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Growth and thermal evolution of submonolayer Pt films on Ru(0 0 0 1) studied by STM
Surface Science 540 (2003) 76–88 www.elsevier.com/locate/susc
Growth and thermal evolution of submonolayer Pt films on Ru(0 0 0 1) studied by STM U. K€ asberger *, P. Jakob Physik-Department E20, Technische Universit€at M€unchen, James-Franck-Straße, 85747 Garching, Germany Received 19 February 2003; accepted for publication 19 May 2003
Abstract The growth of submonolayer Pt on Ru(0 0 0 1) has been studied with scanning tunneling microscopy. We focus on the island evolution depending on Pt coverage hPt , growth temperature TG and post-growth annealing temperature TA . Dendritic trigonal Pt islands with atomically rough borders are observed at room temperature and moderate deposition rates of about 5 · 104 ML/s. Two types of orientation, rotated by 180 and strongly influenced by minute amounts of oxygen are observed which is ascribed to nucleation starting at either hcp or fcc hollow sites. The preference for fcc sites changes to hcp in the presence of about one percent of oxygen. At lower growth temperatures Pt islands show a more fractal shape. Generally, atomically rough island borders smooth down at elevated growth temperatures higher than 300 K, or equivalent annealing temperatures. Dendritic Pt islands, for example, transform into compact, almost hexagonal islands, indicating similar step energies of A- and B-type of steps. Depending on the Pt coverage the thermal evolution differs somewhat: While regular islands on Ru(0 0 0 1) are formed at low coverages, vacancy islands are observed close to completion of the Pt layer. 2003 Elsevier B.V. All rights reserved. Keywords: Growth; Nucleation; Scanning tunneling microscopy; Ruthenium; Platinum; Epitaxy; Low index single crystal surfaces; Metal–metal interfaces
1. Introduction Many modern electronic devices are based on nanometer-scale layer sequences prepared by homo- and heteroepitaxial growth. Examples are high mobility transistors and semiconductor laser structures. In the last decade there has been an
* Corresponding author. Tel.: +49-89-289-12821; fax: +4989-289-12338. E-mail address: [email protected] (U. K€ asberger).
increasing interest in metal-on-metal systems due to the observation of various magnetic effects like giant and colossal magneto resistance (GMR and CMR) and their enormous economical potential in device applications [1]. The properties of these semiconductor and metal structures critically rely on the quality of the interface of adjacent layers. A high level of control of processes such as segregation, intermixing and alloying is desirable and may be achieved by the detailed knowledge of basic physical growth processes. New insight in the growth process on a fundamental level is obtained by studying model
0039-6028/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0039-6028(03)00776-3
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systems. Recently, a large number of material systems have been investigated intensely. In this work, Pt on Ru(0 0 0 1) has been analyzed which combines a moderate lattice mismatch (2.5%) with interesting catalytic properties. The heteroepitaxial growth of Pt on Ru(0 0 0 1) thus is of basic interest within existing growth models and scaling laws [2– 4] and may, in addition, unveil unusual chemical activity. Our growth conditions are typical for nonequilibrium layer growth, i.e. net growth is obtained by supersaturation of the Ru surface with impinging Pt adatoms. Depending on the Pt adatom density and mobility, stable or slowly decaying nuclei eventually form. Nuclei stability thereby is associated with the higher coordination number of atoms within aggregates as compared to single atoms. Once formed, small nuclei are much less mobile than single atoms (for Pt/Pt(1 1 1) see Ref. [5]) and they act as further nucleation centers leading to island formation. This scenario is quite general and holds for homo- and heteroepitaxial metal-on-metal growth as reviewed by Brune [6]. In addition to nuclei formation on terraces, energetically favorable, highly coordinated sites may be found at step edges. In the so-called step-flow growth (which dominates if adatom mobility is sufficiently high for adatoms to reach the steps instead of forming stable nuclei on the flat terraces) adatoms primarily attach to step edges with islands on terraces being virtually absent. This quite basic growth picture needs to be detailed to explain the growth of Y -shaped dendritic islands, observed for many hexagonal surfaces at intermediate growth temperatures (see Section 4). A characteristic property of hcp crystal surfaces like Ru(0 0 0 1) is the occurrence of two different types of step edges, with one or two nearest neighbor atoms at the bottom of the step, which are called A- and B-steps, respectively [7]. These steps can either be formed by misorienting the Ru(0 0 0 1) surface or by creating islands onto the terraces. Due to the different local geometry of the A- and B-type step edges adatom diffusion along these steps differs substantially. This leads to a preferential growth along particular directions and hence to the formation of Y -shaped dendrites [8].
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2. Experimental The STM measurements were performed in a vacuum chamber at a base pressure of 1 · 1010 mbar using a room temperature and a variable temperature STM operated at 180–300 K with a magnetically clamped Pt–Ir tip. Typical parameters for the STM operated in the constant current mode are a tunneling current ITunnel of about 1 nA at a bias voltage UBias of 0.2–2 V. Pt was evaporated by resistively heating a 100 lm thick foil (Goodfellow, purity >99.99%) mounted on a liquid nitrogen cooled holder. During Pt deposition the sample temperature could be set to 130–600 K (DT ±10 K measured with a Chromel/Alumel thermocouple). For growth temperatures TG of about 300 K the base pressure rises by about 5 · 1011 mbar; at TG 500 K it stays within the low 1010 mbar range. The Pt deposition rate may be chosen between 1 · 104 and 4 · 103 ML/s with high reproducibility. Gas exposures were performed using a needle doser (Ø ¼ 2 mm) mounted in front of the sample in the heating stage. Sample cleaning is achieved via standard procedures: Arþ ion-sputtering and oxygen cleaning cycles. The T -shaped Ru crystal has a size of 5 · 5 mm2 (front face) and is oriented in the (0 0 0 1) direction with an accuracy of ±0.5. It is clamped onto a Ta plate which can be transferred from the STM to the Pt source, a heating/cooling stage where it may be cooled to 150 K and heated up to 1600 K, and an additional heating/cooling stage in front of the sputter gun. During the annealing process the temperature could be measured with an infrared-radiation thermometer which was calibrated using a sample with a thermocouple attached. The reproducibility at T > 350 K is DT 6 1 K; systematic deviation of the true temperature is estimated to a few percent of the actual value. The desired annealing temperature TA was typically reached in about 2–3 min and kept fixed at TA for about another 2 min. All images of Figs. 1–3 were obtained after a preparation procedure with Arþ ion-sputtering, annealing, O2 dosing cycles and Pt deposition. Post-annealing of Pt-layers typically lasted 2–3 min; care has been taken not to overshoot the given temperature values during heat-up. All layers were newly prepared, i.e. no successive
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depositions have been applied, in order to avoid accumulation of contaminants from the residual gas during STM acquisition.
3. Results and interpretation The following subsections deal with the morphological and thermal evolution of submonolayer amounts of Pt deposited onto the clean hexagonal close-packed Ru(0 0 0 1) surface. We focus on the dependence of the deposited layers on directly accessible growth parameters such as Pt coverage hPt , growth temperature TG and annealing temperature TA and their immediate consequences illuminated by STM. 3.1. Increasing Pt coverage The STM images in Fig. 1 give an overview of the coverage dependent evolution of pseudomorphic Pt islands on Ru(0 0 0 1) in the range of hPt ¼ 0:10–0:90 ML. The growth temperature during Pt deposition was set to about room temperature (see figure caption for exact values), the deposition rate to R ¼ 5 104 ML/s. In the low coverage regime (hPt ¼ 0:10 ML), (a), well separated trigonal dendritic islands are observed, in accordance with the hexagonal substrate symmetry. A closer inspection of the dendrites in Fig. 1(a) yield average branch diameters dav of the order of . Subtracting the branch widening w 10 A 35 A caused by the tip apex [9] gives a corrected value , corresponding to about of dcorr ¼ dav 2w 15 A five atomic distances. The average branch length . In Fig. 1(b) a Pt ranges from about 90–150 A coverage of hPt ¼ 0:15 ML has been deposited leaving the branch dimensions almost unchanged. The surface island density is slightly higher than in (a): 1.5 · 1011 cm2 vs. 1.1 · 1011 cm2 . 1 Ap-
1
For narrow terrace widths below the mean free diffusion length of Pt adatoms on Ru(0 0 0 1), the formation of stable nuclei is unlikely and deposited Pt atoms predominantly attach to step edges, leaving the terrace almost free of islands (see lower part of image (c) of Fig. 1). These areas and their capture zone are excluded from the determination of the surface island density (see Section 4) [10].
parently, nuclei formation is not yet completed at hPt ¼ 0:1 ML and nucleation still takes place before Pt adatoms attach to existing islands. More so, this tendency continues in (c) with the island density reaching 1.5–2 · 1011 cm2 which is close to the value in (b) and may be the saturation island density. At hPt P 0:5 ML (d), additional Pt adatoms contribute exclusively to further growth of existing dendrites with the onset of partial coalescence. Here, the morphology of Pt islands published in [11] is reproduced. Due to additional Pt and edge diffusion processes the branch length and thickness have increased until hPt 0:6 ML, when coalescence of neighboring islands sets in. The Pt island boundaries on Ru(0 0 0 1) remain atomically rough even for hPt ¼ 0:9 ML. Besides a more compact shape with still atomically rough island boundaries, considerable second layer nucleation has set in for hPt > 0:6 ML (e). By then, the average area of the first layer islands has )2 . As a concurrent proincreased to about (200 A cess to nuclei formation mobile adatoms in the second layer of an island will search the island borders until a low energy barrier is found for a step down transition. For small islands, the rate of collision nc of adatoms with the island borders, i.e. the attempt frequency of these adatoms to overcome the island border barrier is high, so that the average adatom density nad and thus the second layer nuclei formation rate will be low. For large islands nc is low and, due to an enlarged nad , second layer nucleation becomes more and more favorable. This behavior would explain the absence of second layer islands for smaller islands in Fig. 1(a)– (d). For Pt/Pt(1 1 1), experimental [12,13] and theoretical [14] results indicate dissimilar energetic barriers of about 0.2 and 0.08 eV to jump across Aand B-type step edges, respectively. This tendency might hold for Pt/Ru(0 0 0 1) as well. In addition to adatoms jumping across step edges an exchange diffusion process as in the case of Co/Pt(1 1 1) [15] remains imaginable. We note that the pseudomorphic Pt islands on Ru(0 0 0 1) are compressed laterally by 2.5%. STM measurements and EMT investigations (effective medium theory) for Ag self-diffusion on strained and unstrained Ag(1 1 1) surfaces [16] as well as DFT calculations (density functional theory) for
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Fig. 1. STM images of the Pt/Ru(0 0 0 1) bimetallic layer showing the coverage dependent evolution in the submonolayer regime (1000 · 1000 A , UBias 0:5 V, ITunnel 1 nA). The Pt deposition rate is about R ¼ 5 104 ML/s. (a) hPt ¼ 0:10 ML (TG ¼ 295 K), (b) A hPt ¼ 0:15 ML (TG ¼ 297 K), (c) hPt ¼ 0:35 ML (TG ¼ 293 K), (d) hPt ¼ 0:50 ML (TG ¼ 292 K), (e) hPt ¼ 0:60 ML (TG ¼ 291 K), (f) hPt ¼ 0:90 ML (TG ¼ 275 K). Each layer has been prepared separately, starting from a clean Ru(0 0 0 1) surface (see Section 2).
