Scanning tunneling microscopy studies of Ge/Si films on Si(111): From layer by layer to quantum dots

We have followed by scanning tunneling microscopy (STM) the growth of thin Ge films obtained by reactive deposition epitaxy on the Si(111) surface kept at 500 $°$ C. For Ge thickness smaller than 0.45 monolayers (ML), STM images show large $\text{7×7}$ flat regions without protrusions while at higher coverages flat, triangular $\text{5×5}$ islands start nucleating. We have followed the evolution of this wetting layer up to its completion and investigated its surface composition at 3 ML by current imaging tunneling spectroscopy measurements. At larger coverages thick Ge islands (quantum dots) start to nucleate according to the Stranski-Krastanov mechanism. We analyze the evolution of the lattice strain both on the wetting layer and on the islands up to 15 ML coverage. A clear expansion of the lattice parameter as a function of the coverage is evidenced both on the islands’ top and on the wetting layer. The luminescence yield measured at 10 K on samples covered by 40 Å of Ge and capped with 10 Å of Si evidences a structure that could be assigned to Ge quantum dots.