Solution-Processed HfO_x for Half-Volt Operation of InGaZnO Thin-Film Transistors

Solution-processed high-κ gate dielectrics, such as Al2O3 and HfO2, show great potential for use in low-cost electronics. However, high-temperature treatments are generally needed to cure the film, limiting the use in flexible electronics. Here we use anodization, a simple, low-cost, solution-based method, to form thin, conformal, high quality HfOx layers at room temperature. Several anodization voltages were studied, and the relative quantity of Hf–O bonding was found to increase by 10% when increasing anodization voltage from 2 to 3 V, while maintaining almost the same when further increasing the anodization voltage. Conduction mechanisms were analyzed, showing that Schottky emission dominates even for the thinnest oxides. HfOx layers formed by anodization voltages of 2, 3, and 4 V show high capacitance density of 1290, 1160, and 1060 nF/cm2, which are equivalent to 2.68, 2.97, and 3.26 nm of thermally grown SiO2. Using such high capacitance density insulators, we fabricated InGaZnO (IGZO) thin-film transistors (TFTs) at room temperature, showing a low operating voltage of 0.5 V, one of the lowest for oxide–semiconductor-based TFTs. TFTs gated with HfOx anodized at the optimal voltage, 3 V, show a high current on/off ratio of >105, a mobility approaching 10 cm2/(V s), and a subthreshold swing as low as 69 mV/dec, close to the theoretical limit at 300 K. The interface trap density was found to be highly related to the quality of the HfOx film, showing a 50% decrease with the increase of anodization voltage from 2 to 3 V. The devices show negligible change after storage in air for 6 months and favorable stability under both positive and negative gate bias stress. Finally, we demonstrate TFTs on plastic substrates, maintaining high quality performance even under a bending radius of 9 mm. As a result, such devices have potential applications in low-cost, low-voltage-operation, low-power electronics.