Probing Free Carrier Plasmons in Doped Semiconductors using Spatially Resolved Electron Energy Loss Spectroscopy*

Plasmons of charge carriers exist not only in metals, but also in a wide range of doped semiconductors and semimetals. These fundamental collective excitations play important roles in the transport and infrared optical properties of solids. Photon-based measurements and reflection electron energy loss spectroscopy offer great energy resolution to study these low energy plasmons, but the results average from large area. Here, we demonstrate the possibility of measuring free carrier plasmons in a spatially resolved way by kV electrons. Experiments were performed using ∼10 meV resolution electron energy loss spectroscopy (EELS) in a state-of-the-art scanning transmission electron microscope (STEM) with angstrom spatial resolution. The combination of high energy and spatial resolution let us observe the interference patterns of surface carrier plasmons in a freestanding film of indium tin oxide (ITO), from which we extract a dispersion relation. We further show that the energies of these plasmons vary near the surfaces and grain boundaries of the film due to band bending. Modeling based on dielectric theory agrees very well with experimental results.