Modeling of Defect Related Reliability Phenomena
in SiC Power-MOSFETs
3.2 Defects at the Silicon Carbide / Silicon Dioxide Interface
The enhanced Active Energy Region (AER) in the SiC/SiO2 material system compared to the Si/SiO2 case allows additional bulk-SiO2 defects to become charged/discharged in SiC MOSFETs. Additionally, different types of interfacial defects have to be considered. A large number of physical and electrical characterizations as well as theoretical studies on the structural properties of the SiC/SiO2 interface have been published in the past 25 years. Within the sub-stochiometric transition layer between semiconductor and oxide, as shown for an ideal atomistic model in Figure 3.3, the accumulation of carbon in clusters during oxidation has been early suggested to be responsible for deep defects states [182, 183]. However, experimentally no significant amount of excess carbon could be measured by Electron Energy Loss Spectroscopy (EELS), Secondary Ion Mass Spectroscopy (SIMS) (Figure 3.3 (right)) and X-ray Photoelectron Spectroscopy (XPS) studies after re-oxidation and nitridation of the interface . Instead, acceptors like carbon interstitial and oxygen bound dimer states have been suggested as suitable defect candidates by ab-initio calculations, as their trap levels lie close to the SiC conduction band edge . The increased complexity of the chemical composition which is induced by additional elements like C and N at the SiC/SiO2 interface region allows for a number of other potential defect candidates. Some of these have been theoretically studied in the framework of DFT employing a hybrid functional in addition to hydrogen related defects by Devynck et. al . Their results render the Si -C-O structure a suitable defect candidate, which can form when a CO molecule replaces an O atom in the SiO2 network. This defect is expected to occur at high defect densities and exhibit CTLs close to conduction band edge of SiC, as experimentally measured by photon stimulated tunneling and electrical characterization by Afanasev et. al . In later works, carbon-dangling bonds (Pb,C centers), analogously to the Pb,0-centers in Si/SiO2, have been suggested by comparison of EDMR spectra with those calculated for Pb,C defects [187, 188, 189] as interface states with energy levels deep within the SiC bandgap. During Si oxidation, these dangling bonds natively form from strain release during the SiC oxidation process and structurally consist of a carbon atom back-bonded to three silicon atoms with an unsaturated bond directed towards the interface.
The detailed structural interface analysis of Woerle et. al  includes various physical characterization methods, e.g. EELS, Photo-Luminescence (PL) and local- Deep Level Transient Spectroscopy (DLTS) on thermally oxidized SiC/SiO2 samples with different surface roughnesses. Their results suggest that an abrupt transition from SiC to SiO2 is only possible on atomically flat surfaces, which is also supported by EELS and scanning transmission electron microscopy (STEM) measurements on SiC MOSFETs with low-temperature deposited oxide layers and comparison to ab-initio calculations for abrupt transitions [191, 192]. Therefore, in this work, linearly interpolated transition regions for the energetic band-edges within 5 Å, as shown in Figure 3.3 (bottom, left), are used within the simulations conducted.