**2. Microstructure-property relationship of TCO materials**

#### **2.1 Nodule formation**

Nodules, also called "black growths" or "black crystals", are conical defects formed during the sputtering process on the surface of the target material. The influence of nucleation on nucleus growth of nodules was investigated by camera monitoring (Schlott, et al., 1996). It was found, that especially impurities originating from the sputtering process, i.e. SiO2 or Al2O3 particles , were collected on the target material and act as nucleus in the nodule formation process.

There are also other important impurities in the form of inclusions that originates from the target manufacturing process. Target materials that happen to feature the increased densification, are more suitable for sputtering processes since they exhibit fewer nodule failures (Schlott, et al., 1995, Gehman, et al., 1992). The shape of nodules has been investigated by SEM analysis and partly a distinct peak is observed, partly a flattened conical shape is formed (Schlott, et al., 1996). From time to time spherical globule like particles are observed at the nodules peak being identified as SiO2 and Al2O3. In this case the particles could be assigned to impurities of the sputtering process being directly related to the nodule formation process. It was found by further detailed investigations that the external layer of the nodules contains pure ITO phase, the thickness of the layers being up to several tens of microns. Additionally these layers containing fewer oxygen concentrations compared to the bulk target material are supposed to be formed by self-sputtering. Selfsputtering means that the target itself was coated by the sputtering process as it is shown schematically in Figure 1. The oxygen deficiency can be explained by the fact that the atmosphere nearby the target surface contains less oxygen.

Fig. 1. Cross section scheme of a nodule formed on the surface of an ITO sputtering target according to (Schlott, et al., 1996).

Sputtered Al:ZnO thin films have already been used in commercially available flat panel

Among others current research is focussing on sputtering process stability based on ITO target materials, on AZO target materials with improved electrical conductivity, thermal and mechanical stability as well as highest transparency of TCO thin layers by enhanced optimization of chemical composition, microstructure tailoring and advanced sintering and

Nodules, also called "black growths" or "black crystals", are conical defects formed during the sputtering process on the surface of the target material. The influence of nucleation on nucleus growth of nodules was investigated by camera monitoring (Schlott, et al., 1996). It was found, that especially impurities originating from the sputtering process, i.e. SiO2 or Al2O3 particles , were collected on the target material and act as nucleus in the nodule

There are also other important impurities in the form of inclusions that originates from the target manufacturing process. Target materials that happen to feature the increased densification, are more suitable for sputtering processes since they exhibit fewer nodule failures (Schlott, et al., 1995, Gehman, et al., 1992). The shape of nodules has been investigated by SEM analysis and partly a distinct peak is observed, partly a flattened conical shape is formed (Schlott, et al., 1996). From time to time spherical globule like particles are observed at the nodules peak being identified as SiO2 and Al2O3. In this case the particles could be assigned to impurities of the sputtering process being directly related to the nodule formation process. It was found by further detailed investigations that the external layer of the nodules contains pure ITO phase, the thickness of the layers being up to several tens of microns. Additionally these layers containing fewer oxygen concentrations compared to the bulk target material are supposed to be formed by self-sputtering. Selfsputtering means that the target itself was coated by the sputtering process as it is shown schematically in Figure 1. The oxygen deficiency can be explained by the fact that the

Fig. 1. Cross section scheme of a nodule formed on the surface of an ITO sputtering target

10-4 Ω cm at aluminium

displays und thin layer solar cells with an electrical resistance of 1-3 .

doping concentrations between 1.6 to 3.2 at.-% (Anders, et al., 2010).

**2. Microstructure-property relationship of TCO materials** 

densification methodologies of TCO target materials.

atmosphere nearby the target surface contains less oxygen.

according to (Schlott, et al., 1996).

**2.1 Nodule formation** 

formation process.

