Improvements in the device characteristics of IZO-based transparent thin-film transistors with co-sputtered HfO2–Al2O3 gate dielectrics

Transparent oxide thin-film transistors (TFTs) have been fabricated by RF magnetron sputtering at room temperature using amorphous indium zinc oxide (IZO) as both active channel and source/drain electrodes and co-sputtered HfO2–Al2O3 (HfAlO) as gate dielectric. In spite of its high dielectric constant, HfO2 has some drawbacks including a high leakage current and rough surface morphologies due to its small band gap (5.31 eV) and microcrystalline structure. In this work, the incorporation of Al2O3 into hafnium (Hf) was achieved by co-sputtering of HfO2 and Al2O3 without any intentional substrate heating, and its structural and electrical properties were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), and spectroscopic ellipsometry (SE) analyses, depending on the Hf:Al compositional ratio. The XRD studies confirmed that the microcrystalline structures of HfO2 were transformed to amorphous structures of HfAlO at approximately Hf:Al = 3:1 at% ratio. According to the AFM analysis, the HfAlO films (0.54 nm) were considerably smoother than the HfO2 films (4.27 nm) due to their amorphous structure. The band gap (Eg) deduced by spectroscopic ellipsometry was increased from 5.31 eV (HfO2) to 5.58 eV (HfAlO). The electrical performances of the TFTs, which were made of well-controlled active/electrode IZO materials and co-sputtered HfAlO dielectric material, exhibited a field-effect mobility of more than 10 cm2/V s, a threshold voltage of about 1.5 V, a sub-threshold swing of about 0.5 V/decade, and an Ion/off ratio of about 105.