Role of Al doping in structural, microstructural, electrical and optical characteristics of as-deposited and annealed ZnO thin films

The role of the Al dopant (0.0 to 3.0 wt%) in modifying the structural, microstructural, electrical and optical properties of pulsed laser-deposited ZnO thin films is reported in both as-deposited (AD) and annealed (AN) films. Incorporation of Al3+ ions in the ZnO matrix (AZO) created localized lattice distortions in AD thin films. Breakdown in the local translational symmetry resulted in the observation of otherwise Raman inactive modes in the phonon spectra of the AZO thin films. Furthermore, Al doping enhanced the n-type character of the films, with the charge carrier density exceeding the Mott critical density of ZnO. Charge transport study at low temperatures revealed the metal-semiconductor transition. The increase in charge carrier density with Al doping concentration resulted in a blue shift in the absorption band-edge of these films. These well-characterized thin films were annealed at 800 degrees C in ambient air for 4 hours. Subsequent characterization revealed drastic modifications in the properties of the AZO thin films. Annealing resulted in reduced lattice distortions, thus improving the crystalline quality of the thin films. This is also supported by the enhanced intensity of E-2(High) phonon mode and the disappearance of Raman inactive modes in the phonon spectra of AN thin films. AN AZO thin films showed reduced charge carrier density and increased resistivity. These radical changes in characteristics suggest that the segregation of Al3+ ions to grain boundaries is a consequence of annealing.