V O L U M E 70, N U M B E R 4 P H Y S I C A L R E V I E W L E T T E R S 25 J A N U A R Y 1993

Two-Dimensional Microstructures of Submonolayer Ag on Pt( l l l ) Induced by Elastic Strain

Andreas F. Becker, Georg Rosenfeld, Bene Poelsema, and George Comsa Institut fur Grenzflachenforschung und Vakuumphysik, Kernforschungsanlage Julich,

Forschungszentrum Julich, Postfach 1913, W5170 Julich, Germany (Received 28 September 1992)

The growth of Ag on Pt(l 11) has been investigated at submonolayer coverages between 80 and 1000 K. Below about 550 K large-scale metastable uniformly strained 2D Ag islands grow pseudomorphical-ly. Heating of the submonolayer film or direct deposition of Ag above this temperature leads to the for­mation of small, "strain-free" 2D equilibrium structures, possibly heptamers.

PACS numbers: 68.55.Ce, 61.50.Cj, 68.60.Dv

The growth of strained overlayers has been the subject of a large number of experimental and theoretical investi­gations. This interest is driven both by fundamental pur­poses and by the importance of such layers in applica­tions, in particular, in optoelectronic devices. Most stud­ies are dealing with growth modes and their dependence on strain. We have focused here on the growth of strained films in the submonolayer regime. Our experi­mental findings show that large-scale uniformly strained 2D Ag islands grow pseudomorphically at sufficiently low temperatures; upon heating, these films fragment into small 2D clusters, leading to a roughening of the deposit­ed film. The same small clusters also develop spontane­ously during deposition at high temperatures. This phenomenon is in obvious contrast to the results obtained in the absence of strain, where heating leads to smoothen-ing of the deposited film.

The system Ag on P t ( l l l ) is well suited for studying strain-induced features: Ag is strongly bound to P t ( l 11) [1,2], the lattice mismatch amounts to 4.3% at room tem­perature, and interdiffusion has not been observed up to the desorption temperature of Ag [1], Indeed, bulk solu­bility has not been observed at 800 K; from the "uncer­tain and hypothetical" boundaries in the phase diagram [3] it seems that the solubility of Ag into Pt and of Pt into Ag amounts to only about 1% and 10%, respectively. Careful checks at 800 K and 50% Ag coverage have not revealed any indication of interdiffusion at a time scale of 6000 s [4]. For the comparison with the behavior in the absence of strain we also present data for the ho-moepitaxial growth of Pt on P t ( l 11).

The information on the surface morphology has been obtained in situ by means of TEAS (thermal-energy He-atom scattering). The scattering geometry leads to a transfer width of about 450 A for the specular beam. The Ag is deposited from an Ag disk, heated by electron bombardment from the rear and placed almost parallel to the Pt( l 11) sample, thus providing a homogeneous depo­sition rate: inhomogeneities < 1% across the probing He beam spot. Under all conditions the width of the He specular beam is 0.25° as measured with a quadrupole mass analyzer with 0.2° acceptance angle. LEED mea­

surements show a well developed ( l x l ) pattern, indicat­ing pseudomorphic growth of the uniformly strained sub­monolayer Ag on Pt( l 11) as long as large 2D Ag islands are present. No indication of a superstructure is observed in the presence of small 2D Ag clusters (see below).

The results reported here have been obtained under in-phase conditions; i.e., the scattering geometry has been set to make He waves from adjacent terraces interfere constructively. Under these conditions, the attenuation of the normalized He specular peak height is a direct mea­sure of the number of surface defects (e.g., steps), here in particular of the total length of the 2D-island edges [5].

1.0

0.5

0 200 400 0 500 1000 Pt exposure time [s] annealing time [s]

<• 0.5

0.0 0 200 400 600 0 500 1000

Ag exposure time [s] annealing time [s] FIG. 1. Uptake curves of (a) Pt and (c) Ag, taken at 596

and 600 K, respectively. Panels (b) and (d) show the evolution of I/Io after interrupting the exposure at the times indicated by the arrows in (a) and (c), respectively.