Ag/Ag(1 1 1) clearly show that the mobility of adatoms on compressed layers increases significantly
[17]. As a consequence, collisions of second layer Pt atoms with island borders will occur more
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frequently so that second layer nucleation on top of small Pt islands in (a)–(d) becomes even less probable. Image (f) depicts the Ru surface covered with 0.90 ML Pt, more precisely 0.87 ML in the first and 0.03 ML in the second Pt layer. The substrate is still visible in between of the Pt islands. The second layer island morphology does not show any dendritic behavior which is in agreement with the expected enhanced adatom mobility on the Pt islands. In fact, second layer islands are compact and their location is predominantly in the inner part of the underlying islands. The shape of second layer islands thus is dramatically different from the fractal shape of Pt on Ru(0 0 0 1) as well as of Pt on Pt(1 1 1) grown under similar conditions [18]. The ABAB stacking sequence of the hcp Ru crystal results in an alternation of A- and B-type step edges for the (0 0 0 1) plane. Pt adatoms which do not form stable nuclei on the terraces will condense at these step edges which feature intrinsically higher coordination numbers. Once attached, growth proceeds from ascending steps. The orientation of the branches pointing away from step edges (oriented along the Æ 1 1 2æ directions of the Ru substrate) switches between perpendicular and ±30 to the step edges. The observed alteration of branch orientation is attributed to the same diffusion anisotropy as reported for Pt islands on the flat terraces (see above). These branches (in the following called fingers) can be clearly seen in Fig. 1(a)–(c) and they have been already observed for various systems [8,9]. It is assumed that deposited Pt atoms attach to Ru step edges the same way as deposited Ru atoms would do, i.e. by occupying hcp sites to continue the existing Ru single crystal substrate. Further information on the occupied lattice site of Pt islands on flat terraces can be extracted by correlating these fingers with the orientation of Pt dendrites on the flat terrace which is studied in the following section. 3.2. hcp and fcc hollow site occupation The direct determination of nucleation sites (hcp or fcc threefold hollow sites) of Pt dimers or small Pt islands on Ru(0 0 0 1) in their earliest state
using STM is quite difficult or even impossible, because of very similar signatures of both types of site in STM images. The lateral distance between fcc and hcp sites on the Ru(0 0 0 1) surface , which may be distinguished by amounts to 1.56 A adding a grid on the atomically resolved surface as has been done for the dissociation of molecular oxygen on Pt(1 1 1) [19]. Due to the close similarity of both hollow sites in STM images, an adsorbate with a clearcut preference for either fcc or hcp sites is additionally needed to fix the reference frame for the unknown species. In our case, an indirect method is used. As discussed in the previous section, the growth and evolution of Pt fingers at Ru step edges can be explained within the same model as for Pt islands on the Ru(0 0 0 1) terraces. The consequence of alternating A- and B-steps with local diffusion anisotropy [8] are most evident in Fig. 2(a) (see also Fig. 1). Assuming that Pt attaches to the Ru steps the same way as deposited Ru atoms would do, i.e. by continuation of the hexagonal lattice of the upper terrace, fingers perpendicular to step edges can be associated with A-steps, fingers diagonal to step edges with B-steps. This assignment is based on the well-established model of Brune et al. [8] which assumes dissimilar mobilities of adatoms along both types of step and across kinks within step edges. For Pt/Pt(1 1 1) this model has been corroborated by applying kinetic Monte Carlo simulations [8,9]. The fact that the pseudomorphic growth of Pt fingers at Ru step edges starts exclusively at hcp threefold hollow sites for A- as well as B-steps allows an identification of Pt sites for dendritic islands: Dendritic islands on terraces with an orientation of their branches identical to step edge fingers can be attributed to hcp nucleation sites (and hcp Pt islands); branches rotated by 60 represent fcc Pt islands. In Fig. 2(a) a low Pt deposition rate of R ¼ 3 104 ML/s is used to enhance attachment to step edges and to suppress nucleation on the terraces, so that large dendrites grow at the step edges. This way the alternating orientations of Pt fingers at A- and B-type of steps are clearly visible. Fig. 2(b) and (c) refer to ‘‘clean’’ and oxygencontaminated growth conditions, at about R ¼ 1:3 103 ML/s and about TG ¼ 300 K. The
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Fig. 2. Alternating orientations of Pt dendrites at A- and B-type of steps of the Ru(0 0 0 1) surface, (a), and dendritic Pt islands on clean, (b), and oxygen contaminated Ru(0 0 0 1), (c). Pt dendrites at steps are oriented perpendicular to the step edge (A-type of step) or diagonal (B step). The preferential fcc nucleation site of terrace Pt islands on the clean surface, (b), reverses to hcp in the presence of a · 2000 A , hPt ¼ 0:25 ML, TG ¼ 292 K, small amount of oxygen (c). Parameters of sample preparation and imaging: (a) 2000 A · 1000 A , hPt ¼ 0:18 ML, TG ¼ 305 K, R ¼ 1:8 103 ML/s, (c) 1000 A · 1000 A , hPt ¼ 0:18 ML, R ¼ 3 104 ML/s, (b) 1000 A 3 TG ¼ 300 K, R ¼ 1:3 10 ML/s. The different orientation of step edges in (b) and (c) compared to (a) results from a 90 rotated and newly clamped Ru sample.
dendrites attached to A- (or B-) type of steps crossing the STM images identify the hcp sites for the oriented Pt islands. Thus, on the widest terrace in (b) the two types of orientation can be identified and labeled as ‘‘h’’ (hcp) and ‘‘f ’’ (fcc), indicating their nucleation sites. Evaluation of many STM images with and without oxygen contaminations confirm a considerable preference of fcc nucleation on the ‘‘clean’’ Ru(0 0 0 1) surface (fcc:hcp 3:1) while hcp nucleation prevails (fcc:hcp 1:2) in the case of minute oxygen contaminations (hO 1% of a monolayer); if several percent of oxygen atoms are preadsorbed we observe a 100% ratio of hcp islands, oriented the same way as the dendrites growing at Ru step edges. Single oxygen atoms are mobile and show up as black dashes in enlarged STM images [20]. This small oxygen contaminations have no significant influence on the island density; such an effect is in fact observed for Pt growth on O(2 · 2) or O(2 · 1) precovered Ru(0 0 0 1) (not shown here). We conclude that nucleation of Pt on Ru(0 0 0 1) is very sensitive to adsorbates, in our case to small amounts of oxygen. Since the growth at the chosen TG and R takes place in the high supersaturation regime the question arises whether or not the system is in thermal equilibrium: We believe that diffusing Pt adatoms can in fact be considered as a lattice gas (with fcc and hcp hollow sites representing two unequal sites per unit cell) in thermal equilibrium,
at least between two collisions; occupation of fcc and hcp sites therefore should reflect the energetic differences of both binding geometries. 3.3. Increasing growth temperature TG Due to various thermally activated processes such as diffusion, migration, surface alloying or thermodynamic de- and attachment of island atoms, the island morphology is very sensitive to the growth temperature TG . Fig. 3 illustrates the dependence of Pt islands on TG for hPt ¼ 0:10 ML deposited at a rate of about 5 · 104 ML/s. For TG ¼ 191 K (a), the islands show no preferential growth direction, but a diffusion limited aggregation (DLA) character [21,22]. The low adatom mobility at low TG results in a relatively high surface island density of n ¼ 6:8 1011 cm2 with an average number N of about 230 Pt atoms per island. The average branch thickness amounts to , equivalent to 2–3 atomic distances, about 5 A confirming a reduced edge- and corner diffusion. The observed branching for Pt/Ru(0 0 0 1) as well as for Pt/Pt(1 1 1) (up to TG ¼ 300 K) [18] into random directions despite a non-zero edge mobility may be explained within the model developed by Zhang et al. [23]: Pt atoms arriving at an existing island remain mobile as long as their lateral coordination is lower than twofold; only then they become immobilized. In their simulations, fractal growth with arm thicknesses of about four atoms
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Fig. 3. Morphological change of Pt islands on Ru(0 0 0 1) with varying growth temperature TG at a fixed coverage of hPt ¼ 0:10 ML and deposition rate of about R ¼ 5 104 ML/s: (a) TG ¼ 191 K, (b) 242 K, (c) 295 K, (d) 408 K and (e) 517 K. Note the different · 500 A ) and (c)–(e) (1000 A · 1000 A ). image sizes for (a) and (b) (500 A
are found in an extended temperature range (compared to standard DLA growth, with an arm thickness of only a single atom). At TG ¼ 242 K, Fig. 3(b), the islands show three dominant growth directions in accordance with the hexagonal symmetry of the substrate (but there is still some DLA like growth). The branch thickness has slightly and N 580 atoms. The increased to about 10 A surface island density has decreased to n ¼ 2:7 1011 cm2 . At TG ¼ 295 K, (c), n ¼ 1:1 1011 cm2 , while N amounts to about 1400 atoms with . Up to TG 300 K a mean branch width of 15 A the island boundaries remained atomically rough. At higher temperatures there is a gradual transition towards smooth island borders. This effect can be seen by comparing images (3c) and (3d). Infrared absorption spectroscopy measurements of CO adsorbed at various locations, e.g. Pt step edges and on the Ru(0 0 0 1) surface [24] indicate a transition at about 340 K: CO which preferentially binds to (rough) Pt island step edges migrates to Ru(0 0 0 1) terraces as the temperature exceeds 340 K. This behavior has tentatively been attributed to atomically rough Pt island borders becoming smooth upon annealing, which is confirmed by our
data. It seems to hold for the variation of the growth temperature, too. At 408 K, (d), edge- and corner diffusion of Pt atoms are sufficiently fast, leading to perfectly oriented dendrites (n 0.2 · 1011 cm2 ) with a branch thickness of about 70 A and N 8000 atoms. Since dendrites may reach enormous sizes, the evaluation of their surface density requires special attention to step edges and dendrites originating from such surface defects, i.e. step edge fingers. Dendritic islands due to a homogeneous nucleation process only form far from step edges on sufficiently large terraces. The total image area useful to analyze nucleation on terraces therefore is lowered by the amount covered by step edge fingers. The growth conditions at TG ¼ 517 K, (e), are such as to prevent island formation on the flat terraces; instead, all of the deposited Pt atoms have condensed at step edges of the Ru substrate (one might still create Pt islands at this temperature by increasing the incident Pt flux). We note that due to the similar sizes of Pt and Ru atoms, surface areas containing either of these species could not be distinguished; the preferential deco-
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Fig. 4. Growth temperature dependence of the surface island density n (a) and the average number N of Pt atoms per island (b). A constant coverage of hPt ¼ 0:10 ML and a constant deposition rate of about 5 · 104 ML/s has been chosen.
