| AVS 62nd International Symposium & Exhibition | |
| Electronic Materials and Processing | Tuesday Sessions |
| Session EM-TuP |
| Session: | Electronic Materials and Processing Poster Session |
| Presenter: | Hiroki Shirakawa, Graduate School of Engineering Nagoya University, Japan |
| Authors: | H. Shirakawa, Graduate School of Engineering Nagoya University, Japan M. Araidai, Graduate School of Engineering, Nagoya University, Japan K. Kamiya, Center for Basic Education and Integrated Learning, Kanagawa Institute of Technology, Japan K. Shiraishi, Graduate School of Engineering, Nagoya University, Japan |
| Correspondent: | Click to Email |
Metal-Oxide-Nitride-Oxide-Semiconductor (MONOS) memories have attracted a great attention as one of the next generation NAND flash memories. MONOS memories can trap charges inside defect sites in charge trap layers (SiN) that are spatially separated each other, leading to great merits for integration techniques such as 3D-structure and multi-level-cell operation. On the charge trapping layer in MONOS memories, it has been reported that large amounts of O and N atoms are incorporated into SiO2/SiN interfaces, leading to the formation of electron occupied defects near the interfaces [1]. In addition, Vianello et al. have found that a lot of H atoms distribute in SiO2, SiN and interface [2]. Yamaguchi et al. have also reported atomic-scale behavior of such defects in MONOS SiN layers during program/erase (P/E) cycles [3]. However, for SiO2 layer of MONOS memories, the decisive atomic-scale information has not been clarified yet.
In this work, we investigate the behavior of N, H atom and O vacancy in SiO2 during P/E cycles using VASP code [4], which is based on the density functional theory with GGA of Perdew-Wang-91. Core valence interactions were described by the ultrasoft pseudo potential. To simulate N related defects in SiO2 layer near SiN/SiO2 interface, we employed the 72 atoms super-cell of α-quartz including the defects. The defects formed by substituting two N atoms for two O atoms in SiO2. The P/E operations correspond to the electron and hole injections into the defects in our calculation.
We found that O vacancies are induced by the two N-substituted defects, forming the complex defect (VoN2) that consists of one O vacancy and two N atoms. Then, VoN2 defect cannot traps charges when the Fermi energy of MONOS is within the ranges of SiN band gap. We also found that the diffusing H atoms in SiO2 change the characteristics of VoN2 defect. VoN2 defect is transformed to another defect including H atoms (VoN2-H) by adsorbing hydrogen atoms. While VoN2-H defect is energetically unstable compared with VoN2 in the neutral charge state, VoN2-H defect becomes very stable in the negative charge state. This indicates that VoN2-H defect is able to trap electrons.
These results suggest that the amount of trapped charges on MONOS memories strongly depends on the dynamics of H atoms in SiO2 during P/E cycles; namely, the thermal diffusion of H atoms would bring about the threshold voltage shift of MONOS memories.
[1] M. Miura, et al., IEICE Tech. Rep. SDM, pp.17, (2007).
[2] E. Vianello et at., IEDM Tech. Dig., pp.75, (2009).
[3] K. Yamaguchi, et al., IEDM Tech. Dig., p.122, (2010).
[4] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993).