Aluminium oxide film

Aluminium is a very reactive metal with a high affinity for oxygen. The metal is nevertheless highly resistant to most atmospheres and to a great variety of chemical agents. This resistance is due to the inert and protective character of the aluminium oxide film which forms on the metal surface (Section 1.5). In most environments, therefore, the rate of corrosion of aluminium decreases rapidly with time. In only a few cases, e.g. in caustic soda, does the corrosion rate approximate to the linear. A corrosion rate increasing with time is rarely encountered with aluminium, except in aqueous solutions at high temperatures and pressures. 

Schmid M, Shishkin M, esse G, Napetschnig E, Varga P, Kulawik M, Nihus N, Rust H-P, Freund H-J (2006) Oxygen-deficient hne defects in an ultrathin aluminium oxide film. Phys Rev Lett 97 046101... 

We prepared aluminium oxide films by radio frequency (r.f) magnetron sputtering fi om an aluminium oxide target in a dedicated vacuum chamber. To study the growth and structure of these films deposited on silicon oxide and films of DIP we used X-ray reflectivity, cross-sectional transmission electron microscopy (TEM) and atomic force microscopy (AFM) in contact mode. For further details on the preparation of the aluminium oxide films we refer to Refs. [112, 113]. 

Figure 9.11 shows typical atomic force microscopy (AFM) images (contact mode) of a crystalline DIP film of about 360 A thickness deposited on silicon oxide (a), an aluminium oxide film ( 174 A thick) deposited on silicon ox-... 

Growth of Aluminium Oxide Films on Silicon Oxide and Films... 

To address this problem we have prepared highly crystalline films of the organic semiconductor DIP and capped them with r.f magnetron sputtered aluminium oxide films. 

The anodisation of aluminium is a well-established process [6-10]. For the formation of barrier aluminium oxide films, commonly used electrolytes are citric acid, tartaric acid and ammonium adipate [11]. Barrier films with high capacitance values and breakdown field strength were obtained in tartaric acid ofpH7[8, 12]. 

Concerning the two-layer model, the thickness and properties of each layer depend on the nature of the electrolyte and the anodisation conditions. For the application, a permanent control of thickness and electrical properties is necessary. In the present chapter, electrochemical impedance spectroscopy (EIS) was used to study the film properties. The EIS measurements can provide accurate information on the dielectric properties and the thickness of the barrier layer [13-14]. The porous layer cannot be studied by impedance measurements because of the high conductivity of the electrolyte in the pores [15]. The total thickness of the aluminium oxide films was determined by scanning electron microscopy. The thickness of the single layers was then calculated. The information on the film properties was confirmed by electrical characterisation performed on metal/insulator/metal (MIM) structures. 

Aluminium Oxide Film as Gate Dielectric for Organic FETs ... 

The aluminium oxide film was prepared at eonstant eurrent density (poten-tiostat in eonstant eurrent mode) with eurrent densities between 0.3 and... 

SEM was used for morphological studies of anodic aluminium oxide films, formed at various formation current densities up to a formation voltage of 60 V. The total thickness of the films was determined by cross-section SEM micrographs as shown in Figure 23.1, but it should be emphasised that one can not clearly identify barrier and porous layers of the oxide film by using this technique. 

Figure 23.1 High-resolution cross-sectional SEM micrograph of an anodic aluminium oxide film of 66 nm formed at 0.3 mA/cm and a formation voltage of 60 V (Al thickness remaining 633 nm).

The anodisation factor can be considered as an indicator for the quality of the aluminium oxide films. In particular, the barrier aluminium oxide films formed with low anodisation factors exhibit high breakdown field strengths... 

To interpret the impedance data, a model of the anodic aluminium oxide film must be established. The anodic aluminium oxide film is a sandwich film consisting of two layers, a barrier layer and a porous layer (Figure 23.4A [2]). 

Figure 23.3 Bode plots of natural and anodic aluminium oxide films. The aluminium oxide film was formed at 2.5 mA/cm and a formation voltage of 60 V, experimental (points) and fitting (lines).