ration of Ru with oxygen atoms, however, allows such a distinction later on. Furthermore, under high temperature growth conditions with enhanced overall mobility of Pt atoms the oriented dendrites have disappeared and a distinction between A- and B-type step edges is no longer possible. Fig. 4 depicts the surface island density n and the average number N of Pt atoms per island, 2 derived from Fig. 3 and additional images of these layers. On a linear scale n approximately follows an exponential law within the range of TG ¼ 191–517 K (a). In the low temperature regime (high supersaturation) the balance between adatom mobility (and thus of the attachment of adatoms to step sites and other surface defects) and deposition rate is such that attachment to surface structural defects is ineffective and a relatively high density of adatoms (nad ) prevails; consequently, an increasing number of nuclei is
2 The average number N of Pt atoms per island can be expressed as N ¼ hPt N0 =n, where N0 ¼ 1:587 1015 cm2 denotes the number of adsorption sites per unit area on Ru(0 0 0 1) and n the Pt surface island density.
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formed. This situation shifts towards the thermal equilibrium for increasing TG : A larger mobility is related to a more probable attachment of Pt adatoms to steps, thereby reducing nad and thus the island formation rate. Moreover, step edges and islands accumulate Pt atoms from wider capture zones. For those reasons and under the chosen growth parameters, n decreases from 6.8 · 1011 cm2 at TG ¼ 191 K to 0.2 · 1011 cm2 at TG ¼ 408 K. Further island formation at TG > 500 K is limited by our average terrace length of about 400 . To depict the value at TG > 500 K in the graph, A a linear scale is preferred (a) instead of the equivalent logarithmic one as in (b). On a perfect surface (no steps) extrapolation of the values at TG < 408 K to TG ¼ 517 K would give about 5 · 109 cm2 . In the standard nucleation theory the relation v between n and R follows a power law n / ðD=RÞ . D denotes the diffusion constant and v corresponds to i=ði þ 2Þ where i is the critical cluster size (for details see [6] and references herein). Evaluation of double-logarithmic plots of the surface island density vs. R for hPt ¼ 0:2 ML at room temperature (not shown) give evidence that i 3 (v ¼ 0:57 0:08), i.e. trimers or maybe tetramers represent stable units. 3 The behavior of cluster dissociation and cluster diffusion in the case of Pt/ Pt(1 1 1) is still under intensive discussion [26–30]; a more refined description of processes affecting the island density which will evolve out of this dispute probably will be of relevance for our bimetallic system as well. 3.4. Postgrowth thermal evolution In the preceding paragraph we have shown that variation of TG primarily influences the adatom mobility and density nad . The latter quantity results from the competition between the flux of impinging Pt atoms and their attachment to existing structures (steps, small nuclei, islands). In
3
The stable nuclei is an idealization since detachment of island atoms at different TG have to be considered under real growth conditions (see Ref. [25]).
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annealing processes after deposition at low TG , the detachment from islands and the readsorption to existing structures represent the dominant mechanism of island evolution. This way the gradual approach of kinetically limited surface structures formed during growth under high supersaturation conditions towards thermal equilibrium is pursued. The STM images in Fig. 5 arranged in a matrix display the thermal evolution of three layers presented in Fig. 1. A maximum annealing temperature TA ¼ 800 K has been chosen in order to prevent surface alloying [11] which starts at about 900 K. In column (A) the initial dendritic island shape collapses into large hexagonal compact Pt islands for TA ! 600 K. Because of the decreasing island density (note the different image
size of the annealed layers), we deduce a significant detachment of Pt atoms from dendrites and readsorption to step edges of the Ru substrate or to remaining larger islands. Apparently, diffusion along island edges is now sufficiently fast to allow occupation of the energetically most favorable adsorption sites, thereby forming islands with a hexagonal shape. Further annealing to 800 K reduces the island density once again and the Ostwald ripening process, barely noticeable at 600 K, becomes more prominent: large islands survive at the expense of smaller ones. We note that the applied annealing time in Fig. 5(A) probably was too short as some of the shrinking islands are still present along with the enlarged islands.
Fig. 5. Array of STM images illustrating the thermal evolution of submonolayer Pt on Ru(0 0 0 1). The various coverages (deposited at about room temperature) were: (A) hPt ¼ 0:15 ML, (B) 0.60 ML and (C) 0.90 ML. Note, that the image size of annealed layers is 2000 · 2000 A while it is 1000 A · 1000 A for the layers as grown. Typical tunneling parameters were UBias 0:5 V and ITunnel 1 nA. A
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The situation in column (C) is different: The high coverage of hPt ¼ 0:90 ML leaves only a small part of the Ru substrate uncovered. Rearrangement of Pt adatoms at elevated TA then produces vacancy islands, i.e. holes within the mono-atomic Pt layer on the Ru substrate. Minimization of borders seems to be much less of an issue for holes as compared to islands of similar size. More precisely the larger vacancies are somewhat irregularly shaped as compared to the regular islands which display an almost hexagonal shape for TA ! 600 K. Second layer islands still exist at 600 K, but they vanish for TA ! 800 K. Vacancy islands also show Ostwald ripening and a decreasing density analogous to regular islands. On Pt(1 1 1) a similar morphology can be produced by Arþ ionbombardment with subsequent annealing to 760 K [31]. However, to study the properties of Pt vacancies on Ru(0 0 0 1), high coverage Pt deposition with subsequent annealing is preferred to the destructive ion bombardment preparation to avoid Pt incorporation into the Ru substrate during the sputter process. The central column (B) can be considered as a transition regime (hPt ¼ 0:60 ML) and it contains characteristic features of both cases mentioned above. For TA ! 600 K, some of the larger islands or Pt islands attached to step edges of the Ru(0 0 0 1) surface contain small vacancies in their inner areas. The vacancy density after annealing is highest at step edges. Obviously, the morphology of step fingers grown at room temperature favors the formation of small pockets within the Pt film as the temperature is increased. During the annealing process detachment and readsorption takes place leading to bigger islands. Most of them do not merge even for TA ! 800 K indicating a repulsive interaction. This repulsion might be due to a strain field surrounding the Pt islands on Ru(0 0 0 1), or due to the fact that neighboring fcc and hcp islands do not easily merge.
4. Discussion Quite naturally, the occupation of fcc and hcp sites of Ru(0 0 0 1) by Pt atoms is expected to differ energetically; theoretical calculations [32] reveal a
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slight preference of 20 meV per site for hcp site occupation when a full monolayer of Pt on Ru(0 0 0 1) is considered. This finding contradicts our observations of a preferential fcc occupation in the absence of oxygen contaminants. At low hPt , however, when nucleation of Pt islands sets in, the energetics and entropy factors may differ to some extent modifying the kinetics of nuclei formation and their preferred hollow site [33]. In fact, for Ir/ Ir(1 1 1) it has been experimentally observed that single Ir atoms preferentially adsorb on hcp sites changing to fcc sites with lateral coordination of those Ir atoms [34]. Recent tight-binding calculations suggest that d-band filling plays a decisive role for the understanding of the site preference (i.e. adsorption in the ‘‘fault’’ or ‘‘correct’’ site with respect to the crystal stacking sequence) of transition metal atoms in epitaxy [33,35]. In particular, single Pt adatoms on Ru(0 0 0 1) were predicted to occupy fcc sites, which agrees nicely with the found 3:1 preference of fcc-type islands in our experiments. Of course, effects associated with lateral coordination during growth of small Pt nuclei (such as observed for Ir on Ir(1 1 1) [34]) may have additionally affected the site preference of our Pt islands. Nevertheless we find the agreement pleasing. Our results also indicate that epitaxy in the presence of adsorbates leads to a more complex situation. Even small amounts of oxygen are found to change the relative stability of small fcc and hcp Pt islands leading to a reorientation of the triangular dendritic islands. Since this effect occurs already for hO 1% of a monolayer we conclude that oxygen is located preferentially at the islandÕs rim rather than being distributed randomly on the surface. Theoretical as well as experimental data suggest that oxygen atoms prefer sites located at either type of steps of Pt(1 1 1) [35]; for Ru(0 0 0 1) only A-type steps are decorated at low hO [20]. The question which type of hollow site is preferred by step edge oxygen atoms for the Pt/Ru bimetallic surface must remain open, since there is no easy way to decide this matter [36,37]. For Pt/Pt(1 1 1), a similar case of reversal of Pt island orientation has been reported in the presence of CO background partial pressures of 109 mbar at TG ¼ 400 K. It has been attributed to a
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Table 1 Island morphology and saturation surface island density n reported for Co, Ni, Pt, Al and Au on Ru(0 0 0 1) at room temperature and moderate deposition rates R hAd (ML)
Lattice mismatch (%)
hNi ¼ 0:05 hCo ¼ 0:2 hPt ¼ 0:25 hAl ¼ 0:12 hAu ¼ 0:3
)7.9 )7.4 +2.5 +5.8 +6.6
Island shape
R (ML/s) 3
5 · 10 2.5 · 103 1.3 · 103 5 · 104 3.3 · 10-3
Hexagonal Triangular Dendritic Compact Fractal
modification of A- and B-type step free energies by CO molecules and associated changes of the mobilities of Pt adatoms along the island steps [12]. Contrary to our bimetallic system, identical nucleation sites for all Pt islands and uniform island orientations exist on Pt(1 1 1). For Pt grown on Ru(0 0 0 1) the orientation of step edge fingers serves as a reference for Pt occupying hcp sites, for the clean as well as the oxygen precovered Ru(0 0 0 1) surface. Since the appearance of step edge fingers is not altered by small amounts of oxygen a different diffusion process along the step edges (as invoked for Pt homoepitaxy) appears unlikely. The oxygen induced reorientation of dendritic Pt islands reported here is therefore attributed to a change of the preferred Pt adsorption site from fcc to hcp. The stabilization of Pt in hcp sites probably is induced by oxygen atoms located at the edges of small Pt nuclei; upon further Pt deposition growth will proceed from these nuclei to form larger islands with oriented dendrites, while maintaining their hcp sites. Dendritic pattern formation is a familiar phenomenon on hexagonal surfaces for a variety of homo- and heteroepitaxial material systems within a specific temperature range: for example Ag/ Ag(1 1 1) (110 K) [8], Pt/Pt(1 1 1) (245 K) [9] and Ag/Pt(1 1 1) (130 K) [8] or Au/Pd(1 1 1) (300 K) [38]. A complex balancing mechanism between edge- and corner diffusion via hopping and/or exchange processes, corner crossing and energetically favorable step edge formation determines the final shape at a given temperature [6]. Either fractals, dendrites, triangles or even compact islands may form. This also holds for the various metals (Ni, Co, Au, Al [39–42] on Ru(0 0 0 1): for details see
n (cm2 )
Ref.
