Modeling of Defect Related Reliability Phenomena
in SiC Power-MOSFETs

3.3 The Role of Hydrogen

Although, as discussed in Chapter 1, forming gas anneals for interface passivation have not improved the interface of the SiC/SiO₂ system, hydrogen may still play a role in defect formation and inevitably is introduced through other process steps, e.g. through Silan (SiH ) which is used as precursor for the poly-Si gate contact formation, poly-Si/SiO₂ interface passivation after deposition in forming gas or through the precursor TEOS upon oxide formation via CVD. Rescher has shown by measuring CV characteristics of MOSCAPs after certain process steps (process splits), that an accumulation of positive charges, i.e. protons, is especially pronounced after the annealing step which follows the interlayer dielectric deposition [193]. A promising approach to describe the dissociation of hydrogen at passivated interfacial bonds at the poly-Si interface under stress conditions is provided by the gate-sided hydrogen release model [94]. The model is capable of describing both the recoverable and the permanent component of BTI in Si-MOSFETs. At its core, this model is based on the assumption that hydrogen related defects, such as the hydrogen bridge (HB) or the hydroxyl-E center (HE ), can release their hydrogen which can then diffuse through the insulator and eventually create defects at another oxide site or create new dangling bonds by forming H by releasing the H atom of a previously passivated center. Note that both the HB and HE defects form from H atoms which rapidly diffuse through SiO₂ due to small diffusion barriers [194]. In the case of HB, additionally an interaction with an OV [195] is required, thus reducing its formation probability. However, both the formation of HB and HE is likely in the SiO₂ network upon availability of the precursor sites with CTLs for +/0 and 0/- within the SiC bandgap or close to the band edges and relaxation energies distributed between 1 eV to 3 eV [77, 172]. Thus, just like in Si based MOSFETs, the role of hydrogen related defect structures cannot be neglected when electrically active defect candidates in bulk a-SiO₂ are studied in SiC MOSFETs.