: Elucidation of Fundamental Nucleation Localization Mechanisms @ University of Virginia

We have examined the effects of Ga+ dose and resulting topography upon the localization of Ge cluster nucleation, demonstrating that doses as low as 1014 ions cm-2 ??? which will not be expected to create even monolayer topography via sputtering ??? can effectively seed subsequent Ga+ nucleation. We have compared the efficiency of nucleation localization and rates during our standard CVD (where surface reactivity affects pyrolosis rates of the depositing species) and solid source deposition (from an effusion cell in an interconnected UHV chamber), showing that the resultant Ge cluster arrays are essentially identical, and that local variations in surface reactivity are thus not the dominant localization mechanism. We have also examined the role of surface chemical effects from implanted Ga. By chemically homogenizing the surface through deposition of a monolayer of Ga (from an effusion cell in an inter-connected UHV chamber) onto the as-implanted surface, we show that the resultant distribution of Ge clusters retains memory of the underlying implant pattern. This demonstrates that variations in surface chemistry are not the dominant mechanism in localizing nucleation. However, we do observe that cluster shapes and coherent-dislocated transition dimensions are different as a function of the surface Ga+, providing a route to controlling of cluster geometries and sizes. A recent breakthrough has been the discovery that an unexpected nanoscale surface topography plays a key role in the localization of cluster nucleation. While our standard Ga+ ion dose of 1014 cm-2 is insufficient to directly create substantial surface topography, we observe that the post implant anneal that we perform to remove crystalline damage can produce a very subtle surface topography associated with each implanted feature. This initially comprises small annular surface depressions, of order 30 nm in diameter and 0.5 nm deep that, on further annealing, collapse to nano-pits c. 1 nm deep and 10 nm wide, near the center of each implant feature. Under annealing conditions where these depressions form (e.g. 550 o C for a few minutes), the Ge cluster localization is very apparent. Under annealing conditions where depressions do not form (i.e. at higher temperatures and/or longer annealing times) the majority of the Ge clusters do not nucleate on the implant sites.