*2.3.2. Lowering processing temperature for the La-Zr-O dielectric*

temperature of the organic components was affected by the solvothermal treatment, i.e., evolution from multiple, different cluster structures toward a single cluster structure, leading to a more uniform and stable cluster structure. (2) Both the La-only and Zr-only precursors

**Figure 2.** Schematic diagram of synthesis of the LZO precursor solution.

80 Green Electronics

**Figure 3.** TG-DTA analyses of different LZO precursors solution.

As mentioned earlier, a cluster core containing two or more metal elements with a structure similar to that of desired final oxide can be used as a building block for film deposition. In this method, the preferential decomposition of one metal compound over the others that causes compositional segregation is not expected to occur because the different metal elements have already been combined into one core and thermal dynamically stabilized. Hence, the decomposition and densification are similar to those occurring in binary metal oxide system. The structure of the clusters, as well as their uniformity and stability, influences the insulating properties of low-temperature deposited film.

The solvothermally treated solutions were found to have a highly enhanced UV absorption ability because of the structural reorganization of the cluster cores, which may facilitate the decomposition of their organic ligands under UV irradiation. The LZ37 films UV-annealed at 200–300°C had densities in the range of 4.3–4.5 g cm−3, which corresponds to only 70–74% of the density of LZO crystals. An excellent insulating ability (~10−8 A cm−2 at 2 MV cm−1) of the film was achieved (**Figure 4(b)**). Without solvothermal treatment of the solution, the current density is several orders higher, indicating significant effect of solvothermal treatment on the improvement of the film properties. In addition to the improvement of the cluster core, the solvothermal treatment of solutions also enhances the organic ingredient to be stabilized by the UV annealing, which facilitates formation of C─O bonding. Improvements of both the cluster core and the organic ingredient greatly contribute to the enhanced dielectric properties of LZO films.

quantum number (n), and therefore, heavy post-transition metal cations with (n–1)d10 ns<sup>o</sup>

**3.2. Approaches to low-temperature solution-processed AOS-TFTs**

Sn4+, which have the same [Kr](4d10)5so

*3.2.1. Chemical pathways*

neighboring NPs.

ligand, OH−

*3.2.1.2. Sol-gel chemistry*

*3.2.1.1. Nanomaterials-based process*

mobility and poor stability). For example, In<sup>2</sup>

framework with a dense microstructure.

tronic configurations, where n ≥ 5, are ideal oxide semiconductor candidates [42]. Both In3+ and

highly dispersed CBM is also found in ZnO due to the small interaction distances [43]. To date, most amorphous oxide semiconductors (AOSs) are deposited on rigid substrates, such as glass and metal foil, and are processable at a high temperature (>400°C). Recent efforts to lower the annealing temperature have shown that device-quality AOSs, exhibiting high performance and device stability that are not easily achievable in organic semiconductor-based electronics, have been successfully grown on plastic substrates. In the following part, we summarize the recent advances in the development of solution-processed AOS at a low temperature. In particular, we discuss the chemical pathways (colloidal-based process, sol-gel routes, auto-combustion chemistry, and impurity-free precursor-based approach) and physical approaches by newly developed annealing techniques (photo-assisted, microwave, and high pressure), which effectively enable the fabrication of low-temperature, solution-processable, high-performance AOS-TFTs.

The nanoparticles (NPs)-based chemical approach is, in principle, the promising pathway for low-temperature annealed, high-performance semiconducting layers. Nevertheless, whereas the metal NPs, smaller than a few tens of nanometers, can be melted even below 200°C due to the dramatic lowering of the melting point [44, 45], the oxide NPs are not capable of undergoing the structural transformation into a granular film morphology at low temperatures, since the melting point of oxide NPs is not decreased depending on the particle size. This unique physical property of oxide NPs leads to the large surface area, porous, and poorly interconnected particulate film at low annealing temperatures, which limits device performance (low

O3

shape, have been generated through chemical synthesis and implemented into device structures by ink-jet printing even at room temperature. However, the *μ* was confined to 0.8 cm<sup>2</sup> V−1 s−1 [46–48], which is associated with the inefficient carrier transport at junctions between

In metal salt-based sol-gel chemistry, the precursor solution is synthesized by dissolving metal salt precursors in solvents with stabilizing agents and water. The resulting metal complexes undergo a hydrolysis reaction through the loss of a proton by one or more of the water molecules that surround the metal cations in the first solvation shell. As a consequence, the aquo ligand molecule, water, bonded to the metal cation is transformed into either a hydroxo

oxolation generating an oxo-bridge, M-O-M, allowing the formation of a metal oxide skeleton

, or an oxo ligand, O2−. Subsequently, a condensation reaction occurs due to the

electronic configuration, meet this requirement, and a

http://dx.doi.org/10.5772/intechopen.75610

Low-Temperature Solution-Processable Functional Oxide Materials for Printed Electronics

NPs, less than 10 nm in size and spherical in

elec-

83

**Figure 4.** (a) UV-vis absorption spectra of LZO precursor solutions and (b) eakage currents of LZO films with and without solvothermal treatment.

Finally, using the LZ37 film UV/O<sup>3</sup> -annealed at 200°C, we fabricated a TFT with a bottomgate top-contact structure. The TFT exhibited a low-gate leakage current of less than 10 pA at an operating voltage of 15 V, a large "on/off" ratio of near 10<sup>6</sup> , a field-effect mobility (*μ*) of 0.37 cm<sup>2</sup> V−1 s−1, and a subthreshold swing factor (*SS*) of 0.61 V decade−1. The off current of the drain (3–30 pA) and the gate leakage (~10 pA) were extremely low and comparable to those of TFTs with the thermally grown SiO<sup>2</sup> insulator [36], indicating excellent insulating property of the low-temperature-processed LaZrO. The *SS* value is similar to those of high-temperatureprocessed In-Zn-O/LaZrO [35] and In-Zn-O/SiO<sup>2</sup> (channel/gate insulator) TFTs, suggesting the similar channel/gate insulator interface properties.
