ALD/ALE 2022 Session AA1-TuA: ALD for Display Applications

Oxide semiconductors have already been adopted for mass-production of display backpanes because of their advantages of high field effect mobility (10~30 cm²/Vs), large-area uniformity, low-cost manufacturing and low-temperature process. The next generation of display technology such as super high vision and memory/logic technology requires the semiconductor which has electron mobility higher than 30 cm²/Vs with high stability. In addition, for application in 3D structures such as 3D NAND, uniform thickness and composition control in 3D structures are required. Therefore, further than the conventional PVD method, it is necessary to study ALD-based oxide semiconductors that can control the thickness of an atomic level and form a film with low defects based on self-limit reaction. Furthermore, the ALD method facilitates the development of high-performance oxide semiconductors by controlling the homogeneous/heterogeneous vertical structure and composition. Therefore, ALD is suitable as a powerful deposition method candidate for oxide semiconductor development. However, since most research of stacking oxide semiconductor which is the methods to overcome the mobility and reliability trade-off is based on sputter or solution deposition, the ALD-based oxide semiconductor stack studies have rarely been reported. In this study, PEALD-based IZO (back-channel)/IGZO top gate thin film transistor investigated relation between the thickness of the IZO layer and electrical/reliability properties of devices. The mobility increases proportionally according to the IZO thickness. In addition, the PBTS reliability is excellent with an absolute value ΔV_th of less than 0.4 V in all of PEALD based TG-TFTs. In particular, the reliability of PBTS is improved proportionally according to the IZO thickness in IZO/IGZO TFT compared to IGZO TFT. Finally, we fabricated PEALD IZO/IGZO TG-TFTs with high mobility (~40 cm²/Vs) and high stability of PBTS under 10800 s (ΔV_th = -0.07 V) through nano scale thickness control.

In the past decades, aluminum oxide (Al₂O₃) has attracted attention because its unique properties such as a wide band gap (~9 eV), a reasonable breakdown electric field (5-10 MV/cm), excellent dielectric properties (6–9), strong adhesion to various materials, and high thermal/chemical stability. There are various Al₂O₃ deposition methods. Among them, atomic layer deposition (ALD) is regarded as a promising conformal film deposition tool. Although ALD-derived Al₂O₃ films have abundant advantages, the ALD method is not always compatible with industrial needs because of its extremely low growth rate (0.1 nm/s). Therefore, many groups have suggested spatially separated ALD (S-ALD) concept to increase growth rate for mass production. In the S-ALD operation, both precursor and reactant are continuously injected and purged from different zones. A moving substrate crosses each zone for chemical reactions between the adsorbed precursor and reactant, and the time-consuming purge steps are no longer needed. Meanwhile, as market demand increases for flexible display, several researchers developed the S-ALD method that works at atmospheric pressure (AP S-ALD). These include roll-to-roll/sheet-to-sheet processes and low investment costs. Although the AP S-ALD have gained increasing interest, the AP S-ALD-derived Al₂O₃ film properties have not clearly observed as a function of deposition temperatures to date. Furthermore, the possible applications of the oxide-based thin film transistors (TFTs) as an insulator should be evaluated.

In this work, we report AP S-ALD-derived Al₂O₃ growth behaviors with different process parameters. For more in-depth growth temperature studies, we conducted X-ray reflectometry (XRR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) analyses. We fabricated metal-insulator-metal (MIM) devices to evaluate the dielectric constant and breakdown electric field. Based upon results, we used the optimal Al₂O₃ as a B.L and G.I to investigate the application in PEALD IGZO TFT. We evaluated instability of the IGZO TFT as a function of gate field stress time and mechanical bending cycle number to understand the Al₂O₃-adopted PEALD-IGZO TFT flexible display.

Tin monoxide (SnO) is promising p-type material which have low formation energy of tin vacancy (V_Sn) and high hole mobility arise from the delocalization of hole conduction path. However, low thermal stability of p-type SnO and facile phase transition to n-type tin dioxide (SnO₂) is major hardship to achieve superior electrical properties and stability^1,2. In this study, we focused on the effect of Al₂O₃ on SnO film properties especially on the crystal structure and electrical performance. Al₂O₃ is already known for effective materials for the passivation layer of SnO TFT in many reports. However, we figured out that Al₂O­₃ not only passivate the surface defect of SnO but also highly influence on the crystallinity and following electrical properties. Also, this effect is enhanced when Al₂O₃ is deposited as in-situ ALD process. To identify the mechanism of the improvement of crystallinity, the nucleation energy and the chemical potential difference of SnO and Al₂O₃ stacked SnO crystallites is calculated. SnO TFT with in-situ processed Al₂O₃ exhibits 1.14 cm²/Vs of field-effect mobility, 4.4E+05 of on/off ratio and low subthreshold swing as 0.15 V/decade. Our study confirms that the mechanism of the improvement in electrical performance of SnO TFT when Al₂O₃ passivation layer is adopted, and in-situ process is far more effective to achieve high performance.

References
(1) Fortunato, E.; Barquinha, P.; Martins, R. Oxide Semiconductor Thin-Film Transistors: A Review of Recent Advances. Adv. Mater. 2012, 24 (22), 2945−2986.

(2) Luo, H.; Liang, L. Y.; Cao, H. T.; Liu, Z. M.; Zhuge, F.Structural, Chemical, Optical, and Electrical Evolution of SnO x Films Deposited by Reactive Rf Magnetron Sputtering. ACS Appl. Mater. Interfaces 2012, 4 (10), 5673−5677

ZnO thin films were deposited by atomic layer deposition (ALD) using diethylzinc (DEZ) as the Zn source and H₂O and H₂O₂ as oxygen sources. The oxidant- and temperature-dependent electrical properties and growth characteristicsare systematically investigated. Materials analysis results suggest that H₂O₂ provides an oxygen-rich environment so that the oxygen vacancies (V_O) is suppressed, implying a lower carrier concentration and a higher resistivity. The lower growth rate makes it possible for the ZnO thin films to grow along the lower surface energy direction of <002>, leading to a lower Hall mobility. Furthermore, the ZnO semiconductor was integrated into thin film transistor (TFT) devices, and the electrical properties are analyzed. The TFT with H₂O₂-ZnO grown at 150 °C shows good electrical properties, such as a high field-effect mobility of 10.7 cm² V^-1 s^-1, a high ratio I_on/I_off of 2×10⁷, a sharp subthreshold swing (SS) of 0.25 V dec.^-1, and a low trapping state (N_trap) of 2.77×10¹² eV^-1 cm^-2, which provides a new pathway to optimize the performance of metal-oxide electronics.