Retention characteristics of SONOS non-volatile memory (NVM) device is expected to be improved by doping silicon nitride to change the type and number of defects which traps charge. Firstly, cluster models of defects in amorphous silicon nitride with and without doping special element, such as oxygen, sulfur, phosphorus, fluorine or chlorine, are built. By density functional theory (DFT) of first-principles, positions of all atoms of the cluster are optimized, and energiesof these clusters are calculated. Energy changes of charge capturing processes for various defects are obtained. The calculational data can provide references for investigating discharge mechanism and improving charge retention on nonvolatile memory device. The capability of capturing electrons for all defects is usually better than capability of capturing holes. The electron discharge process should be sensitive to temperature, but the hole discharge process may be mainly controlled by tunneling mechanism. Like silicon oxynitride, silicon nitride doped with sulfur or phosphorus is promising for improving charge retention characteristics of SONOS device, due to more defects having better capability of capturing electrons.