Island decay on the anisotropic Ag(110) surface
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Island decay on the anisotropic Ag(110) surface Karina Morgenstern1,2, Erik Lægsgaard1, Flemming Besenbacher1 1 Institute of Physics and Astronomy and CAMP, University of Aarhus, DK - 8000 Aarhus C, Denmark 2 Institut für Experimentalphysik, FU Berlin, D - 14195 Berlin, Germany ABSTRACT We have investigated the decay of two-dimensional islands on the anisotropic Ag(110) surface using variable-temperature scanning tunneling microscopy. Contrary to predictions from traditional Ostwald ripening theory, a quasi-one-dimensional decay mode is observed at low temperatures (175-220 K). A surprisingly sharp transition to the quasi-two-dimensional decay mode is observed around 220 K. This transition is accompanied by a fast equilibration of the island shape. These findings have tentatively been rationalized within a simple model to identify the underlying rate limiting atomistic processes.
INTRODUCTION In recent years the development of fast scanning tunneling microscopes (STM) has triggered the investigation of changes in surface morphologies that were deliberately created far from equilibrium [1-10], as well as thermal fluctuation around equilibrium-shaped structures [11,12] in high temporal resolution. Model studies of the decay of islands, i.e., structures of monatomic height consisting of up to several thousand atoms, on metal surfaces [1-10] allow one to predict the limits of stability of atomic systems using a common theory for both metals and semiconductors which are the main components of microtechnology nowadays. In this paper we discuss fundamental laws of the stability of nanostructures on an anisotropic surface, thereby summarizing earlier publications on this subject [13]. Studies of the decay of quasi-isotropic islands have led to an improved understanding of the physical properties and processes involved in their decay. It has been shown that a common theory based on the classic Ostwald ripening theory [14] can be used for island decay on metal and semiconductor surfaces [2-4]. The importance of short-ranged island interaction for Ostwald ripening has been pointed out (Si(100) [2,3], Ag(111) [10]). Based on this knowledge, energetic barriers have been measured (Si(111) [7], Ag(111) [9]). Finally, the dominating coarsening mechanisms for different coverage (Ag(100) [5]) and the rate limiting step for ripening between islands on the same terrace (Cu(100) [6]) and on top of each other (Cu(111) [8]) have been determined. The majority of these studies were carried out on isotropic surfaces, whereas the material of choice of the semiconductor industry, Si(100), has a higher complexity, being anisotropic due to dimerization. The relevant processes on anisotropic surfaces are however less well understood, because the only studies on this surface [2,3,7] were performed at temperatures well above the predicted crossover temperature from isotropic to anisotropic island growth [15]. Indeed, the anisotropy does not influence the Ostwald ripening in the temperature range investigated [3]. To elucidate the effects of anisot
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