5 · 1010 3 · 1010 43 · 1010 46 · 1010 0.03 · 1010
[39] [40] This work [42] [41]
Table 1). The predominant dendritic growth observed for Pt on Ru(0 0 0 1) at room temperature and moderate deposition rate fits nicely into the diversity of island shapes already observed on this substrate material. By comparing reported island densities (room temperature) of Ni/Ru(0 0 0 1) [39], Co/Ru(0 0 0 1) and especially of Au/Ru(0 0 0 1) [40] with our findings of Pt on Ru(0 0 0 1) we noticed significant differences by up to three orders of magnitude. For these materials, Table 1 summarizes the island morphologies and surface island densities on Ru(0 0 0 1). For better comparison, a larger Pt deposition rate (R ¼ 1:3 103 ML/s), closer to those given in Table 1, has been chosen (note that n 1 1011 cm2 at R ¼ 5 104 ML/s in Fig. 1). Key factors determining the island density of deposited metal atoms are the incident flux U and the adatom mobility. 4 Both factors counteract each other: A high U enhances the adatom density nad , leading to accelerated nuclei formation; a high mobility of the adatoms, on the other hand, favors attachment to steps and defects, thereby reducing nad . With increasing nad during film growth a system therefore changes from step-flow mode to the nucleation regime. Eventually, the island density has reached a (saturation) value which makes at-
4
In addition to growth temperature and incident flux, a weak dependence of the island density on the total coverage is observed as well. In Fig. 1 the surface island density is found to increase with Pt coverage until the saturation density is reached. At higher hPt , the islands start merging and a determination of the island density from STM images is no longer possible. At very low hPt formation of Pt islands might be avoided altogether, e.g. if nad does not reach the critical value nc required for stable nuclei formation.
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tachment to existing islands more probable than nuclei formation, i.e. the nucleation regime transforms into the island growth regime. For deposited Co, Ni and, in particular, in the case of Au, the low island density at room temperature indicates highly mobile adatoms on the Ru(0 0 0 1) surface. While for the former elements the high adatom diffusivity combines with a likewise substantial mobility along island borders, the fractal growth behavior observed for Au [40,41] seems to indicate that gold atoms behave differently. However, they too retain an enormous mobility along island borders giving rise to wide . Pt and Al on Ru(0 0 0 1) branches of about 200 A seem to behave the opposite way: Diffusivities of these adatoms are rather low, leading to a very high saturation island density (even though the deposition rates are low compared to the other systems reported in Table 1). According to the observed dendritic island shape the mobility of attached Pt atoms is moderate; some limitations exist with respect to corner diffusion as well as corner crossing. In particular this latter effect leads to dissimilar orientations of dendrites attached to either A- or B-type of steps. As mentioned in the previous section adjacent Pt islands do not easily merge at elevated hPt . Two reasons may explain this observation: First, nucleation of Pt islands occur at two different possible adsorption sites, hcp and fcc hollow sites which yields Pt dendrites rotated by 180 (see Fig. 1(a) and (b), as well as Fig. 2(b)). Due to the lateral displacement of hcp and fcc sites there will always exist a small gap between both types of island. Second, the number of impinging adatoms onto the bare Ru(0 0 0 1) surface contributing to the narrowing of the gap by condensing at the island borders decreases as the gap closes. Therefore, the growth speed of opposite protrusions or island borders is progressively reduced. This effect is attenuated by the two processes: (i) second layer adatoms may descend the atomically rough island step edges and (ii) diffusion along island edges. We note that interlayer transport, case (i), becomes less probable as straight step edges evolve at elevated temperatures. We deduce this argument from the annealed layer in Fig. 5, column (C), (hPt ¼ 0:90 ML and TA ¼ 600 K) where second
87
layer islands of narrow size distribution have survived because of the formation of straight vacancy island edges. 4.1. Summary The growth of Pt on Ru(0 0 0 1) in the submonolayer regime has been studied using scanning tunneling microscopy. The growth at room temperature leads to the formation of triangular, dendritic islands. Occupation of either hcp or fcc sites by the Pt atoms is held responsible for the occurrence of two type of island orientations. Higher growth temperatures lead to an increase in island size, while the island density is reduced accordingly. In parallel, dendrites attached to step edges of the Ru substrate become more common and island formation on terraces (by means of nucleation) less likely. Eventually, at T > 500 K, a step-flow growth of Pt on Ru(0 0 0 1) is observed (for an incident flux of about 8 · 1011 s1 cm2 ). Annealing of Pt layers grown at room temperature to about 600 K leads to smooth island borders with the initial island density n largely retained. At even higher TA (800 K) the evolution of Pt islands is dominated by an Ostwald ripening process, thereby reducing n and increasing the average island size. Under the chosen growth parameters the shape of Pt islands on Ru(0 0 0 1) at temperatures lower than TG ¼ 240 K can be described as a combination of fractals and dendrites. Well oriented trigonal islands reflecting the hexagonal symmetry of the substrate form between 240 and 300 K. In order to obtain a smooth Pt monolayer on Ru(0 0 0 1) TG has to be set to values higher than 600 K to ensure that all Pt adatoms exclusively condense at step edges and hence at the same adsorption site (hcp) to avoid growth mismatch. The surfaces of the grown Pt islands are smooth without any ripples or other types of defect. We therefore conclude that Pt grows pseudomorphically on Ru(0 0 0 1). Second layer Pt is not observed until hPt > 0:6 ML, when deposited at about 300 K, indicating a negligible barrier for this kind of islands with rough borders. For islands with smooth borders (grown at elevated temperatures), on the contrary, appreciable barriers for
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interlayer transport exist and second layer Pt islands are quite common.
Acknowledgements We thank A. Schlapka and D. Menzel for helpful and stimulating discussion. Financial support of the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 338 is gratefully acknowledged.
References [1] U. Hartmann (Ed.), Magnetic Multilayers and Giant Magnetoresistance: Fundamentals and Applications, vol. 37, Springer Series, 2000, ISBN 3-540-65568-9. [2] M.C. Bartelt, J.W. Evans, Phys. Rev. B 46 (1992) 12675. [3] J.W. Evans, M.C. Bartelt, Phys. Rev. B 63 (2001) 235408. [4] H. Brune, G.S. Bales, J. Jacobsen, C. Boragno, K. Kern, Phys. Rev. B 60 (1999) 5991. [5] K. Kyuno, G. Ehrlich, Phys. Rev. Lett. 84 (2000) 2658. [6] H. Brune, Surf. Sci. Rep. 31 (1998) 121. [7] S.C. Wang, G. Ehrlich, Phys. Rev. Lett. 67 (1991) 2509. [8] H. Brune, H. R€ oder, K. Bromann, K. Kern, J. Jacobsen, P. Stoltze, K. Jacobsen, J. Nørskov, Surf. Sci. 349 (1996) L115. [9] M. Hohage, M. Bott, M. Morgenstern, Z. Zhang, T. Michely, G. Comsa, Phys. Rev. Lett. 76 (1996) 2366. [10] G.S. Bales, Surf. Sci. 356 (1996) L439. [11] F. Buatier de Mongeot, M. Scherer, B. Gleich, E. Kopatzki, R.J. Behm, Surf. Sci. 411 (1998) 249. [12] M. Kalff, G. Comsa, T. Michely, Phys. Rev. Lett. 81 (1998) 1255. [13] K. Kyuno, G. Ehrlich, Phys. Rev. Lett. 81 (1998) 5592. [14] P.J. Feibelman, Phys. Rev. Lett. 81 (1998) 168. [15] E. Lundgren, B. Stanka, G. Leonardelli, M. Schmid, P. Varga, Phys. Rev. Lett. 82 (1999) 5068. [16] H. Brune, K. Bromann, H. R€ oder, K. Kern, J. Jacobsen, P. Stoltze, K. Jacobsen, J. Nørskov, Phys. Rev. B 52 (1995) R14380.
