Iridium (Ir) is a refractory metal commonly seen in industrial applications, but has great potential for optical applications including metasurfaces. Metasurfaces are used to control the optical properties of an interface via sub-wavelength surface structures. These patterns require sharply defined features to create precise optical phase interactions. For high-temperature environments, most materials are insufficient candidates for metasurfaces because the sharpness of the surface structures are lost due to edge-rounding or oxidation. Ir is better suited for metasurface applications in high-temperature environments but the patterning of Ir using nanofabrication techniques has not been thoroughly investigated. In this work, Ir metasurfaces were fabricated and characterized for optical applications in the infrared.
Variable angle spectroscopic ellipsometry (VASE) was used to determine the thicknesses of polymethyl methacrylate (PMMA) on Si before and after etching with two different etchants (CF4 + O2 and Argon). Once a complete optical model for a base PMMA on Si sample was created, it was applied to all etched samples to determine thicknesses. Despite some minor changes to the optical behavior of PMMA caused by the Ar etching, our ability to fit to observed interference peaks remained unaffected. This technique allows for nanometer accurate thickness measurements, which is an improvement from current thickness measurement methods such as stylus profilometry.
Variable angle spectroscopic ellipsometry was used to determine the optical properties of n- and p-type GaAs over a doping range of 4.6×1016 to 9.3×1018 cm-3 and a spectral range of 190 nm to 30 μm. Increased doping concentration was observed to have several distinct effects on the samples’ optical properties: the band edge broadens and shifts to a higher energy; the E1 and (E1 + Δ1) absorption peaks blur together; the E2 absorption peak decreases; sub-bandgap, infrared absorption increases. Additionally, the doping effects are generally stronger for n-type than for p-type GaAs. These findings will help inform future design of optoelectronics.
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