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 chargetransport 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.

Fluoride Doping in Crystalline and Amorphous Indium Oxide Semiconductors

“…Charge transport properties measured in TFTs reveal that as the F – content in F:In–O increases, both I off and I on fall, while I on / I off increases, thus affording superior TFT switching properties versus undoped In–O-based TFTs. Also, in contrast to previous literature reports, − , F – -doping generally reduces the electron mobility in MO matrices . Furthermore, extended X-ray absorption fine structure (EXAFS) analysis revealed that F – -doping disrupts crystallization by enhancing the local and medium-range disorder.…”

“…For this study, we investigated the archetypal MO system In–O because it is widely investigated for TFT applications and one of the compositionally simplest MO semiconductors. ,− Furthermore, any effect of fluoride-doping can be more easily tracked in this system via both theoretical and experimental analyses . The goal is to synthesize pure amorphous and crystalline F:In–O samples in sufficient quantities (∼200 mg) to carry out thorough experiments.…”

“…Recently, this Laboratory investigated the effect of fluoride-doping on structure–function relationships in amorphous indium oxide matrices . Charge transport properties measured in TFTs reveal that as the F – content in F:In–O increases, both I off and I on fall, while I on / I off increases, thus affording superior TFT switching properties versus undoped In–O-based TFTs.…”

Frequency-Agile Low-Temperature Solution-Processed Alumina Dielectrics for Inorganic and Organic Electronics Enhanced by Fluoride Doping

“…Cation doping of gate dielectrics (e.g., La in AlO x ) has been used to mitigate the substantial capacitance–frequency dependence at low frequencies (∼10 Hz) and yields hysteresis-free transistor performance and reliable mobilities . Other than MO cation doping, − anion doping has also been explored to modulate MO properties, especially for MO semiconductors. − Moreover, this laboratory recently found that incorporating fluoride (F) in both the solution-processed semiconductor (InO x ) and dielectric (AlO x ) films promotes metal coordination and impurity removal, leading to high-performance amorphous oxide TFTs . However, we did not explore the details of how anion doping enhances the dielectric stability at low frequencies or study the doping mechanism.…”

Thiazolothiazole‐Based Quinoidal Compounds for High‐Performance n‐Channel Organic Field‐Effect Transistors with Low‐Cost Metal Electrodes

“…The research works in the OFETs are mainly focused on improving the intrinsic electrical properties of organic semiconductors and the quality of OSC layers. [ 11–15 ] Besides organic semiconductor, source/drain electrodes have strong impact on the device performance since they affect the charge carrier injection and the electrodeorganic semiconductor interface. [ 16 ] Currently, nearly all of the high performance OFETs use gold as the source–drain (S–D) electrodes.…”