Morphological Transition of Epitaxial Rhodium (111)
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-0.1 V. Continuous RHEED observations confirm that there is no change in the surface morphology during cooling. RESULTS One of the interesting findings of this study is that the surface morphology of Rh (11) films deposited at low temperature exhibits rapid transitions during in-situ UHV annealing. We monitored the behavior by RT-RHEED. For a 20 ML film deposited at 300 K, the RHEED patterns are broad and spotty indicating a rough 3D surface, as shown in Fig. I (a). As temperature increases, the transverse width of the RHEED specular reflection exhibits sudden drops at - 600 K and - 1000 K, as shown in Fig. 1. Accordingly the diffraction patterns change qualitatively at those temperatures, as shown in the insets of Fig. 1.Below 600 K the RHEED patterns remain unchanged from those of as-deposited. They become sharp streaks above 600 K [Fig. I (b)] corresponding to a smoothening transition to a 2D surface. The streaks become much sharper and shorter above 1000 K [Fig. 1 (c)]. The latter width change corresponds to an increase in terrace length from hundreds of A to several thousands of A. We therefore employ annealing at above 1000 K for the subsequent nucleation and growth studies. The focus of this report is the morphological transition near 600 K. The growth temperature dependent morphological transition of Rh (I ll) is demonstrated in Fig. 2 by the STM images of a series of 10.1 ML films grown at different temperatures. Several important features are immediately apparent: (1) the surface features show a characteristic length scale typified by mound-like islands with well-defined size and separation; (2) the shape of the features exhibits a transition near 600 K from compact at high growth temperatures [Figs. 2 (d-f)] where the step-edges are smooth, to fingered at low temperatures [Figs. 2 (a-c)] where the stepedges have "kinks"; and (3) the height of the features has a minimum near the transition, where layer-by-layer growth is observed. The presence of a layer-by-layer growth regime between 500 and 700 K is supported by persistent RHEED oscillations, as shown in Fig. 3. These observations reinforce the view that surface features are generally not self-affine when diffusion is sufficiently high and that the morphology of layer-by-layer growth is a special case simply because the coherent nature of the process predicates well defined length scales. 50
Fig. 1. Temperature dependence of the transverse RHEED width along the [110] azimuth during annealing for 20 ML Rh grown at 300 K. The heating rate was about 15 K/min. mark the morphological transitions discussed in the text. Inset: characteristic RHEED patterns along S20 the [110] azimuth at three temperatures (a) 300 K, (b) 700 K, and (c) 1100 K.
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T (K) Quantitative analysis of the STM images reveals that the surfaces exhibit sharp ring-like Fourier components in the growth plane and long ranged oscillations in the height-height correlation functions as well, thus confirming the presence of non-se
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