EVIDENCE FOR (100)SI/SIO2 INTERFACIAL DEFECT TRANSFORMATION AFTER IONIZING-RADIATION

The interface traps created by ionizing radiation or hot-electron injection in MOS capacitors fabricated on (100) Si were found to undergo significant changes with time over an extended period (several months). Immediately after radiation or hot-electron damage, an interface-trap peak above midgap appears. The peak (designated peak-1), along with its background, continuously changes with time after damage and is gradually converted to a second peak (peak-2) located below midgap. A significant amount of experimental evidence is presented that shows that this interfacial defect transformation process occurs generally in a variety of samples (e.g., dry and wet oxides, p- and n-type Si, various gate electrodes, etc.). The rate of the process is a function of the device structure, its processing parameters, the details of the radiation treatment, and the gate bias polarity and is thermally activated. When peak-2 achieves a certain magnitude, a reverse transformation process can occur, especially under a positive gate bias. It is shown that peak-2 is not generated directly by ionizing radiation; rather, the pre-existence of peak-1 above midgap is a necessary condition for its formation. It is also shown that only a portion of peak-1 contributes to the defect transformation process.