Formation of Pinhole-Free Epitaxial Yttrium Silicide on Silicon(111)
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FORMATION OF PINHOLE-FREE EPITAXIAL YTTRIUM SILICIDE ON SILICON(111)
Michael P. Siegal*, Jorge J. Santiago and William R. Graham Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104 *presently at AT&T Bell Laboratories, Murray Hill, NJ 07974
ABSTRACT We report the growth of structurally continuous, pinhole-free epitaxial YSi2 films on Si(l1l) substrates as thin as 30A. This is accomplished by depositing both yttrium andsiicon in the appropriate stoichiometric ratio onto substrates held near room temperature, which is apparently below the activation energy for the nucleation of a reaction between a deposited Y film and a Si substrate. Diffusion of Si atoms from an evaporation source into a thin Y layer occurs below this barrier energy, allowing the layer by layer formation of YSi2 _x without the removal of silicon from the substrate, maintaining a relatively low interface free energy between the growing silicide film and the Si~lll) substrate. Samples have been annealed to 500-900 0 C to improve epitaxy without the creation of pinholes. Use of the template method allows for the growth of thicker films also free of pinholes. I.
INTRODUCTION Yttrium silicide is a metal with the hexagonal AIB2 structure and a 0.0% lattice mismatch to Si(1ll).[1,2] The compositions previously reported for yttrium silicide films have ranged from YSi 1 7 to YSi 2 0' with the former representing a vacancy distribution in the Si-sublattice.[2,3] Recent work has shown that highly epitaxial YSi 2 x/Si(111) structures can be formed with chiminima -3% as measured by ion channeling in Rutherford backscattering spectrometry (RBS).[4] However, these films contain large densities of pinholes exposing 20-25% of the Si substrate to the surface. Furthermore, the presence of these pinholes adds strain to the films by allowing the crystalline lattice to slightly expand in the planar direction parallel to the interface, into the pinhole regions. Such strains increase with film thickness, reaching -1% for 500A YSi 2 x films, probably due to different coefficients of thermal expansion.[5] The best films resulted from the growth of a thin, 30-40A template layer, followed by reactive deposition of additional Y onto the substrate heated to a temperature high enough to induce silicide formation (-3000 C). Subsequent annealing at 800-900°C yields the high degree of epitaxy which had previously been reported only for Niand Co- silicide films grown on silicon.[4,6] Epitaxial overlayers are known to exhibit three types of morphologies during the initial stages of film growth: Volmer-Weber (island), Frank-van der Merwe (layer by layer), and Stranski-Krastanov (layer followed by island).[7-11] The mode of the growth mechanism which will occur for a given system is controlled by the surface thermodynamics as determined by the free energy of the substrate surface (y¥), the interface free energy (,i) and the surface free energy of the film (yf). When ys < 'i + y•, Volmer-Weber growth is favored.[7] Previous work has
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