Influence of Al doping on the crystal structure, optical properties, and photodetecting performance of ZnO film

The structural, optical, and electrical properties of undoped and Al-doped ZnO films deposited on p-Si (001) substrate were studied using DC-unbalanced magnetron sputtering. This study is motivated by the absence of detail reports concerning the orbital states inducing the optical bandgap (E-g) blueshift. Besides, the influences of Al on the possible modification of point defect and the photodetecting performance are highlighted to offer guidelines for better development in ZnO-based photodetector. It was found that the Al doping reduced the grain size. The doping increased the lattice parameters and slightly decreased the local-symmetry distortion at ZnO4. From the absorbance spectra, the doping increased E-g of ZnO film (3.28-3.36 eV), induced by the Burstein-Moss effect. From the density-functional calculation, the Burstein-Moss effect induced Eg from the valence band maximum (VBM) to Fermi level located above the lowest Zn 4s conduction state. From the photoluminescence spectra, the undoped film showed the transitions from O vacancy, Zn interstitial, and free-exciton states to the VBM. The doped film showed the transitions from Zn interstitial to O interstitial and few Zn vacancy states at the cost of O vacancies. Moreover, the energy level of free-exciton states slightly decreased. Notably, O interstitials increased the lattice parameters. From the electrical properties, the doping enhanced the ultraviolet-region photo-to-dark-current ratio up to 3.044 V, leading to the photodetecting sensitivity enhancement. This study emphasizes the significant effect of Al doping on the optical absorbance, point-defect evolution, and photodetecting performance of ZnO film for low-voltage ultraviolet photodetector applications.