Abstract
Introduction
Recently, oxide semiconductors have attracted considerable attention as next-generation semiconductor materials. In2O3, one of the oxide semiconductors, is used as a material for low temperature polycrystalline oxide (LTPO) because it is a single crystal with high mobility of 160 cm2/Vs [1] and can crystallize at low temperatures below 200 ℃ and has conductivity. LTPO is generally used as a switching thin-film transistor (TFT) in organic light-emitting diode (OLED) displays because of its low off-leakage current and is expected to be applied to TFTs to drive OLEDs by improving their mobility. However, because of the trade-off between mobility and operational stability, achieving both has been challenging. Therefore, we focused on InGaO (IGO), which is a mixture of In2O3 with high mobility, and Ga2O3 with excellent insulating properties. We fabricated TFTs using a solution method combining a carbon-free aqueous precursor solution and deep ultraviolet excimer light and confirmed the superiority of the excimer light-assisted process by evaluating various characteristics [2, 3]. In this report, we described the details of the excimer light-assisted process, the surface observation results obtained by atomic force microscopy (AFM) with and without excimer light irradiation after In2O3, Ga2O3, and IGO thin films were fabricated using aqueous precursor solutions, and the characterization of TFT characteristics fabricated using the excimer light-assisted process.
Experimental method
In2O3, Ga2O3, and In0.5Ga0.5O precursor solutions were prepared to form the thin films, and the In2O3 precursor solution was prepared by mixing indium nitrate trihydrate (In(NO3)3・3H2O) and ultrapure water at 0.3 mol/L. The Ga2O3 precursor solution was prepared by mixing gallium nitrate octahydrate (Ga(NO3)3・8H2O) and ultrapure water at 0.3 mol/L. The In0.5Ga0.5O precursor solution was prepared by mixing In(NO3)3・3H2O, Ga(NO3)3・8H2O, and ultrapure water at a composition ratio of 1:1. Next, the solutions were applied to the hydrophilized EAGLE XG glass substrates to form In2O3, Ga2O3, and In0.5Ga0.5O thin films using the spin-coating method, and the glass substrates were rotated at 2000 rpm for 30 s to form the thin films. Subsequently, the samples were prepared with and without excimer light irradiation for 90 min.
Results and discussion
Fig. 1 illustrates the principle of the excimer light-assisted process and the results of surface observations of the In2O3, Ga2O3, and In0.5Ga0.5O thin films obtained by AFM. An aqueous solution of indium nitrate is decomposed by excimer light irradiation. Free radicals (・OH) generated during decomposition modify indium ions, which are converted to In2O3 by heat treatment in air. This effect is expected to promote the formation of In2O3 from the aqueous indium nitrate solution (Fig. 1(a)). Comparing the root mean square (RMS) roughness of the samples without and with excimer light irradiation, the RMS roughness of the samples without excimer light irradiation ranged from 2 to 6 nm, while the RMS roughness of the samples with excimer light irradiation was less than 1 nm (Fig. 1(b)).
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