High-Performance Thin-Film Transistors with an Atomic-Layer-Deposited Indium Gallium Oxide Channel: A Cation Combinatorial Approach

The effect of gallium (Ga) concentration on the structural evolution of atomic-layer-deposited indium gallium oxide (IGO) (In1–xGaxO) films as high-mobility n-channel semiconducting layers was investigated. Different Ga concentrations in 10–13 nm thick In1–xGaxO films allowed versatile phase structures to be amorphous, highly ordered, and randomly oriented crystalline by thermal annealing at either 400 or 700 °C for 1 h. Heavy Ga concentrations above 34 atom % caused a phase transformation from a polycrystalline bixbyite to an amorphous IGO film at 400 °C, while proper Ga concentration produced a highly ordered bixbyite crystal structure at 700 °C. The resulting highly ordered In0.66Ga0.34O film show unexpectedly high carrier mobility (μFE) values of 60.7 ± 1.0 cm2 V–1 s–1, a threshold voltage (VTH) of −0.80 ± 0.05 V, and an ION/OFF ratio of 5.1 × 109 in field-effect transistors (FETs). In contrast, the FETs having polycrystalline In1–xGaxO films with higher In fractions (x = 0.18 and 0.25) showed reasonable μFE values of 40.3 ± 1.6 and 31.5 ± 2.4 cm2 V–1 s–1, VTH of −0.64 ± 0.40 and −0.43 ± 0.06 V, and ION/OFF ratios of 2.5 × 109 and 1.4 × 109, respectively. The resulting superior performance of the In0.66Ga0.34O-film-based FET was attributed to a morphology having fewer grain boundaries, with higher mass densification and lower oxygen vacancy defect density of the bixbyite crystallites. Also, the In0.66Ga0.34O transistor was found to show the most stable behavior against an external gate bias stress.