Experimental and theoretical study of ultra-thin oxides

We report on an experimental and theoretical study of transport through thin oxides. The experimental study was carried out on the tunnel switch diode (TSD) which consists of an MOS junction on top of a pn junction. The properties of the TSD depends critically on the properties of the tunnel oxide layer. Our results indicate that these devices can exhibit two different modes of behaviour depending on the stress history of the oxide. An unstressed device exhibits a thyristor-like I –V characteristic with fairly low current density. As the oxide is stressed, however, the I –V characteristic discontinuously shifts into a higher-current thyristor-like mode in which current transport appears to be highly non-uniform and depends strongly on stress history. This suggests a possible structural change in the oxide layer which is not completely destructive in that the device continues to function. We present a possible theoretical model of such a structural change in which microscopic filaments are generated in the oxide. Calculations of J –V curves for such structures with varying filament heights qualitatively match stressed MOS I –V curves found in the literature and qualitatively explain the dual-mode behaviour of the TSD.