Evidence for Collective Motion in LEEM Measurements of Metals on Semiconductors*

We review evidence for collective motion from LEEM measurements of three metal on semiconductor systems: Pb/Ge(111), Au/Ge(111), and Ag/Ge(110). Pb/Ge(111) shows a novel phase separation with fluctuating domains of $\sim$ 100nm size which spontaneously switch back and forth from the (1x1) to $\beta $ phase in the region of the phase diagram where the two phases coexist. This striking mechanism occurs because nm-scale domains can have thermally-induced density fluctuations comparable to the density difference between the two phases (PRL, \underline {99}, 096103 (2007)). Au/Ge(111) also shows evidence for fluctuating domains between the ($\surd $3x$\surd $3)R30 and (1x1) phases, both for small domains of 100nm diameter, and at the edges of large domains on a sample with low step density. LEEM movies also show ``hopping'' of large islands (tens to hundreds of nm in diameter) of Au on Ge(111). Self-assembly of large one dimensional (1D) islands (1-10 $\mu $m x 70-140 nm, $\sim$ 7nm high, for 7ML) along the [1,-1,0] direction occurs for Ag grown from 300-530 $^{\circ}$C on Ge(110). During the growth process, such 1D islands have been observed to collapse into other islands and defects in \textless\ 1sec.