Far-UV Annealed Inkjet-Printed In₂O₃ Semiconductor Layers for Thin-Film Transistors on a Flexible Polyethylene Naphthalate Substrate

Abstract

The inkjet-printing process of precursor solutions containing In nitrate dissolved in 2-methoxyethanol is optimized using ethylene glycol as a cosolvent that allows the stabilization of the droplet formation, leading to a robust, repeatable printing process. The inkjet-printed precursor films are then converted to InO semiconductors at flexible-substrate-compatible low temperatures (150-200 °C) using combined far-ultraviolet (FUV) exposure at ∼160 nm and thermal treatment. The compositional nature of the precursor-to-metal oxide conversion is studied using grazing incidence X-ray diffraction (GIXRD), X-ray reflectivity (XRR), and Fourier transform infrared (FTIR) spectroscopy that indicate that amorphous, high density (up to 5.87 g/cm), and low impurity InO films can be obtained using the combined annealing technique. Prolonged annealing (180 min) at 150 °C yields enhancement-mode TFTs with saturation mobility of 4.3 cm/(Vs) and ∼1 cm/(Vs) on rigid Si/SiO and flexible plastic PEN substrates, respectively. This paves the way for manufacturing relatively high-performance, printed metal-oxide TFT arrays on cheap, flexible substrate for commercial applications.

Printed, Highly Stable Metal Oxide Thin‐Film Transistors with Ultra‐Thin High‐κ Oxide Dielectric

“…Printed TFTs that were fabricated with thermal annealing combined with FUV irradiation show excellent performance at low temperature (≤200 °C), as depicted in Figure a. The semiconductor layer (In 2 O 3 ) was inkjet‐printed on top of the flexography‐printed dielectric layer (Al 2 O 3 ) using a nitrate‐based semiconductor ink reported earlier 36. The annealing time of the semiconductor was varied from 30 to 90 min at both 180 and 200 °C annealing temperatures.…”

“…Since the second peak in the combustion xerogel occurs at a higher temperature, (233 °C) than the annealing temperature used to form the dielectric thin films, UV treatment was applied. When low‐wavelength far‐UV (FUV) irradiation (160 nm) is combined with thermal annealing the required process temperature is decreased as the high‐energy UV photons enhance the degradation of organic residuals and improve the M‐O‐M densification through radical mediated reactions 28,35,36…”

“…When the films are exposed to high‐energy photons, the cleavage of alkoxy groups into active metals and oxygen atoms occurs to simplify the metal–oxygen–metal (M‐O‐M) network formation through radical‐mediated chemical reactions. Consequently, the thin films' densification is improved 28,35,36. Even higher efficiency can be achieved by combining solution combustion synthesis (SCS) with UV curing at low temperature as previously reported by our group.…”

“…The flexographic printing of the Al 2 O 3 thin films onto flexible substrates was investigated by assessing the effect of number of layers and annealing process conditions (combined with low‐temperature and UV irradiation) on the printing quality. Printed low‐voltage solution‐based flexible oxide TFTs were pursued by inkjet printing an indium oxide (In 2 O 3 ) semiconductor layer that was annealed at low‐temperature using combined UV and thermal annealing 36. The operational stability under bias‐stress, ageing during storage, and mechanical bending stress were studied to probe the usability of the printed oxide TFTs in flexible applications.…”

The self-activated radical doping effects on the catalyzed surface of amorphous metal oxide films

“…Recently, these AOSs were used in various flexible devices 4 and a sensor array 5 as the active layer. However, the AOSs devices still suffer from instability issues such as illumination, bias, and temperature stress.…”