Direct current electrical degradation of iron-doped titania ceramics
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Direct current electrical degradation of iron-doped titania ceramics Jianxin Sheng, Tatsuo Fukami, and Junich Karasawa Faculty of Engineering, Shinshu University, 500 Wakasato, Nagano 380, Japan (Received 20 October 1997; accepted 9 February 1998)
An anomalous increase of current was found in Fe-doped titania ceramics subjected to a constant field of 105 Vym. It is suggested that the space charge rises from blockage of O2 (g) ! O22 (s) ion transfer at the cathode. This leads to an increase of n-conductivity in the cathodic region and p-conductivity in the anodic region according to the specific defect equilibrium. This viewpoint was reinforced by two newly observed phenomena: (i) the I-V plot shows a linear feature at the initial stage, but it gradually becomes a rectifying feature with time; and (ii) an edge-located electrode shows lower current density and faster current saturation compared with a normal electrode.
A steady increase of conduction current with time induced by a constant electric field stress has been reported in some capacitors using perovskite titanates.1–5 This behavior exerts severe influences on the reliability of those components, because a slow increase of the leakage currents will result in dielectric breakdown, especially when integration of the device requires a reduction in the thickness of the dielectric layer. On the other hand, this detrimental behavior seems promising for the preparation of an analog memory device6 or synapse element,7 because every electrical stress duration or voltage pulse number corresponds to an electronic storage state. A model for the voltage-induced resistance degradation is based on the defect structure and transport aspects. The present paper describes the degradation in Fe-doped TiO2 ceramics and elucidates the processes responsible for the conduction behavior with new evidences. Ceramics of 1.0 mol % Fe2 O3 -doped TiO2 (f16 3 1.0 mm) were prepared by mixing the desired proportions of TiO2 and Fe2 O3 followed by heating the mixture at 1350 ±C for 2 h. The as-fired ceramics were brown-colored, having 96% theoretical density. X-ray diffraction and scanning electron microscopy showed a single rutile structure having an average grain size of around 50 mm. On one side of the specimen, a platinum layer (f 14 mm) was sputter deposited, which served as the anode. The other side was coated with an ensemble of spot cathodes (platinum or aluminum) of 1 mm diameter each. In a particular case, the specimen was fractured in such a way that some of the spots were located on the fractured edge as shown in Fig. 1. Under an electric stress of 105 Vym, the time dependence of resistance was measured in air at 200, 250, 300, and 350 ±C, respectively. Every spot cathode was used only once to avoid the interference of stored charges. The I-V characteristic was tested immediately after degradation, J. Mater. Res., Vol. 13, No. 7, Jul 1998
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where the bias voltage was increased
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