X-ray structural studies of epitaxial yttrium silicide on Si(111)
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P. A. Heiney Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania 19104
M. P. Siegal Sandia National Laboratory, Albuquerque, New Mexico 87185 (Received 30 July 1993; accepted 23 February 1994)
We performed a series of glancing angle and reflection x-ray diffraction experiments to study both the in-plane and out-of-plane structure of epitaxial YSi2_x films grown on S i ( l l l ) , with thicknesses ranging from 85 A to 510 A. These measurements allowed us to characterize the mean film lattice constants, the position correlation lengths of the film, and the presence and extent of strain as a function of film thickness. We find that the strain along the basal plane increases as a function of increasing thickness to approximately 1% in the 510 A film; the corresponding out-of-plane strain is such that the film unit cell volume increases as a function of thickness. The corresponding in-plane microscopic strain varies from 0.5% for the 85 A film to 0.3% for the 510 A film. We relate our results to the mode of film growth and the presence of pinholes in the films.
I. INTRODUCTION The study of epitaxial metal silicide-silicon interfaces is of interest for both technological and fundamental reasons. The further development of small-scale electronic devices requires improvements in metallic contact properties, which depends on a better understanding of the microstructure of the interfacial region. This metal-semiconductor interface microstructure in itself is of interest for the study of the physical and chemical processes of epitaxial growth.1 Efforts in the study of the properties and of the methods of preparation of epitaxial silicides have concentrated on the rare earth silicides, RESi2 *, where RE = Y, Er, or Tb.2~7 All of the rare earth silicides have a hexagonal phase based on the A1B2 structure. They are of particular interest because the arrangement of the silicon atoms is almost identical to that of the S i ( l l l ) substrate. This results in lattice mismatch percentages which vary from -2.5 to 0.83%. 3 Thus, they are ideal model systems for the study of pseudomorphic film growth.8 YSi2_^ has a reported lattice mismatch of 0.0% with Si(lll), 3 with bulk lattice parameters a = 3.842 A and c = 4.144 A, respectively. Previous studies set x between 0 and 0.3, with the latter concentration corresponding to a vacancy distribution in the silicon sublattice, with an in-plane lattice structure twice the size of the YSi2-^.3'4 Films of YSi2_^ and ErSi2_x can be grown under ultrahigh vacuum conditions.2'6'9 These films are highly epitaxial, with a chi-minimum of 3-4% as measured by ion channeling in Rutherford backscattering spectrometry (RBS), a result in the same order as that 1434
J. Mater. Res., Vol. 9, No. 6, Jun 1994
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obtained with films of NiSi2 and CoSi2. The high degree of crystallinity observed in these films is achieved by growing the films on a thin silicide template deposited at 300 °C. This method yields a high quality surface
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