Nodules have to be avoided, since they modify the sputtering process and it is therefore necessary to interrupt the process and to clean or even to exchange the target material. Nodule formation is a severe problem since nodules show a reduced sputtering voltage compared to the surrounding material. Thereby the nodules prevent the sputtering of the material being covered by the nodule layers. Furthermore when the nodule formation is avoided, arc discharge is not needed and the sputtering process is excecuted at increased sputtering voltages enabling to operate the facility at higher efficiencies (Nadaud, et al., 1995). The nodule formation was observed for metallic indium-tin targets during the reactive sputtering process (Schlott, et al., 1996) and also for sputtering of oxide ceramic ITO targets (Schlott, et al., 1995). The elimination of inclusions and metallic phases is predominant for the effective avaoidance of nodule formation (see Figure 2).

Fig. 2. Cold isostatically pressed ITO body (a, left) and sintered ITO body with a shrinkage of 15 % (a, right), SEM topography of fractured ITO surface after sintering with a mean grain size of 25 µm (b) and outbreak (dark) and segregations (bright) analysed by SEM at polished ITO surfaces (c).

A trouble-free microstructure is important to prevent the chipping (Schlott, et al., 1996). Impurties and metallic InSn eutectica have to be avoided in order to guarantee sufficient thin film qualities (Schlott, et al., 1996).

Once being formed the nodules grow contineously during the sputtering process. They do not dissolve unless they explode due to thermal stresses or due to the power of micro arc discharge effects (Schlott, et al., 1996). If that is done the desintegrated nodule particles are scattered and form nucleus of new nodules or they form small holes, so called pinholes, in the growing layer contributing to a significant quality degradation of the sputtered thin film (Kukla, et al., 1998).

Nodule formation is observed particularly frequently when the target material consists of several tiles being composed to enlarge the target surface. The split between the tiles are considered as collecting sites for impurites and dust. Furthermore the split could be coated

Sintering of Transparent Conductive Oxides:

Fig. 4. Cubic In2O3 unit cell.

oxygen is as follows according to (Chopra, et al., 1983):

1991). The density of SnO2 is 6.95 g/cm3 (Bates, et al., 1986).

Fig. 5. Tetragonal SnO2 unit cell.

From Oxide Ceramic Powders to Advanced Optoelectronic Materials 591

The unit cell has 80 atoms or 16 molecular weights (Warschkow, et al., 2003) and has two unequal cation positions (Granqvist & Hultaker, 2002). The lattice constant is 10.117 ± 0.001 Å (Marezio, 1966). The space group is referred to as Ia3 or Th7. Pure In2O3 has a density of 7.17 g/cm3 (Bates, et al., 1986) and the theoretical density of the cubic structure is 7.12 g/cm3 (Marezio, 1966). The electron subshell configuration of both atoms indium and

The oxygen atoms need two more p-electrons to reach a stable 8-electron configuration. The indium atoms have three electrons in addition to a stable electron configuration. As a result the stoichiometry of the oxide is In2O3 resulting in a transition of electrons from In to O and a crystal structure with In3+ and O2- ions in the lattice (Mayr, 1998). The unit cell has two unequal In-positions, the first space-diagonally (d position) and the other surface-diagonally (b position). With the d positions the oxygen atoms are located in the corners of an slightly distorted cube with two space-diagonal empty sites. In the second cube the oxygen atoms are located in a slightly differently distorted cube with two surface-diagonal empty sites. The characteristics of the defect structure are determined by this special arrangement in atoms (Hwang, et al., 2000). In2O3 exhibit a cubic bixbyite structure (Marezio, 1966), SnO2 in contrast has a tetragonal structure (see Fig. 4) similarly to the rutile structure (Enoki, et al.,

O: 1s2 ⏐ 2s2 2p4 - - ⏐ (2)

In: 1s2 ⏐ 2s2 2p6 ⏐ 3s2 3p6 3d10 ⏐ 4s2 4p6 4d10 ⏐ 5s2 5p (3)

by self-sputtering. If an Ar atom is smashed onto these coated splits, the scattered particles composed of In, Sn and O are able to be deposited onto the sputtered thin films (Schlott, et al., 1996). A key component for the suppression of undesired nodule formation is the basic understanding of most important ITO characteristics as well as specific sintering techniques and its effectiveness to tailor distinct microstructural properties of TCO target materials. In the following sections an overview of the state of the art and future trends of the related topics are given.