[17] C. Ratsch, A.P. Seitsonen, M. Scheffler, Phys. Rev. B 55 (1997) 6750. [18] T. Michely, RWTH Themen, Berichte aus der Rheinisch Westf€alischen Technischen Hochschule Aachen, 1/2001, 56–60, ISSN-NR. 0179-079X. [19] B.C. Stipe, M.A. Rezaei, W. Ho, S. Gao, M. Persson, B.I. Lundqvist, Phys. Rev. Lett. 78 (1997) 4410. [20] T. Zambelli, J. Wintterlin, J. Trost, G. Ertl, Science 273 (1996) 1688. [21] T.A. Witten, L.M. Sander, Phys. Rev. Lett. 47 (1981) 1400. [22] T.A. Witten, L.M. Sander, Phys. Rev. B 27 (1983) 5686. [23] Z. Zhang, X. Chen, M.G. Lagally, Phys. Rev. Lett. 73 (1994) 1829. [24] A. Schlapka, U. K€asberger, D. Menzel, P. Jakob, Surf. Sci. 502–503 (2002) 129. milauer, A. Zangwill, D.D. Vvedensky, [25] C. Ratsch, P. S Surf. Sci. 329 (1995) L599, and references herein. [26] K. Kyuno, A. G€ olzh€auser, G. Ehrlich, Surf. Sci. 397 (1998) 191. [27] K. Kyuno, G. Ehrlich, Surf. Sci. 437 (1999) 29. [28] P.J. Feibelman, T. Michely, Surf. Sci. 492 (2001) L723. [29] K. Kyuno, G. Ehrlich, Surf. Sci. 511 (2002) 459. [30] K. Kyuno, G. Ehrlich, Surf. Sci. 511 (2002) 461. [31] T. Michely, Ph.D. Thesis, Bonn (1991). [32] M. Lischka, A. Gross, private communications. [33] S. Papadia, B. Piveteau, D. Spanjaard, M.C. Desjonqueres, Phys. Rev. B 54 (1996) 14720. [34] S.C. Wang, G. Ehrlich, Phys. Rev. Lett. 62 (1989) 2297. [35] B. Piveteau, D. Spanjaard, M.C. Desjonqueres, Phys. Rev. B 46 (1992) 7121. [36] P.J. Feibelman, Phys. Rev. B 56 (1997) 10532. [37] P.J. Feibelman, Phys. Rev. B 59 (1999) 2327. [38] B. Gleich, M. Ruff, R.J. Behm, Surf. Sci. 386 (1997) 48. [39] J.A. Meyer, P. Schmid, R.J. Behm, Phys. Rev. Lett. 74 (1995) 3864. [40] R.Q. Hwang, C. G€ unther, J. Schr€ oder, S. G€ unther, E. Kopatzki, R.J. Behm, J. Vac. Sci. Technol. A 10 (4) (1992) 1970. [41] J. Schr€ oder, C. G€ unther, R.Q. Hwang, R.J. Behm, Ultramicroscopy 42–44 (1992) 475. [42] E. Kopatzki, H.-G. Keck, I.D. Baikie, J.A. Meyer, R.J. Behm, Surf. Sci. 345 (1996) L11.
Growth and thermal evolution of submonolayer Pt films on Ru(0 0 0 1) studied by STM U. K€ asberger *, P. Jakob Physik-Department E20, Technische Universit€at M€unchen, James-Franck-Straße, 85747 Garching, Germany Received 19 February 2003; accepted for publication 19 May 2003
Abstract The growth of submonolayer Pt on Ru(0 0 0 1) has been studied with scanning tunneling microscopy. We focus on the island evolution depending on Pt coverage hPt , growth temperature TG and post-growth annealing temperature TA . Dendritic trigonal Pt islands with atomically rough borders are observed at room temperature and moderate deposition rates of about 5 · 104 ML/s. Two types of orientation, rotated by 180 and strongly influenced by minute amounts of oxygen are observed which is ascribed to nucleation starting at either hcp or fcc hollow sites. The preference for fcc sites changes to hcp in the presence of about one percent of oxygen. At lower growth temperatures Pt islands show a more fractal shape. Generally, atomically rough island borders smooth down at elevated growth temperatures higher than 300 K, or equivalent annealing temperatures. Dendritic Pt islands, for example, transform into compact, almost hexagonal islands, indicating similar step energies of A- and B-type of steps. Depending on the Pt coverage the thermal evolution differs somewhat: While regular islands on Ru(0 0 0 1) are formed at low coverages, vacancy islands are observed close to completion of the Pt layer. 2003 Elsevier B.V. All rights reserved. Keywords: Growth; Nucleation; Scanning tunneling microscopy; Ruthenium; Platinum; Epitaxy; Low index single crystal surfaces; Metal–metal interfaces
1. Introduction Many modern electronic devices are based on nanometer-scale layer sequences prepared by homo- and heteroepitaxial growth. Examples are high mobility transistors and semiconductor laser structures. In the last decade there has been an
* Corresponding author. Tel.: +49-89-289-12821; fax: +4989-289-12338. E-mail address: [email protected] (U. K€ asberger).
increasing interest in metal-on-metal systems due to the observation of various magnetic effects like giant and colossal magneto resistance (GMR and CMR) and their enormous economical potential in device applications [1]. The properties of these semiconductor and metal structures critically rely on the quality of the interface of adjacent layers. A high level of control of processes such as segregation, intermixing and alloying is desirable and may be achieved by the detailed knowledge of basic physical growth processes. New insight in the growth process on a fundamental level is obtained by studying model
0039-6028/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0039-6028(03)00776-3
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systems. Recently, a large number of material systems have been investigated intensely. In this work, Pt on Ru(0 0 0 1) has been analyzed which combines a moderate lattice mismatch (2.5%) with interesting catalytic properties. The heteroepitaxial growth of Pt on Ru(0 0 0 1) thus is of basic interest within existing growth models and scaling laws [2– 4] and may, in addition, unveil unusual chemical activity. Our growth conditions are typical for nonequilibrium layer growth, i.e. net growth is obtained by supersaturation of the Ru surface with impinging Pt adatoms. Depending on the Pt adatom density and mobility, stable or slowly decaying nuclei eventually form. Nuclei stability thereby is associated with the higher coordination number of atoms within aggregates as compared to single atoms. Once formed, small nuclei are much less mobile than single atoms (for Pt/Pt(1 1 1) see Ref. [5]) and they act as further nucleation centers leading to island formation. This scenario is quite general and holds for homo- and heteroepitaxial metal-on-metal growth as reviewed by Brune [6]. In addition to nuclei formation on terraces, energetically favorable, highly coordinated sites may be found at step edges. In the so-called step-flow growth (which dominates if adatom mobility is sufficiently high for adatoms to reach the steps instead of forming stable nuclei on the flat terraces) adatoms primarily attach to step edges with islands on terraces being virtually absent. This quite basic growth picture needs to be detailed to explain the growth of Y -shaped dendritic islands, observed for many hexagonal surfaces at intermediate growth temperatures (see Section 4). A characteristic property of hcp crystal surfaces like Ru(0 0 0 1) is the occurrence of two different types of step edges, with one or two nearest neighbor atoms at the bottom of the step, which are called A- and B-steps, respectively [7]. These steps can either be formed by misorienting the Ru(0 0 0 1) surface or by creating islands onto the terraces. Due to the different local geometry of the A- and B-type step edges adatom diffusion along these steps differs substantially. This leads to a preferential growth along particular directions and hence to the formation of Y -shaped dendrites [8].
77
2. Experimental The STM measurements were performed in a vacuum chamber at a base pressure of 1 · 1010 mbar using a room temperature and a variable temperature STM operated at 180–300 K with a magnetically clamped Pt–Ir tip. Typical parameters for the STM operated in the constant current mode are a tunneling current ITunnel of about 1 nA at a bias voltage UBias of 0.2–2 V. Pt was evaporated by resistively heating a 100 lm thick foil (Goodfellow, purity >99.99%) mounted on a liquid nitrogen cooled holder. During Pt deposition the sample temperature could be set to 130–600 K (DT ±10 K measured with a Chromel/Alumel thermocouple). For growth temperatures TG of about 300 K the base pressure rises by about 5 · 1011 mbar; at TG 500 K it stays within the low 1010 mbar range. The Pt deposition rate may be chosen between 1 · 104 and 4 · 103 ML/s with high reproducibility. Gas exposures were performed using a needle doser (Ø ¼ 2 mm) mounted in front of the sample in the heating stage. Sample cleaning is achieved via standard procedures: Arþ ion-sputtering and oxygen cleaning cycles. The T -shaped Ru crystal has a size of 5 · 5 mm2 (front face) and is oriented in the (0 0 0 1) direction with an accuracy of ±0.5. It is clamped onto a Ta plate which can be transferred from the STM to the Pt source, a heating/cooling stage where it may be cooled to 150 K and heated up to 1600 K, and an additional heating/cooling stage in front of the sputter gun. During the annealing process the temperature could be measured with an infrared-radiation thermometer which was calibrated using a sample with a thermocouple attached. The reproducibility at T > 350 K is DT 6 1 K; systematic deviation of the true temperature is estimated to a few percent of the actual value. The desired annealing temperature TA was typically reached in about 2–3 min and kept fixed at TA for about another 2 min. All images of Figs. 1–3 were obtained after a preparation procedure with Arþ ion-sputtering, annealing, O2 dosing cycles and Pt deposition. Post-annealing of Pt-layers typically lasted 2–3 min; care has been taken not to overshoot the given temperature values during heat-up. All layers were newly prepared, i.e. no successive
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depositions have been applied, in order to avoid accumulation of contaminants from the residual gas during STM acquisition.
