Growth and surface structure of Ge–Si alloy films on Si(111)-(7×7)

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Abstract
Thin epitaxial films of a Gex–Si1−x (50-50) alloy for x=0.5 have been grown in ultrahigh vacuum on a Si(111)-(7×7) 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 (1×1), (5×5), and (7×7) LEED patterns were observed with increasing annealing temperature. The (5×5) structure which forms near 500 °C is indicative of the Ge0.5Si0.5 alloy. Alloy films up to 1000 Å were then grown by Ge deposition onto the Si substrate held at 560 °C where intermixing readily occurs. For film thicknesses greater than two monolayers, the (5×5) 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 Å, the 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 Å, the lattice constant of the surface of the film relaxes to about 1.5% larger than the Si as compared to the value for the bulk Ge0.5Si0.5 alloy which is about 2% larger than Si.

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