#### **2.2 The system ITO and related defect structures**

Enoki (Enoki, et al., 1991) proposed the following In2O3-SnO2 phase diagram shown in Figure 3.

Fig. 3. Phase diagram of the pseudo-binary system In2O3-SnO2 (χSn = atom concen-tration of tin (%)).

Hereby the abbriviations C1 and C2 represent the cubic ITO phase and the orthorhombic intermediate respectively, and T is the rutile type SnO2. It was found that the C2 phase is formed in the concentration range between 47.9 and 59.3 mole-% Sn at temperatures exceeding 1573 K.

Morover it was oberserved that the intermediate (C2) falls apart into In2O3 (C1) and the Tphase according to the following eutectic reaction:

$$\mathbf{C\_2} \leftrightarrow \mathbf{C\_1} + \mathbf{T} \tag{1}$$

Finally it was observed that the solubility limit of SnO2 in In2O3 phase is between about 12.4 to 15.0 mole-% which is independent of the temperature (Enoki, et al., 1991). For some basic understanding it is helpful to consider the crystal structure of solid In2O3. The C type rare earth sesquioxide-or bixbyite crystal structure of Indium oxide is a variation of the cubically body-centred crystal structure (Marezio, 1966).

Fig. 4. Cubic In2O3 unit cell.

590 Sintering of Ceramics – New Emerging Techniques

by self-sputtering. If an Ar atom is smashed onto these coated splits, the scattered particles composed of In, Sn and O are able to be deposited onto the sputtered thin films (Schlott, et al., 1996). A key component for the suppression of undesired nodule formation is the basic understanding of most important ITO characteristics as well as specific sintering techniques and its effectiveness to tailor distinct microstructural properties of TCO target materials. In the following sections an overview of the state of the art and future trends of the related

Enoki (Enoki, et al., 1991) proposed the following In2O3-SnO2 phase diagram shown in

Fig. 3. Phase diagram of the pseudo-binary system In2O3-SnO2 (χSn = atom concen-tration of

Hereby the abbriviations C1 and C2 represent the cubic ITO phase and the orthorhombic intermediate respectively, and T is the rutile type SnO2. It was found that the C2 phase is formed in the concentration range between 47.9 and 59.3 mole-% Sn at temperatures

Morover it was oberserved that the intermediate (C2) falls apart into In2O3 (C1) and the T-

 C2 ↔ C1 + T (1) Finally it was observed that the solubility limit of SnO2 in In2O3 phase is between about 12.4 to 15.0 mole-% which is independent of the temperature (Enoki, et al., 1991). For some basic understanding it is helpful to consider the crystal structure of solid In2O3. The C type rare earth sesquioxide-or bixbyite crystal structure of Indium oxide is a variation of the cubically

topics are given.

Figure 3.

tin (%)).

exceeding 1573 K.

phase according to the following eutectic reaction:

body-centred crystal structure (Marezio, 1966).

**2.2 The system ITO and related defect structures** 

The unit cell has 80 atoms or 16 molecular weights (Warschkow, et al., 2003) and has two unequal cation positions (Granqvist & Hultaker, 2002). The lattice constant is 10.117 ± 0.001 Å (Marezio, 1966). The space group is referred to as Ia3 or Th7. Pure In2O3 has a density of 7.17 g/cm3 (Bates, et al., 1986) and the theoretical density of the cubic structure is 7.12 g/cm3 (Marezio, 1966). The electron subshell configuration of both atoms indium and oxygen is as follows according to (Chopra, et al., 1983):

