High-performance amorphous (α−) InGaZnO-based thin film transistors (TFTs) were fabricated on flexible polyethylene terephthalate substrates coated with indium oxide (In2O3) films. The InGaZnO films were deposited by rf magnetron sputtering with the presence of O2 at room temperature. The n-type carrier concentration of InGaZnO film was ∼2×1017 cm−3. The bottom-gate-type TFTs with SiO2 or SiNx gate dielectric operated in enhancement mode with good electrical characteristics: saturation mobility 11.5 cm2 V−1 s−1 for SiO2 and 12.1 cm2 V−1 s−1 for SiNx gate dielectrics and drain current on-to-off ratio >105. TFTs with SiNx gate dielectric exhibited better performance than those with SiO2. This is attributed to the relatively high dielectric constant (i.e., high-k material) of SiNx. After more than 500 h aging time at room temperature, the saturation mobility of the TFTs with SiO2 gate dielectric was comparable to the as-fabricated value and the threshold voltage shift was 150 mV.
Ti ( 200 Å ) ∕ Au ( 800 Å ) Ohmic contacts to n-type amorphous indium zinc oxide (IZO) films with carrier concentrations of (1×1015)–(5×1020)cm−3 showed as-deposited specific contact resistances in the range of (3×10−1)–(1×10−4)Ωcm2. Postgrowth annealing from 200to500°C resulted in significant improvement in contact resistances due to increase of the carrier concentration in the near surface region of IZO layer, which can be attributed to the formation of Ti–O alloy phases that induce oxygen vacancies in the IZO. After annealing at 500°C, the lowest contact resistance of 8×10−6Ωcm2 was achieved in the sample with carrier concentration of 5×1020cm−3. Temperature dependent measurement showed that tunneling was dominant transport mechanism in the contacts on the most highly doped films (n∼5×1020cm−3) and thermionic emission on the most lightly doped films (n∼1×1015cm−3).
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The performance of amorphous InGaZnO4 thin film transistors with three different gate dielectrics (SiO2, SiON, and SiNx) is reported. The gate dielectric films were prepared by plasma enhanced chemical vapor deposition with SiH4, N2O, and NH3 gases at 250°C. Current-voltage (C-V) and refractive index characterization showed that different film compositions were achieved when changing the flow rate of N2O in the precursor gas mixture. The use of SiNx films reduced the interfacial roughness of the channel/gate dielectric by a factor of 2.3 compared to SiO2. This results in an improvement of saturation mobility of the thin film transistors (TFTs) by a similar factor. An enhancement of subthreshold gate-voltage swing and drain current on-to-off ratio for TFTs with SiNx was also attributed to the reduction of the trap density at the channel/gate-dielectric interface.
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