Half-volt operation of IGZO thin-film transistors enabled by ultrathin HfO2 gate dielectric

Amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) enabled by an ultrathin, 5 nm, HfO2 film grown by atomic-layer deposition were fabricated. An ultra-low operation voltage of 1 V was achieved by a very high gate capacitance of 1300 nF/cm2. The HfO2 layer showed excellent surface morphology with a low root-mean-square roughness of 0.20 nm and reliable dielectric properties, such as low leakage current and high breakdown electric field. As such, the a-IGZO TFTs exhibit desirable properties such as low power devices, including a small subthreshold swing of 75 mV/decade, a low threshold voltage of 0.3 V, and a high on/off current ratio of 8 × 106. Furthermore, even under an ultralow operation voltage of 0.5 V, the on/off ratio was also up to 1 × 106. The electron transport through the HfO2 layer has also been analyzed, indicating the Poole-Frenkel emission and Fowler-Nordheim tunneling mechanisms in different voltage ranges.