$$\text{O: } \text{ } \text{1s}^2 \mid \text{ } \text{2s}^2 \text{ } \text{2p}^4 \text{ } \text{ - } \mid \tag{2}$$

$$\text{In: } \text{ } 1\text{s}^2 \mid \text{ } 2\text{s}^2 \text{ } 2\text{p}^6 \mid \text{ } 3\text{s}^2 \text{ } 3\text{p}^6 \text{ } 3\text{d}^{10} \mid \text{ } 4\text{s}^2 \cdot 4\text{p}^6 \cdot 4\text{d}^{10} \mid \text{ } 5\text{s}^2 \cdot 5\text{p} \tag{3}$$

The oxygen atoms need two more p-electrons to reach a stable 8-electron configuration. The indium atoms have three electrons in addition to a stable electron configuration. As a result the stoichiometry of the oxide is In2O3 resulting in a transition of electrons from In to O and a crystal structure with In3+ and O2- ions in the lattice (Mayr, 1998). The unit cell has two unequal In-positions, the first space-diagonally (d position) and the other surface-diagonally (b position). With the d positions the oxygen atoms are located in the corners of an slightly distorted cube with two space-diagonal empty sites. In the second cube the oxygen atoms are located in a slightly differently distorted cube with two surface-diagonal empty sites. The characteristics of the defect structure are determined by this special arrangement in atoms (Hwang, et al., 2000). In2O3 exhibit a cubic bixbyite structure (Marezio, 1966), SnO2 in contrast has a tetragonal structure (see Fig. 4) similarly to the rutile structure (Enoki, et al., 1991). The density of SnO2 is 6.95 g/cm3 (Bates, et al., 1986).

Fig. 5. Tetragonal SnO2 unit cell.

Sintering of Transparent Conductive Oxides:

following reaction:.

given in (Chopra, et al., 1983).

(2Sn•

function of oxygen partial pressure (pO2

constant K:

From Oxide Ceramic Powders to Advanced Optoelectronic Materials 593

In2O3 = 2InxIn + 3OxO ↔ 2Ing + 3/2Og (5)

 2Ing + (1/2O2)g ↔ (In2O)g (6) Furthermore In2O3 is produced from gaseous In20g in oxygen atmospheres according to the

 In2O3 ↔ 2InxIn + 3OxO ↔ (In2O)g + (O2)g (7) Taking account of mass action law it is obvious that the oxygen partial pressure controles the number of oxygen vacancies VxO. The number of charge carriers in In2O3 is thus depending very much on oxygen partial pressure. Theoretically the number of charge carriers would increase at decreasing oxygen partial pressures associated with an increase of electrical conductivity (Mayr, 1998). However, the mechanismen is more complicated in reality since scattering mechanisms can occur consequently decreasing the charge carrier mobility and ion conductivity. More detailed information about scattering mechanisms are

Oxygen vacancy concentration is a function of the defect structure of indium tin oxide (Freeman, et al., 2000). At the beginning first oxygen vacancies start to form as soon as the

gaseous oxygen phase (1/2O2g) (Freeman, et al., 2000). The following equation describes the

Only under reducing conditions (~pO2-1/8) oxygen vacancies are formed by diffusing into the bulk from their former lattice sites (Oxo) (Hwang, et al., 2000). Under extreme reducing

(González, et al., 2001). The equation (9) decribes the transformation of gaseous species whereas two electrons for any oxygen vacancy. Equation (10) shows that the number of

Oxo ↔ (1/2O2)g + V••o + 2e′ (9)

If one oxygen atom is missing in the unit cell the valance electrons of the surroun-ding atoms have a reduced ionisation energy, which then is provided by thermal lattice vibrations. These electrons are in a quasi-free state and act as conduction electrons. That means that oxygen vacancies provide electrons for the conduction bands (Mayr, 1998). Even in the undoped state small oxygen deficencies can be detected. In this case oxygen vacancies appear in reduced concentrations compared to other defect structures (Hwang, et al., 2000).

