Effects of Tantalum Doping on Electrical Characteristics of Indium-Gallium-Zinc Oxide Thin-Film Transistors

Abstract

Thin-film transistors (TFTs) that utilize oxide semiconductors as channel materials have been widely used in various electronic device applications, such as displays, sensors, and memory devices. Among the oxide semiconductors, transparent amorphous indium-gallium-zinc oxides (a-IGZO) have superior characteristics, namely, high field-effect mobility (~10cm2/Vs), good uniformity, low processing temperatures, optical transparency, and low-cost fabrication processability compared with conventional amorphous/poly-silicon TFTs. However, oxide TFTs including a-IGZO TFTs have bias stress-induced instabilities, which are still critical issues in launching commercial oxide TFT products. These issues can be attributed to the oxygen vacancy-related defects, which are known to act as a major source of electrons and as carrier traps. To suppress such oxygen vacancy-related defects, many researchers reported various techniques, such as ozone treatment, oxygen annealing on oxide TFTs, passivation, and metal-doped oxide semiconductors.In this study, we investigated the device performance of tantalum (Ta)-doped IGZO TFTs as a function of Ta content. Ta was used as a dopant because of its bond-dissociation-energy (BDE, 805kJ/mol) with oxygen higher than those of In, Ga, and Zn. The Ta-doped IGZO, which was adopted as a channel layer of bottom-gate TFTs, was fabricated using a magnetron co-sputtering method. Heavily doped p-type Si wafer with thermally grown SiO2 (300nm) was used as substrates. The RF power for the a-IGZO (In2O3 :Ga2O3 : ZnO = 1:1:1)target was fixed at 100 W while the power for the Ta target increased from 0 to 50 W. Aluminum (Al) was deposited as source/drain electrodes using an electron beam evaporator. With increasing Ta content in the IGZO films, the field-effect mobility (μFE) of the Ta-doped IGZO TFTs decreased, the threshold voltage (Vth) increased from negative to positive values, and the subthreshold swing (SS) became smaller with higher on/off ratio. Especially, Ta (10W)-doped IGZO TFTs exhibited μFE of 7.11 cm2/Vs, a SS of 0.195V/decade, and an on/off ratio ~1x108. These results show that the Ta-doping improves the switching characteristics of the Ta-IGZO TFTs. Furthermore, to evaluate the bias-stress stability, positive bias stress (PBS), negative bias stress (NBS) tests were conducted.