*2.3.1. Hybrid cluster precursors of the La-Zr-O insulator for transistors*

dielectrics are high-dielectric constant and large breakdown field [21]. However, selection of a binary oxide as an insulator generally involves a compromise between these two characteristics. That is because binary oxides with high-dielectric constants normally have small band

One approach to the production of high-performance dielectrics relies on the use of mixed multiple-component oxides. These oxides provide convenient means for controlling the dielectric constant and breakdown field through incorporation of components that specifically contribute to either dielectric constant or breakdown. Furthermore, amorphous phase can be stabilized by mixing multi-components, resulting in the films with extremely flat surfaces. Common binary oxides used for tuning these properties are listed in **Table 1** [20, 22–26]. To meet the performance requirements of gate dielectrics in TFTs, multi-component oxides can be produced by strategies as illustrated in **Figure 1(b)**. A single homogeneous dielectric can be produced by combining selected wide band-gap materials with those exhibiting smaller gaps

and higher dielectric constants. For example, the mixtures of Hf-Si-O [27–29] and HfO<sup>2</sup>

and Al2

material at the dielectric-semiconductor interface provides an additional alternative.

SiO<sup>2</sup> 3.9 9.0 3.2 4.7

O3 9.0 8.8 2.8 4.9

O3 30 6.0 2.3 2.6 ZrO<sup>2</sup> 25 5.8 1.5 3.2 HfO<sup>2</sup> 25 5.8 1.4 3.3

O5 22 4.4 0.35 2.95 TiO<sup>2</sup> 80 3.5 0 2.4

**Table 1.** Summary of some relevant characteristics of major binary oxides.

[30, 31] have been extensively studied as gate dielectrics in Si CMOS devices. Alternatively, wide- and small-gap materials can be interleaved to form multilayered structures, as dem-

O3

The presence of sharp dielectric interfaces in such structured materials provides a means to improve dielectric-breakdown fields. Finally, a compositionally graded material dominated by a high-dielectric-constant material at the metal-insulator interface and a high-band-gap

The fundamental challenges in depositing oxide thin films from solution are associated with the processes of conversion of soluble precursors into dense solids. Thus, understanding the structure of metal-organic precursors in solution and their effects on processability and on the final structure and properties of the oxide is the key to the production of high-quality oxides. Although improvements of solution-processed oxide dielectrics reported so far are impressive, many of them exhibit porous structures with coarse morphologies indicating that proper

 **(eV) CBO (eV) VBO (eV)**

) and vice versa (SiO<sup>2</sup>

and Al2

O3 ).

produced through atomic layer deposition.


gaps (HfO<sup>2</sup>

78 Green Electronics

, Ta<sup>2</sup> O5 , ZrO<sup>2</sup>

onstrated by stacked layers of TiO<sup>2</sup>

**Material κ Eg**

Al2

La<sup>2</sup>

Ta<sup>2</sup>

**2.3. Producing high-quality films from solution**

, La<sup>2</sup> O3

, and TiO<sup>2</sup>

Both lanthanum oxide and zirconium oxide are typical high-κ materials, having dielectric constant values in the range of 20–30. However, lanthanum oxide is hygroscopic, and both oxides are polycrystalline. Addition of Zr to La-O to create insulating LZO system with a dielectric constant in the range of 20–25 exhibits diverse chemistries in solution and resists crystallization in the solid phase. The LZO dielectric has shown some excellent properties in all-oxide TFTs [33–35], but leakage needs to be further suppressed and a processing temperature that is compatible with plastic substrates is highly desirable. Analyses of structures of solutions, gels, and solids by various characterizations have revealed a close structural relationship between the clusters in the solutions and the final solids even after annealing at high temperatures [32].

The synthesis of LZO precursor solution is summarized in **Figure 2**. First, lanthanum (III) acetate (La(OAc)) and zirconium (IV) butoxide solution (Zr(BtO)) were each dissolved in appropriate amounts of propionic acid (PrA) to produce La and Zr solutions. After that, the two solutions were mixed to obtain LZO mixtures with La/Zr molar ratios of 3/7 (LZ37) or 5/5 (LZ55). For solvothermal treatment, the LZO mixtures were sealed in an autoclave (AC) container and heated at 160–180°C for 2–5 h with magnetic stirring. The precursor solutions were spin coated on Pt/Ti/SiO<sup>2</sup> /Si substrates, followed by annealing at 200–500°C in oxygen.

The thermal behaviors of precursor solutions were analyzed by thermal-gravity differential thermal analysis (TG-DTA) (**Figure 3**). Comparing LZO solutions with and without solvothermal treatment, we have observed three key features: (1) the decomposition

showed decomposition temperatures higher than those of LZO precursors, indicating that the La and Zr components were not simply mechanically mixed, even before solvothermal treatment. (3) The residual masses after TG analysis were quite different for solvothermally treated precursor solutions, representing the variation in the inorganic and organic contents in the precursors. Therefore, we speculate that the cluster development underwent two stages. The first stage is likely to be associated with the decomposition of the initial Zr clusters during their transition into La-Zr clusters. The second stage involved further growth/condensation of the clusters.

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Interestingly, the solvothermal treatment impacted not only on the solution structure but also on the final material structure. It was turned out that solvothermal treatment led to a uniform structure in the precursor, which was even inherited by the final annealed oxide. Without pre-formation of a uniform cluster structure, the final material remained non-uniform even if the metal components in the precursors were well mixed on the cluster scale but not inside the clusters. This suggests that the inorganic core of the cluster in the precursor remained the structural unit after annealing, during which the organic ligands around it decomposed and the cores compacted, without significantly reacting with the surrounding components. Therefore, the solvothermally treated La-Zr solutions resulted in more uniform oxide solids with improved insulating and dielectric properties. The reorganization of the clusters under solvothermal conditions also enhanced UV light absorption and enabled film deposition under UV light at low temperature, which is presented in more detail in the

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

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

following part.

of LZO films.

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

properties of low-temperature deposited film.

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

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

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 showed decomposition temperatures higher than those of LZO precursors, indicating that the La and Zr components were not simply mechanically mixed, even before solvothermal treatment. (3) The residual masses after TG analysis were quite different for solvothermally treated precursor solutions, representing the variation in the inorganic and organic contents in the precursors. Therefore, we speculate that the cluster development underwent two stages. The first stage is likely to be associated with the decomposition of the initial Zr clusters during their transition into La-Zr clusters. The second stage involved further growth/condensation of the clusters.

Interestingly, the solvothermal treatment impacted not only on the solution structure but also on the final material structure. It was turned out that solvothermal treatment led to a uniform structure in the precursor, which was even inherited by the final annealed oxide. Without pre-formation of a uniform cluster structure, the final material remained non-uniform even if the metal components in the precursors were well mixed on the cluster scale but not inside the clusters. This suggests that the inorganic core of the cluster in the precursor remained the structural unit after annealing, during which the organic ligands around it decomposed and the cores compacted, without significantly reacting with the surrounding components. Therefore, the solvothermally treated La-Zr solutions resulted in more uniform oxide solids with improved insulating and dielectric properties. The reorganization of the clusters under solvothermal conditions also enhanced UV light absorption and enabled film deposition under UV light at low temperature, which is presented in more detail in the following part.
