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

• Published in: Current applied physics Vol. 11; no. 4S; pp. S135 - S138
• Main Authors: Son, Heegeun, Kim, Jeongsoo, Yang, Jungil, Cho, Dongkyu, Yi, Moonsuk
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.2011
• Subjects: aluminum oxide; atomic force microscopy; Dielectrics; electrical properties; Electrodes; Gates; Hafnium; Hafnium oxide; heat; indium; Semiconductor devices; spectroscopy; temperature; Thin film transistors; Transistors; X-ray diffraction; zinc oxide
• ISSN: 1567-1739; 1878-1675
• DOI: 10.1016/j.cap.2011.03.080

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.