© 1993 The American Physical Society 477

V O L U M E 70, N U M B E R 4 P H Y S I C A L R E V I E W L E T T E R S 25 J A N U A R Y 1993

Figure 1 shows the evolution of the normalized He specular peak height, I/Io, during the exposure of P t ( l l l ) to Pt [Fig. 1(a)] and to Ag [Fig. 1(c)]. The be­havior is characteristic of 2D layer growth [5,6]. The ini­tial decay of I/Io evidences the increase of the integral length of the developing 2D islands. When coalescence of the 2D islands sets in, the rim length starts to decrease and consequently I/Io rises until it reaches a maximum value. The maximum value of I/Io (roughly) corresponds to the minimum of the rim length, i.e., to completion of the first layer. For the case of Pt on Pt ( l 11) the value of the maximum of I/Io at monolayer completion is sig­nificantly lower than its initial value. This indicates that the layer-by-layer growth is not ideal: Nucleation in the next layer sets in before the previous one has been comp­leted [6]. This observation is consistent with recent scan­ning tunneling microscopy findings [7]. In contrast, in the case of Ag on Pt ( l 11) the value of I/Io in the max­imum is close to the initial level obtained for the clean Pt( l 11) substrate. This fact reveals the growth of an al­most perfect first layer of Ag with virtually no Ag nu­cleation in the second layer before completion of the first one. Note that the Debye-Waller attenuation for the first layer of Ag on P t ( l l l ) is quite similar to that for the bare Pt( l 11) surface (cf. Fig. 2).

Ag uptake curves like the one in Fig. 1(c) have been measured at various temperatures. Between 300 and 900 K they all look quite similar. Only the minimum varies substantially: At higher temperatures the minimum is, most surprisingly, lower; i.e., the smaller adstructures are formed at higher temperatures. Careful He scattering and LEED measurements [4] demonstrate that the large Ag structures grown on P t ( l l l ) at lower temperatures are pseudomorphic, thus uniformly strained. The 2D is­lands reach linear dimensions of several hundreds of A at 200 K.

An interesting question now is how the two systems

cti

100 300 500 700 900

sur face t e m p e r a t u r e [K]

FIG. 2. Height of the specular He beam, for clean P t ( l l l ) ( x ) and with 0.2 monolayer Ag, put down at 200 K, during heating (o) and cooling (•), respectively. The triangle has been taken from an Ag uptake curve measured at 800 K. The asterisk shows the result of an annealing measurement (see text).

P t / P t ( l l l ) and Ag/PT(111) relax towards equilibrium. For this purpose the exposure to Pt and to Ag has been interrupted after deposition of about half a monolayer [arrows in Figs. 1(a) and 1(c)]. Subsequently, the evolu­tion of I/Io has been measured as a function of time [Figs. 1(b) and 1(d)]. After stopping the deposition of Pt, I/Io steadily recovers [Fig. 1(b)]: The total length of the rims bordering the 2D Pt islands decreases. Obvious­ly, the system strives to reach its equilibrium configu­ration involving a few large islands with a low concentra­tion of step atoms. This annealing process corresponds to Ostwald ripening in 2D as reported recently [8]. The same explanation has been put forward to describe coar­sening experiments for Cu grown on Cu(001) [9]. Surprisingly, in the case of Ag deposition on P t ( l l l ) [Fig. 1(c)], instead of increasing, I/Io decreases mono-tonically with time. Like the 2D Pt structures on Pt( l 11), the 2D Ag structures developing during growth at 600 K do not constitute equilibrium structures. How­ever, unlike the homoepitaxial case, the 2D Ag equilibri­um structures towards which the system strives after in­terruption of the Ag deposition are smaller than the metastable ones created during exposure at 600 K. This behavior is a consequence of the relief of strain by frag­mentation. Isothermal annealing above about 600 K leads to strain release by the formation of small, almost strain-free Ag clusters. These small clusters do not devel­op in 3D but stay two dimensional. This has been in­ferred from the facts that the Ag exposures needed to complete the first monolayer are temperature indepen­dent (300-800 K) and that this Ag monolayer is as flat as the Pt substrate [4]. In addition, we have directly determined the height of the small clusters with respect to the substrate by LEED measurements. From the sys­tematic extinction of the central part of the specular peak (and its corresponding) broadening upon variation of the electron energy we have obtained a height of 2.3 A [4].

The results of further experiments evidencing addition­al aspects of the system are shown in Fig. 2. The loga­rithm of the He specular peak height is plotted versus the surface temperature. The crosses show the normal rever­sible Debye-Waller behavior for clean P t ( l l l ) . They serve as a reference for the unchanged surface morpholo­gy of the substrate. The circles have been obtained after deposition at 200 K of 20% of an Ag monolayer on Pt( l 11). The decrease (A— B) of the He specular peak height is small, indicative of the growth of large 2D Ag islands. Subsequent slow heating (-^0.1 K/s) yields a de­cay of the He peak height, which within experimental ac­curacy parallels that for clean PtCl 11). Thus no significant change of the film morphology occurs up to about 550 K. However, between 550 and 700 K the He specular peak height decays dramatically, indicating a major evolution of the film morphology. The data be­tween 700 and 900 K again parallel the reference data for clean P t ( l l l ) , indicating that no further structural

