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.
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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.