*3.2.2. Physical pathways*

Elimination of impurities such as hydroxides, anions, and carbon along with the formation of oxygen vacancies is critical for determining properties of AOS because they not only hinder the sol-gel oxide framework formation reaction but also interrupt the efficient transport of charge carriers [49]. To enable low-temperature processable products, the thermal decomposition of metal precursors should be completed at a low temperature, as evidenced by the use of metal nitrate precursors, the anion of which is almost completely decomposed even at 250°C, instead of other metal precursors (chloride, acetate) [50]. In addition, the chemical structure should be tailored toward a framework containing less hydroxide, which can be

A "sol-gel on chip" hydrolysis approach from soluble metal alkoxide precursors was reported [52], which affords unprecedented high *μ* of ~10 cm<sup>2</sup> V−1 s−1, reproducible and stable turn-on voltage (~0 V) at maximum process temperature as low as 230°C. The approach uses the *insitu* hydrolysis and condensation of transition metal alkoxides when they are exposed to an aqueous environment by nucleophilic substitution, thus affording the M-O-M framework at low temperature. The process is applicable to a broad range of AOS that are of immediate

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.

A redox-based combustion synthetic approach was applied to oxide thin films using acetylacetone or urea as a fuel and metal nitrates as oxidizers [55]. The self-energy-generating combustion chemistry provides a localized energy supply, eliminating the need for high, externally applied processing temperatures. Furthermore, the atomically local oxidizer supply can efficiently remove organic impurities without coke formation in combustion reactions with balanced redox

and Al2

O3

annealed at temperature as low as 200°C yielded TFTs

gate dielectrics, respectively.

O3

realized by doping an element that has a high electronegativity [51].

interest in TFT applications.

84 Green Electronics

*3.2.1.4. Auto-combustion synthesis*

chemistry. The semiconducting In<sup>2</sup>

with *μ* approaching 1.0 and 13.0 cm<sup>2</sup> V−1 s−1 on SiO<sup>2</sup>

*3.2.1.3. Impurity-free precursor-based approach*
