Tunable Infrared Plasmon Response of Lithographic Sn‐doped Indium Oxide Nanostructures

Transparent conductive oxides (TCOs) have strong potential for plasmonic applications. Given their easily tunable properties and low energy response, significant challenges in the controlled fabrication and precise characterization of TCOs must be better understood before this potential can be realized. Here, the mid- to near-infrared plasmonic response of Sn-doped In2O3(ITO) nanostructures is presented, fabricated top-down using electron beam lithography and radio-frequency sputtering. These equilateral ITO triangles of different side lengths are imaged at high spatial and energy resolution with monochromated electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope. Applying the Richardson-Lucy (RL) deconvolution algorithm to experimental EELS spectra reveals localized surface plasmon (LSP) excitations between 150 and 550 meV and a 730 meV bulk plasmon. This very-low-energy response to an electron beam is compared with boundary element method simulations of nanostructures. These simulations use the dielectric functions of continuous thin films of the same materials, characterized by ellipsometry, 4-point probe, and Hall effect tests. Additionally, upon rapid thermal annealing of ITO, blue-shifts in LSP energy, and longer LSP lifetimes are examined as a consequence of an amorphous-to-polycrystalline transformation and an increase in the free carrier density.