STM studies of Ge-Si thin layers epitaxially grown on Si(111)

Ge/Si alloys were prepared in UHV by solid phase epitaxy on Si(lll) substrates. The alloy formation, as a function of the evaporation rate and the Ge layer thickness has been followed in situ by RHEED and scanning tunneling microscopy. The 5 x 5 surface reconstruction appeared after annealing at 450 degrees C Ge layers (up to 10 Angstrom thick), obtained from a low rate Knudsen cell evaporator. In this case a nearly flat and uniform layer of reconstructed alloy was observed. When using an e-gun high rate evaporator we needed to anneal the Ge layer up to 780 degrees C to obtain a 5 x 5 reconstruction. The grown layer was not flat, with many steps and Ge clusters; at high coverages (10 Angstrom and more) large Ge islands appeared. Moreover, we then succeeded in visualizing at atomic resolution the top of some of these Ge islands which displayed a 2 x 1 reconstruction, probably induced from the high compressive strain due to the lattice mismatch with the substrate, We suggest that this unusual behavior could be connected to the high evaporation rate, which helped the direct formation of Ge microcrystals on the Si substrate during the deposition process.