TEM investigation of barrier-like anodic oxide films on aluminium

In a previous work, the authors investigated the formation of thin barrier-like oxide films on aluminium using pulse anodizing [1]. The electrochemical experiments have shown differences in the properties and behaviour of the anodic films depending on the pulse frequency. Furthermore, the results suggest a relationship to the microstructure (e.g. thickness or composition) of the oxide films. Therefore, the authors have completed the former investigation by new research using transmission electron microscopy (TEM) and ellipsometry. All results were compared with electrochemical measurements. Again, the anodic films were formed at 3V_Ag/AgCl with various pulse frequencies in an acetate buffer solution (pH5.9). The aluminium was chemical vapour deposited on silicon wafers to get a smooth and well-defined surface which is favourable for subsequent analytical analysis (TEM, ellipsometry). The TEM investigation confirms the tendency that the oxide thickness decreases with increasing pulse frequency. The film thicknesses determined by coulometry and microscopy fit very well assuming a native oxide layer of approximately 1nm. Additionally, the Al:O ratio across the film thickness clearly depends on the pulse frequency. A model concept explaining this fact will be presented. Furthermore, there are no indications to the formation of crystalline domains within the oxide layers deposited at various pulse frequencies.
[1] M. Schneider, C. Lämmel, C. Heubner, A. Michaelis: Surface and Interface Analysis 45 (2013) 1497-1502