Effect of annealing on photoluminescence and optical properties of porous anodic alumina films formed in sulfuric acid for solar energy applications

Photoluminescence and optical properties of porous oxide films formed by two-step aluminum anodization at a fixed current 200 mA have been investigated. It was found that the crystallographic structure depend strongly on the annealing temperature. X-ray diffraction (XRD) reveals an amorphisation of the porous oxide films after annealing. This evolution has been confirmed by Raman spectroscopy measurement. Spectroscopic ellipsometry (SE) in the UV–vis and near infra red (IR) spectra shows that refraction index n increases and the extinction coefficient k decreases with annealing temperature. This observation has been confirmed with reflectivity measurements. As a consequence the reflectivity reaches 97% when porous alumina films were annealed at 650 °C. Photoluminescence (PL) measurements show two PL peaks in the emission and excitation spectra. The first emission peak is centered at 460 nm (α-band) and the second (β-band) shifts from 500 to 525 nm, depending on excitation wavelength. For excitation spectra, one spectral peak is located at 271 nm and the second shifts to longer wavelengths with increasing emission wavelength. The results indicate the existence of two PL centers. One is associated with oxygen adsorption at the pore wall and oxygen vacancies inside the alumina. The other is related to the adsorption of water and/or OH groups at the surface of the pore wall and to structure defects and sulfur inclusion inside the films.