**4.4 Sintering behavior and electrical conductivity**

The electrical conductivity of both pure indium oxide and pure tin oxide is a result of stoichiometry disturbance due to formation of oxygen vacancies. In case of In2O3 this structure can be described by the complex In2O3-x(Vo)xe′2x (Mayr, 1998). By tin doping, having a higher valence number compared to indium, negative charge carriers are incorporated into the lattice contributing to an additional increase of electrical conductivity.

Freeman and co-workers (Freeman, et al., 2000) have calculated the theoretical distribution of energy bands in tin doped and undoped In2O3. In the case of tin doped indium oxide the s-band in the lower section of the conduction band is broadened. Consequently a high mobility of electrons is achieved explaining the high electrical conductivity. Further investigation of electronic ITO structure was elaborated in (Odaka, et al., 2001).

In general the electrical conductivity of Sn doped ITO is lower than theoretically predicted. It was observed that high Sn doping concentrations even reduce the electrical conductivity. The In4Sn3O12 phase mentioned before is not or slightly electrically conductive (Nadaud, et al., 1998). This fact can be explained by the inactivity of Sn-cations located in M1-positions due to the ternary coordiation of the surrounding oxygen ions. This is similar to the structure of SnO2 which is neutral. Enventhough In and Sn are located in close vicinity of the periodic system of elements the additional electron of the Sn atom causes a higher affinity to oxygen compared to indium.

Sintering of Transparent Conductive Oxides:

different defect structures.

**5. Conclusions and future trends** 

sintered TCO target materials.

From Oxide Ceramic Powders to Advanced Optoelectronic Materials 605

1773 K is correlated with the oxygen content. These results also indicate that the typical ITO characteristics not only depend on a specific oxygen concentration range since a hysteresis is present. Investigations of electrical In2O3 characteristics at elevated temperatures have been elaborated by De Wit (Wit, 1975). Hwang (Hwang, et al., 2000) proposed three different regimes of electrical conductivity as a function of oxygen partial pressure and tin concentration. These three regimes have been ascertained experimentally by electrical conductivity and thermoelectric power measurements. The thermoelectric power is a measurement for the thermal diffusion current which is achieved by a temperature gradient. The first regime is characterized by low oxygen partial pressure (~pO2-1/6) and low tin doping concentrations. The second regime is distinguished by mean oxygen partial pressure (~pO20) and mean tin concentrations and the third regime by high oxygen partial pressure (~pO2-1/8) and high tin doping concentrations. The different doping concentrations result in

The electrical conductivity of bulk nano-ITO is significantly lower compared to the electrical conductivity of bulk µ-ITO. The explanation seems to be that the charge carrier density and the mobility of charge carriers is much lower in nano-ITO (Hwang, et al., 2000). Modeling of optical and electrical characteristics of ITO-thin films made of nano-ITO have been

Ceramic transparent conductive oxides are widely used for the processing of thin transparent conductive oxides films by vacuum sputtering techniques. These thin film layers are used in liquid crystal display technologies and various application fields such as energy conservation, information storage, electrophotography, electromagnetic radiation shielding and optoelectronic industry. In order to achieve maximum electrical and thermal conductivities and high sputtering efficiencies usually TCO target materials with distinct degrees of oxygen deficiencies are being used. The transparent semiconductor indium-tinoxide with its high transmission for visible light, its high electrical conductivity and its strong plasma reflection in the near infrared is one of the most common transparent conductive materials. A simplified description of basic understanding of most important ITO characteristics are given and correlated to the desired microstructural properties of

Specific sintering techniques, i.e. hot isostatic pressing of vacuum pre-sintered, compacted and capsuled ITO bodies results in a distinct consolidation of microstructure and a homogenisation of ITO phase and thus to an increase in HIP sintered density close to the theoretical density. In order to achieve homogeneous microstructures with small mean grain sizes of the pure ITO phase elaborate demands on specific sintering methodologies and adopted sintering processing chains have to be applied. It is important to note that the characteristics of industrially available raw powders in terms of phase composition, particle size distribution, powder density, degree of oxygen deficiency and concentrations of free metal species such as In, Sn and InSn intermetallics could have considerable impact on the quality of the later ITO product. The assessment of superior powder quality in the framework of stream lined and rationalized powder synthesis and processing can therefore be seen as a way forward to realize further optimization of ITO target materials for

elaborated by Granqvist and co-workers (Ederth, et al., 2003).

