Source-drain resistance characteristics of back-channel etched amorphous InGaZnO thin film transistors with TiO2:Nb protective layer

This work investigates the source-drain (S-D) parasitic resistance (RSD) characteristics of the back-channel-etched (BCE) a-IGZO TFTs with ultra-thin Nb doped TiO2 (TNO) protective layer. It is shown that RSD is strongly related to the thickness of the TNO protective layer although the electrical performances of the BCE a-IGZO TFTs with different TNO thickness are similar to each other. The BCE TFT with 3 nm TNO shows an unusually large RSD value (300 Ω cm). It is suggested that a ~3 nm TNO depletion layer should be formed at the TNO/a-IGZO interface in the S-D region in this case. In addition, RSD of the BCE TFTs with 1 and 5 nm TNO is 11 and 26 Ω cm, respectively. The low RSD of these two devices is caused by much thinner TNO depletion layers in the S-D region. Besides, a moderate RSD of 53 Ω cm for the S-D lift-off device can be ascribed to a lower a-IGZO band bending at the Mo/a-IGZO interface than that of the BCE devices at the TNO/a-IGZO interface.