Structural and optical investigation of pure and Al doped ZnO annealed at different temperatures

The structural and optical properties of ZnO and Zn0.9Al0.1O (AlZnO) samples annealed in air at different temperatures of (400–900 °C) for 12 h are investigated. It is found that cell parameters, average crystalline diameter and average grain size are affected by both Al and annealing. The ultra violet (UV) absorption coefficient (α) is increased by Al and can be represented by the relation ;(α (AlZnO)~2α (ZnO), but it is generally decreased by annealing. Two different types of band gap are obtained for each sample in which their values are higher for AlZnO than that of ZnO. The first band gap is called fundamental energy gap (Egh) and its value above 3 eV, while the second is called optical energy gap (EgL) and its value below 2.75 eV. In addition, the values of EgL (ZnO), Egh (AlZnO) and EgL (AlZnO) are increased by annealing up to 700 °C, followed by a decrease at 900 °C, while the values of Egh (ZnO) are increased by annealing up to 900 °C. Furthermore, a unique energy gap of 3.33 eV is obtained at 312 °C for ZnO and AlZnO. On the other hand, the lattice dielectric constant (eL), free carrier concentration (N/m*), dielectric loss (tan δ), optical conductivity (σopt) and electrical conductivity (σele) are obtained for both series. It is found that eL is increased by annealing up to 900 °C for both series, while (N/m*) is increased by annealing for ZnO, but it is decreased for AlZnO. But their values are higher for ZnO than that of AlZnO. Further, the value of eL (11.26) of the as-prepared ZnO is nearly similar to the value of eL (11.76) for AlZnO at 900 °C. Although tan δ is similar for both series, it is decreased by annealing up to 900 °C for ZnO, and increased for AlZnO up to 700 °C followed by decrease at 900 °C. Similar behavior is obtained for σopt, while the vice is versa for σele. The obtained results are interpreted in terms of the variance of carrier concentration, oxygen vacancies and interatomic spacing against annealing temperature and Al content. The present results confirm the semiconducting nature as well as assure that the high photo-response of the investigated samples.