Rapid low-temperature processing of metal-oxide thin film transistors with combined far ultraviolet and thermal annealing

Abstract

We propose a combined far ultraviolet (FUV) and thermal annealing method of metal-nitrate-based precursor solutions that allows efficient conversion of the precursor to metal-oxide semiconductor (indium zinc oxide, IZO, and indium oxide, In2O3) both at low-temperature and in short processing time. The combined annealing method enables a reduction of more than 100 °C in annealing temperature when compared to thermally annealed reference thin-film transistor (TFT) devices of similar performance. Amorphous IZO films annealed at 250 °C with FUV for 5 min yield enhancement-mode TFTs with saturation mobility of ∼1 cm2/(V·s). Amorphous In2O3 films annealed for 15 min with FUV at temperatures of 180 °C and 200 °C yield TFTs with low-hysteresis and saturation mobility of 3.2 cm2/(V·s) and 7.5 cm2/(V·s), respectively. The precursor condensation process is clarified with x-ray photoelectron spectroscopy measurements. Introducing the FUV irradiation at 160 nm expedites the condensation process via in situ hydroxyl radical generation that results in the rapid formation of a continuous metal-oxygen-metal structure in the film. The results of this paper are relevant in order to upscale printed electronics fabrication to production-scale roll-to-roll environments.

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

“…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.…”

Flexography‐Printed In2O3 Semiconductor Layers for High‐Mobility Thin‐Film Transistors on Flexible Plastic Substrate

“…The process leads into high-mobility In 2 O 3 TFTs on ALD-grown amorphous Al 2 O 3 with µ sat ≈ 8 cm 2 V −1 s −1 which show enhancement-mode operation with reproducible turn-on voltages at ≈0 V after a lowtemperature post-contact-annealing step. By combining the demonstrated process with available UV-based low-temperature annealing schemes, [ 8,15,21 ] roll-printed metal-oxide TFT devices and circuits on low-cost plastic substrates can be achieved in the future. [ 22 ] the solution was stirred at 75 °C for more than 12 h and fi ltered prior using with a 0.45 µm pore size glass fi bre fi lter.…”

“…[ 36 ] The formation of thin (<1 nm) interface oxide layer between the Al-contacts and the semiconductor can be observed in the close-up TEM image in Figure 2 f, indicating the formation of a compound interface, [ 36 ] and giving direct evidence of AlO x formation during evaporation as also postulated by Xu et al [ 35 ] In addition, oxygen depletion from the surface of the In 2 O 3 fi lm during the exposure to low oxygen partial pressure ( p O2 ≈ 1 × 10 −9 atm, by assuming similar pumping rates for N 2 and O 2 ) can occur due to the relatively low formation entalphy ΔH (V O ) of oxygen vacancies at the surface of the In 2 O 3 fi lm. [ 37 ] However, by performing a low temperature post-contact-annealing step in air at 150 °C for 30 min, [ 8,35 ] the oxygen vacancies of the In 2 O 3 fi lm are brought closer to equilibrium with the oxygen partial pressure in normal conditions ( p O2 ≈ 0.2 atm) and the turn-on voltage, V on , of the In 2 O 3 TFTs can be reproducibly positioned to ≈0 V for enhancement-mode operation, as shown in Figure S6a, Supporting Information, for spin-coated reference devices on Si/SiO 2 .…”

“…A large amount of work has concentrated on lowering the conversion temperature of the precursor solutions to semiconducting metal-oxide layers. Functional TFTs have been obtained at ≈200 °C or below (1) via the careful design of precursor chemistry utilizing metal alkoxides with controlled hydrolysis, [ 14,15 ] combustion process, [ 11,19 ] or the addition of oxidizing agents, [ 20 ] (2) via the use of additional energy in conversion such as various wavelengths of UV light, [ 8,15,21 ] or microwaves, [ 22 ] or (3) by employing conditions during annealing that promote effi cient precursor conversion such as ozone or vacuum. [ 6,7,20 ] After the fi rst report on inkjet-printed metal oxide layers from metal chloride precursors by Lee et al in 2007, [ 16 ] the deposition of metal-oxide semiconductor layers for TFTs has been successfully performed via several printing techniques.…”

“…[ 2,3 ] Although undoped In 2 O 3 possess high bulk charge-carrier concentration limiting its usability as semiconductor channel, thin (≈10 nm) In 2 O 3 semiconductor layers have recently been successfully utilized as semiconductor layers for high-mobility solution-processed TFTs, albeit in some cases with nonoptimal depletion-mode operation. [6][7][8][9][10][11][12] Metal-oxide semiconductor layers can be solution processed from stabilized nanoparticle dispersions or from sol-gel-precursor solutions. [ 3,4 ] Nanoparticles typically require high temperature sintering (>400 °C) to allow a connected network of particles and to remove dispersion stabilizing encapsulants.…”