The primary model system for studying the heteroepitaxial growth on vicinal surfaces in our group has been
CaF₂ on stepped Si(111). This combination exhibits several properties important for a good model system:

• Atomically straight steps on Si(111) (minimize influence of defects on the growth)
• Lattice mismatch only 0.6% at room temperature
• Atomically sharp interface between the two materials (no alloying)
• Materials distinguishable in STM images
• CaF₂ has a potential as a mask material for selective deposition/etching

Observed Growth Modes of CaF₂ on Stepped Si(111)

Random Islands	Step Flow	Array of Dots

More images and some ideas about possible practical applications of CaF₂ nanostructures
can be found on UW-Madison MRSEC page.

Apart from the possible technological importance for making arrays of quantum wires and quantum dots the step flow and dots-at-steps modes are of interest for fundamental understanding of the growth mechanisms. A particularly interesting effect that we have investigated in collaboration with Feng Liu and Adam Li from Prof. Lagally's group is the existence of the critical coverage which determines the transition from growth of dots and broken stripes to continuous stripes. Image on the left shows the broken stripe growth below the critical coverage (a) and almost continuous stripes close to critical coverage (b). Note that in case (a) almost all stripes are shorter than 80 nm (colored green), whereas in case (b) almost all stripes are longer then 80 nm (colored red).

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