**4.1. Transparent conducting oxides (TCOs) in general**

**Precursor type Semi. Ann.** 

*3.2.2.4. Laser-assisted annealing*

**3.3. Conclusions and outlook**

Alkoxide IZO 210 Al2

In2 O3

Nanoparticle In2

given in **Table 2**.

86 Green Electronics

Metal salt + fuel In2

Metal salt In2

Metal nitrate In2

Metal salts IGZO

Metal nitrate In2

Aqueous carbon-free **temp. (°C)**

O3 200 Al2

O3 140 Al2

<80 Al2

O3 300 200

O3 180 200

ZnO 80–180 Al2

Ammonia complex Li-ZnO 300 SiO<sup>2</sup> 11.45 10<sup>7</sup>

O3 SiO<sup>2</sup>

O3

**Table 2.** Summary of low-temperature (<300°C) solution-processed oxide semiconductors.

Metal salt IZO 220 SiO<sup>2</sup> 1.78 10<sup>6</sup> HPA [62] Metal salt IZO — — 9.0 — Laser anneal [63]

**Insulator Mobility** 

A femtosecond (fs) laser annealing was applied for an effective and rapid fabrication of AOS thin films [63]. The method led to improvements in *μ* (9.0 cm<sup>2</sup> V−1 s−1) and in "on/off" ratio due

A summary of solution-processed AOS-TFTs fabricated at a low temperature (<300°C) is

During the past decade, there has been significant active research regarding the development of high-performance semiconductors that can be generated by a low-cost, large-scale solution process. The recent progress with solution-processed, high-performance AOS confirms their potential feasibility in electronic applications. For practical realization of high-performance electronic devices even on plastic substrates, the high mobility at annealing temperature below 150°C

to the efficient removal of impurities and enhanced metal-oxide composition.

Nanorod ZnO 230 SiO<sup>2</sup> 0.6 <10<sup>6</sup> Colloidal [64] Ammine-hydroxo ZnO 150 SiO<sup>2</sup> 0.4 10<sup>6</sup> Impurity-free [53]

SiO<sup>2</sup> 22.1

O3 8.76

Hydroxo ZnO 140 SiO<sup>2</sup> 1.7 10<sup>7</sup> MW [56]

**(cm2 V−1 s−1)**

O3 RT — 0.8 2 × 10<sup>3</sup> Colloidal [48]

13.0 1.0

0.85

11.29

/ZrO<sup>2</sup> 11.0 10<sup>4</sup>

3.2 7.5 **On/off Note**

10<sup>3</sup> 10<sup>5</sup>

10<sup>5</sup>

10<sup>8</sup>

–10<sup>6</sup> O2

O3 6.0 10<sup>8</sup> Sol-gel on chip [52]

O3 2.0 10<sup>8</sup> Precursor soaking

–10<sup>8</sup> Alkali doping [65]

/O<sup>3</sup>

[67]

–10<sup>9</sup> DUV [36]

>10<sup>6</sup> Far UV [68]

–10<sup>5</sup> DUV [54]

Combustion [55]

anneal [66]

The use of highly conductive and highly transparent thin films in the visible range of the spectrum is of great importance for a variety of optoelectronic device applications such as displays, solar cells, opto-electrical interfaces, and circuitries. Transparent conducting oxides (TCOs), in contrast to glass fiber, silicon, and compound semiconductors, are highly flexible intermediate states, whose conductivity can be tuned from insulating through semiconducting to conducting as well as their transparency adjustable. Furthermore, main carriers can be switched between n-type and p-type, opening a wide range of new technological applications.

So far, TCOs including binary, ternary, and quaternary oxide systems are mainly based on indium (III) oxide (In<sup>3</sup> O3 ), tin (IV) oxide (SnO<sup>2</sup> ), zinc (II) oxide (ZnO), and their mixtures with some dopants to tune structural and opto-electrical properties (**Figure 6**). The plot of In<sup>2</sup> O3 includes results for Sn-doped c (ITO) and other dopants, and the plot for various deposition methods is shown in **Figure 7** [69]. The slopes of the plots for improvement versus time in lowering doped In<sup>2</sup> O3 and doped SnO<sup>2</sup> resistivities appear to plateau at approximately 10−4 and 2 × 10−4 Ω cm, respectively. Thus, it is likely that further technological improvement in lowering resistivity of these materials is limited. However, the doped ZnO plot still presents a descending slope of improvement versus time. Therefore, it can be considered that further improvement of conductivity of ZnO-based systems would be possible.

