Phase transformation in self-assembled Gd silicide nanostructures on Si(001)
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Jun Nogamib) Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario, Canada M5S 3E4 (Received 25 January 2011; accepted 17 May 2011)
Gd silicide nanostructures epitaxially grown on Si(001) are studied by plan-view transmission electron microscopy and associated nanobeam electron diffraction, as well as scanning tunneling microscopy. The nanobeam diffraction measurements show a direct correlation between the nanostructure morphology, either nanowires or islands, and the silicide crystal structure. Scanning tunneling microscopy shows a phase transformation from nanowires to islands that nucleate at nanowire intersections. A specific mechanism for this transformation is proposed that explains nanowire growth behavior previously observed on vicinal Si surfaces.
I. INTRODUCTION
Many rare-earth (RE) metal silicides form self-assembled nanowires (NWs) on the Si(001) surface.1–3 NW formation is due to an anisotropy in the lattice mismatch between the silicides and the silicon substrate. Nanosized compact silicide islands with much smaller aspect ratios are also observed under some growth conditions.4 In many of these RE silicide systems, there are several different polymorphs that exist very close to the composition RESi2. These phases have different lattice constants and, thus, have different lattice mismatches with the substrate.5 For NW-forming RE silicides, it is generally the hexagonal phase that has one lattice constant very closely matched with the substrate. NW growth occurs along the direction of small lattice mismatch, whereas lateral growth is constrained by a strong lattice mismatch in the perpendicular direction. Annealing NWs can lead to the transformation of the NWs into rectangular islands.6 Previous cross-sectional transmission electron microscopy (TEM) studies on such samples show the existence of both hexagonal and nonhexagonal silicide phases.7–9 From scanning tunneling microscopy (STM) and TEM measurements on the same sample, it can be inferred that the nanostructure morphology is correlated with the underlying silicide crystal structure.8 In this study, nanobeam electron diffraction (NBD) is used in plan-view TEM to study the epitaxy of GdSi2 nanostructures grown on Si(001). In NBD, information
can be collected from single selected nanostructures, allowing direct correlation between the morphology of the nanostructures and the crystal structures. Consistent with prior work in other RE silicide systems, the NBD data presented show that the Gd silicide NWs have the hexagonal crystal structure, whereas the islands have the orthorhomic/tetragonal structure (these two phases have very similar lattice parameters, making them difficult to distinguish, and will be referred to as orth/tet hereafter). This analysis also confirms the relationship between silicide morphology and lattice mismatch, showing that the nanostructure morphology is controlled by the magnitude and direction of the lattice mismatch with respect to the silicon. STM data show that phase transformation from
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