This replacement was necessary to adapt the equivalent circuit to the nonideal behaviour of the aluminium oxide film. The exponent n of the CPE element can be regarded as a measure of the inhomogeneity of the film structure [17]. For an ideal capacitor the exponent n is one. For the calculation of the CPE values, the fitting program in Ref [18] was used. 

The capacitance values of the aluminium oxide films formed at various formation current densities are shown in Figure 23.5. The capacitance CPE ) decreases from 0.19 pF/cm, for the film formed at 0.3 mA/cm, to 0.13 pF/cm, for the film formed at 2.5 mA/cm. At higher current densities, the capacitance of the films increases. The capacitance of the aluminium oxide films is much... 

Figure 23.4 Two-layer model of the anodic aluminium oxide film (A) and associated equivalent circuit (B)...

The thickness of the initial aluminium oxide film is about 1.4 nm. 

The variation of the thickness of the barrier layer with the current density is also shown in Figure 23.5. Increasing the current density, a thicker aluminium oxide film is formed. However, the thickness ratio of the barrier layer to the porous layer is almost unchanged for the films formed at low current densities. It reduces significantly for the films formed at current densities higher than... 

For all aluminium oxide films the exponent n in the CPE was nearly constant, approximately 0.99 0.04, determined for current densities between 2.5 and 8.5 rciAJcnC. This value is almost one and significantly higher than values found in the literature [15]. The n value is related to the layer inhomogeneity. The high value of n confirms the formation of homogeneous aluminium oxide films. 

A significant influence of the formation voltage on the film properties was found. The aluminium oxide films were formed at a constant current density of 0.5 mA/cm. The formation voltage was raised in steps to 5, 10, 15, 30, 60 and 100 V. When the formation voltage was reached, the potentiostat was switched... 

Figure 23.6 Bode plots of aluminium oxide films formed at 0.5 inA/cm, different formation voltages 5, 10 and 15 V, experimental points and fitting lines.

The transformation from an amorphous film to a crystalline film at 30 V was also observed for Ti02 films [20]. The Raman spectrum of anatase was observed when the formation voltage was raised to 30 V. The Raman intensities increased with increasing formation voltages. A similar phenomenon can explain the change of the structural properties for the aluminium oxide films formed at voltages > 30 V. 

The significant reduction of the resistance i 2 of the aluminium oxide film fi om 166 MQ/cm at 30 V to 8.2 MQ/cm at 100 V could be explained by increasing crystallinity inside the amorphous phase. The resistance of the film decreases from 2.8 MQ/cm at 30 V to 1.6M 2/cm at 100 V. Figure 23.9 shows the effect of the formation voltage on the capacitance and the barrier layer thickness. The capacitance decreases with increasing formation voltage, while the barrier layer thickness approaches a constant value around 40% of total thickness. 

The dependence of the aluminium oxide film properties on the anodisation time was studied. Figure 23.10 shows the current-time transients during anodisation at the formation current density of 2.5 mA/cm. In the constant current... 

The aluminium oxide films formed for different times (marked in Figure 23.10) were characterised by electrochemical impedance spectroscopy. For the aluminium oxide film anodised for 160 s, the open circuit potential (OCP) is not stable. This can be explained by instability of the film structure. The processes of the film formation were not yet completed. The OCP is more stable and positive for films anodised for more than 700 s. This can be explained by the formation of the compact barrier aluminium oxide layer. 

The impedance response of the films was observed with just one time constant (Ri, CPEi), except for the film formed for 10900 s. This film showed two time constants similar to the films formed at voltages > 30 V. In this case, the aluminium oxide film might be contaminated by anions migrating from the surface into the film. 

During the period of the current decay, there are two competitive processes, densification to form the barrier layer and dissolution of barrier layer to form the porous layer. Under the high electrical field strength (constant voltage mode), the densification of the aluminium oxide films is favoured for process durations shorter than 3700 s. When the constant voltage was kept for a long anodisation time (beyond 10900 s), the dissolution of the aluminium oxide film becomes more dominant. Thus, the film could be contaminated by inward migration of the electrolyte into the film or by formation of micro-voids. 

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