STM-induced formation of Ag islands on Ag(111)

Abstract

The formation of triangular-shaped adatom islands and vacancy islands on Ag(111) at room temperature is observed in a local growth experiment. The tip of a scanning tunneling microscope (STM) vibrating normal to the surface leads to a local modification. Atoms are removed from flat terraces and renucleate as adatom islands. Simultaneously with the modification process the surface is imaged with the STM. In the case of triangular islands only one out of two possible types of close-packed 〈110〉 step edges is found in the STM images. This observation is explained by differences in the step formation energy, rather than by diffusion. The homoepitaxial growth of metal atoms on (111) surfaces has been studied from various aspects, both experimentally and theoretically. Besides the fundamental understanding of the involved atomic processes the main interest stems from the technologically important control of layer-by-layer growth. Layers are usually grown by molecular beam deposition techniques (MBE) in which material from the gas phase nucleates on the substrate. The temperature dependence of island growth has been investigated for the deposition of Pt on Pt(111) by Michely et al. [1]. The authors found a rich variation of growth modes, from fractal growth for low temperatures to various types of triangular and hexagonal island shapes at different elevated temperatures. For coverages exceeding one monolayer Kunkel et al. [2] reported a layer-by-layer growth regime for T> 600 K. In the case of Ag/Ag(111), however, no layer-bylayer growth was observed in the temperature range between 175 K and 575 K [3]. However, a smooth layer growth can be generated either by precovering the surface with a submonolayer of Sb [3] or by an artificially high island density, produced for example by short sputter pulses [4]. Theoretical work to elucidate growth processes on stepped (111) surfaces has investigated the step formation energies and diffusion along or perpendicular to steps. Stumpf and Scheffler [5] have performed density-functional calculations elucidating the formation energy of densely packed steps on Al(111). The calculations show a difference of 0.016 eV for the two different types of step along the 〈110〉 directions (Fig. 1). Island morphologies are often explained by kinetic processes. Interlayer mass transport on Ag(111) has been studied in detail by Li and DePristo [6], revealing a strong anisotropy between the two different types of densely packed step edges. Recently the formation of vacancy islands on Ag(111) has been reported by applying a higher tunneling current (gap resistance R = 0.1 M) [7]. In that experiment the STM tip was fixed at a certain position for 60 s and induced islands were observed by subsequent imaging. Here we report the growth of Ag islands on a flat Ag surface initiated by an STM tip vibrating normal to the surface. The STM is used to initiate the atom transport process as well as simultaneously imaging the surface. Ag atoms are removed from the surface and nucleate again along crystalline axes. No additional Ag atoms are added from the gas phase. All of the redistribution processes and the imaging occur at the Fig. 1. On the (111) surface of an f.c.c. packed lattice two non-equivalent types of step edges are possible, forming either {100} facets (type A) or {111} facets (type B). Vacancy islands are rotated by 180◦ relative to the adatom islands within the same type of steps