*3.2.1.3. Impurity-free precursor-based approach*

The presence of impurities has a negative impact on the performance of oxide semiconductors. In this regard, the metal-hydroxide nanocluster is a viable alternative to metal-salt precursors due to the absence of impurity-containing chemical species. The metal hydroxide is converted into a metal oxide framework by a thermally activated reaction. Aqueous Zn hydroxide solution was shown to drastically lower the annealing temperature to 150°C, producing ZnO-TFT with a *μ* of 0.4 cm<sup>2</sup> V−1 s−1 [53]. Another approach for the impurity-free precursor is the use of aqueous carbon-free metal-oxide precursors such as zinc oxide hydrate dissolved in ammonium hydroxide to generate Zn ammonium complex precursor [54]. This route allows for the growth of ultra-thin (4–5 nm), high-quality polycrystalline ZnO films on arbitrary substrates. Transistors fabricated using this simple process at 180°C showed an electron mobility of up to 11.0 cm<sup>2</sup> V−1 s−1.

Formation of the ammine-hydroxo complex and its low-temperature conversion to corresponding oxide, however, are generally not convenient, except for Zn. Thus, the design of a new chemical complex with the easy accessibility to other elements and the low-temperature processability for chemical transformations are essential in the impurity-free precursor-based approach.
