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Optical and electrical properties of indium tin oxide (ITO) films on Si substrates in the thickness range from 10 nm to 100 nm were investigated. Spectroscopic ellipsometry was used to obtain the complex permittivity of the ultra-thin films in the spectral range from visible to long-wave infrared. It was found that as the thickness decreases, the Drude component of the electric permittivity becomes vanishingly small, eventually leaving only positive permittivity values. This coincides with an epsilon-near-zero wavelength red-shift from 1.5 to 2.1 um for the films that retain negative permittivities. Hall measurements were conducted to determine that the mobility of the films correspondingly decreased from 35 cm2/Vs to single digits. This decreasing mobility is the result of a non-electrical dead layer that was determined to be ~14 nm thick, which occurs at the ITO/Si interface and is due predominately to interfacial defects. The thickness of this dead layer depends on the deposition process and substrate. The optical and electrical properties of ultra-thin ITO films are useful for the precise design of infrared plasmonic modulators, perfect light absorbers and other photonic devices.
Cleary et al. (2018) studied this question.