Role of Fluoride Doping in Low-Temperature Combustion-Synthesized ZrO Dielectric Films

Zirconium oxide (ZrO) is an attractive metal oxide dielectric material for low-voltage, optically transparent, and mechanically flexible electronic applications due to the high dielectric constant (κ ∼ 14-30), negligible visible light absorption, and, as a thin film, good mechanical flexibility. In this contribution, we explore the effect of fluoride doping on structure-property-function relationships in low-temperature solution-processed amorphous ZrO. Fluoride-doped zirconium oxide (F:ZrO) films with a fluoride content between 1.7 and 3.2 in atomic (at) % were synthesized by a combustion synthesis procedure. Irrespective of the fluoride content, grazing incidence X-ray diffraction, atomic-force microscopy, and UV-vis spectroscopy data indicate that all F:ZrO films are amorphous, atomically smooth, and transparent in visible light. Impedance spectroscopy measurements reveal that unlike solution-processed fluoride-doped aluminum oxide (F:AlO), fluoride doping minimally affects the frequency-dependent capacitance instability of solution-processed F:ZrO films. This result can be rationalized by the relatively weak Zr-F versus Zr-O bonds and the large ionic radius of Zr⁺⁴, as corroborated by EXAFS analysis and MD simulations. Nevertheless, the performance of pentacene thin-film transistors (TFTs) with F:ZrO gate dielectrics indicates that fluoride incorporation reduces I-V hysteresis in the transfer curves and enhances bias stress stability versus TFTs fabricated with analogous, but undoped ZrO films as gate dielectrics, due to reduced trap density.