Fluorine-doped tin oxides for mid-infrared plasmonics

Abstract

Fluorine-doped tin oxides (FTO) were investigated for infrared plasmonic applications. Nano-crystalline FTO thin films were grown by the SPEED chemical-spray deposition method. Complex permittivity spectra were measured from 1.6 to 12 μm wavelength. These spectra were used to calculate materials parameters, which compared well with values from transport measurements, and to predict characteristics of surface plasmon polaritons (SPP). Reflectivity spectra for lamellar FTO gratings revealed SPP coupling resonances in good agreement with predictions. The FTO film studied here is well suited for plasmonic applications in the important 3-5 μm wavelength range.

Electrodynamic properties of aqueous spray-deposited SnO2:F films for infrared plasmonics

“…[3][4][5] More recently, FTO has been investigated as a host for surface plasmon polaritons (SPPs) in proposed infrared plasmonic devices. [6][7][8] SnO 2 is a wide band-gap semiconductor with natural ntype conductivity due to oxygen vacancies 9 and interstitial Sn. 10 Since tin exists in the lattice as an ion with charge 4 þ , each oxygen atom in the chemical formula accepts two electrons from Sn.…”

“…30 Our approach is an aqueous spray method, which has been shown to give perfectly conformal coatings with plasma frequencies up to the near infrared. 6 The perfect conformity is important because many plasmonic applications require structured devices, such as slot waveguides, 31,32 transmission gratings, 33 and lamellar gratings. 6 There have been several other studies of aqueous spray deposition of FTO with similar chemistry and conditions.…”

“…6 The perfect conformity is important because many plasmonic applications require structured devices, such as slot waveguides, 31,32 transmission gratings, 33 and lamellar gratings. 6 There have been several other studies of aqueous spray deposition of FTO with similar chemistry and conditions. 15,[34][35][36] These studied electrical properties, optical transparency, and their dependence on F doping and growth conditions, with emphasis on application as transparent conductors.…”

“…6 We began this study with a coarse sampling that spanned a wide range of F concentrations at the more moderate temperature 450°C using borofloat glass substrates. Results presented below suggest the need for higher temperatures, so a second set of samples was processed at 480°C, but with finer sampling of the F concentration.…”

“…SEM images of typical polycrystalline FTO surfaces have been published. 6,7 Contactless rf-eddy-current method (Lehighton Electronics, Inc.) was used to map sheet resistance R s , whose values were also determined by a four-point probe method. Resistivity ρ was found by multiplying R s and film thickness.…”