The metal/high-k gate stack added new degrees of freedom for controlling the threshold voltage (V_TH) of CMOS devices. The V_TH is altered depending on the materials of high-k oxide, which is attributed to an electric dipole layer formed at the interface between high-k oxide and underling interfacial SiO₂ layer [1]. Several mechanisms were proposed to explain the origin of the interface dipole layer, but a unified view has not yet been reached.

The proposed mechanisms is roughly classified into two groups: (1) electronic redistribution around the interface and (2) redistribution of ions across the interface. The second one, proposed by Kita and Toriumi [2] has gained popular support because of its applicability to a wide range of high-k materials. The model states that the interface dipole is formed by the movement of negatively charged O ions from higher oxygen-density side to a lower side. This model is referred to as “oxygen-density difference accommodation,” hereafter.

The picture of oxygen-density difference accommodation model was well reproduced in our MD simulation (Fig.1) of Al₂O₃/SiO₂ system [3-5], which employs a simple two-body ionic interaction model of the Born-Mayer-Huggins potential. A dipole layer is formed by the migration of O ions from the Al₂O₃ side to the SiO₂ side (Fig.2). The direction and magnitude of the dipole coincides well with experimental results. The driving force of the O- ions migration is found to be the short-range repulsion between ionic cores. The repulsive force becomes larger in the higher oxygen density materials, so that the O- ions is pushed into the lower oxygen density side (Fig.3).

In my previous talk in this conference held in 2014 [4], I discussed that an imbalance of multipole potentials between both oxide layer is a possible origin of the O^- ion migration. However, the subsequent analysis [6] clarified that the dominant force acting on O^- ions in near the interface is not originated from the charge-to-charge interaction, so that my previous hypothesis was denied nowadays.

Furthermore, the author’s group succeeded in reproducing the direction of dipoles at MgO/SiO₂ and SrO/SiO₂ interfaces (Fig.2) [5], which is opposite to that of the Al₂O₃/SiO₂ system. The MgO/SiO₂ system is an irregular case of the oxygen-density difference accommodation model, because the oxygen density of MgO is higher than that of SiO₂, but the experimentally observed dipole is opposed to the direction of the density gradient of O ions. In spite of the fact, our MD simulation can correctly reproduce the direction of the dipole at the MgO/SiO₂ interface. The dipoles at the MgO/SiO₂ and SrO/SiO₂ interfaces are formed by a preferential migration of metal cations from the high-k oxide toward the SiO₂ layer, and silicate layers are formed at these interfaces.

In conclusion, the dipole formation at high-k/SiO₂ interfaces can be explained by the ion redistribution model, as shown in Fig.4. The negative dipoles at the MgO/SiO₂ and SrO/SiO₂ interfaces are formed by a preferential migration of metal cations from the high-k oxide toward the SiO₂ layer during the formation of a stable silicate phase. Thus, the migration of metal cations as well as the migration of oxygen ions must be taken into account when considering the mechanics of the dipole layer formation.

 Acknowledgement

This work was supported by JST-CREST and a Grant-in-Aid for Scientific Research (B) (15H03979) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

References

[1] A. Toriumi and T. Nabatame, in High Permittivity Gate Dielectric Materials, ed. S. Kar (Springer, Heidelberg, 2013) Vol. 43, Chap. 6.

[2] K. Kita, and A. Toriumi, Appl. Phys. Lett. 94, 132902 (2009).

[3] R. Kuriyama, M. Hashiguchi, R. Takahashi, K. Shimura, A. Ogura, S. Satoh, and T. Watanabe, Jpn. J. Appl. Phys. 53, 08LB02 (2014).

[4] T. Watanabe, R. Kuriyama, M. Hashiguchi, R. Takahashi, K. Shimura, A. Ogura, and S. Satoh, ECS Trans. 64 [8], 3 (2014).

[5] K. Shimura, R. Kunugi, and T. Watanabe, Jpn. J. Appl. Phys. 55, 04EB03 (2016).

[6] R. Kunugi, N. Nakagawa, T. Watanabe, Applied Physics Express 10, 031501 (2017).