Investigations on solution derived aluminium doped zinc oxide thin films

Abstract

Aluminium (Al) doped transparent zinc oxide thin films have been successfully grown on sapphire (0 0 0 1) substrates by an economical chemical solution deposition technique. These films were characterized in terms of their structural, optical, and electrical properties. Detailed XPS analysis of the O1s core level spectra has been carried out for the ZnO films annealed at different temperatures. Lower binding energy oxygen peak (O1) is related to ZnO bond, whereas higher binding energy peaks (O2 and O3) are related to the presence of OH and H 2O species. Zn is bonded mainly to oxygen, however presence of metallic Zn was also detected by XPS. Decrease in O2 and O3 contents with the increase in annealing temperature has been correlated with the microstructure of the film. Higher annealing temperature was found to be effective to densify the film and thereby reduced the contents of the hydrated species. An optical transmittance 80–90% in the visible range and an optical band gap ∼3.25 eV was measured in case of undoped ZnO thin film. There is no significant change in band gap energy with Al doping. The resistivity of undoped ZnO was measured to be about 3.8 Ω cm. The resistivity of ZnO films decreased and remained in the range of 0.27–0.32 Ω cm for up to 4 at.% Al doping. Al doping content beyond 4 at.% was found to increase the resistivity of the films probably due to the segregation of aluminium as oxide resulting scattering of the electrons.