A Study on the Crystallized IGZO via Plasma-enhanced Atomic Layer Deposition for Emerging Display Applications

Abstract

Since the InGaZnO (IGZO) materials were discovered, numerous research has been reported due to their favorable characteristics for applying TFT applications such as low-off current and relatively high mobility. In particular, sputter-based IGZO using the target metal composition of In: Ga: Zn = 1: 1: 1 has been extensively studied to enhance electrical properties because of the advantages of its uniformity and high deposition rates. However, most recently, as the developing and expanding application fields, conventional IGZO has a challenging aspect because higher mobility and excellent step-coverage are required to be applied to high-resolution displays and 3-dimensional NAND. In this regard, Atomic layer deposition (ALD) has attractive potential as a deposition method for next-generation applications because of dense film growth and excellent step-coverage due to self-limiting reaction. From a materials approach, the crystallized IGZO has the potential to enhance device properties because it has stable oxygen binding states due to its crystal structure. However, despite these advantages, conventional deposition methods usually require high temperatures to grow crystallized IGZO, resulting it challenging to apply to devices. In this aspect, the approach to PEALD for IGZO growth is a promising method because it could adjust crystal structure while having the advantages of the ALD. Firstly, we studied the effect of crystal structure on the plasma energy of mono or multi-component metal oxide. As the plasma energy increases, it is confirmed that the crystallinity of mono and multi-component metal oxide increases even at a low growth temperature. Secondly, we systematically synthesized IGZO films with various compositions by the supercycle technique of plasma enhanced ALD (PEALD) to determine the optimum metal cation composition range of the IGZO system for high mobility thin film transistors (TFTs). The trends in the metal composition-dependent electrical properties of ALD-processed IGZO films are comparable to the previously reported results, while the structural properties were exclusive. As a crystalline-aligned structure, TFT characteristics and reliability were robust due to its high film density. Lastly, the optimal In-Ga-Zn region is newly demonstrated via PEALD because both microstructure and carrier concentration affect device characteristics complexly. In this region, the device exhibited a remarkably high µFE of 41.4-43.7 cm2/Vs, a low S.S of 0.24-0.25 V/decade, an initial Vth of -0.9 ~ -1.0 V, and a slight Vth shift (0.01 V) under PBTS.