Flexible Amorphous Indium-Gallium-Zinc Oxide Thin-Film Transistor with Aluminum Oxide Gate Insulator on Polyimide Substrate By Spray Pyrolysis for Foldable Display
Authors: Bae Jinbaek , Jang Jin 
Published: November 2022

Research interest in thin-film transistor (TFT)-based flexible electronics for next-generation foldable and rollable displays is increasing, centering on flexible plastic substrates such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyimide (PI). For flexible electronics using plastic substrates, it is important to select suitable semiconductor materials that can be deposited at low substrate temperature. Metal oxide (MO) semiconductors such as indium-gallium-zinc oxide (IGZO), zinc oxide (ZnO) are attracting attention as suitable candidates for plastic substrate electronics because of their higher mobility and optical transmittance than hydrogenated amorphous silicon (a-Si:H). In addition, since it can be processed at a lower temperature than low-temperature polysilicon (LTPS), it can be easily applied to flexible plastic substrates. One of the greatest advantages of MO is that it can be easily fabricated by conventional semiconductor processing methods such as sputtering, atomic laser deposition, and solution processes. Among them, solution-processed deposition method is particularly advantageous in terms of manufacturing efficiency because it does not require a separate vacuum system, and can efficiently utilize raw materials, so the fabrication cost is inexpensive. In particular, spray pyrolysis can guarantee environmental stability, high mobility, and large area production than other solution processes. In this work, we developed the high-performance, flexible a-IGZO/Al2O3 TFTs using spray pyrolysis fabricated on the flexible PI substrate. In a previous study, we developed a method to reduce curling and electrostatic discharge (ESD) damage to PI substrates by using a mixture of carbon nanotubes (CNTs) and graphene oxide (GO) between carrier glass and PI substrate. Therefore, it was possible to mechanically detach the TFTs on PI substrate and handle them easily even after detachment. The spray-pyrolyzed Al2O3 film is one of the high-k dielectric materials, which can lower the operating voltage, so the low-power consumption of the TFT backplane can be realized. The flexible spray-pyrolyzed a-IGZO/Al2O3 TFT exhibits the field-effect mobility (µFE) of 9.75 cm2 V−1 s−1, threshold voltage (VTH) of −0.8 V, subthreshold swing (SS) of 0.29 V dec−1, and on/off current ratio of 6.2 × 107. Even without TFT passivation layer, it shows environmental stability. The TFTs show robust electrical performance even under tensile strain with a bending radius of 1 mm. In addition, it showed stable electrical characteristics even in the repeated bending test of 30,000 cycles under compressive strain with a bending radius of 1 mm. This is due to fewer defects resulting from the smooth interface of the a-IGZO film and the Al2O3 film deposited through spray pyrolysis. Therefore, spray-pyrolyzed a-IGZO/Al2O3 TFT fabricated on PI substrate is a promising candidate for next-generation foldable display application.

Figure 1