Keywords:Silicon nitride, First-Principles Calculation, Non-volatile memory
The multi-level cell technology in NAND flash memory demands a large threshold voltage shift, which depends upon the amount of charge stored by deep trap levels distributed in silicon nitride. Thus, the introduction of point defects capable of inducing deep trap levels in silicon nitride is needed to realize high-bit density flash memories. In this work, we investigate the energy levels caused by metal defects (Mg, Ti, Hf, V, Mn, and Al) in silicon nitride using first-principles calculations. A 168-atom supercell (Si72N96) was formed by expanding a unit cell of β-Si3N4 phase in a 2x2x3 matrix on a-axis, b-axis, and c-axis, respectively. The six different supercells were formed by the substitution of a Si-atom in the Si72N96 with each metal listed earlier. In the case of Mn, we have found five different defect levels of energy 1.59, 1.91, 2.03, 3.00, and 3.12 eV below the bottom of the conduction band. From Atomic Layer DOS calculation, we have observed that all five trap levels are mainly localized at the Mn atom. Some of these trap levels would be able to behave as deep electron trap levels.