Effect of surface topography on reflection electron energy loss plasmon spectra of group III metals

In situ reflection electron energy loss spectroscopy (REELS) and reflection high energy electron diffraction employing a $20keV$ electron beam at a 2° grazing angle were used to characterize the surface properties of molecular beam epitaxy (MBE) grown Al, Ga, and In metals on silicon and sapphire substrates. In our study we found that the surface topography strongly influences the REELS plasmon spectra. Smooth Al films with $<1nm$ rms roughness exhibited surface plasmon peaks. Both surface and bulk plasmons are seen from an Al film with a rms roughness of $3.5nm$⁠. Aluminum surfaces with $>5nm$ rms roughness yielded only bulk plasmon peaks. To understand the EELS spectrum for the Ga and In films, the rms roughness alone is not the relevant figure of merit as the electron beam interaction with the surface is influenced most by the shape of the tops of the surface grains and the grain size. Indium films on Si with a rms roughness of $52nm$ were found to excite predominantly surface plasmons as the grazing angle electron beam scattered mostly off the flat top surface of each grain and was not strongly influenced by the crevices between the grains. The rounded tops of the Ga topography with $31nm$ rms roughness facilitated transmission through the grains and therefore excited a combination of bulk and surface plasmons. This experimental method is very surface sensitive, as a probe depth of $0.8nm$ was inferred from the diminishing intensity of the substrate peak with increasing coverage of a flat metal surface. The techniques and methods discussed here can be readily applied to other thin film systems such as MBE-grown III-V semiconductors, sputtered oxides, and other vacuum deposited materials.