Scanning tunneling microscopy of Ag growth on GaAs(110) at 300 K: From clusters to crystallites

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Abstract
Scanning-tunneling-microscopy (STM) studies of Ag overlayer growth on GaAs(110) at 300 K show cluster nucleation at low coverage followed by conversion to nanocrystallites with distinct facets. Clusters formed by depositions below ∼0.5 monolayer (ML) Ag exhibit no apparent preferential substrate orientation, and individual atoms within the clusters cannot be resolved. These are three-dimensional (3D) clusters, as shown by calculations of cluster volumes and areas based on the STM images, and they contain up to ∼250 atoms. Continued deposition yields new small clusters in addition to the growth of existing clusters and the coalescing of clusters in close proximity. By ∼5 ML deposition the Ag structures exhibit crystalline order and expose {111} facets. The evolution from clusters to crystallites also involves a preferential orientation so that [11¯0] of Ag is parallel to [11¯0] of GaAs(110) and the Ag overlayer (111) plane is tilted 25° around the [11¯0] direction of GaAs(110). The overlayer derived from these 3D crystallites is then highly irregular, and contact with the partially relaxed GaAs(110) surface is achieved through regularly stepped Ag(110) planes. These results demonstrate weak substrate interaction and cooperative Ag rearrangement to minimize surface energies, even at 300 K.