Using scanning tunneling microscopy and thermal He diAraction we have studied the morphology of the vicinal Pt(997) surface, close to the hexagonal close-packed (111)face. While oxygen adsorption induces a step-doubling transition, the clean surface is thermally unstable towards faceting. Above T = 0.5T~the surface is found to undergo partial phase separation into large {II I i facets and regions of undisturbed step regions. The faceted phase is stabilized by a reduction of surface stress through reconstruction of the (111)faces. The faceting is found to proceed via a nucleation-and-growth mechanism.The structure of vicinal surfaces, i.e. , regularly stepped surfaces which are generated by a slight miscut with respect to a low-index plane, has been the subject of many recent studies [1][2][3][4]. They constitute ideal model systems to study the relation between surface structure and surface energy. Nominally, vicinal surfaces are composed of terraces of the low-index orientation separated by a superlattice of parallel steps accommodating the misorientation 8. The total surface energy y(8, T) of the vicinal is given byThe first term yo is the surface energy of the low-index terraces while the second term adds the contribution from each of the monatomic steps, with P being the energy per unit length to form an isolated monatomic step of height h. The last term B accounts for interactions between neighboring steps and aII is the unit vector along the step edge. The nominal structure is only stable if the vicinal orientation is a tangent in the Wulff plot (y versus 8) in polar form [5]. Unstable orientations phase separate into a "hill and valley structure" of coexisting regions of stable low-index orientations (facets).Because of the delicate balance between step and terrace energies also stable vicinals can be subject to morphological changes as a function of temperature or impurity concentration. The ordered superlattice of equally spaced steps is stabilized by the repulsive step-step interaction between neighboring steps. At elevated temperatures thermal disorder through excitation of kinks competes with the order established by the repulsive step-step interaction. This thermal kink proliferation eventually results in a roughening transition of the vicinal surface [6]. Changes in temperature might also modify the terrace and step-free energies, and thereby induce an orientational instability. So far only three examples of thermal faceting of clean surfaces are known: vicinal Si surfaces close to (111) and vicinal Pt surfaces close to (100) where the orientational instability is induced by reconstruction [2,7] and vicinal Pb surfaces close to (111) where faceting is driven by surface melting [8]. More frequently observed are faceting transitions induced by impurity adsorption [9]. In this Letter we report measurements of temperature and impurity induced changes in the surface morphology of the vicinal Pt(997) surface. Nominally this vicinal surface is composed of close-packed (111) terraces separated each 20.2 A b...

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We report a quantitative atomic scale study of nucleation kinetics on an inhomogeneous substrate. Our model system, Al͞Au͑111͒-͑ p 3 3 22͒, reveals a distinct nucleation transition due to the repulsive nature of surface dislocations. Whereas for T , 200 K Al adatoms are confined to quasipseudomorphic stacking areas experiencing a very small diffusion barrier ͑30 6 5 meV͒, at T . 200 K surface dislocations, representing repulsive barriers of DE ഠ 560 meV, can be surmounted. The results illustrate the significance of surface dislocations as repulsive line defects in nucleation and growth.[ S0031-9007(99)

We report a novel mechanism, internal (111) faceting, of strain relief at heterointerfaces with square symmetry. The mechanism has been revealed for thin Cu films on Ni(100) by scanning tunneling microscopy. In the first monolayer monatomic chains of Cu atoms are shifting laterally by 1͞ p 8 lattice constant along ͗110͘ and thereby protrude from the surface layer. With each Cu layer added, the protrusion stripes grow in width by one atom, forming internal ͕111͖ facets in the Cu film. This picture is in marked contrast to the widely accepted continuum theory of epitaxial growth, which predicts a pseudomorphic film growth up to a critical thickness of 8 monolayers.

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