A density-functional study of defect volatility in amorphous silicon dioxide

Hole trapping in the gate insulator of pMOS transistors has been linked to a wide range of detrimental phenomena, including random telegraph noise (RTN), 1/ f noise, negative bias temperature instability (NBTI), stress-induced leakage currents (SILC) and hot-carrier degradation. Recently we were able to show that the hydrogen bridge (HB) and hydroxyl E´ centers (H-E´centers) are likely candidates for BTI defects in amorphous silicon dioxide (a-SiO₂). In time-dependent defect spectroscopy (TDDS) measurements, it was observed that defects tend to dis- and reappear in the measurements. This so called volatility is not a rare event, but occurs for a majority of the defects. In this work we investigate whether this particular behavior could be explained by an extension of our four-state model. As both of the investigated defect candidates contain hydrogen, we propose that the behavior could be explained by the hydrogen atom moving away from the defect site onto a neighboring oxygen atom and back again. Our results show that the suggested mechanism is likely to occur for hydroxyl E´ centers, but not for the hydrogen bridges.