Solution processed IGZO TFT depending Ga concentration
Electrical characteristics of solution-processed InGaZnO thin film transistors depending on Ga concentration
We investigated the role of Ga in solution-processed InGaZnO thin film transistors (TFTs). The incorporation of Ga into a InZnO compound system results in a decrease in the carrier concentration of the films and an off-current of TFTs. This is a result of the Ga ions forming stronger chemical bonds with oxygen, as compared to the Zn and In ions, acting as a carrier suppressor. It was verified, using X-ray photoelectron spectroscopy (XPS), that the vacancy-related oxygen 1s peak was decreased when the Ga content increased.
MoW, SiNx, and IZO layers were used as gate electrode, gate insulator, and source and drain electrodes, respectively. IGZO active channel layers of ~45 nm were deposited at 450 °C using a spin‐coating method. It was observed that the TFT without any Ga content operates in the depletion mode with a deep threshold voltage (Vth) of −10.32 V. The spin-coated active channel film, without any Ga, is very effective as a conductor. As the Ga content is increased in the IGZO, the Vth is shifted in the positive direction and the off current of the TFT dropped below 10−10A of IDS. This results in an improvement of the on‐to‐off current ratio (Ion/off). At a Ga/Zn ratio of 0.5 (below 1016 cm−3), the TFT device displays optimal performance. The linear mobility (µlin), Vth, Ion/off, and subthreshold voltage (S) were estimated to be 0.86 cm2/Vs, 6.89 V, ∼106, and 0.63 V/decade, respectively. From these results, it is apparent that the Ga content in the IGZO is a critical factor in determining the IGZO TFT operating characteristics as well as the electrical properties of solution IGZO films. Although it is not demonstrated here, for a Ga/Zn ratio above 0.5, the positive shift of turn on the voltage clearly occurs and the on current and mobility of the TFT was drastically reduced, much like a semi‐insulator.