Optical and Electronic Analysis of the Two Fluorite-Related Phases in the Ga-In-Sn-O System

Optical transparency and electrical conductivity, traditionally mutually-exclusive materials properties, are both critical for the performance of electrode materials in applications such as flat-panel displays and organic photovoltaics (OPVs). Owing to their wide band gaps which are amenable to degenerate doping, transparent conducting oxides (TCOs) are widely utilized for organic optoelectronic applications. The two fluorite-related phases in the Ga-In-Sn-O (GITO) pseudo-ternary system---Ga,Sn co-substituted bixbyite 2O3 and the so-called "T-phase" Ga3--xIn5+xSn2O 16---have the potential to overcome the issues associated with Sn-doped 2O3 (ITO) by reducing the indium content, utilizing compositions that are more stable in acidic conditions, and offering work function levels that are in better alignment with adjacent layers in organic optoelectronic devices.;In this work, the composition and processing effects on the optical and electronic properties of the bixbyite phase and the T-phase in the GITO system were determined. Electronic properties were analyzed by conductivity and Seebeck coefficient measurements, and the combination of these two properties revealed carrier characteristics for these two phases. The optical band gaps of the phases were determined from absorption data by methods developed in this work, which account for the band-altering effects of doping in TCOs. The optical characterization methods were experimentally confirmed through a combination of diffuse-reflectance and photoluminescence measurements of un-doped and Sn-doped In2O3.;The bixbyite phase's conductivity, Seebeck coefficient, and optical band gap are dominated by the concentration of Sn-on-In substitution defects, as expected. Remarkably, Ga substition was found to have a negligible effect on the electronic properties. A conductivity of 3000 S/cm was achieved in bulk specimens containing a relatively small Sn content of 1 atomic percent and 4.5 atomic percent Ga. Ga behaves as an isovalent substitution of In in the bixbyite lattice, in contrast to other TCOs, where the substitution defect serves as an electron trap. The finding that the Ga content in Sn,Ga co-substituted 2O3 has a minimal, if any, effect on optoelectronic properties opens the possibility of In replacement in devices that utilize ITO as the transparent electrode.;Under optimized processing conditions, the T-phase was found to exhibit a conductivity greater than 1800 S/cm after reduction annealing (over 900 S/cm in un-reduced specimens) and an optical band gap over 3 eV. Processing and compositional effects on conductivity were found to mainly change the carrier concentration, while leaving the mobility relatively constant throughout the range of synthesis conditions. Measurements of the work function and optical band gap reveal an unchanging Fermi level across the T-phase composition range. Ab-initio calculations of band structure indicate that the Fermi level conservation is likely due to a compensation of the Burstein-Moss shift by a "self-renormalization" of the fundamental band gap.