isotropic diffusionthe island densityN is given by [20]N , expfsiEd 1 Eidysi 1 2dkBT g , (1)where Ed is the activation energy for diffusion, andEiis the binding energy for the critical cluster sizei, abovewhich clusters form stable islands; e.g.,i ­ 1 correspondsto stable dimers, etc. Models based on critical clussize predict that for eachi there is a unique distributionfunction [16–19]. Scaling functions shown in Figs. 3(b3165VOLUME 76, NUMBER 17 P H Y S I C A L R E V I E W L E T T E R S 22 APRIL 1996ehfre()(Kasarlofarinoiarevbeuloctss-lidlylueslyse-ceheya)–n-gr-rgi-nt.thoftese ofionents.K.s,ÅftFIG. 3. (a) Arrhenius plot of island density,N in Å22, forRh coverageu of 0.1 ML as a function of inverse temperaturThe slopes indicate respective activation energies of0.06 60.02 eV at the low temperature branch (right side of tgraph) and0.25 6 0.08 eV at high temperatures (left side othe plot). Scaling of island size distributions for temperatu(b) below 600 K and (c) above 600 K. Temperatures are600 K (circles), 580 K (up triangles), 530 K (down triangles430 K (squares), 390 K (hexagons), and 350 K (diamonds);770 K (up triangles), 730 K (diamonds), 700 K (circles), 680(squares), and 630 K (hexagons). Number of islands perat sizes and coverageu, Nssud, is normalized byS2yu with Sthe average island size, ands is scaled byS. Lines in (b) and(c) are to guide the eye.and 3(c) yield the critical cluster size ofi ­ 1 below600 K andi ­ 2 above [16–19]. From Eq. (1) this leadto Ed ­ 0.18 6 0.06 eV andE2 ­ 0.6 6 0.4 eV. Notethat these are average activation energies for a neisotropic (111) surface. The measured “one-bond” eneis comparable to the reported experimental estimate0.5 6 0.5 eV for Fe(100) [15] as well as the Monte Carand rate equation estimates of0.6 6 0.1 and 0.7 eV,respectively [16,18]. The measuredEd also is comparableto a previously published value for Rh(111) of0.15 60.02 eV [21].While the diffusion along step edges is clearlycrucial factor in determining the final morphology othe system [13], it is the initial “seed” clusters thset up the edge-diffusion process. In the stable dimregime (i ­ 1 case), the presence of thickened finge[Fig. 1(c)] indicates some edge diffusion. However,the regime of stable trimers (i ­ 2 case) additional pro-cesses become available, including one-bond detachm(unstable dimers) which can trigger more edge diffusiThis can lead to the formation of compact islandsthe i ­ 2 regime. A similar transition is predicted forsquare lattice even without edge diffusion [16,19]. Thefore the observed change in critical cluster size is edently large enough to give rise to the observed glomorphological transition accompanied by layer-by-laygrowth. We have also observed a similar transition ding the annealing of films deposited at temperatures be600 K [22].Finally we examine the evolution of mounds as a funtion of film thickness at temperatures above and below3166.esb),c)rearlygyofatersentn.n-i-alrr-w-hemorphological transition. Figure 4 illustrates the proceat 725 K. The initial growth is layer-by-layer accompanied by RHEED oscillations. After the initial severamonolayers the aspect ratio of the surface features rapincreases, within a couple of monolayers, to a stable va(the so-called “magic slope” [1–4]) [Fig. 4(d)]. Theseparation, however, evolves smoothly and continuoufrom the initial nucleation at the submonolayer level, ashown in Fig. 4(e), similar in appearance to those rported in Fe(001) [4]. As growth proceeds, the surfafeatures maintain their shape but increase in size, so tboth scale and coarsen, as demonstrated in Figs. 4(4(c). The coarsening process follows a power-law depedence on film thicknesst with an exponent of0.33 6 0.03[Fig. 4(e)]. This exponent is not predicted by existinmodels, and it may have implications for the undelying physics of the asymmetric diffusion barrier. Fogrowth temperatures above and below the morpholocal transition the coarsening exhibits the same exponeThe fact that the same exponent is obtained for growwithin regimes where the strength and spatial extentthe asymmetric step-edge barriers are different indicathat the exponent is independent of the size and shapthe barrier.A splitting of RHEED streaks occurs for growth below350 K, which corresponds to the characteristic separatof the mounds [4,14]. This gives a second independmeasure of the separation in addition to STM imageFIG. 