3. Results and interpretation The following subsections deal with the morphological and thermal evolution of submonolayer amounts of Pt deposited onto the clean hexagonal close-packed Ru(0 0 0 1) surface. We focus on the dependence of the deposited layers on directly accessible growth parameters such as Pt coverage hPt , growth temperature TG and annealing temperature TA and their immediate consequences illuminated by STM. 3.1. Increasing Pt coverage The STM images in Fig. 1 give an overview of the coverage dependent evolution of pseudomorphic Pt islands on Ru(0 0 0 1) in the range of hPt ¼ 0:10–0:90 ML. The growth temperature during Pt deposition was set to about room temperature (see figure caption for exact values), the deposition rate to R ¼ 5 104 ML/s. In the low coverage regime (hPt ¼ 0:10 ML), (a), well separated trigonal dendritic islands are observed, in accordance with the hexagonal substrate symmetry. A closer inspection of the dendrites in Fig. 1(a) yield average branch diameters dav of the order of . Subtracting the branch widening w 10 A 35 A caused by the tip apex [9] gives a corrected value , corresponding to about of dcorr ¼ dav 2w 15 A five atomic distances. The average branch length . In Fig. 1(b) a Pt ranges from about 90–150 A coverage of hPt ¼ 0:15 ML has been deposited leaving the branch dimensions almost unchanged. The surface island density is slightly higher than in (a): 1.5 · 1011 cm2 vs. 1.1 · 1011 cm2 . 1 Ap-
1
For narrow terrace widths below the mean free diffusion length of Pt adatoms on Ru(0 0 0 1), the formation of stable nuclei is unlikely and deposited Pt atoms predominantly attach to step edges, leaving the terrace almost free of islands (see lower part of image (c) of Fig. 1). These areas and their capture zone are excluded from the determination of the surface island density (see Section 4) [10].
parently, nuclei formation is not yet completed at hPt ¼ 0:1 ML and nucleation still takes place before Pt adatoms attach to existing islands. More so, this tendency continues in (c) with the island density reaching 1.5–2 · 1011 cm2 which is close to the value in (b) and may be the saturation island density. At hPt P 0:5 ML (d), additional Pt adatoms contribute exclusively to further growth of existing dendrites with the onset of partial coalescence. Here, the morphology of Pt islands published in [11] is reproduced. Due to additional Pt and edge diffusion processes the branch length and thickness have increased until hPt 0:6 ML, when coalescence of neighboring islands sets in. The Pt island boundaries on Ru(0 0 0 1) remain atomically rough even for hPt ¼ 0:9 ML. Besides a more compact shape with still atomically rough island boundaries, considerable second layer nucleation has set in for hPt > 0:6 ML (e). By then, the average area of the first layer islands has )2 . As a concurrent proincreased to about (200 A cess to nuclei formation mobile adatoms in the second layer of an island will search the island borders until a low energy barrier is found for a step down transition. For small islands, the rate of collision nc of adatoms with the island borders, i.e. the attempt frequency of these adatoms to overcome the island border barrier is high, so that the average adatom density nad and thus the second layer nuclei formation rate will be low. For large islands nc is low and, due to an enlarged nad , second layer nucleation becomes more and more favorable. This behavior would explain the absence of second layer islands for smaller islands in Fig. 1(a)– (d). For Pt/Pt(1 1 1), experimental [12,13] and theoretical [14] results indicate dissimilar energetic barriers of about 0.2 and 0.08 eV to jump across Aand B-type step edges, respectively. This tendency might hold for Pt/Ru(0 0 0 1) as well. In addition to adatoms jumping across step edges an exchange diffusion process as in the case of Co/Pt(1 1 1) [15] remains imaginable. We note that the pseudomorphic Pt islands on Ru(0 0 0 1) are compressed laterally by 2.5%. STM measurements and EMT investigations (effective medium theory) for Ag self-diffusion on strained and unstrained Ag(1 1 1) surfaces [16] as well as DFT calculations (density functional theory) for
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Fig. 1. STM images of the Pt/Ru(0 0 0 1) bimetallic layer showing the coverage dependent evolution in the submonolayer regime (1000 · 1000 A , UBias 0:5 V, ITunnel 1 nA). The Pt deposition rate is about R ¼ 5 104 ML/s. (a) hPt ¼ 0:10 ML (TG ¼ 295 K), (b) A hPt ¼ 0:15 ML (TG ¼ 297 K), (c) hPt ¼ 0:35 ML (TG ¼ 293 K), (d) hPt ¼ 0:50 ML (TG ¼ 292 K), (e) hPt ¼ 0:60 ML (TG ¼ 291 K), (f) hPt ¼ 0:90 ML (TG ¼ 275 K). Each layer has been prepared separately, starting from a clean Ru(0 0 0 1) surface (see Section 2).
Ag/Ag(1 1 1) clearly show that the mobility of adatoms on compressed layers increases significantly
[17]. As a consequence, collisions of second layer Pt atoms with island borders will occur more
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frequently so that second layer nucleation on top of small Pt islands in (a)–(d) becomes even less probable. Image (f) depicts the Ru surface covered with 0.90 ML Pt, more precisely 0.87 ML in the first and 0.03 ML in the second Pt layer. The substrate is still visible in between of the Pt islands. The second layer island morphology does not show any dendritic behavior which is in agreement with the expected enhanced adatom mobility on the Pt islands. In fact, second layer islands are compact and their location is predominantly in the inner part of the underlying islands. The shape of second layer islands thus is dramatically different from the fractal shape of Pt on Ru(0 0 0 1) as well as of Pt on Pt(1 1 1) grown under similar conditions [18]. The ABAB stacking sequence of the hcp Ru crystal results in an alternation of A- and B-type step edges for the (0 0 0 1) plane. Pt adatoms which do not form stable nuclei on the terraces will condense at these step edges which feature intrinsically higher coordination numbers. Once attached, growth proceeds from ascending steps. The orientation of the branches pointing away from step edges (oriented along the Æ 1 1 2æ directions of the Ru substrate) switches between perpendicular and ±30 to the step edges. The observed alteration of branch orientation is attributed to the same diffusion anisotropy as reported for Pt islands on the flat terraces (see above). These branches (in the following called fingers) can be clearly seen in Fig. 1(a)–(c) and they have been already observed for various systems [8,9]. It is assumed that deposited Pt atoms attach to Ru step edges the same way as deposited Ru atoms would do, i.e. by occupying hcp sites to continue the existing Ru single crystal substrate. Further information on the occupied lattice site of Pt islands on flat terraces can be extracted by correlating these fingers with the orientation of Pt dendrites on the flat terrace which is studied in the following section. 3.2. hcp and fcc hollow site occupation The direct determination of nucleation sites (hcp or fcc threefold hollow sites) of Pt dimers or small Pt islands on Ru(0 0 0 1) in their earliest state
using STM is quite difficult or even impossible, because of very similar signatures of both types of site in STM images. The lateral distance between fcc and hcp sites on the Ru(0 0 0 1) surface , which may be distinguished by amounts to 1.56 A adding a grid on the atomically resolved surface as has been done for the dissociation of molecular oxygen on Pt(1 1 1) [19]. Due to the close similarity of both hollow sites in STM images, an adsorbate with a clearcut preference for either fcc or hcp sites is additionally needed to fix the reference frame for the unknown species. In our case, an indirect method is used. As discussed in the previous section, the growth and evolution of Pt fingers at Ru step edges can be explained within the same model as for Pt islands on the Ru(0 0 0 1) terraces. The consequence of alternating A- and B-steps with local diffusion anisotropy [8] are most evident in Fig. 2(a) (see also Fig. 1). Assuming that Pt attaches to the Ru steps the same way as deposited Ru atoms would do, i.e. by continuation of the hexagonal lattice of the upper terrace, fingers perpendicular to step edges can be associated with A-steps, fingers diagonal to step edges with B-steps. This assignment is based on the well-established model of Brune et al. [8] which assumes dissimilar mobilities of adatoms along both types of step and across kinks within step edges. For Pt/Pt(1 1 1) this model has been corroborated by applying kinetic Monte Carlo simulations [8,9]. The fact that the pseudomorphic growth of Pt fingers at Ru step edges starts exclusively at hcp threefold hollow sites for A- as well as B-steps allows an identification of Pt sites for dendritic islands: Dendritic islands on terraces with an orientation of their branches identical to step edge fingers can be attributed to hcp nucleation sites (and hcp Pt islands); branches rotated by 60 represent fcc Pt islands. In Fig. 2(a) a low Pt deposition rate of R ¼ 3 104 ML/s is used to enhance attachment to step edges and to suppress nucleation on the terraces, so that large dendrites grow at the step edges. This way the alternating orientations of Pt fingers at A- and B-type of steps are clearly visible. Fig. 2(b) and (c) refer to ‘‘clean’’ and oxygencontaminated growth conditions, at about R ¼ 1:3 103 ML/s and about TG ¼ 300 K. The
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81
Fig. 2. Alternating orientations of Pt dendrites at A- and B-type of steps of the Ru(0 0 0 1) surface, (a), and dendritic Pt islands on clean, (b), and oxygen contaminated Ru(0 0 0 1), (c). Pt dendrites at steps are oriented perpendicular to the step edge (A-type of step) or diagonal (B step). The preferential fcc nucleation site of terrace Pt islands on the clean surface, (b), reverses to hcp in the presence of a · 2000 A , hPt ¼ 0:25 ML, TG ¼ 292 K, small amount of oxygen (c). Parameters of sample preparation and imaging: (a) 2000 A · 1000 A , hPt ¼ 0:18 ML, TG ¼ 305 K, R ¼ 1:8 103 ML/s, (c) 1000 A · 1000 A , hPt ¼ 0:18 ML, R ¼ 3 104 ML/s, (b) 1000 A 3 TG ¼ 300 K, R ¼ 1:3 10 ML/s. The different orientation of step edges in (b) and (c) compared to (a) results from a 90 rotated and newly clamped Ru sample.
