Abstract
Near-infrared absorption in transparent conducting oxides (TCOs) is usually caused by electronic intraband transition at high doping levels. Improved infrared transparency is commonly explained by enhanced drift mobility in these TCOs. Here, an alternative cause behind the high infrared transparency of La-doped barium stannate (LBSO) transparent electrodes is presented. Following the Drude model formalism, we reconstructed spectrally resolved dielectric permittivity for a set of thin films with different free electron concentrations. A comparison of optical properties of LBSO with the tin-doped indium oxide thin films with identical carrier concentrations suggests that the redshift of the screened plasma wavelength for LBSO originates from its large high-frequency dielectric constant of 4.4, one of the highest reported for the s-orbital-based TCOs. Moreover, our measurements confirm an optical mobility significantly higher (>300 cm2/V s) than the drift mobility, effectively suppressing the free carrier absorption. These factors enable high infrared transparency of LBSO films and motivate further exploration of LBSO as broadband TCOs for solar cells and nanophotonics.
Highlights
Transparent conducting oxides (TCOs) combine low optical absorption in the visible spectral range with high electrical conductivity, which is achieved by the combination of their large bandgap energy (>3 eV) and ability to support high doping densities (Ne) of 1020–1021 cm−3
These properties led to the extensive usage of TCOs as electrodes for optoelectronic applications,1 which require transparency in visible and/or near-infrared regions (NIR) of the spectrum
This study presents a quantitative analysis of dielectric permittivity for a set of LBSO thin films with different Ne
Summary
Transparent conducting oxides (TCOs) combine low optical absorption in the visible spectral range with high electrical conductivity, which is achieved by the combination of their large bandgap energy (>3 eV) and ability to support high doping densities (Ne) of 1020–1021 cm−3. These properties led to the extensive usage of TCOs as electrodes for optoelectronic applications, which require transparency in visible and/or near-infrared regions (NIR) of the spectrum (e.g., tandem photovoltaics). Reference refractive index (n) and extinction coefficient (k) of sputtered ITO films for different carrier densities measured by spectroscopic ellipsometry were obtained from Holman et al.
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