Comparative Study of Atomic Layer Deposited Indium-Based Oxide Transistors with a Fermi Energy Level-Engineered Heterojunction Structure Channel through a Cation Combinatorial Approach.

Amorphous indium-gallium-zinc oxide ( a -IGZO) has become a standard channel ingredient of switching/driving transistors in active-matrix organic light-emitting diode (AMOLED) televisions. However, mobile AMOLED displays with a high pixel density (≥500 pixels per inch) and good form factor do not often employ a -IGZO transistors due to their modest mobility (10-20 cm 2 /(V s)). Hybrid low-temperature polycrystalline silicon and oxide transistor (LTPO) technology is being adapted in high-end mobile AMOLED devices due to its ultralow power consumption and excellent current drivability. The critical issues of LTPO (including a complicated structure and high fabrication costs) require a search for alternative all-oxide thin-film transistors (TFTs) with low-cost processability and simple device architecture. The atomic layer deposition (ALD) method is a promising route for high-performance all-oxide TFTs due to its unique features, such as in situ cation composition tailoring ability, precise nanoscale thickness controllability, and excellent step coverage. Here, we report an in-depth comparative investigation of TFTs with indium-gallium oxide (IGO)/gallium-zinc oxide (GZO) and indium-zinc oxide (IZO)/GZO heterojunction stacks using an ALD method. IGO and IZO layers with different compositions were tested as a confinement layer (CL), whereas the GZO layer was used as a barrier layer (BL). Optimal IGO/GZO and IZO/GZO channels were carefully designed on the basis of their energy band properties, where the formation of a quasi-two-dimensional electron gas (q2DEG) near the CL/BL interface is realized by rational design of the band gaps and work-functions of the IGO, IZO, and GZO thin films. To verify the effect of q2DEG formation, the device performances and stabilities of TFTs with CL/BL oxide heterojunction stacks were examined and compared to those of TFTs with a single CL layer. The optimized device with the In 0.75 Zn 0.25 O/Ga 0.80 Zn 0.20 O stack showed remarkable electrical performance: μ FE of 76.7 ± 0.51 cm 2 /(V s), V TH of -0.37 ± 0.19 V, SS of 0.13 ± 0.01 V/dec, and I ON/OFF of 2.5 × 10 10 with low operation voltage range of ≥2 V and excellent stabilities (Δ V TH of +0.35, -0.67, and +0.08 V for PBTS, NBIS, and CCS, respectively). This study suggests the feasibility of using high-performance ALD-derived oxide TFTs (which can compete with the performance of LTPO transistors) for high-end mobile AMOLED displays.