(image) (image) [Previous] [Next]

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 \( _\mathrm {2} \) 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 \( N_\mathrm {C}V_\mathrm {Si} \) has been suggested as deep level defect responsible for dopant-deactivation [197]. 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 [198], 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 [199]. 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 [200]. 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 [201]. However, over-nitridation leads to an increased EDMR signal that has been related to the so called \( K_\mathrm {N} \)-center, which is a silicon dangling bond that forms on a Si atom that is bonded to three N atoms. These \( K_\mathrm {N} \)-centers are also observed in plasma nitrided oxides (PNO), while a similar EDMR spectrum is observed for such defects in Si \( _\mathrm {3} \)N \( _\mathrm {4} \) and referred to as \( K \)-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.