InOi′′)x ↔ 2Sn•

conditions (pO2 ~ 10-14 atm) non-reducing defects are formed such as (2Sn•

oxygen vacancies (V••o) formed and the number of doped lattice sites (n = [D•

InOi′′)x) and are transformed to

In3OOOi′′)x

In]) is a

In + (1/2O2)g + 2e′ (8)

1/2). This relation is described by the equilibrium

K = pO21/2[V••o]n2 (10)

The formation of Indium oxide is described according to the following reaction equation:

In parallel to this reaction the formation of gaseous In2O can occur:

oxygen atoms leave their interstitial lattice sites ((2Sn•

formation of oxygen vacancies and the release of two electrons:

Indium atoms are incorporated in the lattice as In3+ and the tin atoms as Sn2+. The tin atoms have the following electron configuration (Mayr, 1998):

$$\left| \text{Sn} : 1 \text{s}^2 \mid \text{2s}^2 \text{2p}^6 \mid \text{3s}^2 \text{3p}^6 \text{3d}^{10} \mid \text{4s}^2 \text{4p}^6 \text{4d}^{10} \mid \text{5s}^2 \text{5p}^2 \tag{4}$$

It was shown by means of X-ray diffraction analysis (Frank & Köstlin, 1982) that the cubic In2O3 structure is preserved by doping with SnO2 up to the solubility limit of Sn in In2O3. The exact solubility limit of Sn in In2O3 is not exactly known and varies between 6 ± 2 at.-% of Sn. Up to this concentration every tin atom is substituted by an indium atom. The solubility in thin layers is even higher (Hwang, et al., 2000). The maximum solubility of Indium in the SnO2 lattice is as low as 1 at.-%. Thereby Sn4+ ions are substituted by In3+ ions significantly decreasing the electrical conductivity. The ion radius of the Sn4+ is 0.71 Å and should lead to a linear reduction of the latticed constant with increasing doping of Sn4+ according to the Vegard law (Nadaud, et al., 1998) since the ion radius of In3+ is 0.81 Å. However, this is not observed. Udawatte (Udawatte, et al., 2000) reports on a maximum lattice constant reached at 5 mole-% of Sn content dropping below the maximum solubility of 6 mole-% of Sn in the In2O3 lattice reported by Nadaud (Nadaud, et al., 1998). These authors have calculated a lattice constant of 10.1247 Å at the maxium solubility limit of Sn compared to a lattice constant of 10.1195 Å that is observed for pure In2O3. The presence of Sn strongly changes the behaviour of the oxygen ions. Due to Sn doping the general distance between oxygen and cation increases, but the distance between oxygen and Sn decreases (Nadaud, et al., 1998). Typical phase modifications of indium-tin oxide are listed in the following table.


Table 1. ITO phases, unit cells, structures, lattice constants and theoretical desities.

The formation of Indium oxide and the subsequent reaction contributing, inter alia , to the defect structure can be described by Kröger-Vink notation (Rahaman, 1995) according to the following structure elements of the chemical reactions (see Table 2).


Table 2. Structure elements of chemical reactions according to Kröger-Vink notation.

Indium atoms are incorporated in the lattice as In3+ and the tin atoms as Sn2+. The tin atoms

