Electrical conductivity and dynamics of electroforming in Al-SiOx-Al thin film sandwich structures

Recent work has established that in evaporated SiOx thin film sandwich structures with gold electrodes, which show the Poole-Frenkel effect (enhanced conductivity at high electric fields due to the lowering of the potential barrier at donor-like centres), the value of the Poole-Frenkel field-lowering coefficient *b increases with voltage cycling prior to the onset of electroforming. These enhanced *b values were associated with the establishment of a high-field region during the electroforming process. In the present work, aluminium electrodes were used in order to explore the characteristics of contrasting system. Electroforming occurred in some samples, but with a maximum current value of less than 1 mA, considerably less than with the gold electrodes. Poole-Frenkel conductivity was observed in the initial voltage cycles, with a *b value of typically 4.5x10-5 eV m1/2 V-1/2, moderately exceeding the theoretical value. Electroforming normally took place after several voltage cycles, but was not permanent, with a reversion to Poole-Frenkel conduction during some cycles. As for the case with gold electrodes, the value of *b increased in the first few cycles, but after further voltage cycling, the behaviour became less stable with varying values of *b. After further voltage cycling, electroforming disappeared and *b reduced to the order of 2x10-5 eV m1/2 V-1/2. This behaviour can be understood in terms of a filamentary conduction model, in which the stability of samples with aluminium electrodes is inferior to those with gold electrodes, as a consequence of the lower melting point.