**2.2. Materials selection for oxide dielectrics**

Conventionally, silicon dioxide (SiO<sup>2</sup> ) has been used as dielectric layer in metal-oxidesemiconductor (MOSFET) due to its excellent insulating property and perfect Si/SiO<sup>2</sup> interfacial properties. Advanced technology has enabled fabrication of sub-22 nm channellength MOSFET, which requires the thickness of SiO<sup>2</sup> to be less than 1 nm [8–10]. However, it is not possible to maintain small leakage currents, which increase dramatically through tunneling for very thin films. In this regard, it is useful to consider the important film features and performance metrics that will produce an optimal gate dielectric. The following are general requirements for a good dielectric film: large relative dielectric constant (>10) and small leakage current density (<10 nA/cm<sup>2</sup> at 1 MV/cm), low-dielectric loss (tanσ < 0.01), and large breakdown field (>4 MV/cm) to preserve the device function as shown in **Figure 1(a)**. In addition, high-mechanical strength, low-thermal expansion, low-water adsorption quality, and high-chemical inertness properties are highly desired. It is worth noting that the transistor mobility strongly depends on the quality of semiconductor/dielectric interface. A dielectric with a rough surface would result in irregular semiconductor/dielectric interface, impeding the flow of charges through the semiconductor. Thus, atomically flat dielectric film surface is essentially required. To achieve the necessary leakage current and breakdown field, films must be as dense as possible and exhibit no pores or cracks. Both from the perspective of surface smoothness and the need for high-breakdown field and low-leakage current, films with amorphous structure are generally preferred for the fabrication of gate dielectric layers.

analysis and understanding of the physicochemical properties of the oxides are prerequisites

Among the various process technologies, solution process has many advantages, not only simple and low-cost process but also homogeneity and excellent composition control and high throughput [7]. Oxide solutions are generally synthesized using functional metal precursors in solvents and deposited on substrates by various coating methods. The coated oxide gels were pre-annealed to remove the solvents and post-annealed to develop active layers. Design of metal oxide precursor solutions (metal composition, metal precursor, chelating agents, etc.), treatment of intermediate oxide gels, and annealing techniques are of paramount importance

for controlling and improving structural and opto-electronic properties of final oxides.

**2. Low-temperature solution-processed oxide dielectric thin films**

standing and practical implementation of complex oxides in devices.

oxide dielectric thin films have become possible now.

length MOSFET, which requires the thickness of SiO<sup>2</sup>

**2.2. Materials selection for oxide dielectrics**

Conventionally, silicon dioxide (SiO<sup>2</sup>

**2.1. Introduction**

76 Green Electronics

In this chapter, we review the progress in solution-processed functional oxide thin films produced at a low temperature and highlight the critical challenges for the fundamental under-

In this section, we focus on the low-temperature solution deposition of high-quality oxide dielectric thin films for the fabrication of active electronic devices such as thin-film transistors (TFTs). Highly stable oxide dielectric materials are of paramount importance as implemented through sophisticated additive processing with the other components of a TFT. However, compared to vacuum-based deposited oxide films, solution-processed counterparts are generally found to be inferior due to their deposition, metal precursors, morphological characteristics, and performance limitations. Although realizing capabilities to print and integrate solution-processed device-quality oxide dielectrics poses a very significant challenge, success is very likely to open many opportunities in fabricating active electronics as well as in providing new approaches to the production of various unique optical and optoelectronic devices. In addition, solution processing of oxide dielectric thin films at a temperature well below 300°C, which likely opens the door to flexible electronics, has remained challenging. By various approaches including tailoring precursor solution or functional solution chemistry, deposition, annealing/heating profile, activation of amorphous film, such as photo-assisted annealing (UV), solvothermal synthesis, and so on, low-temperature solution-processed

semiconductor (MOSFET) due to its excellent insulating property and perfect Si/SiO<sup>2</sup> interfacial properties. Advanced technology has enabled fabrication of sub-22 nm channel-

it is not possible to maintain small leakage currents, which increase dramatically through

) has been used as dielectric layer in metal-oxide-

to be less than 1 nm [8–10]. However,

to improve their properties and to spur development of new oxide materials.