dendrites attached to A- (or B-) type of steps crossing the STM images identify the hcp sites for the oriented Pt islands. Thus, on the widest terrace in (b) the two types of orientation can be identified and labeled as ‘‘h’’ (hcp) and ‘‘f ’’ (fcc), indicating their nucleation sites. Evaluation of many STM images with and without oxygen contaminations confirm a considerable preference of fcc nucleation on the ‘‘clean’’ Ru(0 0 0 1) surface (fcc:hcp 3:1) while hcp nucleation prevails (fcc:hcp 1:2) in the case of minute oxygen contaminations (hO 1% of a monolayer); if several percent of oxygen atoms are preadsorbed we observe a 100% ratio of hcp islands, oriented the same way as the dendrites growing at Ru step edges. Single oxygen atoms are mobile and show up as black dashes in enlarged STM images [20]. This small oxygen contaminations have no significant influence on the island density; such an effect is in fact observed for Pt growth on O(2 · 2) or O(2 · 1) precovered Ru(0 0 0 1) (not shown here). We conclude that nucleation of Pt on Ru(0 0 0 1) is very sensitive to adsorbates, in our case to small amounts of oxygen. Since the growth at the chosen TG and R takes place in the high supersaturation regime the question arises whether or not the system is in thermal equilibrium: We believe that diffusing Pt adatoms can in fact be considered as a lattice gas (with fcc and hcp hollow sites representing two unequal sites per unit cell) in thermal equilibrium,
at least between two collisions; occupation of fcc and hcp sites therefore should reflect the energetic differences of both binding geometries. 3.3. Increasing growth temperature TG Due to various thermally activated processes such as diffusion, migration, surface alloying or thermodynamic de- and attachment of island atoms, the island morphology is very sensitive to the growth temperature TG . Fig. 3 illustrates the dependence of Pt islands on TG for hPt ¼ 0:10 ML deposited at a rate of about 5 · 104 ML/s. For TG ¼ 191 K (a), the islands show no preferential growth direction, but a diffusion limited aggregation (DLA) character [21,22]. The low adatom mobility at low TG results in a relatively high surface island density of n ¼ 6:8 1011 cm2 with an average number N of about 230 Pt atoms per island. The average branch thickness amounts to , equivalent to 2–3 atomic distances, about 5 A confirming a reduced edge- and corner diffusion. The observed branching for Pt/Ru(0 0 0 1) as well as for Pt/Pt(1 1 1) (up to TG ¼ 300 K) [18] into random directions despite a non-zero edge mobility may be explained within the model developed by Zhang et al. [23]: Pt atoms arriving at an existing island remain mobile as long as their lateral coordination is lower than twofold; only then they become immobilized. In their simulations, fractal growth with arm thicknesses of about four atoms
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Fig. 3. Morphological change of Pt islands on Ru(0 0 0 1) with varying growth temperature TG at a fixed coverage of hPt ¼ 0:10 ML and deposition rate of about R ¼ 5 104 ML/s: (a) TG ¼ 191 K, (b) 242 K, (c) 295 K, (d) 408 K and (e) 517 K. Note the different · 500 A ) and (c)–(e) (1000 A · 1000 A ). image sizes for (a) and (b) (500 A
are found in an extended temperature range (compared to standard DLA growth, with an arm thickness of only a single atom). At TG ¼ 242 K, Fig. 3(b), the islands show three dominant growth directions in accordance with the hexagonal symmetry of the substrate (but there is still some DLA like growth). The branch thickness has slightly and N 580 atoms. The increased to about 10 A surface island density has decreased to n ¼ 2:7 1011 cm2 . At TG ¼ 295 K, (c), n ¼ 1:1 1011 cm2 , while N amounts to about 1400 atoms with . Up to TG 300 K a mean branch width of 15 A the island boundaries remained atomically rough. At higher temperatures there is a gradual transition towards smooth island borders. This effect can be seen by comparing images (3c) and (3d). Infrared absorption spectroscopy measurements of CO adsorbed at various locations, e.g. Pt step edges and on the Ru(0 0 0 1) surface [24] indicate a transition at about 340 K: CO which preferentially binds to (rough) Pt island step edges migrates to Ru(0 0 0 1) terraces as the temperature exceeds 340 K. This behavior has tentatively been attributed to atomically rough Pt island borders becoming smooth upon annealing, which is confirmed by our
data. It seems to hold for the variation of the growth temperature, too. At 408 K, (d), edge- and corner diffusion of Pt atoms are sufficiently fast, leading to perfectly oriented dendrites (n 0.2 · 1011 cm2 ) with a branch thickness of about 70 A and N 8000 atoms. Since dendrites may reach enormous sizes, the evaluation of their surface density requires special attention to step edges and dendrites originating from such surface defects, i.e. step edge fingers. Dendritic islands due to a homogeneous nucleation process only form far from step edges on sufficiently large terraces. The total image area useful to analyze nucleation on terraces therefore is lowered by the amount covered by step edge fingers. The growth conditions at TG ¼ 517 K, (e), are such as to prevent island formation on the flat terraces; instead, all of the deposited Pt atoms have condensed at step edges of the Ru substrate (one might still create Pt islands at this temperature by increasing the incident Pt flux). We note that due to the similar sizes of Pt and Ru atoms, surface areas containing either of these species could not be distinguished; the preferential deco-
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Fig. 4. Growth temperature dependence of the surface island density n (a) and the average number N of Pt atoms per island (b). A constant coverage of hPt ¼ 0:10 ML and a constant deposition rate of about 5 · 104 ML/s has been chosen.
ration of Ru with oxygen atoms, however, allows such a distinction later on. Furthermore, under high temperature growth conditions with enhanced overall mobility of Pt atoms the oriented dendrites have disappeared and a distinction between A- and B-type step edges is no longer possible. Fig. 4 depicts the surface island density n and the average number N of Pt atoms per island, 2 derived from Fig. 3 and additional images of these layers. On a linear scale n approximately follows an exponential law within the range of TG ¼ 191–517 K (a). In the low temperature regime (high supersaturation) the balance between adatom mobility (and thus of the attachment of adatoms to step sites and other surface defects) and deposition rate is such that attachment to surface structural defects is ineffective and a relatively high density of adatoms (nad ) prevails; consequently, an increasing number of nuclei is
2 The average number N of Pt atoms per island can be expressed as N ¼ hPt N0 =n, where N0 ¼ 1:587 1015 cm2 denotes the number of adsorption sites per unit area on Ru(0 0 0 1) and n the Pt surface island density.
83
formed. This situation shifts towards the thermal equilibrium for increasing TG : A larger mobility is related to a more probable attachment of Pt adatoms to steps, thereby reducing nad and thus the island formation rate. Moreover, step edges and islands accumulate Pt atoms from wider capture zones. For those reasons and under the chosen growth parameters, n decreases from 6.8 · 1011 cm2 at TG ¼ 191 K to 0.2 · 1011 cm2 at TG ¼ 408 K. Further island formation at TG > 500 K is limited by our average terrace length of about 400 . To depict the value at TG > 500 K in the graph, A a linear scale is preferred (a) instead of the equivalent logarithmic one as in (b). On a perfect surface (no steps) extrapolation of the values at TG < 408 K to TG ¼ 517 K would give about 5 · 109 cm2 . In the standard nucleation theory the relation v between n and R follows a power law n / ðD=RÞ . D denotes the diffusion constant and v corresponds to i=ði þ 2Þ where i is the critical cluster size (for details see [6] and references herein). Evaluation of double-logarithmic plots of the surface island density vs. R for hPt ¼ 0:2 ML at room temperature (not shown) give evidence that i 3 (v ¼ 0:57 0:08), i.e. trimers or maybe tetramers represent stable units. 3 The behavior of cluster dissociation and cluster diffusion in the case of Pt/ Pt(1 1 1) is still under intensive discussion [26–30]; a more refined description of processes affecting the island density which will evolve out of this dispute probably will be of relevance for our bimetallic system as well. 3.4. Postgrowth thermal evolution In the preceding paragraph we have shown that variation of TG primarily influences the adatom mobility and density nad . The latter quantity results from the competition between the flux of impinging Pt atoms and their attachment to existing structures (steps, small nuclei, islands). In
3
The stable nuclei is an idealization since detachment of island atoms at different TG have to be considered under real growth conditions (see Ref. [25]).
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annealing processes after deposition at low TG , the detachment from islands and the readsorption to existing structures represent the dominant mechanism of island evolution. This way the gradual approach of kinetically limited surface structures formed during growth under high supersaturation conditions towards thermal equilibrium is pursued. The STM images in Fig. 5 arranged in a matrix display the thermal evolution of three layers presented in Fig. 1. A maximum annealing temperature TA ¼ 800 K has been chosen in order to prevent surface alloying [11] which starts at about 900 K. In column (A) the initial dendritic island shape collapses into large hexagonal compact Pt islands for TA ! 600 K. Because of the decreasing island density (note the different image
size of the annealed layers), we deduce a significant detachment of Pt atoms from dendrites and readsorption to step edges of the Ru substrate or to remaining larger islands. Apparently, diffusion along island edges is now sufficiently fast to allow occupation of the energetically most favorable adsorption sites, thereby forming islands with a hexagonal shape. Further annealing to 800 K reduces the island density once again and the Ostwald ripening process, barely noticeable at 600 K, becomes more prominent: large islands survive at the expense of smaller ones. We note that the applied annealing time in Fig. 5(A) probably was too short as some of the shrinking islands are still present along with the enlarged islands.
Fig. 5. Array of STM images illustrating the thermal evolution of submonolayer Pt on Ru(0 0 0 1). The various coverages (deposited at about room temperature) were: (A) hPt ¼ 0:15 ML, (B) 0.60 ML and (C) 0.90 ML. Note, that the image size of annealed layers is 2000 · 2000 A while it is 1000 A · 1000 A for the layers as grown. Typical tunneling parameters were UBias 0:5 V and ITunnel 1 nA. A
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The situation in column (C) is different: The high coverage of hPt ¼ 0:90 ML leaves only a small part of the Ru substrate uncovered. Rearrangement of Pt adatoms at elevated TA then produces vacancy islands, i.e. holes within the mono-atomic Pt layer on the Ru substrate. Minimization of borders seems to be much less of an issue for holes as compared to islands of similar size. More precisely the larger vacancies are somewhat irregularly shaped as compared to the regular islands which display an almost hexagonal shape for TA ! 600 K. Second layer islands still exist at 600 K, but they vanish for TA ! 800 K. Vacancy islands also show Ostwald ripening and a decreasing density analogous to regular islands. On Pt(1 1 1) a similar morphology can be produced by Arþ ionbombardment with subsequent annealing to 760 K [31]. However, to study the properties of Pt vacancies on Ru(0 0 0 1), high coverage Pt deposition with subsequent annealing is preferred to the destructive ion bombardment preparation to avoid Pt incorporation into the Ru substrate during the sputter process. The central column (B) can be considered as a transition regime (hPt ¼ 0:60 ML) and it contains characteristic features of both cases mentioned above. For TA ! 600 K, some of the larger islands or Pt islands attached to step edges of the Ru(0 0 0 1) surface contain small vacancies in their inner areas. The vacancy density after annealing is highest at step edges. Obviously, the morphology of step fingers grown at room temperature favors the formation of small pockets within the Pt film as the temperature is increased. During the annealing process detachment and readsorption takes place leading to bigger islands. Most of them do not merge even for TA ! 800 K indicating a repulsive interaction. This repulsion might be due to a strain field surrounding the Pt islands on Ru(0 0 0 1), or due to the fact that neighboring fcc and hcp islands do not easily merge.
