Explosive Nucleation and Collective Diffusion during Low Temperature Growth of Pb/Ge(111) Islands

Lead deposited on Ge(111) at 220-283K first formed an amorphous wetting layer before undergoing explosive nucleation forming Pb(111) islands at critical coverage of 1.33±0.07ML (w.r.t substrate). LEEM real time images revealed evidence of collective diffusion as the main driving force during nucleation and growth. The islands exhibited anisotropic growth with superlinear growth rate in island size vs time dependence during the early growth stage, involving transport of millions of atoms in the wetting layer over mesoscale distances within the LEEM acquisition speed, fueled by slight compression of the wetting layer at the critical coverage time. Analysis of island growth after the nucleation event showed the number of islands eventually saturating, reducing the growth rate to linear. A Kronig-Penney model was used to predict the locations of LEEM I-V peaks to infer experimental island heights.[1] Similar behavior was previously seen for Pb/Si(111) at low temperatures, with unusually fast diffusion and island heights determined by quantum size effects.[2] To explain the explosive nucleation and critical coverage of Pb/Ge(111), we also present a first-principles DFT computation of the structure and chemical potential of Pb overlayers on Ge(111) as a function of coverage.

[1] M. S. Altman et al, Surf. Rev. Lett. 5, 1129, (1998).

[2] M. T. Hershberger et al., Phys. Rev. Lett. 1113, 236101 (2014).