478

VOLUME 70, NUMBER 4 P H Y S I C A L R E V I E W L E T T E R S 25 JANUARY 1993

changes occur. Subsequent slow cooling of the surface at approximately the same rate yields the behavior repre­sented by the squares. Obviously, the He reflectivity does not resume its initial value upon cooling to 200 K. The data just parallel the Debye-Waller behavior for clean Pt(l 11) (crosses) showing that the substantial change of the surface morphology during heating from 500 to 700 K is irreversible. Note that similar curves have been ob­tained after deposition of submonolayer coverages of Ag at various temperatures below 500 K. Always we find that large 2D islands are growing below 500 K, and the transition towards smaller structures during subsequent heating always appears in the same temperature range. We have also checked the irreversible nature of the frag­mentation shown in Fig. 2 just below the transition tem­perature. For this purpose, after initial heating to 900 K, we have cooled the system [20% Ag on Pt(l 11)] to 450 K and kept it there (asterisk in Fig. 2) for as long as 4500 s. Even after such a long period no sign of a recovery of the He specular peak height could be detected. Obviously, the small 2D Ag structures are the stable ones.

The small Ag clusters can also be produced directly by deposition of Ag at elevated ( > 700 K) substrate temper­atures. These structures are indistinguishable from those created after fragmentation of the low-temperature de­posit as is exemplified by the triangle (Fig. 2), corre­sponding to 20% of an Ag monolayer, which has been ob­tained during Ag uptake at 800 K. An experiment like that shown in Fig. 1(d), but this time above 700 K, i.e., monitoring I/IQ upon interruption of the Ag deposition and keeping the temperature constant, shows no sign of recovery whatsoever: 1/1$ stays constant. This demon­strates that the structures grown above 700 K, and of course also the identical ones obtained through fragmen­tation by heating to 700 K, are true equilibrium struc­tures [10].

A crude estimate of the size of the equilibrium small 2D Ag clusters can be obtained from the magnitude of their cross section for diffuse scattering of the probing He atoms. From the initial slope of the Ag uptake curves, taken at sufficiently high temperature ( > 700 K), the effective cross section is determined to 45 A2 per Ag atom. Assuming a reasonable value of 130 A2 for an iso­lated Ag monomer and applying the overlap approach [5] we infer that the equilibrium structures probably are symmetrical heptamers [11]. This allows the following plausible physical picture: The central Ag atom resides at a threefold hollow site on Pt(l 11). The remaining six Ag atoms are all accommodated in the first shell sur­rounding it. They can easily relax somewhat in outward directions to relieve most of the strain.

The establishment of equilibrium apparently occurs via a major rearrangement of the structures developed during growth at low temperatures. In homoepitaxy, as illustrat­ed for Pt/Pt(l 11) [Fig. 1(b)], annealing occurs via Ostwald ripening, i.e., large 2D islands grow at the ex­

pense of smaller ones [8]. Obviously, this mechanism re­quires evaporation of Pt adatoms from the edges of small 2D islands onto the (111) terraces. The development of nonequilibrium structures during growth, therefore, occurs only when the detachment rate of Pt atoms from small islands is not significant at the time scale set by the deposition rate. By analogy, large nonequilibrium 2D Ag structures on P t ( l l l ) will develop as long as the attach­ment of monomers to 2D Ag islands is probable, i.e., pro­vided the lifetime of an added Ag atom is large with respect to the times set by the deposition rate.

The attachment of Ag atoms to the equilibrium struc­tures, i.e., to the heptamers with their outward-relaxed outer atoms, may be suppressed. Indeed, the latter speculation is consistent with the result of measurements in which we have prepared 2D equilibrium submonolayer Ag films at 800 K, which subsequently have been exposed to Ag at temperatures below 550 K. The small equilibri­um structures exposed to Ag in the temperature range where large, uniformly strained Ag islands normally grow do not grow further into smooth Ag monolayers. Rather further roughening is observed, involving nucleation and growth in the second layer before completion of the first one. This would explain why equilibrium structures, once formed, do not grow further into large islands below 550 K.

The decomposition of large 2D uniformly strained Ag islands must not necessarily proceed via the release of monomers. It is conceivable that strain relief starts at the island edges by the formation of ruptures or dislocations, allowing the splitting off of mobile heptamers.

