Nucleation and Growth of Pb Islands on Ge(111) at Low to Room Temperatures via Collective Diffusion*

Pb deposited on Ge(111) at 220-283K showed unusual collective diffusion behavior after critical coverage of ~1.5ML. Depending on sample temperature during deposition, the area and number density of the Pb islands vary. From LEEM images, a high rate of mass transport of Pb atoms occurred during island formation, too fast to be explained by thermal random walk diffusion. To measure the diffusion barrier, experiments followed the growth of these islands above critical coverage at constant temperature. Using a Kronig-Penney model to predict the locations of the LEEM I-V peaks,[1] I-V curves were used to infer island heights as a function of temperature and coverage. Similar behavior was seen for Pb on Si(111) at low temperatures: unusually fast diffusion speed and island heights determined by quantum size effects.[2] First-principles DFT study of the structure of Pb/Ge(111) showed favorable Pb binding sites. Ab initio thermodynamic calculations of the coverage dependent Pb chemical potential on Ge(111) agree qualitatively with experiments.

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

[2] T. Jaroch et al., J. Cryst. Growth 523, 125-137 (2019).