In–Ga–Zn–O (IGZO) sputtering in an Ar, O2, and H2 atmosphere followed by annealing represents an effective method for defect reduction of the resulting films. The carrier density of Ar+O2+H2-sputtered IGZO films increases with increasing H2 amount during sputtering; however, we found that the increased carrier density was markedly decreased by annealing even at 150 °C. The Ar+O2+H2-sputtered IGZO was used as the active channel in thin-film transistors (TFTs), which led to markedly improved electrical properties after annealing at 150 °C, compared with those obtained using conventional Ar+O2-sputtered films. The proposed method is very promising for low-temperature-processed oxide TFTs.

High-performance In–Ga–Zn–O (IGZO) Schottky diodes (SDs) were fabricated using hydrogenated IGZO (IGZO:H) at a maximum process temperature of 150 °C. IGZO:H was prepared by Ar + O2 + H2 sputtering. IGZO:H SDs on a glass substrate exhibited superior electrical properties with a very high rectification ratio of 3.8 × 1010, an extremely large Schottky barrier height of 1.17 eV, and a low ideality factor of 1.07. It was confirmed that the hydrogen incorporated during IGZO:H deposition increased the band gap energy from 3.02 eV (IGZO) to 3.29 eV (IGZO:H). Thus, it was considered that the increase in band gap energy (decrease in electron affinity) of IGZO:H contributed to the increase in the Schottky barrier height of the SDs. Angle-resolved hard X-ray photoelectron spectroscopy analysis revealed that oxygen vacancies in IGZO:H were much fewer than those in IGZO, especially in the region near the film surface. Moreover, it was found that the density of near-conduction band minimum states in IGZO:H was lower than that in IGZO. Therefore, IGZO:H played a key role in improving the Schottky interface quality, namely, the increase of Schottky barrier height, decrease of oxygen vacancies, and reduction of near-conduction band minimum states. Finally, we fabricated a flexible IGZO:H SD on a poly­(ethylene naphthalate) substrate, and it exhibited record electrical properties with a rectification ratio of 1.7 × 109, Schottky barrier height of 1.12 eV, and ideality factor of 1.10. To the best of our knowledge, both the IGZO:H SDs formed on glass and poly­(ethylene naphthalate) substrates achieved the best performance among the IGZO SDs reported to date. The proposed method successfully demonstrated great potential for future flexible electronic applications.

We developed low-temperature (150 °C) processed top-gate and coplanar metal-semiconductor field-effect transistors (MES-FETs) with a stacked In-Ga-Zn-O (IGZO) channel consisting of a hydrogenated IGZO (IGZO:H) on conventional IGZO (IGZO). The IGZO and IGZO:H films were prepared by Ar + O 2 and Ar + O 2 + H 2 sputtering, respectively. By applying an IGZO:H on IGZO (IGZO:H/IGZO) stacked channel, the on-current of the MES-FET significantly increased while maintaining a low off-current. An on-off current ratio of 4.2 × 10 8 , a turn-on voltage of −5.9 V and a subthreshold swing of 155 mV decade −1 were achieved by the IGZO:H/IGZO = 25/25 nm stacked channel. The carrier concentration of the IGZO: H/IGZO stacked film dramatically increased to 1.4 × 10 19 cm −3 by stacking the IGZO:H (4.4 × 10 17 cm −3 ) film on the IGZO (7.6 × 10 17 cm −3 ) film. It was suggested that diffused hydrogens from IGZO:H to IGZO enhanced carrier concentration in the IGZO near the IGZO:H/IGZO interface, which acts as a pseudo two dimensional electron gas. Moreover, a potential barrier was formed at the IGZO:H/IGZO interface, which could have helped to maintain the low off-current of the MES-FETs. Therefore, the IGZO:H/IGZO stacked channel plays an important role in improving the performance of the MES-FETs.

Heterojunction channel engineering is discussed as a means of enhancing the performance and bias stress stability of In-Ga-Zn-O thin-film transistors (IGZO TFTs). A heterojunction channel was formed by depositing In-rich IGZO on IGZO111 with an atomic ratio of In:Ga:Zn=1:1:1, whose energy band lineup was type-II with a potential well of 0.4 eV for electrons. The thickness of the bottom IGZO111 layer strongly affected the field-effect mobility (μ FE ) of the TFT owing to electron confinement effect at the heterojunction interface. μ FE was enhanced from 12.4 cm 2 V −1 s −1 for homogeneous IGZO111 TFT to 24.7 cm 2 V −1 s −1 for the heterojunction TFT. Device simulation results revealed that electron confinement in the heterojunction channel played an important role in determining μ FE in the TFT. A channel engineering approach will provide a method of overcoming the trade-off between μ FE and the positive-bias stress stability of oxide TFTs.

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