Because of the challenges in producing such insulators through solution methods, most solution-processed oxide TFTs [11–17] have been fabricated by using binary oxide gate insulators formed through vacuum-based depositions. Although binary oxides will continue to be used for TFT gate dielectric applications, they do not represent an optimal approach to realizing high-performance devices. Generally, binary oxides tend to crystalize [18–20] at relatively low-process temperatures, resulting in enhanced impurity interdiffusion and high-leakage currents due to formation of grain boundaries. Two most important prerequisites of oxide

**Figure 1.** (a) General requirements of a dielectric layer and (b) major designs of device-quality dielectric layer.

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 gaps (HfO<sup>2</sup> , Ta<sup>2</sup> O5 , ZrO<sup>2</sup> , La<sup>2</sup> O3 , and TiO<sup>2</sup> ) and vice versa (SiO<sup>2</sup> and Al2 O3 ).

chemistries have not been utilized. By placing greater emphasis on conversion pathways from precursor to oxide, low-energy reactions should be devised that allow condensation to proceed uniformly. Especially for electronic applications, thin oxide films must retain density,

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

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

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It was found that *hybrid clusters, having inorganic cores coordinated by organic ligands*, are the typical form of metal-organic precursor structures [32]. The study has shown that solvothermal synthesis of the precursor results in significantly improved insulating properties (e.g., two orders lower leakage current) of high-temperature annealed oxide films. We put emphasis on the structural analysis of the cluster precursors and annealed solids and relate the results to the significant improvement of properties by solvothermal treatment of solutions. A change in the cluster core toward structural unification can be brought about by the solvothermal treatment, leading to higher uniformity and higher stability of clusters. The final structure of the material maintained the features of the core structure in solution, even after annealing at high temperatures. These results demonstrate the key role played by designing cluster structure in solution. In addition, improved synthesis of the cluster precursor under solvothermal conditions leads to low-temperature deposition of oxide insulating films at below 200°C. Based on these strategies, we have designed and succeeded in producing various highquality oxide dielectric films including lanthanoids-zirconium-oxide systems (Ln-Zr-O, Ln = La, Ce, Nd, Sm, Gd, Ho, Tm), Hf-Zr-O, Y-Zr-O, Hf-Ta-O, La-Ti-O, and Hf-La-O. As a typical example, detailed studies on La-Zr-O system will be shown in the following part with a focus

on the structural analysis of the cluster precursor under solvothermal conditions.

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

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

/Si substrates, followed by annealing at 200–500°C in oxygen.

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

were spin coated on Pt/Ti/SiO<sup>2</sup>

homogeneity, and uniformity during condensation.

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> -Al<sup>2</sup> O3 [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 demonstrated by stacked layers of TiO<sup>2</sup> and Al2 O3 produced through atomic layer deposition. 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 material at the dielectric-semiconductor interface provides an additional alternative.

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

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


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

chemistries have not been utilized. By placing greater emphasis on conversion pathways from precursor to oxide, low-energy reactions should be devised that allow condensation to proceed uniformly. Especially for electronic applications, thin oxide films must retain density, homogeneity, and uniformity during condensation.

It was found that *hybrid clusters, having inorganic cores coordinated by organic ligands*, are the typical form of metal-organic precursor structures [32]. The study has shown that solvothermal synthesis of the precursor results in significantly improved insulating properties (e.g., two orders lower leakage current) of high-temperature annealed oxide films. We put emphasis on the structural analysis of the cluster precursors and annealed solids and relate the results to the significant improvement of properties by solvothermal treatment of solutions. A change in the cluster core toward structural unification can be brought about by the solvothermal treatment, leading to higher uniformity and higher stability of clusters. The final structure of the material maintained the features of the core structure in solution, even after annealing at high temperatures. These results demonstrate the key role played by designing cluster structure in solution. In addition, improved synthesis of the cluster precursor under solvothermal conditions leads to low-temperature deposition of oxide insulating films at below 200°C. Based on these strategies, we have designed and succeeded in producing various highquality oxide dielectric films including lanthanoids-zirconium-oxide systems (Ln-Zr-O, Ln = La, Ce, Nd, Sm, Gd, Ho, Tm), Hf-Zr-O, Y-Zr-O, Hf-Ta-O, La-Ti-O, and Hf-La-O. As a typical example, detailed studies on La-Zr-O system will be shown in the following part with a focus on the structural analysis of the cluster precursor under solvothermal conditions.
