**3. Results and discussion**

*Solid State Physics - Metastable, Spintronics Materials and Mechanics of Deformable...*

very important in oxide semiconductors [6].

**1.5 Thermal expansion coefficient**

understand the mechanism [10].

**2. Experimental**

600 to 800°C for 30 min.

The mobility of carriers in thermoelectric materials has played a vital role in the tuning of Seebeck coefficient and power factor. The power factor is strongly dependent on the conductivity which has strong dependence on mobility. Therefore, the modulation of mobility to achieve highest value of power factor is necessary. The mobility of the carrier can be controlled by the scattering mechanisms. Two scattering mechanisms, that is, lattice scattering and impurity scattering mechanisms, are

Diffusion is a process of movement of particles from hot junction to cold junction in thermoelectric material. The diffusion of charge carriers has fundamental importance to tune the thermoelectric properties of oxide semiconductors. The diffusion in compound semiconductors is more complex than in elemental semiconductors because of the larger number of possible native point defects that can, in principle, mediate self-diffusion [7, 8]. Oxide semiconductors have high density of intrinsic defects, which in principal affect the diffusion of charge carriers. Therefore, for the effective use of oxide semiconductors for thermoelectric properties, the control of intrinsic defects has fundamental importance and should be studied further. Again, we propose that annealing will be a very effective method of

Thermal expansion is critical, as the devices for high-temperature applications will be subjected to extreme temperature fluctuations. The CTE of TE materials is of critical importance because the shear stress is proportional to the temperature gradient, and the larger the heterogeneity in the thermal expansion coefficient of a material is, the larger is the shear stress that will result [9]. It is also reported that thermal expansion coefficient of semiconductor for low- and high-temperature region is almost the same but different for medium temperature. Therefore, a comprehensive study on the thermal expansion coefficient is still needed to completely

In this study, experiment is held under thermal vapor deposition technique using single-stage horizontal glass tube furnace. In this experiment, 99.9% pure magnesium, Zinc, and Copper powders along with Ge, In, and Al powders are used under the ratio 1:1 as source material. This source material is being kept in the center of a glass tube in ceramic boat. The silicon substrate is placed at substrate holder, and the distance between source and the substrate is about 15 cm. The temperature of the furnace is tuned at 950°C for 30 min, whereas the oxygen flow is kept constant at 100 sccm. After the growth of thin film, the substrate of silicon is divided into different pieces for annealing purpose at different temperatures from

X-ray diffraction has been performed for the structural analysis of grown thin film. Raman spectroscopy has been also performed to study the rotational and vibrational modes of thin film. Surface morphology is being assessed by the scanning electron microscope (SEM). The most important characterization to calculate

studying the diffusion properties of carriers in oxide semiconductors.

**1.3 Mobility**

**1.4 Carrier diffusion**

**52**

**Figure 3**(**a–c**) represents the XRD patterns of Cu2InO4, CuAlO2, and Zn2GeO4 thin films annealed at different temperature from 600 to 800°C, respectively. The XRD graph of Cu2InO4 thin films in **Figure 3**(**a**) demonstrated that unannealed and low temperature (600°C) would not be able to make the grown material crystalline due to low thermal energy for bonding. But as we increased the temperature above 600°C, the sample is converted into crystalline structure with preferred orientation (006) at 2θ = 33.086° [11, 12]. It is also observed that the intensity of this plane is increased as we further increase the annealing temperature, which suggested that carriers now get enough energy to sit down at a particular position in planes of the crystal. **Figure 3**(**b**) shows the XRD graph of CuAlO2. The unannealed sample consists of one major phase at 2θ = 32.05 which belongs to CuAlO2 (0 0 6) plane [13]. Annealing resulted in the development of new phases at 2θ = 35.4, 42.4, and 48.4 related to CuO (1 1 1) and CuAlO2 (1 0 4) and (0 0 9) orientations, respectively. We have observed that (0 0 6) plane has the strongest intensity which is oxygen sensitive; therefore, enhancement of intensity of this plane with annealing temperature is understandable.

**Figure 3**(**c**) shows the XRD pattern of grown and annealed samples of Zn2GeO4 thin films at various temperatures. The unannealed and annealed samples consist of eight diffraction peaks which are related to Zn2GeO4, Si, Au, and ZnO, respectively. The diffraction peak at 25.8, 42.9, 44.7, 58.4, 58.9, and 65° are belonging to Zn2GeO4

**Figure 3.** *(a–c) XRD patterns of Cu2InO4, CuAlO2, and Zn2GeO4, respectively.*

**Figure 4.**

*(a–c) Effect of annealing temperature on the Seebeck coefficient of Cu2InO4, CuAlO2, and Zn2GeO4, respectively.*

which are indexed as (2 2 0), (6 0 0), (0 0 6), (6 3 0), and (7 1 3), respectively [14]. The peak which appears at 28.6° and 38.4° is belonging to (1 1 1) plane of Si and Au [15, 16], respectively, along with peak of ZnO at 34.6° having (0 0 2) plane [17]. It is observed that substrate and pre-deposited gold shows maximum intensity in all the samples which is attributed to the thin porous layer deposited over the substrate. The XRD data also demonstrated that the sample annealed at 900o C has three additional peaks at 45.7° and 65° which are related to the secondary phases of Zn2GeO4.

