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

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 (I_on) and electron mobility due to scattering from loosely bound F^- centers but enhances important TFT performance parameters such as the I_on/I_off 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.