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) subst rates 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 increas ing coverage, flat, triangular 5 x 5 islands start nucleating while th e Si substrate retains the 7 x 7 reconstruction. The islands' evolutio n. up to the completion of the wetting layer, is described in the fram ework of a statistical model of growth. At 3 ML, the composition and o rdering of the wetting layer are investigated by Current Imaging Tunne ling Spectroscopy (CITS) measurements. revealing small differences in the atomic corrugations. The analysis of topographic and current image s supports an elemental composition for the topmost layer. At coverage s larger than 3 ML. thick Ge islands nucleate according to the Stransk i-Krastanov mechanism of growth. We analyze the evolution of the latti ce strain up to 15 ML coverage. A clear expansion of the lattice param eter as a function of coverage is found both on the islands' top and o n the wetting layer. (C) 1998 Elsevier Science B.V. All rights reserve d.