Mechanisms of ZnO buffer layer in bottom gate ZnO:Al transparent thin film transistors

The structure of Al-doped ZnO (ZnO:Al) transparent thin-film transistors (TTFTs) were deposited at room temperature using a radio frequency magnetron cosputter system. The performances of the ZnO:Al TTFTs were improved by inserting a ZnO buffer layer between the ZnO:Al channel layer and the SiO2 gate insulator. The ZnO:Al TTFTs with 80-nm-thick ZnO buffer layer exhibited a higher field-effect mobility of 90.1 cm2 (V s)−1, a lower subthreshold slope of 0.24 V/decade and a lower maximum surface state density of 2.69 × 1011 eV−1 cm−2. The associated on-to-off current ratio of the TTFTs was 1.2 × 108. The performance improvement of the ZnO:Al TTFTs was attributed to crystalline improvement and the releasing functions of lattice mismatching and strain between the ZnO:Al channel layer and the SiO2 insulator layer.