Ab Initio Study of Transition Levels for Intrinsic Defects in Silicon Nitride

The charge trapping properties of intrinsic defects in β-Si 3N4 are investigated by means of density functional theory calculations using a hybrid exchange correlation functional that properly reproduces the band gap of the material. The following defects have been considered: an N vacancy, VN, consisting of a N3 - Si * and a N3 - Si-Si - N3 adjacent units; an N vacancy saturated with H, VN(H) (N3 - Si-H and N 3 - Si-Si - N3); and a Si atom substitutional to N, SiN, where Si is bound to three other Si atoms, Si3 - Si*. The analysis of the charge trapping ability is performed by computing the optical and thermodynamic transition levels between the 0 and -1 charge states of the defects, with an approach that goes beyond the single-particle approximation provided by the Kohn-Sham levels. The N vacancy turns out to be a deep trap for electrons, whereas the H impurity in the N vacancy and a Si atom substitutional for N are relatively shallow defects with thermodynamic 0/-1 transition levels at ∼1 eV below the bottom of the conduction band. The results suggest that Si-Si bonds or tiny Si clusters formed inside Si3N4 are responsible for the charge trapping behavior of this material in charge trapping memory devices.