Growth of Ge-Si(111) epitaxial layers: intermixing, strain relaxation and island formation

We have followed by scanning tunneling microscopy (STM) the growth of thin Ge films obtained by reactive deposition epitaxy on Si(lll) substrates kept at 500 degrees C. For a Ge coverage less than 0.45 ML. STM images show large 7 x 7 flat areas without any protrusion. For increasing coverage, flat, triangular 5 x 5 islands start nucleating while the Si substrate retains the 7 x 7 reconstruction. The islands' evolution. up to the completion of the wetting layer, is described in the framework of a statistical model of growth. At 3 ML, the composition and ordering of the wetting layer are investigated by Current Imaging Tunneling Spectroscopy (CITS) measurements. revealing small differences in the atomic corrugations. The analysis of topographic and current images supports an elemental composition for the topmost layer. At coverages larger than 3 ML. thick Ge islands nucleate according to the Stranski-Krastanov mechanism of growth. We analyze the evolution of the lattice strain up to 15 ML coverage. A clear expansion of the lattice parameter as a function of coverage is found both on the islands' top and on the wetting layer. (C) 1998 Elsevier Science B.V. All rights reserved.