Self-Assembled GdSi 2 Nanostructures Grown on Si(001) Studied by TEM and STM
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Self-Assembled GdSi2 Nanostructures Grown on Si(001) Studied by TEM and STM Gangfeng Ye1, Martin A. Crimp1, and Jun Nogami2 1 Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI, 48824-1226 USA 2 Department of Materials Science and Engineering, University of Toronto, Toronto, ON, Canada M5S 3E4 ABSTRACT Self-assembled gadolinium silicide (GdSi2) nanostructures grown on Si(001) were studied by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). Crosssectional TEM and plan-view TEM moiré fringe pattern analyses show that the GdSi2 nanostructures can be divided into two classes: elongated nanowires with hexagonal crystal structure and rectangular islands with orthorhombic or tetragonal crystal structure.
INTRODUCTION Rare-earth (RE) metal silicides have attracted interest because they can form self-assembled nanowires (NW) when grown on the Si(001) surface [1-7]. The NW formation is due to an anisotropy in lattice mismatch between the hexagonal form of the RE silicide and the substrate [2, 3], as outlined in Table I [8]. However, TEM and STM studies show that not only NWs, but also nanostructures with much smaller aspect ratios (referred as islands from this point on), are present. It is proposed that the islands may have the orthorhombic or tetragonal crystal structures (referred as orth/tet in the rest of the paper since they have similar crystal structures), which have lower anisotropies in lattice mismatch compared to that of the hexagonal phase [9]. Nevertheless, to date no experimental results have confirmed this. In this report, TEM and STM are used to study the crystal structure and morphology of GdSi2 nanostructures grown on Si(001). Cross-sectional TEM shows that hexagonal and orth/tet polymorphs of the silicide co-exist in these nanostructures. Moiré fringes observed in plan-view TEM show that the morphology of the silicide nanostructures is related to crystal structure: the hexagonal phase forms NWs while the orth/tet phase forms islands, thereby confirming the relationship between silicide morphology lattice mismatch.
EXPERIMENTAL DETAILS The GdSi2 nanostructures were grown by metal deposition onto the Si(001) surface at 650oC in ultrahigh vacuum (UHV), followed by 650oC annealing for 20 minutes. The base pressure of the vacuum system was 2.0×10-10 torr. The silicon substrates were pre-cleaned by flashing up to 1175oC in UHV. The Gd coverage was calibrated by a quartz crystal thickness monitor. The typical metal coverage was 1.5 monolayer (ML) [1 ML=6.78×1014 atoms/cm2 surface atomic density on Si(001)]. STM was carried out at room temperature in the UHV chamber. The sandwich technique was used to prepare cross-sectional TEM samples, which were sectioned perpendicular to the Si direction. Both cross-sectional and plan view TEM samples were
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Table 1. Lattice parameters for bulk GdSi2 and lattice mismatches with respect to Si(001) structure
a (nm)
hexagonal tetragonal orthorhombic
0.3877 0.410
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