The fact that Sn is deteriorating the electrical conductivity becomes more reasonable when high doping levels exceeding 9 at.-% Sn doping concentrations at increased oyxgene partial pressures (p02 = 1 atm) are applied as it was experimentally proven by Nadaud (Nadaud, et al., 1998).

With the exception of the In4Sn3O12-phase formation there are mainly two reasons explaining the decrease of electrical conductivity of ITO as soon as doping concentrations exceeding limiting values of 6 at.-% of tin. First the electrical conductivity is reduced due to the formation of the neutral irreducable clusters (Frank & Köstlin, 1982) as for example (2Sn• InOi)x complex or the strongly attached and neutral (Sn2O4)x complex (Frank & Köstlin, 1982). Second the lattice is progressively distored as soon as the doping concentration are increased. The atoms are displaced from their original positions and In2O3 is similar to SnO2 crystalline structure. Consequently Sn2Oi′′-cluster are formed acting as neutral lattice defects (Nadaud, et al., 1998). Furthermore these formed clusters are able to restrict doped charge carrier concentrations und decrease charge carrier mobility by causing "*un-ionized impurity scattering"* (Hwang, et al., 2000).

At increasing oxygen vacancy concentrations a compensation process is initiated resulting in a further decrease of the electrical conductivity according to the following equation (Mayr, 1998):

$$\rm O\_{l}^{\prime\prime} + V\_{o} \Leftrightarrow O\_{o}^{\circ} \tag{16}$$

The electrical characteristics of ITO –phases have been experimentally investigated by Bates and co-workers (Bates, et al., 1986). After sintering of ITO the cubic body centred In2O3 phase, being able to incorporate tin concentrations up to 20 mole-% by solid solution process, the rhomboedric In4Sn3O12 as well as the tetragonal SnO2 have been detected. It was found that the electrical conductivity is increasing with increased In2O3 phase content up to a phase concentration of about 30 mole-%, passing a constant conductivity level up to about 50 mole-% In2O3 and reaching a maximum conductivity level at In2O3 phase concentration of about 80 mole-%.

This maximum electrical conductivity is a factor of 20 to 25 times higher than the electrical conductivity of pure In2O3 (1.6 up to 2.7⋅103 /Ωcm) and a factor of 6 to 20 times higher than the electrical conductivity of In4Sn3O12 (100 up to 300 /Ωcm). These values are clearly lower than those of thin layers which are in the range of up to 104 /Ωcm (Nadaud, et al., 1994).

Studies of the cyclical heating and subsequent cooling of 10 up to 70 mole-% In2O3 in air resulted in a reproducible hysteresis of the electrical conductivity and the thermoelectric power. These characteristics are connected to the formation of the high temperature In4Sn3O12 phase according to the authors (Bates, et al., 1986).

It is also claimed that there are other phase transformations at elevated temperatures, contributing to additional explanations of the fluctuations in electrical conductivity. In continuous thermogravimetrical studies of ITO targets a hysteresis of oxygen uptake and release was observed from multiple cyclical heating and cooling in atmospheres with controlled oxygen partial pressure (Otsuka-Matsua-Yao, et al., 1997). The conclusion is that the microstructural transformation of ITO in the temperature range between 1273 K and

The fact that Sn is deteriorating the electrical conductivity becomes more reasonable when high doping levels exceeding 9 at.-% Sn doping concentrations at increased oyxgene partial pressures (p02 = 1 atm) are applied as it was experimentally proven by Nadaud (Nadaud, et

With the exception of the In4Sn3O12-phase formation there are mainly two reasons explaining the decrease of electrical conductivity of ITO as soon as doping concentrations exceeding limiting values of 6 at.-% of tin. First the electrical conductivity is reduced due to the formation of the neutral irreducable clusters (Frank & Köstlin, 1982) as for example

