Anodization of aluminium thin films on p++Si and annihilation of strong luminescence from Al2O3

Photoluminescence (PL) of Al2O3 films obtained by anodization of thermally evaporated and annealed thin Al films on p++Si in 0.3 M oxalic acid has been investigated. Thermal annealing at 200-950 °C under the dry nitrogen atmosphere was used for deactivation of luminescence centres. Luminescence from as grown films was broad and located at 425 nm. This luminescence reached to highest level after annealing at 600 °C. Maximum 10 min was required for full optical activation and prolonged annealing up to 4 h did not change the luminescence intensity. Because of deep levels, absorption band edge of as grown films was shifted to the lower energy which is 3.25 eV. Annealing above 800 °C reduced the PL intensity and this observation was correlated with the blue shift of band edge as the defects annealed out. Disappearing PL intensity and blue shift of band edge absorption after annealing at 950 °C was mainly attributed to the oxygen-related defects and partly to impurities that may be originated from oxalic acid. AFM results did not show any hexagonally ordered holes but uniformly distributed nanosized Al2O3 clusters that were clearly seen. XRD measurements on as grown Al2O3 showed only [1 1 0] direction of α phase. Debye-Scherer calculation for this line indicates that cluster size is 35.7 nm. XRD and AFM pictures suggest that nanocrystalline Al2O3 are embedded in amorphous Al2O3.