Effect of Al doping on structural and optical properties of atomic layer deposited ZnO thin films

In this work, zinc oxide (ZnO) and aluminum-doped ZnO (AZO) thin films with varying Al contents were grown via atomic layer deposition on Si and glass substrates. The structural and optical properties of ZnO and AZO thin films were investigated using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), photoluminescence (PL), and ultraviolet-visible spectroscopy. While undoped ZnO had a wurtzite crystal structure, Al2O3 layers altered the crystal structure depending on the Al content. With the increasing number of Al2O3 monolayers, the gamma- Al2O3 structure became more prominent, and ZnO peaks were completely suppressed when the Zn ratio reached 10:1 and beyond. SEM images revealed that the thin films homogeneously covered the entire substrate surface. EDS mapping showed a uniform distribution of Zn, Al, and O in the structure. According to the XPS results, the thin films displayed significant peak intensities corresponding to the main components Zn, O, and Al. Zinc atoms were present only in their oxidized state, not as interstitial atoms. The O 1 s peak could be deconvoluted into three components: the oxygen-zinc bond, the oxygen-aluminum bond, and chemisorbed or OH species. In the AZO samples, a direct correlation was observed between the intensity of the Al 2p peak and the number of Al2O3 layers. Additionally, the calculated elemental ratio of Al increased from 2.582 to 23.955 at.% with more Al2O3 layers. Optical measurements revealed that ZnO and AZO thin films exhibit high transmittance in the visible region. Moreover, introducing Al2O3 layers into the supercycle led to an increasing trend in transmittance. Another improvement in the optical properties was observed as a blue shift in the absorption edge with Al doping, indicating band gap widening. The optical band gap of the ZnO thin films increased from 3.2 to 3.5 eV with Al doping. The origin and concentration of defects were evaluated by deconvoluting the PL spectra. A comparative investigation of the PL spectrum and XPS results revealed variations in defect concentration with deposition parameters and defect formation mechanism was discussed.