**Figure 6.** Unit structure of typical binary oxides: bixbyite In<sup>2</sup> O3 , wurtzite ZnO, and rutile SnO<sup>2</sup> .

#### **4.2. N-type conductive oxides**

#### *4.2.1. Indium-tin-oxide (ITO)*

ITO is the most widely used TCOs because of its two key characteristics: conductivity and optical transparency. Conventionally, ITO films are produced by vacuum-based deposition techniques. The best resistivity of ITO film deposited by sputtering was reported as low as 1.2 × 10−4 Ω cm [70]. As an alternative to vacuum-based deposition, recently much efforts have been focused on production of solution-based deposition for high-performance ITO films using various approaches such as nanomaterials-based [71–75], aqueous metal salts solution [76], combustion synthesis [77, 78], and advanced annealing techniques (microwave annealing [79] and ultraviolet laser annealing [80]. A summary of properties of ITO films prepared by various solution-based deposition processes is given in **Table 3**.

A redox-based combustion synthetic approach was applied to ITO thin film using acetylacetone as a fuel and metal nitrate as an oxidizer, which enabled production of high-quality ITO film at a temperature as low as 250°C [77]. The redox reaction between the fuel and oxidizer in the precursor solution generates internal heat, facilitating conversion from metal precursor to metal oxide at low temperature. The SCS-ITO thin film exhibited high-crystalline quality, atomically smooth surface (RMS ~ 4.1 Å), and low-electrical resistivity (4.2 × 10−4 Ω cm). The TFT using SCS-ITO film as the S/D electrodes showed excellent electrical properties with neg-

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

0.7 V, and 2.1 cm<sup>2</sup> V−1 s−1, respectively (**Figure 8**). The performance and stability of the SCS-ITO TFT are comparable to those of the sputtered-ITO TFT, emphasizing that the SCS-ITO film is

There are increasing emphases on not only developing a TCO with higher conductivity but also with a material other than ITO. Two major concerns with ITO are the cost and the poten-

Current candidate materials for a higher conductivity TCO are summarized in **Table 4**, which

ants, and combinations of these binaries, plus Ga2O3, in ternary, quaternary, and even qui-

As a promising binary TCO material, ZnO is considered the prime candidate for a higher conductivity and to be low cost. However, ZnO is more sensitive to oxygen than ITO, and process control is more difficult. Work continues on improving ZnO with alternative dopants, especially Al (AZO) and Ga (GZO). As for sol-gel AZO film, the lowest reported resistivity is

1.5 × 10−4 Ω cm [83], which is close to that of film prepared by sputtering [84].

**Figure 8.** (a) Cross-sectional structure and (b) transfer curves of ZIZO-TFT using SCS-ITO as S/D electrodes.

, In<sup>2</sup> O3 , 0.43 V/decade,

89

, and ZnO, with alternative dop-

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

ligible hysteresis. The obtained "on/off" ratio, *SS*-factor, *Vth*, and *μ* were 5 × 10<sup>7</sup>

a promising candidate for totally solution-processed oxide TFTs.

include the conventional binary oxides of CdO, SnO<sup>2</sup>

nary combination of these five oxides.

*4.2.2. Alternatives to ITO*

tially limited resource.

**Figure 7.** The plot of resistivity improvement versus time for various TCOs.


**Table 3.** Properties of ITO films prepared by various solution-deposition processes.

A redox-based combustion synthetic approach was applied to ITO thin film using acetylacetone as a fuel and metal nitrate as an oxidizer, which enabled production of high-quality ITO film at a temperature as low as 250°C [77]. The redox reaction between the fuel and oxidizer in the precursor solution generates internal heat, facilitating conversion from metal precursor to metal oxide at low temperature. The SCS-ITO thin film exhibited high-crystalline quality, atomically smooth surface (RMS ~ 4.1 Å), and low-electrical resistivity (4.2 × 10−4 Ω cm). The TFT using SCS-ITO film as the S/D electrodes showed excellent electrical properties with negligible hysteresis. The obtained "on/off" ratio, *SS*-factor, *Vth*, and *μ* were 5 × 10<sup>7</sup> , 0.43 V/decade, 0.7 V, and 2.1 cm<sup>2</sup> V−1 s−1, respectively (**Figure 8**). The performance and stability of the SCS-ITO TFT are comparable to those of the sputtered-ITO TFT, emphasizing that the SCS-ITO film is a promising candidate for totally solution-processed oxide TFTs.