4. Coarsening of surface features for growth at 725(a)–(c) STM images of 30, 150, and 300 ML Rh surfacerespectively. The horizontal white bars correspond to 400for images (a) and (b), and 1000 Å for (c). (d) Log-log plot othe aspect ratio of surface features,WyR, and (e) log-log plotof the feature separationR in Å as a function of film thicknesstin ML. The line in (e) indicates a power law with an exponenof 0.33 6 0.03.VOLUME 76, NUMBER 17 P H Y S I C A L R E V I E W L E T T E R S 22 APRIL 1996arctohend.aehsieonnalnesapasagethsithngthiongseIncaelsnd.rDarn,.tt.o,e,ill,ds,d,gkov,a,,v.ev..,b-It is interesting to point out that the rounded triangulshape of the mounds shown in Figs. 4(a)–4(c) reflethe threefold fcc symmetry, indicating the absencestacking faults; in fact, we do see hexagonal features wimpurities are present.In summary, we have studied the nucleation agrowth of epitaxial Rh(111) using STM and RT-RHEEDThe Rh surface exhibits a morphological transition ne600 K, about1y4 of the Rh melting temperature, whersurface features change from fingered to compact. Tis accompanied by a minimum in surface roughneand by persistent layer-by-layer growth. Detailed studof the nucleation statistics indicate that a transitiin critical cluster size from one to two atoms catrigger a morphological transition by influencing locgeometry. A second characteristic feature is the preseof mounds that exhibit well defined shapes and sizBoth compact and fingered mounds maintain their shbut increase in size as growth proceeds followingpower-law dependence on film thickness with a univerexponent of1y3. While the presence of kinks alonthe step edges modifies the diffusion barrier, it donot alter the nature of the coarsening process, andobserved exponent seems to be independent of theand shape of the step-edge barrier. Our results revealthe occurrence of a global morphological transition duriMBE growth can lead to layer-by-layer growth on bosides of the transition, a special regime in the evolutof non-self-affine surface morphologies. Our findinfor epitaxial Rh should be generally applicable to othepitaxial systems, particularly metallic (111) surfaces.a wider context, the appearance of global morphologitransitions triggered by local symmetry breaking raisseveral interesting questions for atomistic growth modeWe are grateful to C. P. Flynn for discussions afor providing synthetic mica substrates. We thank L. MSander, B. G. Orr, A. Zangwill, and J. G. Amar fostimulating discussions, and C. Taylor, D. Barlett, andLitvinov for assistance. The work was supported in pby ONR Grant No. N00014-92-J-1335.*Permanent address: Department of Physics aAstronomy, University of North Carolina, Chapel HillNC 27599-3255.sfnrisssce.elsezeatrls..td[1] M. D. Johnson, C. Orme, A. W. Hunt, D. Graff, J. LSudijono, L. M. Sander, and B. G. Orr, Phys. Rev. Le72, 116 (1994); C. Orme, M. D. Johnson, J. L. SudijonK. T. Leung, and B. G. Orr, Appl. Phys. Lett.64, 860(1994).[2] H. J. Ernst, E. Fabre, R. Folkerts, and J. L. LapujouladPhys. Rev. 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Kunkel, B. Poelsema, L. L. Verheij, and G. ComsPhys. Rev. Lett.65, 733 (1990).[12] J. Tersoff, A. W. D. van der Gon, and R. M. TrompPhys. Rev. Lett.72, 266 (1994).[13] H. Röder, K. Bromann, H. Brune, and K. Kern, Phys. ReLett. 74, 3217 (1995).[14] J. A. Stroscio, D. T. Pierce, and R. A. Dragoset, Phys. RLett. 70, 3615 (1993).[15] J. A. Stroscio and D. T. Pierce, Phys. Rev. B49, 8522(1994).[16] J. G. Amar and F. Family, Phys. Rev. Lett.74, 2066(1995); (to be published).[17] G. S. Bales and D. C. Chrzan, Phys. Rev. B50, 6057(1994).[18] M. C. Bartelt and J. W. Evans, Phys. Rev. B46, 12 675(1992); Surf. Sci.298, 421 (1993); J. W. Evans and M. CBartelt, J. Vac. Sci. Technol. A12, 1800 (1994).[19] C. Ratsch, A. Zangwill, P.Šmilauer, and VvedenskyPhys. Rev. Lett.72, 3194 (1994).[20] J. A. Venables, Philos. Mag.27, 697 (1973).[21] G. Ayrault and G. Ehrlich, J. Chem. Phys.60, 281 (1974).[22] F. Tsui, J. Wellman, C. Uher, and R. Clarke (unpulished).3167

Morphology Transition and Layer-by-Layer Growth of Rh(111)

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Morphology Transition and Layer-by-Layer Growth of Rh(111)

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