Impact of atomic layer deposition supercycle design on the reliability of heterogenous IGZO channel TFTs

Abstract

Oxide semiconductor thin-film transistors (TFTs) are promising candidates for application in display backplanes and memory devices. To ensure both high performance and stable electrical properties of oxide TFTs over long-term use, superior mobility and device stability need to be obtained. In this regard, atomic layer deposition (ALD) has gained attention as a facile method for obtaining conformal oxide films under defect-free conditions and adjusting the composition or distribution for targeted TFT performance. Here, we propose that, even with the same bulk composition, variations in the local composition using an ALD supercycle design can significantly affect the TFT performance, particularly the bias-temperature stability. To ascertain the influence of the local cation distribution on the performance, we compared two supercycle designs by reversing the sub-cycles of indium, gallium, and zinc with identical bulk In–Ga–Zn ratios. Despite minimal changes in the electrical characteristics, such as the mobility and threshold voltage, of the two TFTs designed with ALD supercycles, there was a significant difference in their reliability. Devices with an indium-rich IGZO active layer on the back channel exhibited a hump phenomenon and approximately five times greater Vth degradation (ΔVth: −0.67 V to −3.36 V). By conducting an additional case study of IGZO TFTs and film analysis, we confirmed that mobility is largely governed by the bulk composition, whereas the local surface composition, serving as the back channel, is the key to reliability. The device performance results could be confirmed through the surface and bulk analyses of heterogeneous films engineered through the ALD supercycle design. Furthermore, it has been proposed that optimizing the ALD supercycle design is the key to obtaining high-performance oxide semiconductor TFTs.