Polarization and Charge Motion in Metal-Al[sub 2]O[sub 3]-SiO[sub 2]-Si Structures

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Abstract
An investigation of the flatband voltage shifts of double insulator MIS capacitors under bias temperature stress has been performed in order to assess and characterize the stability of double insulator IGFET's. These shifts were opposite to the applied bias voltage (“ionic” direction), and it was found that they are composed of two components: (a) A “fast” component which is linear with gate bias voltage and symmetric with bias polarity. Most of the shift occurs in less than 1 min at all temperatures used in this work (65°–260°C). Its magnitude is temperature activated (0.15 eV). This part of the flatband voltage shift is explained by a bulk polarization of the layer with a polarizability that is independent of thickness, (b) A “slow” component which is observed under positive bias. It is characterized by a linear dependence of flatband voltage shift on the logarithm of stress time for times larger than a delay time, which is thermally activated (1.2 eV). This component is interpreted to be due to motion of mobile, ionic charge, such as sodium, across the and layers. This charge motion is greatly reduced by increasing the thickness.

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