Paper SS1-ThA10
Adatom- and Addimer-Mediated Exchange Diffusion of Embedded Ge Atoms in the Ge/Si(001) Surface Alloy

Thursday, November 12, 2009, 5:00 pm, Room M

Using low-bias empty-state STM images, we can resolve the location of embedded Ge atoms in the Ge/Si(001) surface alloy. We directly observe the diffusion of these embedded atoms at elevated temperatures (>100°C). That the diffusion of the embedded Ge atoms occurs in time bursts, is spatially correlated, and results in long displacements implies that the process is defect mediated. The responsible defects are adsorbed monomers and dimers. We have identified two exchange-diffusion pathways for the movement of the embedded Ge atoms, both of which are consistent with previous first-principles calculations [1, 2]. Adsorbed monomers of Si or Ge can readily place exchange with surface atoms as they anisotropically diffuse along the substrate dimer rows. These monomer exchange events are strongly correlated with common trap sites for monomers, such as, the ends of dimer rows of islands. Less frequent events are associated with adsorbed dimers that can exchange one of their atoms with a surface atom. That is, an adsorbed Si-Si dimer can exchange one of its Si atoms with an embedded Ge atom to become a Si-Ge dimer, which can subsequently re-exchange the Ge atom into the surface at a different location. Because the barrier for exchange in both of these pathways is only slightly higher than that for diffusion, Ge deposition on Si(001) leads to intermixing and surface-alloy formation at any temperature where diffusion is active. Sandia is a Lockheed Martin Company, operated for the U.S. DOE under Contract DE-AC04-94AL85000. This work was supported in part by the Division of Materials Science and Engineering, Office of Science, U.S. DOE, and was performed at the Center for Integrated Nanotechnologies, a U.S. DOE-BES user facility.

[1] Lu et al., Surf. Sci. Lett., 506, L282 (2002).

[2] Zipoli et al., APL, 92, 191908 (2008).