At increasing oxygen vacancy concentrations a compensation process is initiated resulting in a further decrease of the electrical conductivity according to the following equation (Mayr,

 Oi′′ + Vo ⇔ Oxo (16) The electrical characteristics of ITO –phases have been experimentally investigated by Bates and co-workers (Bates, et al., 1986). After sintering of ITO the cubic body centred In2O3 phase, being able to incorporate tin concentrations up to 20 mole-% by solid solution process, the rhomboedric In4Sn3O12 as well as the tetragonal SnO2 have been detected. It was found that the electrical conductivity is increasing with increased In2O3 phase content up to a phase concentration of about 30 mole-%, passing a constant conductivity level up to about 50 mole-% In2O3 and reaching a maximum conductivity level at In2O3 phase

This maximum electrical conductivity is a factor of 20 to 25 times higher than the electrical conductivity of pure In2O3 (1.6 up to 2.7⋅103 /Ωcm) and a factor of 6 to 20 times higher than the electrical conductivity of In4Sn3O12 (100 up to 300 /Ωcm). These values are clearly lower than those of thin layers which are in the range of up to 104 /Ωcm (Nadaud, et al., 1994).

Studies of the cyclical heating and subsequent cooling of 10 up to 70 mole-% In2O3 in air resulted in a reproducible hysteresis of the electrical conductivity and the thermoelectric power. These characteristics are connected to the formation of the high temperature

It is also claimed that there are other phase transformations at elevated temperatures, contributing to additional explanations of the fluctuations in electrical conductivity. In continuous thermogravimetrical studies of ITO targets a hysteresis of oxygen uptake and release was observed from multiple cyclical heating and cooling in atmospheres with controlled oxygen partial pressure (Otsuka-Matsua-Yao, et al., 1997). The conclusion is that the microstructural transformation of ITO in the temperature range between 1273 K and

In4Sn3O12 phase according to the authors (Bates, et al., 1986).

InOi)x complex or the strongly attached and neutral (Sn2O4)x complex (Frank & Köstlin, 1982). Second the lattice is progressively distored as soon as the doping concentration are increased. The atoms are displaced from their original positions and In2O3 is similar to SnO2 crystalline structure. Consequently Sn2Oi′′-cluster are formed acting as neutral lattice defects (Nadaud, et al., 1998). Furthermore these formed clusters are able to restrict doped charge carrier concentrations und decrease charge carrier mobility by causing "*un-ionized impurity* 

al., 1998).

(2Sn•

1998):

*scattering"* (Hwang, et al., 2000).

concentration of about 80 mole-%.

1773 K is correlated with the oxygen content. These results also indicate that the typical ITO characteristics not only depend on a specific oxygen concentration range since a hysteresis is present. Investigations of electrical In2O3 characteristics at elevated temperatures have been elaborated by De Wit (Wit, 1975). Hwang (Hwang, et al., 2000) proposed three different regimes of electrical conductivity as a function of oxygen partial pressure and tin concentration. These three regimes have been ascertained experimentally by electrical conductivity and thermoelectric power measurements. The thermoelectric power is a measurement for the thermal diffusion current which is achieved by a temperature gradient. The first regime is characterized by low oxygen partial pressure (~pO2-1/6) and low tin doping concentrations. The second regime is distinguished by mean oxygen partial pressure (~pO2 0) and mean tin concentrations and the third regime by high oxygen partial pressure (~pO2 -1/8) and high tin doping concentrations. The different doping concentrations result in different defect structures.

The electrical conductivity of bulk nano-ITO is significantly lower compared to the electrical conductivity of bulk µ-ITO. The explanation seems to be that the charge carrier density and the mobility of charge carriers is much lower in nano-ITO (Hwang, et al., 2000). Modeling of optical and electrical characteristics of ITO-thin films made of nano-ITO have been elaborated by Granqvist and co-workers (Ederth, et al., 2003).