4. Discussion Quite naturally, the occupation of fcc and hcp sites of Ru(0 0 0 1) by Pt atoms is expected to differ energetically; theoretical calculations [32] reveal a
85
slight preference of 20 meV per site for hcp site occupation when a full monolayer of Pt on Ru(0 0 0 1) is considered. This finding contradicts our observations of a preferential fcc occupation in the absence of oxygen contaminants. At low hPt , however, when nucleation of Pt islands sets in, the energetics and entropy factors may differ to some extent modifying the kinetics of nuclei formation and their preferred hollow site [33]. In fact, for Ir/ Ir(1 1 1) it has been experimentally observed that single Ir atoms preferentially adsorb on hcp sites changing to fcc sites with lateral coordination of those Ir atoms [34]. Recent tight-binding calculations suggest that d-band filling plays a decisive role for the understanding of the site preference (i.e. adsorption in the ‘‘fault’’ or ‘‘correct’’ site with respect to the crystal stacking sequence) of transition metal atoms in epitaxy [33,35]. In particular, single Pt adatoms on Ru(0 0 0 1) were predicted to occupy fcc sites, which agrees nicely with the found 3:1 preference of fcc-type islands in our experiments. Of course, effects associated with lateral coordination during growth of small Pt nuclei (such as observed for Ir on Ir(1 1 1) [34]) may have additionally affected the site preference of our Pt islands. Nevertheless we find the agreement pleasing. Our results also indicate that epitaxy in the presence of adsorbates leads to a more complex situation. Even small amounts of oxygen are found to change the relative stability of small fcc and hcp Pt islands leading to a reorientation of the triangular dendritic islands. Since this effect occurs already for hO 1% of a monolayer we conclude that oxygen is located preferentially at the islandÕs rim rather than being distributed randomly on the surface. Theoretical as well as experimental data suggest that oxygen atoms prefer sites located at either type of steps of Pt(1 1 1) [35]; for Ru(0 0 0 1) only A-type steps are decorated at low hO [20]. The question which type of hollow site is preferred by step edge oxygen atoms for the Pt/Ru bimetallic surface must remain open, since there is no easy way to decide this matter [36,37]. For Pt/Pt(1 1 1), a similar case of reversal of Pt island orientation has been reported in the presence of CO background partial pressures of 109 mbar at TG ¼ 400 K. It has been attributed to a
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Table 1 Island morphology and saturation surface island density n reported for Co, Ni, Pt, Al and Au on Ru(0 0 0 1) at room temperature and moderate deposition rates R hAd (ML)
Lattice mismatch (%)
hNi ¼ 0:05 hCo ¼ 0:2 hPt ¼ 0:25 hAl ¼ 0:12 hAu ¼ 0:3
)7.9 )7.4 +2.5 +5.8 +6.6
Island shape
R (ML/s) 3
5 · 10 2.5 · 103 1.3 · 103 5 · 104 3.3 · 10-3
Hexagonal Triangular Dendritic Compact Fractal
modification of A- and B-type step free energies by CO molecules and associated changes of the mobilities of Pt adatoms along the island steps [12]. Contrary to our bimetallic system, identical nucleation sites for all Pt islands and uniform island orientations exist on Pt(1 1 1). For Pt grown on Ru(0 0 0 1) the orientation of step edge fingers serves as a reference for Pt occupying hcp sites, for the clean as well as the oxygen precovered Ru(0 0 0 1) surface. Since the appearance of step edge fingers is not altered by small amounts of oxygen a different diffusion process along the step edges (as invoked for Pt homoepitaxy) appears unlikely. The oxygen induced reorientation of dendritic Pt islands reported here is therefore attributed to a change of the preferred Pt adsorption site from fcc to hcp. The stabilization of Pt in hcp sites probably is induced by oxygen atoms located at the edges of small Pt nuclei; upon further Pt deposition growth will proceed from these nuclei to form larger islands with oriented dendrites, while maintaining their hcp sites. Dendritic pattern formation is a familiar phenomenon on hexagonal surfaces for a variety of homo- and heteroepitaxial material systems within a specific temperature range: for example Ag/ Ag(1 1 1) (110 K) [8], Pt/Pt(1 1 1) (245 K) [9] and Ag/Pt(1 1 1) (130 K) [8] or Au/Pd(1 1 1) (300 K) [38]. A complex balancing mechanism between edge- and corner diffusion via hopping and/or exchange processes, corner crossing and energetically favorable step edge formation determines the final shape at a given temperature [6]. Either fractals, dendrites, triangles or even compact islands may form. This also holds for the various metals (Ni, Co, Au, Al [39–42] on Ru(0 0 0 1): for details see
n (cm2 )
Ref.
5 · 1010 3 · 1010 43 · 1010 46 · 1010 0.03 · 1010
[39] [40] This work [42] [41]
Table 1). The predominant dendritic growth observed for Pt on Ru(0 0 0 1) at room temperature and moderate deposition rate fits nicely into the diversity of island shapes already observed on this substrate material. By comparing reported island densities (room temperature) of Ni/Ru(0 0 0 1) [39], Co/Ru(0 0 0 1) and especially of Au/Ru(0 0 0 1) [40] with our findings of Pt on Ru(0 0 0 1) we noticed significant differences by up to three orders of magnitude. For these materials, Table 1 summarizes the island morphologies and surface island densities on Ru(0 0 0 1). For better comparison, a larger Pt deposition rate (R ¼ 1:3 103 ML/s), closer to those given in Table 1, has been chosen (note that n 1 1011 cm2 at R ¼ 5 104 ML/s in Fig. 1). Key factors determining the island density of deposited metal atoms are the incident flux U and the adatom mobility. 4 Both factors counteract each other: A high U enhances the adatom density nad , leading to accelerated nuclei formation; a high mobility of the adatoms, on the other hand, favors attachment to steps and defects, thereby reducing nad . With increasing nad during film growth a system therefore changes from step-flow mode to the nucleation regime. Eventually, the island density has reached a (saturation) value which makes at-
4
In addition to growth temperature and incident flux, a weak dependence of the island density on the total coverage is observed as well. In Fig. 1 the surface island density is found to increase with Pt coverage until the saturation density is reached. At higher hPt , the islands start merging and a determination of the island density from STM images is no longer possible. At very low hPt formation of Pt islands might be avoided altogether, e.g. if nad does not reach the critical value nc required for stable nuclei formation.
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tachment to existing islands more probable than nuclei formation, i.e. the nucleation regime transforms into the island growth regime. For deposited Co, Ni and, in particular, in the case of Au, the low island density at room temperature indicates highly mobile adatoms on the Ru(0 0 0 1) surface. While for the former elements the high adatom diffusivity combines with a likewise substantial mobility along island borders, the fractal growth behavior observed for Au [40,41] seems to indicate that gold atoms behave differently. However, they too retain an enormous mobility along island borders giving rise to wide . Pt and Al on Ru(0 0 0 1) branches of about 200 A seem to behave the opposite way: Diffusivities of these adatoms are rather low, leading to a very high saturation island density (even though the deposition rates are low compared to the other systems reported in Table 1). According to the observed dendritic island shape the mobility of attached Pt atoms is moderate; some limitations exist with respect to corner diffusion as well as corner crossing. In particular this latter effect leads to dissimilar orientations of dendrites attached to either A- or B-type of steps. As mentioned in the previous section adjacent Pt islands do not easily merge at elevated hPt . Two reasons may explain this observation: First, nucleation of Pt islands occur at two different possible adsorption sites, hcp and fcc hollow sites which yields Pt dendrites rotated by 180 (see Fig. 1(a) and (b), as well as Fig. 2(b)). Due to the lateral displacement of hcp and fcc sites there will always exist a small gap between both types of island. Second, the number of impinging adatoms onto the bare Ru(0 0 0 1) surface contributing to the narrowing of the gap by condensing at the island borders decreases as the gap closes. Therefore, the growth speed of opposite protrusions or island borders is progressively reduced. This effect is attenuated by the two processes: (i) second layer adatoms may descend the atomically rough island step edges and (ii) diffusion along island edges. We note that interlayer transport, case (i), becomes less probable as straight step edges evolve at elevated temperatures. We deduce this argument from the annealed layer in Fig. 5, column (C), (hPt ¼ 0:90 ML and TA ¼ 600 K) where second
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layer islands of narrow size distribution have survived because of the formation of straight vacancy island edges. 4.1. Summary The growth of Pt on Ru(0 0 0 1) in the submonolayer regime has been studied using scanning tunneling microscopy. The growth at room temperature leads to the formation of triangular, dendritic islands. Occupation of either hcp or fcc sites by the Pt atoms is held responsible for the occurrence of two type of island orientations. Higher growth temperatures lead to an increase in island size, while the island density is reduced accordingly. In parallel, dendrites attached to step edges of the Ru substrate become more common and island formation on terraces (by means of nucleation) less likely. Eventually, at T > 500 K, a step-flow growth of Pt on Ru(0 0 0 1) is observed (for an incident flux of about 8 · 1011 s1 cm2 ). Annealing of Pt layers grown at room temperature to about 600 K leads to smooth island borders with the initial island density n largely retained. At even higher TA (800 K) the evolution of Pt islands is dominated by an Ostwald ripening process, thereby reducing n and increasing the average island size. Under the chosen growth parameters the shape of Pt islands on Ru(0 0 0 1) at temperatures lower than TG ¼ 240 K can be described as a combination of fractals and dendrites. Well oriented trigonal islands reflecting the hexagonal symmetry of the substrate form between 240 and 300 K. In order to obtain a smooth Pt monolayer on Ru(0 0 0 1) TG has to be set to values higher than 600 K to ensure that all Pt adatoms exclusively condense at step edges and hence at the same adsorption site (hcp) to avoid growth mismatch. The surfaces of the grown Pt islands are smooth without any ripples or other types of defect. We therefore conclude that Pt grows pseudomorphically on Ru(0 0 0 1). Second layer Pt is not observed until hPt > 0:6 ML, when deposited at about 300 K, indicating a negligible barrier for this kind of islands with rough borders. For islands with smooth borders (grown at elevated temperatures), on the contrary, appreciable barriers for
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interlayer transport exist and second layer Pt islands are quite common.
Acknowledgements We thank A. Schlapka and D. Menzel for helpful and stimulating discussion. Financial support of the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 338 is gratefully acknowledged.
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