Growth and surface structure of Ge-Si alloy films on Si(111)- (7x7).

Thin films of Ge(x)-Si(1-x) alloys for x=0.5 have been grown in ultra high vacuum on a Si(111)-(7x7) surface and examined using Auger electron spectroscopy and digital LEED. The alloy films were prepared by thermal evaporation of Ge onto a Si (111) substrate. Following deposition onto a room temperature Si surface, a sequence of (1x1), (5x5) and (7x7) LEED patterns were observed with increasing annealing temperature. The (5x5) structure which forms near 500C is indicative of the Ge(0.5)Si(0.5) alloy. Alloy films up to 1000 Angstroms were then grown by Ge deposition onto the Si substrate held at 560C where intermixing readily occurs. For film thicknesses greater than two monolayers, the (5x5) surface periodicity of the alloy was always found. The lattice constant of the alloy was determined as a function of film thickness by digital analysis of the LEED patterns using a vidicon camera detector. Results show that for thicknesses less than 150 angstroms, the (50-50) Ge- Si alloy grows pseudomorphically on the Si substrate, i.e., the lattice constant of the alloy matches that of the substrate. At thicknesses near 1000 Angstroms, the lattice constant of the surface of the film relaxes to about 1.5 percent larger than the Si as compared to the value for the bulk Ge(0.5) Si (0.5) alloy which is about two percent larger than Si.