Fluoride Doping in Crystalline and Amorphous Indium Oxide Semiconductors

Abstract

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.

Remarkable Bias‐Stress Stability of Ultrathin Atomic‐Layer‐Deposited Indium Oxide Thin‐Film Transistors Enabled by Plasma Fluorination

“…Based on the above analysis, we can conclude that, as the fluorine doping level increases in the defective In 2 O 3 , the F dopants would first occupy V O (forming F O ) and then form the F O F i pairs. Notably, the formation of the F O F i pairs has been reported in a previous publication, [ 38 ] where their DFT calculations also showed a significant reduction in the F i defect formation energy in the presence of a nearby F O .…”

“…Fluorine anion (F − ) has a similar size to oxygen anion (O 2− ), so it can well fill V O (forming an F O substitution) without causing significant lattice distortion. In fact, the fluorine doping strategy has been previously reported to modify many metal oxides for various purposes, [ 31–35 ] and the fluorine doping effects were found to depend much on the material structure, [ 36,37 ] crystallinity, [ 38 ] and doping concentration. [ 39 ] As for the crystalline In 2 O 3 in particular, its cubic bixbyite crystal structure can be viewed as the fluorite structure with one fourth of the anions removed, and therefore, In 2 O 3 naturally has a plenty of structural vacancies for fluorine to form interstitials (F i ).…”

“…[ 39 ] As for the crystalline In 2 O 3 in particular, its cubic bixbyite crystal structure can be viewed as the fluorite structure with one fourth of the anions removed, and therefore, In 2 O 3 naturally has a plenty of structural vacancies for fluorine to form interstitials (F i ). [ 38 ] Owing to the strong electronegativity of fluorine, F i can possibly trap an electron to form a stable bound state (F i − ), thereby reducing the free electron density in the In 2 O 3 conduction band. Moreover, the lowest conduction band of In 2 O 3 is mainly composed of the In 5s states and the antibonding O 2p states.…”

Thermal Stability of Amorphous Metal Oxides: The Interplay of Secondary Cations, Degree of Substitution, and Local Structure

“…One common type of AMO used for channel layer materials is In 2 O 3 films doped with Sn 2+ , Zn 2+ , etc. The ternary or even quaternary oxides have been explored and can be synthesized via different thin-film deposition methods, such as sputtering, pulsed laser deposition (PLD), and solution processing. − The interplay of secondary cations and growth methods enables the tunability of material characteristics, such as carrier concentration, conductivity/mobility, stress stability, photoresistivity, and degree of crystallinity. For example, the indium–gallium–zinc oxide (a-IGZO) potentially used in commercial optical displays is an excellent candidate for representing the industrial significance of AMOs. , …”