The capture and release of charge carriers to and from point defects greatly affect semiconductor devices. Trapping centers in the oxides of MOS transistors have been shown to cause several important parasitic effects such as Stress-Induced Leakage Currents (SILC), Bias Temperature Instability (BTI) or Random Telegraph Noise (RTN).
The detailed capture and release process in these degradation effects is understood as being a Non-radiative MultiPhonon (NMP) transition, i.e. a transition involving the emission or absorption of multiple phonons but without any radiative contribution. NMP transitions have been used by several groups in the context of semiconductor device reliability. Usually, the employed descriptions require a number of parameters that have to be calibrated to experimental data. However, the physically meaningful range for some of these parameters is unknown and cannot be obtained from experimental data.
The central concept of NMP transitions is the Line Shape Function (LSF), which describes the influence of the atomic vibrations on a particular electronic transition. A simple method was developed to extract the LSF from ab-initio Density Functional Theory (DFT) calculations, which do not require any parametrization. This method was used to investigate the NMP properties of two defects commonly proposed to be relevant for Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) degradation: the oxygen vacancy and the hydrogen bridge in SiO2. Both defects were modeled using a crystalline SiO2 host lattice and the electronic structure was calculated using the PBE functional. The influence of strain, level alignment scheme, and exact exchange on the results was investigated.
The calculated LSF were then used in a semiconductor device simulation based on Non-Equilibrium Green's Functions (NEGFs), to predict the hole capture properties of the defects in a realistic device situation.
The calculations showed that the commonly proposed defects can not fully explain the degradation behavior observed in MOSFET degradation experiments. Some of the shortcomings are assumed to arise from the high symmetry of the crystalline host structure employed in the DFT calculations. To obtain a more realistic description of the unstructured gate oxide, amorphous SiO2 structures have been generated that will be used for more sophisticated defect studies.