Island Diffusion and Coarsening on Metal (100) Surfaces

Abstract

The diffusion and coarsening of two-dimensional homoepitaxial islands on Cu(100) and Ag(100) surfaces have been studied at room temperature with time-sequenced scanning tunneling microscopy. Quantitative analyses of the dependence of island diffusion coefficient D vs the island side length L, D~L 2a , yield noninteger scaling exponents which are consistent with island coalescence. Moreover, the near absence of island decay shows that the island diffusion occurs via mass transport along the island periphery. [S0031-9007(97)04302-0] PACS numbers: 68.35.Fx, 61.16.Ch, 61.46. + w, 82.65.Dp Diffusion of two-dimensional (2D) epitaxial islands on a substrate is an important subject in surface science and thin-film growth. Information about the elementary atomistic processes involved in thin-film growth may be extracted by simply studying the macroscopic island migration. A number of studies [1][2][3][4][5] have aimed at establishing a general scaling relation, D~L 2a , between the diffusion coefficient D of a large island and the island side length L. Three dominant mechanisms have been proposed. If the island motion is caused by evaporation and condensation events as the island exchanges atoms randomly with the 2D (or 3D) gas (EC mechanism), then a 1. If the evaporation and condensation events are correlated, or equivalently, if the island motion is terracediffusion limited (TD mechanism), then a 2. If the island moves as a result of atom diffusion along the island periphery (PD mechanism), then a 3. These models predicting integer exponents describe, however, highly simplified or limiting cases. In fact, noninteger scaling behaviors or size-dependent scaling exponents have been observed in several Monte Carlo simulations [6-8] and detailed theoretical treatments [5,9].Recently, the diffusion and coarsening of very large 2D islands (containing hundreds to thousands of atoms or vacancies) have been investigated with scanning tunneling microscopy (STM) [3,4,[10][11][12], with quantitative measurements reported for Ag(100) [3,11] and Ag(111) [4,12]. For vacancy island diffusion on Ag(111), a value of a ഠ 2 was found and the TD mechanism was suggested to be dominant [4]. Nevertheless, as reported earlier [13], the evaporation rate of atoms from such island edges is nearly 3 orders of magnitude too small to account for the observed island diffusion coefficient. For adatom islands on Ag(100), a very weak (if any) dependence, a ഠ 1, was found, and the EC mechanism was suggested to be dominant [3]. The near integer exponents in both studies seem to support the simplified TD and EC models, and both experiments downplay possible contributions from the PD mechanism. This is questionable, because periphery diffusion is expected to be a faster atomistic process than evaporation of atoms [14].In this Letter, we present the first known example where the periphery diffusion is, in fact, the main mechanism for island diffusion. We carry out a combined study of island diffusion, island coalescence, and island decay for two model syste...

Activation energy for the decay of two-dimensional islands on Cu(100)

“…We therefore study the decay of small islands in an environment of large ones, a situation that differs from the work of Pai et al 9 The images were recorded using a tunneling current of 1.0 nA with the tip biased negatively by 0.10-…”

“…For the Cu͑100͒ and the Ag͑100͒ surface this random-walk-induced coalescence can be the prevailing mechanism in the coarsening process. [8][9][10] Until recently it was tacitly assumed that atom exchange between islands and steps on metal surfaces is mediated via adatoms on the terraces as the diffusing species. In a recent study of Ostwald ripening on Cu͑100͒ it was proposed that single atom vacancies rather than adatoms are responsible for the mass transport between the islands.…”

“…We therefore study the decay of small islands in an environment of large ones, a situation that differs from the work of Pai et al 9 The images were recorded using a tunneling current of 1.0 nA with the tip biased negatively by 0.10- In order to analyze the STM images we used a special purpose computer code that allowed the simultaneous evaluation of the sizes of all islands in an image. The program fits a spline to the gray values of each scan line and searches for the largest slopes in this spline.…”

Diffusion and submonolayer island growth during hyperthermal deposition on Cu(100) and Cu(111)

“…One of the most important questions that still remains open is the role of island diffusion, since for mobile islands the aggregation rates in the RE approach depend explicitly on the diffusion coefficients D s of islands of different sizes s. Island diffusion on surfaces has been studied both theoretically [7,8,9] and experimentally [10,11,12,13,14,15,16]. While in the large island limit the size dependence of the island diffusion coefficients can be classified based on simple basic processes [17], for smaller islands D s depends on the geometric and energetic details of the underlying surface, and can be a complicated, non-monotonic function of s [18,19].…”

Energetics and many-particle mechanisms of two-dimensional cluster diffusion on Cu(100) surfaces

“…4 Recent studies have shown that in metal-on-metal systems even large clusters can have significant mobility at moderate temperatures. 5 This is due to such mechanisms as evaporation-adsorption, terrace ͑or vacancy͒ diffusion, and single-atom diffusion along the periphery. 4,6 Smaller clusters can diffuse with more complicated mechanisms, however.…”