It was shown by means of X-ray diffraction analysis (Frank & Köstlin, 1982) that the cubic In2O3 structure is preserved by doping with SnO2 up to the solubility limit of Sn in In2O3. The exact solubility limit of Sn in In2O3 is not exactly known and varies between 6 ± 2 at.-% of Sn. Up to this concentration every tin atom is substituted by an indium atom. The solubility in thin layers is even higher (Hwang, et al., 2000). The maximum solubility of Indium in the SnO2 lattice is as low as 1 at.-%. Thereby Sn4+ ions are substituted by In3+ ions significantly decreasing the electrical conductivity. The ion radius of the Sn4+ is 0.71 Å and should lead to a linear reduction of the latticed constant with increasing doping of Sn4+ according to the Vegard law (Nadaud, et al., 1998) since the ion radius of In3+ is 0.81 Å. However, this is not observed. Udawatte (Udawatte, et al., 2000) reports on a maximum lattice constant reached at 5 mole-% of Sn content dropping below the maximum solubility of 6 mole-% of Sn in the In2O3 lattice reported by Nadaud (Nadaud, et al., 1998). These authors have calculated a lattice constant of 10.1247 Å at the maxium solubility limit of Sn compared to a lattice constant of 10.1195 Å that is observed for pure In2O3. The presence of Sn strongly changes the behaviour of the oxygen ions. Due to Sn doping the general distance between oxygen and cation increases, but the distance between oxygen and Sn decreases (Nadaud, et al., 1998). Typical phase modifications of indium-tin oxide are listed in the

Sn : 1s2 ⏐ 2s2 2p6 ⏐ 3s2 3p6 3d10 ⏐ 4s2 4p6 4d10 ⏐ 5s2 5p2 (4)

constant (Å)

charged

density (g/cm3)

have the following electron configuration (Mayr, 1998):

phase unit cell Structure lattice

following structure elements of the chemical reactions (see Table 2).

e electron g gas phase

abbreviaton subscript term superscript term V vacancy In In lattic site x neutral In In atom O O lattice site • positively

In2O3 cubic Bixbyite 10.117 7.17 SnO2 tetragonal Rutile - 6.95 In4Sn3O12 rhomboedric Fluorite - 7.30 Table 1. ITO phases, unit cells, structures, lattice constants and theoretical desities.

The formation of Indium oxide and the subsequent reaction contributing, inter alia , to the defect structure can be described by Kröger-Vink notation (Rahaman, 1995) according to the

O O atom i Interstitial ′ negatively charged

Table 2. Structure elements of chemical reactions according to Kröger-Vink notation.

following table.

h hole

The formation of Indium oxide is described according to the following reaction equation:

$$\mathrm{In}\_2\mathrm{O}\_3 = 2\mathrm{In}^{\mathrm{x}}\mathrm{In} + 3\mathrm{CO}\_\mathrm{O} \leftrightarrow 2\mathrm{In}\_\mathrm{g} + 3/2\mathrm{O}\_\mathrm{g} \tag{5}$$

In parallel to this reaction the formation of gaseous In2O can occur:

$$\text{Zn} \text{g} + (\text{1}/\text{2O})\_{\text{f}} \leftrightarrow (\text{InxO})\_{\text{f}} \tag{6}$$

Furthermore In2O3 is produced from gaseous In20g in oxygen atmospheres according to the following reaction:.

$$\text{InzO3} \leftrightarrow 2\text{Inr}\_{\text{In}} + 3\text{O} \\ \text{vO} \leftrightarrow (\text{InzO})\_6 + (\text{O})\_6 \tag{7}$$

Taking account of mass action law it is obvious that the oxygen partial pressure controles the number of oxygen vacancies VxO. The number of charge carriers in In2O3 is thus depending very much on oxygen partial pressure. Theoretically the number of charge carriers would increase at decreasing oxygen partial pressures associated with an increase of electrical conductivity (Mayr, 1998). However, the mechanismen is more complicated in reality since scattering mechanisms can occur consequently decreasing the charge carrier mobility and ion conductivity. More detailed information about scattering mechanisms are given in (Chopra, et al., 1983).

