5. Conclusion

switching ratio up to 103 times/s. When ZnO is prepared at high temperature, the resistivity of the films becomes comparable to the resistivity of the Si substrates. There is no proper variation observed as a function of the preparation temperature

photoresponsivity increases. Table 4 gives the photoresponsivity of the ZnO/Si heterojunction for various grown temperatures and various wavelengths of the

The Co-doped ZnO thin films show the semiconducting nature, and it is confirmed from the relationship between the conductivity and temperature [1]. The conductivity increases as the temperature increases. The conductivity is measured from 340 K, not from room temperature. The non-linear behaviour of the electrical conductivity is due to the lattice defects. The activation energy decreases due to the increase in the donor carrier density as the doping concentration of Co increases in the temperature limit of 363–403 K. But the activation energy increases due to decrease in Fermi level as the doping concentration increases in the temperature limit 408–473 K. The reported

In the case of Y-doped ZnO thin films, the electrical resistivity of the film first decreases and then increases as the doping concentration of Y increases [26]. The minimum electrical resistivity of the films is 7.25 ohm-cm, and this value corresponds to the doping concentration of 0.5%. Due to the scattering from grain boundaries and ionized impurities, the Hall mobility decreases gradually from 15.6

higher than IZO films [18]. This is due to the substitution of fluorine ions at oxygen ion site and aluminium ions at zinc site resulting in one free electron per site. Hence the conductivity of both films increases than IZO films. IFZO films have the highest

λ = 350 nm 37 mAW<sup>1</sup> 30 mAW<sup>1</sup> 30 mAW<sup>1</sup> 35 mAW<sup>1</sup> λ = 475 nm 74 mAW<sup>1</sup> 80 mAW<sup>1</sup> 80 mAW<sup>1</sup> 74 mAW<sup>1</sup> λ = 585 nm 85 mAW<sup>1</sup> 90 mAW<sup>1</sup> 90 mAW<sup>1</sup> 84 mAW<sup>1</sup>

Co doping concentration % Activation energy

 0.405 eV 0.044 eV 0.383 eV 0.077 eV 0.271 eV 0.343 eV 0.119 eV 0.439 eV

mobility, and IAZO films have the lowest mobility (Figure 12).

. The carrier concentration of both IAZO and IFZO films are

Grown temperature of the heterostructure 80°C 150°C 200°C 250°C

363–403 K 403–473 K

of the films. But as the input wavelength of the photon increases, the

incident photon at 0.5 V reverse bias condition.

activation energy is given in Table 5.

1

Photoresponsivity of the ZnO/Si heterojunction.

Activation energy of Co-doped ZnO films.

to 6.1 cm2 V<sup>1</sup> s

2D Materials

Table 4.

Table 5.

26

This chapter provides an insight into the property tailoring by parameter variations and doping that gives interesting variation in low dimensions for ZnO. Interesting basic properties and wide range of applications encourage the research about undoped and doped ZnO thin film synthesis by both physical and chemical methods. They provide amazing opportunity to include a design with ease to prepare ZnO thin film structures possessing desired electrical and optical properties. Representative works by the author group and few others are reviewed to indicate the variation in structural, optical and electrical properties of undoped and metaldoped ZnO thin films. The co-doped ZnO films are also discussed.
