Fluoride Doping in Crystalline and Amorphous Indium Oxide Semiconductors

In this contribution, the structural and electronic effects of fluoride doping in both crystalline and amorphous indium oxides are investigated by both experimental and theoretical techniques. Pristine crystalline and amorphous fluoride-doped indium oxide (F:In–O) phases were prepared by solution-based combustion synthesis and sol–gel techniques, respectively. The chemical composition, environment, and solid-state microstructure of these materials were extensively studied with a wide array of state-of-the-art techniques such as UV–vis, X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, ¹⁹F and ¹¹⁵In solid-state NMR, high-resolution transmission electron microscopy (HR-TEM), and extended X-ray absorption fine structure (EXAFS) as well as by density functional theory (DFT) computation combined with MD simulations. Interestingly, the UV–vis data reveal that while the band gap increases upon F^–-doping in the crystalline phase, it decreases in the amorphous phase. The ¹⁹F solid-state NMR data indicate that upon fluorination, the InO₃F₃ environment predominates in the crystalline oxide phase, whereas the InO₄F₂ environment is predominant in the amorphous oxide phase. The HR-TEM data indicate that fluoride doping inhibits crystallization in both crystalline and amorphous In–O phases, a result supported by the ¹¹⁵In solid-state NMR, EXAFS, and DFT-MD simulation data. Thus, this study establishes fluoride as a versatile anionic agent to induce disorder in both crystalline and amorphous indium oxide matrices, while modifying the electronic properties of both, but in dissimilar ways.