Nanocrystallites of Si Embedded in CaF 2 by Molecular Beam Epitaxy (MBE)
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NANOCRYSTALLITES OF Si EMBEDDED IN CaF 2 BY MOLECULAR BEAM EPITAXY (MBE) A. P. Taylor*, K. Stokes*, Z. C. Wu**, P. D. Persans*, L. J. Schowalter*, and F. K. LeGoues"" * Physics Department and Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York 12180 ** Rensselaer and Oakridge National Lab., Oakridge, TN *** IBM, Yorktown Heights, New York 10598 ABSTRACT Thin films of CaF 2 containing layers of Si nanocrystals were grown epitaxially on Si(111) substrates by MBE. When Si is deposited epitaxially on CaF2 it forms islands and, by choosing appropriate growth conditions, a composite of Si nanocrystals embedded in a CaF 2 host crystal is obtained which we have observed with TEM. We varied the effective Si concentration in the composite region from 1% to 25%. The CaF 2 could be grown with high crystal quality as demonstrated by ion channeling results which gave Ca x,. ,'s as low as 4.7%. Raman spectra of the CaF 2 containing Si nanocrystallites showed significant shifts toward lower energies. Ellipsometry measurements were performed on a number of samples over a range of effective Si concentrations and Maxwell-Garnett effective medium theory was used to fit the data. A key technological advantage to this approach is that the epitaxial Si/CaF 2 composite material is readily integrable into VLSI processing. INTRODUCTION In recent years there has been a flurry of activity in fabricating and characterizing the properties of semiconductor nanocrystallites[1,2]. In particular, the discovery of photoluminescence from porous Si has enticed many researchers into this field[3]. There is an ongoing debate as to the source of luminescence from porous Si. Possibilities include quantum confinement, siloxene, hydrogenated amorphous Si, surface polymers, or an SiHl, alloy, to name a few[4]. By fabricating nanocrystallites of Si embedded in CaF 2 by molecular beam epitaxy, we will have the ability to probe the optical properties of Si nanocrystallites existing in a much cleaner environment than in porous Si. Si nanocrystallites in CaF2 is an attractive system for a number of reasons. Due to the level of control over growth conditions inherent to MBE, we should have more control over the nanocrystallite size distribution than with porous Si. Also, the Si nanocrystallites are protected against oxidization in atmosphere which is not the case in the porous Si system. The Si nanocrystallites may be F terminated which could be important for their optical properties. The CaF 2 host is an excellent medium for waveguiding since the 12 eV band gap of CaF 2 allows for transmission over a broad range of wavelengths. From a technological view point, if this system is found to have promising optical properties then it is readily compatible with Si VLSI. FABRICATION METHOD Molecular beam epitaxy was used to make composite layered structures of Si nanocrystallites interleaved with CaF 2 . The CaF 2 was thermally evaporated and the Si was e- beam evaporated. Typical base pressures were in the mid 10-"mb range and in the 10-9
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