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Kartik Venkatraman1 Peter Rez2 Katia March3 Peter Crozier1

1, Arizona State University, Tempe, Arizona, United States
2, Arizona State University, Tempe, Arizona, United States
3, Arizona State University, Tempe, Arizona, United States

Widely used vibrational spectroscopies like Raman and Fourier-transform infrared (FTIR) spectroscopy provide chemical fingerprints of bonding arrangements for materials characterization. The spatial resolution using photons as probes is limited by the photon wavelength. Advances in monochromation in the modern scanning transmission electron microscope (STEM) has made it possible to detect vibrational excitations using electron energy-loss spectroscopy (EELS) [1]. This new capability can be used to investigate the delocalized behavior of bulk as well as the localized behavior of surfaces and interfaces. For a comprehensive understanding of the technique, experiments need to be performed on simple model systems. We explore the spatial variation in the vibrational stretch signals from the bulk, surface, and interface when an electron beam is scanned across a SiO2/Si interface. The results are interpreted it in terms of the classical dielectric theory. This investigation provides baseline data which can be used to further explore the influence of more complex geometries on vibrational modes in materials.
A 3 μm layer of SiO2 on a Si wafer was prepared for STEM-EELS analysis by lifting out a focused ion beam (FIB) lamella using a Nova 200 NanoLab (FEI) FIB. A NION UltraSTEM 100 aberration-corrected electron microscope operated at 60 kV and equipped with a monochromator was used to perform EELS linescans across the SiO2/Si interface. The energy-loss spectrum from SiO2 shows the Si-O bond-stretch mode at 144 meV and bond-bend mode at 100 meV. An initial drop in the bond-stretch signal is observed when the electron probe is 200 nm from the Si due to the long-range nature of the electrostatic interaction. However, the distance from the interface at which this signal intensity drops to half its maximum value is 5 nm. Calculations show that surface effects influence the energy position of a vibrational signal, and the surface contribution dominates the energy-loss spectrum over bulk for typical TEM sample thicknesses (≤ 100 nm). We show that, in practice, nanometer resolution is possible when selecting a part of the SiO2/Si interface signal that is at a different energy position than the bulk signal. Calculations also show that, at 60 kV, the signal in the SiO2 can be treated non-relativistically (no retardation) while the signal in the Si, not surprisingly, is dominated by relativistic effects. The surface effect allows local thickness in thin SiO2 films to be determined based on the peak energy. Further geometrical effects on the vibrational modes of the system from edges and corners will also be presented.
References:
[1] O.L. Krivanek et al., Nature, 514 (2014), p. 209.
[2] The support from National Science Foundation CHE-1508667 and the use of (S)TEM at John M. Cowley Center at Arizona State University is gratefully acknowledged.

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