| AVS 64th International Symposium & Exhibition | |
| Surface Science Division | Wednesday Sessions |
| Session SS+HC+NS-WeA |
| Session: | Dynamical Processes at Surfaces |
| Presenter: | Matt Hershberger, Iowa State University Ames Laboratory –USDOE |
| Authors: | M.T. Hershberger, Iowa State University Ames Laboratory –USDOE M. Hupalo, Iowa State University Ames Laboratory –USDOE P.A. Thiel, Iowa State University Ames Laboratory –USDOE M.K.L. Man, Hong Kong University of Science and Technology, Hong Kong M.S. Altman, Hong Kong University of Science and Technology, Hong Kong C.H. Mullet, University of California-Davis S. Chiang, University of California-Davis M.C. Tringides, Iowa State University Ames Laboratory –USDOE |
| Correspondent: | Click to Email |
Surface diffusion is the main process controlling mass transport of many important phenomena such as nucleation, nanostructure growth, pattern formation, and chemical reactions. In practically all cases, it is described as a random walk of independently moving adatoms. Such process is inherently stochastic and therefore very slow as a route to self-organization in nature.
A series of experiments in different epitaxially grown metallic films over the last 5 years using different techniques has challenged the classical picture. The experiments have shown unusually fast, collective diffusion is present in nature, observed in a range of experiments over different length and time scales.
STM experiments show that fully completed Pb crystalline islands emerge “explosively” out of the compressed wetting layer on Si(111) after a critical coverage qc=1.22ML is reached. The unexpectedly high island growth rates and directional correlations show that mass transport is through the correlated motion of the wetting layer.[1] Additional deposition of Pb shows island density that does not reach steady state; it shows abrupt jumps in island density with new generations of smaller islands continuing to nucleate (in contrast to classical nucleation). Real time experiments with LEEM, monitoring the refilling of an initial vacant area in Pb/Si(111), show that the initial steep profile does not disperse and that the profile propagates at constant velocity x~t. The profile follows a non-Fickian form with two moving highly correlated fronts, one inward and the other outward.[2] The formation of long anisotropic multi-height Ag islands on Ge(110) is exceedingly fast, when compared to the rates expected from random walk Ag diffusion barriers. A wetting layer is also present prior to the crystallization and is responsible for the fast growth rates, although the temperature is above room temperature.[3] Evidence for collective diffusion has been seen in Pb/Si(100), Pb/Ge(111), Pb/Ni(111), Pb/W(110), Ag/Si(110).
A better understanding of these processes can guide the search of collective transport in other systems, especially to identify the relevant growth “window” (of temperature and coverage). It can further clarify the role of stress since the compression of the non-crystalline wetting layer is critical for these effects.[4]
References:
1. M. T. Hershberger et al., .Phys. Rev. Lett. 113, 236101 (2014)
2. K. L. Man et al., Phys. Rev. Lett. 110, 036104 (2013)
3. C. H. Mullet et al., manuscript submitted.
4. E. Granato et al., Phys. Rev. Lett. 111, 126102 (2013)