High-Performance Thin-Film Transistor with Atomic Layer Deposition (ALD)-Derived Indium–Gallium Oxide Channel for Back-End-of-Line Compatible Transistor Applications: Cation Combinatorial Approach

In this paper, the feasibility of an indium-gallium oxide (In_2(1-x)Ga_2xO_y) film through combinatorial atomic layer deposition (ALD) as an alternative channel material for back-end-of-line (BEOL) compatible transistor applications is studied. The microstructure of random polycrystalline In₂O_y with a bixbyite structure was converted to the amorphous phase of In_2(1-x)Ga_2xO_y film under thermal annealing at 400 °C when the fraction of Ga is ≥29 at. %. In contrast, the enhancement in the orientation of the (222) face and subsequent grain size was observed for the In_1.60Ga_0.40O_y film with the intermediate Ga fraction of 20 at. %. The suitability as a channel layer was tested on the 10-nm-thick HfO₂ gate oxide where the natural length was designed to meet the requirement of short channel devices with a smaller gate length (<100 nm). The In_1.60Ga_0.40O_y thin-film transistors (TFTs) exhibited the high field-effect mobility (μ_FE) of 71.27 ± 0.98 cm²/(V s), low subthreshold gate swing (SS) of 74.4 mV/decade, threshold voltage (V_TH) of -0.3 V, and I_ON/OFF ratio of >10⁸, which would be applicable to the logic devices such as peripheral circuit of heterogeneous DRAM. The in-depth origin for this promising performance was discussed in detail, based on physical, optical, and chemical analysis.