**Figure 4**(**a**–**c**) demonstrated the effect of annealing temperature on the Seebeck coefficient of Cu2InO4, CuAlO2, and Zn2GeO4 thin films grown by thermal evaporation technique, respectively. All graphs showed that the value of Seebeck coefficient increases as the annealing temperature increased. It is also observed that the value of Seebeck coefficient also increases as the measurement temperature increases from 25 to 100°C. Zn2GeO4 has the highest value of the Seebeck coefficient (1470 μV/K) as compared to Cu2InO4 and CuAlO2. The observed result can be explained as post-growth annealing enhances the density of oxygen atoms and also provides more thermal energy to Ge atoms which resulted in the creation of GeObased secondary phases. These newly developed secondary phases act as barrier for charge carriers at the interface of secondary phases. Due to this barrier, the lowenergy carriers are filtered out at the interface and caused the enhancement in the Seebeck coefficient. As other two samples have no secondary, therefore have lower value of the Seebeck coefficient as evident by XRD data (**Figure 5).**

To further probe the effect of annealing temperature on the thermoelectric properties of grown oxide semiconductors, we have calculated the power factor using the following formula:

$$\mathbf{P} = \mathbf{S}^2.\mathbf{a} \tag{5}$$

**55**

**4. Conclusion**

*thermal evaporation, respectively.*

**Figure 5.**

*Thermoelectric Properties of Oxide Semiconductors DOI: http://dx.doi.org/10.5772/intechopen.88709*

where S is Seebeck coefficient and α is electrical conductivity. The power factor is enhanced significantly with increasing annealing and measurement temperature

*(a–c) Effect of annealing temperature on the power factor of CuInO, CuAlO, and ZnGeO thin films grown by* 

This chapter described the effect of annealing temperature on the thermoelectric properties of oxide semiconductors. All samples were grown by thermal evaporation technique using tube furnace under vacuum using similar growth conditions. After growth, oxide semiconductors were annealed in oxygen environment at various temperatures. The reported results have suggested that Zn2GeO4 has good potential to be used as thermoelectric material because it has the highest

because both Seebeck coefficient and electrical conductivity increases.

value of Seebeck coefficient and power factor.

*Thermoelectric Properties of Oxide Semiconductors DOI: http://dx.doi.org/10.5772/intechopen.88709*

**Figure 5.**

*Solid State Physics - Metastable, Spintronics Materials and Mechanics of Deformable...*

which are indexed as (2 2 0), (6 0 0), (0 0 6), (6 3 0), and (7 1 3), respectively [14].

observed that substrate and pre-deposited gold shows maximum intensity in all the samples which is attributed to the thin porous layer deposited over the substrate.

**Figure 4**(**a**–**c**) demonstrated the effect of annealing temperature on the Seebeck coefficient of Cu2InO4, CuAlO2, and Zn2GeO4 thin films grown by thermal evaporation technique, respectively. All graphs showed that the value of Seebeck coefficient increases as the annealing temperature increased. It is also observed that the value of Seebeck coefficient also increases as the measurement temperature increases from 25 to 100°C. Zn2GeO4 has the highest value of the Seebeck coefficient (1470 μV/K) as compared to Cu2InO4 and CuAlO2. The observed result can be explained as post-growth annealing enhances the density of oxygen atoms and also provides more thermal energy to Ge atoms which resulted in the creation of GeObased secondary phases. These newly developed secondary phases act as barrier for charge carriers at the interface of secondary phases. Due to this barrier, the lowenergy carriers are filtered out at the interface and caused the enhancement in the Seebeck coefficient. As other two samples have no secondary, therefore have lower

is belonging to (1 1 1) plane of Si and Au

. α (5)

which are related to the secondary phases of Zn2GeO4.

having (0 0 2) plane [17]. It is

C has three addi-

and 38.4°

*(a–c) Effect of annealing temperature on the Seebeck coefficient of Cu2InO4, CuAlO2, and Zn2GeO4,* 

The XRD data also demonstrated that the sample annealed at 900o

value of the Seebeck coefficient as evident by XRD data (**Figure 5).**

To further probe the effect of annealing temperature on the thermoelectric properties of grown oxide semiconductors, we have calculated the power factor

P = S<sup>2</sup>

[15, 16], respectively, along with peak of ZnO at 34.6°

and 65°

The peak which appears at 28.6°

using the following formula:

tional peaks at 45.7°

**Figure 4.**

*respectively.*

**54**

*(a–c) Effect of annealing temperature on the power factor of CuInO, CuAlO, and ZnGeO thin films grown by thermal evaporation, respectively.*

where S is Seebeck coefficient and α is electrical conductivity. The power factor is enhanced significantly with increasing annealing and measurement temperature because both Seebeck coefficient and electrical conductivity increases.

### **4. Conclusion**

This chapter described the effect of annealing temperature on the thermoelectric properties of oxide semiconductors. All samples were grown by thermal evaporation technique using tube furnace under vacuum using similar growth conditions. After growth, oxide semiconductors were annealed in oxygen environment at various temperatures. The reported results have suggested that Zn2GeO4 has good potential to be used as thermoelectric material because it has the highest value of Seebeck coefficient and power factor.

*Solid State Physics - Metastable, Spintronics Materials and Mechanics of Deformable...*
