Structure–Charge Transport Relationships in Fluoride-Doped Amorphous Semiconducting Indium Oxide: Combined Experimental and Theoretical Analysis

Abstract

Anion doping of transparent amorphous metal oxide (a-MO) semiconductors is virtually unexplored but offers the possibility of creating unique optoelectronic materials owing to the chemical tuning, modified crystal structures, and unusual charge-transport properties that added anions may impart. We report here the effects of fluoride (F–) doping by combustion synthesis, in an archetypical metal oxide semiconductor, indium oxide (In–O). Optimized fluoride-doped In–O (F:In–O) thin films are characterized in depth by grazing incidence X-ray diffraction, X-ray reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure (EXAFS). Charge-transport properties are investigated in thin-film transistors (TFTs), revealing that increasing fluoride content (0.0 → 1.57 atom %) slightly lowers the on-current (Ion) and electron mobility due to scattering from loosely bound F– centers but enhances important TFT performance parameters such as the Ion/Ioff ratio, subthreshold swing, and bias stress stability, yielding superior TFT switching versus undoped In–O. These results are convincingly explained by ab initio molecular dynamics simulations and density functional theory electronic structure calculations. Combined with the EXAFS data, the experimental and theoretical results show that F– hinders crystallization by enhancing the local and medium-range disorder, promotes a uniform film morphology, and favors the formation of deeper, more localized trap states as compared to F–-free In–O. These data also show that the local organization and electronic structure of amorphous F–-doped oxide semiconductors are significantly different from those of F–-doped crystalline oxide semiconductors and suggest new avenues to further modify a-MOs for enhanced optoelectronic properties.