Oxygen vacancy concentration is a function of the defect structure of indium tin oxide (Freeman, et al., 2000). At the beginning first oxygen vacancies start to form as soon as the oxygen atoms leave their interstitial lattice sites ((2Sn• InOi′′)x) and are transformed to gaseous oxygen phase (1/2O2g) (Freeman, et al., 2000). The following equation describes the formation of oxygen vacancies and the release of two electrons:

$$(\text{Zn}^{\bullet}\text{In}\_{\text{In}}\text{O}\_{\text{i}}\prime)^{\text{x}} \leftrightarrow \text{Zn}^{\bullet}\text{In}\_{\text{In}} + (1/2\text{O}\_{2})\_{\text{R}} + 2\text{e}^{\prime} \tag{8}$$

Only under reducing conditions (~pO2-1/8) oxygen vacancies are formed by diffusing into the bulk from their former lattice sites (Oxo) (Hwang, et al., 2000). Under extreme reducing conditions (pO2 ~ 10-14 atm) non-reducing defects are formed such as (2Sn• In3OOOi′′)x (González, et al., 2001). The equation (9) decribes the transformation of gaseous species whereas two electrons for any oxygen vacancy. Equation (10) shows that the number of oxygen vacancies (V••o) formed and the number of doped lattice sites (n = [D• In]) is a function of oxygen partial pressure (pO21/2). This relation is described by the equilibrium constant K:

$$\text{O}^{\text{x}} \leftrightarrow (1/2\text{O})\_{\text{f}} + \text{V}^{\bullet \text{\textquotedblleft}} \text{ o} + 2\text{e}^{\prime} \tag{9}$$

$$\mathbf{K} = \mathbf{p} \mathbf{O} \mathbf{2}^{1/2} [\mathbf{V}^{\bullet \bullet} \mathbf{o}] \mathbf{n}^2 \tag{10}$$

If one oxygen atom is missing in the unit cell the valance electrons of the surroun-ding atoms have a reduced ionisation energy, which then is provided by thermal lattice vibrations. These electrons are in a quasi-free state and act as conduction electrons. That means that oxygen vacancies provide electrons for the conduction bands (Mayr, 1998). Even in the undoped state small oxygen deficencies can be detected. In this case oxygen vacancies appear in reduced concentrations compared to other defect structures (Hwang, et al., 2000).

Sintering of Transparent Conductive Oxides:

phase is limited (Bates, et al., 1986).

**3.1 ITO powder synthesis** 

**powders** 

600 °C.

From Oxide Ceramic Powders to Advanced Optoelectronic Materials 595

The atoms of the rhomboedric cells are more densely packed compared to the cubic structure. The rhomoedric phase was fist discovered by Bates et. al. and the density was calculated to 7.303 g/cm3 (Bates, et al., 1986). It was found by X-ray diffraction experiments that the conformation is a densely packed M'mM''nO3m defect structure typically observed for the compositions Yb7O2 and Pr7O12. The In2O3 as well as SnO2 solubility in the In4Sn3O12-

**3. Synthesis of nano- and microcrystalline ITO (Sn:In2O3) and AZO (Al:ZnO2)** 

In most cases indium-tin-oxide powders are synthesized by hydrothermal processes. Gel formation is based on the co-precipitation of InCl3⋅H2O and SnCl2 educts (Udawatte & Yanagisawa, 2000). The microcrystalline powder were homogeneous and reveal the composition of tin doped indium oxide [In(OH)3:Sn] and tin doped indium hydroxide [InOOH:Sn]. Calcination of In(OH)3:Sn at 300 °C resulted in cubic tin doped indium oxide [In2O3:Sn]. At calcination temperatures above 500 °C the InOOH:Sn phase is transformed to a solid solution of the formula (In2Sn1-xO5-y). Both powders have been calcinated in air atmosphere. It was found by Mössbauer analysis that the Sn4+-ion coordination number is 8 because each tin atom has 8 neighbouring oxygen atoms which bear opposite charge. A similar synthesis scheme is presented in (Yanagisawa, et al., 2000). In this case indium-tinoxide has been synthesized from a In-Sn-hydrogel. The hydrothermal treatment of the gel at 300 °C resulted in the formation of InOOH:Sn with mean particle sizes in the range of 80 nm. The subsequent calcination of the product at different calcination temperatures led to different microstructural vacancy configurations of indium tin oxide solid solutions. Calcination at 700 °C lead to a powder with primary particles sizes in the range of about 160 nm. Another approach is the processing of aqueous In(NO3)3⋅H2O solutions, subsequent heating and calcination at 500 °C in order to achieve nanosized ITO powders (Sorescu, et al., 2004). The solution of metallic tin and indium in HCl is precipitated by [NH4OH] (Nam, et al., 2001) and the resulting indium-tin-hydroxide gel is dried, grinded and calcinated at

