Chapter 5
Level Shift Model

The ETM has been based on the concept that electrons tunneling into and out of a defect have the same energy. This implies that the corresponding trap level remains at the same energy within the oxide bandgap regardless of whether the defect has captured a charge or not. However, as pointed out in Section 2.3, this level is subject to a shift after each charging or discharging event. This is due to the fact that the defect undergo structural relaxation, which involves strengthening, weakening, or even disrupting bonds. With this deformation, the defect orbitals change which is accompanied by a shifts of the corresponding defect levels. This concept of the level shift has not received much attention so far but has important implications for the tunneling kinetics in a - SiO2  .

In the first part of this chapter it will be demonstrated that the charging and discharging of defects involves a non-negligible structural relaxation, which results in an appreciable level shift and thus impacts the trapping dynamics. Using DFT, the corresponding switching levels will be determined for a number of defects suspected to be present in a- SiO2  . Based on the position of these levels, the defects will be tentatively classified according to their expected trapping behavior in NBTI experiments. In order to allow for quantitative predictions, a new model accounting for the level shift will be developed in the second part of this chapter. This model will then be tested whether it can explain the NBTI experiments.

 5.1 Defects in Amorphous Silicon Dioxide
  5.1.1 Oxygen Vacancy
  5.1.2 E′γ  Center and Variants
  5.1.3 Hydrogen Atom
  5.1.4 Hydrogen Bridge
 5.2 The Level Shift Model
  5.2.1 Model Evaluation
 5.3 Conclusion