**1. Introduction**

Sn-doped indium oxide or indium tin oxide (ITO) plays important roles in optoelectronics applications by providing contact layer with high transparency in the visible. Currently, ITO remains as the transparent conducting oxide (TCO) with the lowest resistivity on a commercial scale (1–2 × 10−4 Ω cm) [1]. In order to obtain both conductivity and transparency, the carrier density should not be too high to maintain its transparency with sufficiently low resistivity/ sheet resistance. In addition, the carrier mobility can be maximized, which will then provide active carriers for conduction. **Figure 1** shows the band structure of the undoped In2 O3 and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 1.** Schematic illustration of the assumed parabolic band structure of (a) undoped and (b) heavily Sn-doped In2 O3 in the vicinity of the top of the valence band and the bottom of the conduction band with a band gap *E*go. A vertical shift in the conduction and valence bands is apparent after heavy Sn doping in (b) assumed to have the sole effect of widening the band gap by a Burstein-Moss shift *W* due to filling up of the lowest states in the conduction band. Shaded areas denote occupied states. Band gaps, Fermi wave number *kF* , and dispersion relations are indicated [2].

heavily Sn-doped In2 O3 that was proposed by Hamberg et al. [2]. The band structure for In2 O3 has a parabolic band that is characterized by effective mass *mc \** for the conduction band and *mv \** for the valence band. The direct band gap (*E*go) is around 3.75 eV. After optimized Sn doping, the band gap (*E*<sup>g</sup> ), the carrier density, and mobility are increased, giving rise to the increase in carriers for conduction while maintaining the transparency. The change in carrier density, defect states, and crystallinity thus affects the final quality of the films.

ITO films can be grown by various methods. **Table 1** gives an overview of the requirements and performance of the different deposition techniques assessed against various different fundamental criteria. The deposition conditions of ITO crucially affect the dopant density, crystallinity, nanostructures formation, and defects in the films. Some of the common thin film deposition techniques are thermal evaporation, sputtering, pulsed laser deposition (PLD), chemical vapor deposition (CVD), and atomic layer deposition (ALD) which are becoming popular in recent years. These techniques can be broadly compared in terms of the energy of depositing specie, substrate temperature and possibly the annealing process after the film is deposited, and some specific features of the techniques.

The energy of depositing species, or the kinetic energy, is determined by the method for vapor creation. For TE, CVD, and ALD, the vapor is largely created via heating effect, which involves thermalization among the vapor species. Generally, a thermalized vapor is


**Table 1.** Different deposition techniques.

heavily Sn-doped In2

doping, the band gap (*E*<sup>g</sup>

*mv \** O3

denote occupied states. Band gaps, Fermi wave number *kF*

has a parabolic band that is characterized by effective mass *mc*

deposited, and some specific features of the techniques.

that was proposed by Hamberg et al. [2]. The band structure for In2

), the carrier density, and mobility are increased, giving rise to the

for the valence band. The direct band gap (*E*go) is around 3.75 eV. After optimized Sn

increase in carriers for conduction while maintaining the transparency. The change in carrier

**Figure 1.** Schematic illustration of the assumed parabolic band structure of (a) undoped and (b) heavily Sn-doped In2

86 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

in the vicinity of the top of the valence band and the bottom of the conduction band with a band gap *E*go. A vertical shift in the conduction and valence bands is apparent after heavy Sn doping in (b) assumed to have the sole effect of widening the band gap by a Burstein-Moss shift *W* due to filling up of the lowest states in the conduction band. Shaded areas

ITO films can be grown by various methods. **Table 1** gives an overview of the requirements and performance of the different deposition techniques assessed against various different fundamental criteria. The deposition conditions of ITO crucially affect the dopant density, crystallinity, nanostructures formation, and defects in the films. Some of the common thin film deposition techniques are thermal evaporation, sputtering, pulsed laser deposition (PLD), chemical vapor deposition (CVD), and atomic layer deposition (ALD) which are becoming popular in recent years. These techniques can be broadly compared in terms of the energy of depositing specie, substrate temperature and possibly the annealing process after the film is

The energy of depositing species, or the kinetic energy, is determined by the method for vapor creation. For TE, CVD, and ALD, the vapor is largely created via heating effect, which involves thermalization among the vapor species. Generally, a thermalized vapor is

density, defect states, and crystallinity thus affects the final quality of the films.

*\**

, and dispersion relations are indicated [2].

O3

O3

for the conduction band and

ideal for better control of the thin film morphology and microstructures; hence, a uniform thin film or coating is always achieved. The TE method remains one of the popular choices for metallic thin film deposition, such as the metal contact or electrodes. The disadvantage of TE method is always the contaminations due to out-diffusion from the walls of deposition chamber as a result of high temperature required to vaporize the targets. To reduce these contaminations, thin film deposition by TE always necessitates high vacuum condition, typically 10−4 Pa or better. On the other hand, CVD is operated at relatively lower temperatures, owing to dissociation energy of the precursors, but requires high substrate heating. As the precursors are constantly supplied from external sources, the film growth is a continuous process, easily multi-element control, good stoichiometry, and epitaxial. The precursors are usually volatile and/or toxic in gaseous state, and thus, the requirement for safety and environmental contamination is important. To obtain high quality and purity of the epitaxial thin films, the high vacuum requirement of CVD is usually more stringent than that for TE.

For deposition of metal oxide–based thin films, however, TE is generally not used due to refractory high temperature for vaporization from solid targets. However, the CVD methods are usually applicable for metal oxide thin film deposition from the metal organic precursors at relatively low temperature of about 200°C. In this respect, the ALD method has gained high popularity for ultrathin oxide deposition for encapsulation of devices against moisture and oxygen contaminations in recent years. With advent of nanoscale devices, which demand higher degrees of accuracy and control in terms of conformity and film thickness at Angstrom level, ALD has superseded the capability of CVD. In addition, ALD has established to be a better method for film composition for a wide range of materials, which include metals, semiconductor, and insulators of both crystalline and amorphous phase for different electronic applications in industries and manufacturing. However, for car industry and energy-efficiency buildings, smart windows with photocatalytic functions are proposed, which require not just cost-effective but also high throughput of coating method. Thus, the atmospheric pressure CVD or APCVD method is applied for transition metal oxide thin film deposition.

PLD [3] and sputtering are considered more "versatile" methods for normal thin film deposition mainly because they employ direct vaporization to produce the required deposition flux. In particular, PLD has attracted much interest mainly due to its ability to deposit of high Tc superconducting films with necessary stoichiometry. While PLD can be operated between high vacuum and low pressure, the sputtering method is usually operated at low pressures with a continuous gas flow. For both methods, the production of a wide range of kinetic energy for the depositing specie ensures thin film depositions from different types of solid targets including doped target such as ITO. The film stoichiometry or multicomponent control is often obtained with the use of background gases for moderation of kinetic energy. While a suitable energy range can facilitate the growth of more uniform and crystalline thin films, the presence of background gas often prevents the film damage by energetic ion bombardment. For PLD, the background gases at suitable pressures may also introduce gas-phase reactions which may generate new species and/or formation of nanoclusters prior to the deposition on the substrate.