The processing of tin doped indium oxide crystalites from a direct indium-tin smelting enriched with oxygen is described in (Frank, et al., 1976). The hetero-geneous nucleation process was initiated by small In2O3 crystalites. Different compositions and concentration ranges of the smelting process resulted in different tin doping concentrations. Indium tin oxide synthesis by a chemical transport process is reported in (Werner, et al., 1996). Starting from metallic indium and tin and dissolution of the educts in nitric acid the solution is dried and calcinated at 900 °C to achieve In2O3 and SnO2. The mixture of both oxides were doped with transport media iodine, or sulfur, or chlorine. Indium oxide crystals doped with 8.2 mole-% tin and tin oxide crystals doped with 2.4 mole-% indium were attained. These powders have been characterized and the synthesis reactions based on chlorine transport

In case the seperate oxides are provided as educt materials for the synthesis reaction, indium doped tin oxide, as reported in (Nadaud, et al., 1994) could be processed. In this case In2O3 und SnO2 powder (purity of 99,99 wt.-%) have been mixed in ethanol and calcinated at

media have been thermodynamically modeled (Patzke, et al., 2000).


Table 3. Overview of reactions on formation and self-compensation of vacancies as well as formation of donators of the system In2O3:Sn according to Kröger-Vink notation.

Nadaud and co-workers investiated oxygen concentration of bulk-ITO by neutron diffraction and Rietveld analysis (Nadaud, et al., 1998). After sintering at 1400 °C in reducing nitrogen atmospheres stoiciometric oxygen concentrations were detected for both undoped and 6 at.-% Sn doped In2O3. On the other hand sintering in oxygen atmospheres resulted in bulk oxygen excess of about 3 %. This could be explained by the stoiciometry of the neutral (2Sn• InO''i)x complexes. Another explanation is the formation of large Sn-based oxygen complexes being difficult to get reduced in the intermediate temperature regime. The same authors investigated these large complexes by Mössbauer, EXAFS and neutron diffraction.

ITO Mössbauer- and EXAFS data von ITO reveal a relaxation of the Sn-O shell similar to the observed relaxation in Sn rhich In4Sn3O12 (Nadaud, et al., 1998). The analytical data allow to extend the explanation of inefficacy doping above 6 at.-% Sn to the effect that Sn atoms are incorporated at cation sites where they are inactive.

The scientific community is controversial discussing the precipitation of the rhomboedric phase In4Sn3O12 at Sn concentrations exceeding 6 at.-% Sn. The In4Sn3O12 structure has been analysed by neutron diffraction experiments and it was ascertaiend that the structure is similar to the Fluorite structure and that a sixfold occupation of M1-sites by Sn cations and sevenfold occupation of M2-sites by In- and Sn cations occurs (Nadaud, et al., 1998). The following figure shows the unit cell of the rhomboedric In4Sn3O12 phase. The lattice constant is 6.2071 Å and the mismatch angle ø is 99.29°.

Fig. 6. Unit cell of the In4Sn3O12 phase.

The atoms of the rhomboedric cells are more densely packed compared to the cubic structure. The rhomoedric phase was fist discovered by Bates et. al. and the density was calculated to 7.303 g/cm3 (Bates, et al., 1986). It was found by X-ray diffraction experiments that the conformation is a densely packed M'mM''nO3m defect structure typically observed for the compositions Yb7O2 and Pr7O12. The In2O3 as well as SnO2 solubility in the In4Sn3O12 phase is limited (Bates, et al., 1986).
