Modeling of Defect Related Reliability Phenomena
in SiC Power-MOSFETs
3.4 Nitrogen Related Defects
Optimizing the SiC/SiO2 interface by POA or PDA in nitrogen containing ambients, i.e. NO, NO or NH3 , has lead to a considerable improvement of the mobility and reduction of deep interface states, as discussed in Chapter 1. However, a significant amount of N can directly be observed at the interface after annealing as reported in several studies [196, 114, 192]. This accumulation of N in the transition region between SiC and SiO2 is correlated with an increased defect density close to the SiC conduction band [196, 51]. Another source of N is its implantation as an electron donor in the source and drain region. In N implanted regions, the has been suggested as deep level defect responsible for dopant-deactivation . This motivated theoretical studies on the incorporation of N close to the SiC/SiO2 interface using ab-initio methods. These first-principle studies have suggested that N can passivate large amounts of both silicon and carbon dangling bonds effectively , however, with the trade-off that states in the lower half of the band-gap are formed by a resulting threefold N, leading to positive charge accumulation. Increased hole trap densities have also been reported due to the incorporation of NO at the interface from capacitance measurements and first-principle calculations . Both studies suggest an increase of positive charge accumulation in an Si-C-N-O or even Si-C-N-O-H transition layer, consistent with the shift of the ideal device characteristics towards more negative gate bias, c.f. Figure 2.3.
The incorporation of NO or NH into well known bulk-SiO2 defects such as the intrinsic electron trap and the oxygen vacancy has been modeled with DFT by Mistry et. al . These studies revealed that the incorporation of NO can passivate the intrinsic electron trap, thereby potentially reducing the available concentration of these defects for electron trapping.
The studies of Higa et. al on dry-oxidized and nitrided interfaces revealed a drastic reduction of the Pb,C-center‘s EDMR signal for short term POA and is especially pronounced on a- and m-face interfaces . However, over-nitridation leads to an increased EDMR signal that has been related to the so called -center, which is a silicon dangling bond that forms on a Si atom that is bonded to three N atoms. These -centers are also observed in plasma nitrided oxides (PNO), while a similar EDMR spectrum is observed for such defects in Si N and referred to as -center [131, 202].
In summary, nitrogen plays a two-fold role for the stability of the SiC/SiO2 interface. Its importance for the passivation of interfacial defects is widely acknowledged and POA in N containing ambients has been established for industrial production. On the other hand, potential defect candidates that could arise from N incorporation at the interface are not clearly identified.