Due to the high charge-retention power, nanometer-scale metal dots are promising systems for near-future memory devices when they are embedded in insulating oxides such as SiO2. However, there exist serious issues of charge-retention degradation [1]; during the repeated charge injection, a metal nanodot is broken by the dissociation of metal atoms from the nanodot surface into insulator under the charge-induced electric field. In addition, the charge leakage from the metal nanodot to dissociated metal atoms decreases the charge-retention power of the metal nanodot. However, there have been no microscopic studies on how the metal nanodot is broken (structural stability) and how the charge-retention power decreases (charging stability). In this work, by the first-principles calculations, we answer these questions and clarify what kind of metal is suitable for a metal nanodot memory in SiO2.

Twenty kinds of metals are considered as candidate elements of metal nanodots. We calculate the dissociation energy of a metal atom from nanodot surface into amorphous SiO2 (a-SiO2) and clarify the structural stability [2]. On the other hand, the charge-retention power is estimated using the ionization energy of a dissociated metal atom in a-SiO2 [3]. Atomic and electronic structures of various metal atoms in a-SiO2 are calculated by the standard first-principles method using VASP code in the density-functional theory.

Main results are summarized as follows: (1) we found that the structural stability is determined by the competition between the metal-metal and metal-SiO2 interactions (bondings). For example, the Au and Ti nanodots are unstable because the Au-Au bonding is basically weak and the Ti-Ti bonding is weaker than Ti-SiO2 interaction, while the W nanodot is stable because the W-W bonding is much stronger than W-SiO2 bonding. It is shown that such strength of bonding reflects both the electron occupancy of d-orbital states and the electron negativity of metal atoms. (2) The degradation of charge-retention power occurs by the charge leakage from a metal nanodot to dissociated metal atoms. It is interesting to note that Ti and W dissociated atoms are difficult to be ionized positively although they have small electron negativity compared to Au. This is because these atoms are electronically bounded strongly to surrounding O and Si atoms in a-SiO2. As a result, the W and Ti nanodots show little charge-retention degradation. (3) From these considerations, we found that the W nanodot has the highest structural and charging stability, thus being suitable for a metal-nanodot memory usage.

These results are discussed in details, together with reviewing recent experiments and considering other degradation processes.

<References>

[1] M. S. Lee et al., Jpn. J. Appl. Phys. 46 (2007) 6202.

[2] R. Nagasawa, T.Nakayama et al., Jpn. J. Appl. Phys. 57 (2018) 04FB05.

[3] M. Y. Yang, T. Nakayama et al, J. Appl. Phys. 114 (2013) 063701.