For submonolayer growth of Ag on P t ( l l l ) we have established that below about 550 K Ag grows in large, pseudomorphic, uniformly strained 2D islands. These is­lands are metastable as demonstrated by the irreversible fragmentation during heating between 550 and 700 K. The fragmentation implies a strong increase of the total length of the rims bordering the 2D Ag islands, i.e., an increase of the step concentration. The energy required for the creation of step edges appears to be overcompen-sated by the relief of strain.

The growth of uniformly strained films and the release of strain by temperature increase has been investigated for many years (e.g., [12-18]). Strain relief has been shown to be possible by introducing dislocations upon exceeding a critical thickness or by the formation of 3D islands. In the latter case, strain can be relieved in a nonuniform way, gradually decreasing with increasing distance from the interface [17]. The overall reduction of strain energy pays for increased total film-vacuum sur­face area. The 3D clusters tend to grow in the third di­mension via Ostwald ripening, driven by a further reduc­tion of the total interface and surface area. In the sub­monolayer case of perfect film-substrate wetting, escape into the third dimension is energetically unfavorable. For the present example of Ag on Pt(l 11) this is reflected by

479

V O L U M E 70, N U M B E R 4 P H Y S I C A L R E V I E W L E T T E R S 25 J A N U A R Y 1993

the fact that the Ag-Pt bond is much stronger than the Ag-Ag one [1,4].

In summary, we have shown that below 550 K large, uniformly strained, metastable 2D Ag islands grow on P t ( l l l ) . Upon heating, submonolayer islands irreversi­bly fragment into small 2D Ag clusters, probably hepta-mers, which also develop directly during growth above 700 K. This "anticoarsening" is induced by the relief of strain enabled by sufficient diffusivity and contrasts with the smoothening of homoepitaxial films at enhanced dif­fusivity as has been illustrated for P t /P t ( l 11).

[1] P. W. Davies, M. A. Quinlan, and G. A. Somorjai, Surf. Sci. 121,290 (1982).

[2] B. Schmiedeskamp, B. Kessler, B. Vogt, and U. Heinzmann, Surf. Sci. 223, 465 (1989).

[3] Binary Alloy Phase Diagrams, edited by B. Massalski (American Society for Metals, Metals Park, OH, 1986), p. 57.

[4] A full account will be given in a forthcoming full paper. [5] B. Poelsema and G. Comsa, in Scattering of Thermal En­

ergy Atoms from Disordered Surfaces, edited by G. Hohler, Springer Tracts in Modern Physics Vol. 115 (Springer, Heidelberg, 1989).

[6] B. Poelsema, R. Kunkel, A. F. Becker, G. Rosenfeld, L. K. Verheij, and G. Comsa, Appl. Phys. A 53, 369 (1991).

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

[8] B. Poelsema, A. F. Becker, G. Rosenfeld, R. Kunkel, N. Nagel, L. K. Verheij, and G. Comsa, Surf. Sci. 272, 269 (1992).

[9] H.-J. Ernst, F. Fabre, and J. Lapujoulade, Phys. Rev. Lett. 69, 458 (1992).

[10] It is interesting to remind oneself that the structures grown at 600 K [Fig. 1(c)], i.e., within the "fragmenta­tion temperature range," are larger than the equilibrium structures. This is due to kinetic effects, i.e., due to the increased Ag adatom density during deposition. This has been verified by varying the deposition rate: The higher rates result in larger island sizes.

[11] Note that the estimate of the size of the small equilibrium clusters is based on an assumed cross section for diffuse He scattering from a monomer. It is conceivable that the clusters are, e.g., symmetrical dodecamers, i.e., 12-atom clusters having edges consisting of alternatingly two and three atoms. The longer edges may have {ill}, {ill}, and {ill} microfacet orientations allowing easy-out­ward-relaxation cross-bridge positions. By again applying the geometrical overlap principle [5], the corresponding cross section of a monomer would yield 190 A2. A final decision on this minor issue is possible only by providing STM pictures with atomic resolution, by no means an easy task.

[12] J. W. Matthews and W. A. Jesser, Acta. Metall. 15, 595 (1967).

[13] R. Vincent, Philos. Mag. 19, 1127 (1969). [14] T. N. Taylor, R. E. Muenchausen, and M. A. Hoffbauer,

Surf. Sci. 243, 65 (1991). [15] T E. Madey, K.-J. Song, C.-Z. Dong, and R. A. Dem-

min, Surf. Sci. 247, 175 (1991). [16] J. H. van der Merwe, J. Appl. Phys. 41, 4725 (1970). [17] E. Bauer and J. G. van der Merwe, Phys. Rev. B 33, 3657

(1986). [18] B. G. Orr, D. Kessler, C. W. Snyder, and L. Sander, Eu-

rophys. Lett. 19,33 (1992).

480