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) (In 1– x Ga x O) films as high-mobility n-channel semiconducting layers was investigated. Different Ga concentrations in 10–13 nm thick In 1– x Ga x O 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 In 0.66 Ga 0.34 O film show unexpectedly high carrier mobility (μ FE ) values of 60.7 ± 1.0 cm 2 V –1 s –1, a threshold voltage ( V TH ) of −0.80 ± 0.05 V, and an I ON/OFF ratio of 5.1 × 10 9 in field-effect transistors (FETs). In contrast, the FETs having polycrystalline In 1– x Ga x O films with higher In fractions ( x = 0.18 and 0.25) showed reasonable μ FE values of 40.3 ± 1.6 and 31.5 ± 2.4 cm 2 V –1 s –1, V TH of −0.64 ± 0.40 and −0.43 ± 0.06 V, and I ON/OFF ratios of 2.5 × 10 9 and 1.4 × 10 9, respectively. The resulting superior performance of the In 0.66 Ga 0.34 O-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 In 0.66 Ga 0.34 O transistor was found to show the most stable behavior against an external gate bias stress.