Investigation of Al-Doped ZnO Channel Layer in ZnO-Based Transparent Thin-Film Transistors

The enhancement mode and bottom gate Al-doped ZnO transparent thin-film transistors (ZnO:Al TTFTs) were investigated. To provide a suitable amount of free carriers and reduce the associated resistance, the ZnO:Al channel layer with Al imperceptibly doped into the ZnO was deposited using a cosputter system. To investigate the function and optimal thickness of the ZnO:Al channel layer, ZnO:Al layers of various thicknesses were deposited in the TTFTs. The maximum effective field-effect mobility (in the linear region) and the on/off current ratio of the ZnO:Al TTFTs with a 30-nm-thick ZnO:Al channel layer were 32.5 cm2 V-1 s-1 and larger than 107, respectively. In this work, the effective field-effect mobility of 32.5 cm2 V-1 s-1 is larger than the previous published performances of the ZnO TTFTs. To investigate the mechanisms of the optimal 30-nm-thick channel layer, the induced thickness of the channel layer was estimated. The estimated induced thickness is about 27 nm. The other 3-nm-thick ZnO:Al channel layer is used to passivate the induced channel layer.