Effect of Al concentration on Al-doped ZnO channels fabricated by atomic-layer deposition for top-gate oxide thin-film transistor applications

Al-doped ZnO (AZO) thin films were prepared by atomic layer deposition, in order to optimize their characteristics as active channel materials for oxide thin-film transistors (TFTs). The crystalline phases of the AZO films deposited at 100 °C appeared as mainly (100) planes, and their surface morphologies were homogeneous and smooth. The electrical conductivities of AZO films with varying Al concentrations were measured in-situ with a temperature sweep from 50 to 250 °C. From the conductivities, the activation energies for the carrier transport of each composition were calculated at some specified temperature ranges. A critical value of incorporated Al increased electrical conductivity by means of the donor doping effect: beyond that value, additional Al was found to act as a typical carrier suppressor. Top-gate-structured AZO TFTs were fabricated and characterized. The maximum field-effect mobility at the saturation region was found to be 0.64 cm² V^-1 s ^-1 when 1-at. %-Al was incorporated into the ZnO channel. Mobility showed a decreasing trend with increasing Al concentration. Negative and positive bias-stress (NBS and PBS) stabilities of the AZO-TFTs were also evaluated. The threshold voltage shifts during the NBS and PBS for 10 ⁴ s were as low as -1.8 and -0.9 V, respectively. Raising the Al concentration increased the NBS instabilities, whereas the PBS stabilities were improved by higher Al concentrations.