In:
ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2014-02, No. 44 ( 2014-08-05), p. 2084-2084
Abstract:
Implementation of high-k/metal gate stack poses a challenge in controlling the threshold voltage (V TH ) of MOSFET devices. Recent studies have shown that an electric dipole layer formed at high-k/SiO 2 interface is responsible for anomalous V TH shifts. The origin of the dipole layer has been explained by several mechanisms, such as the areal density difference of O atoms [1], electronegativity difference [2] , and contact induced gap state [3]. Among these models, the first model proposed by Kita and Toriumi [1] is noteworthy because of its simplicity and potential to be applicable to wide variety 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 one. Recently, we performed molecular dynamics (MD) simulations of high-k/SiO 2 interfaces and succeeded in reproducing the O atom migration across the Al 2 O 3 /SiO 2 interface [4]. The O ion moves one-sidedly from Al 2 O 3 side to SiO 2 one, but opposite movement never occurs. The simulation result implies that the O ion migration is induced by some drift forces, but not by a simple diffusion due to the O density gradient across the high-k/SiO 2 . In this paper, we discuss the driving force of the O ion migration across the high-k/SiO 2 in terms of the multipole moment around oxide cation. The MD simulation was performed by a commercial simulation package SCIGRESS ME from Fujitsu Ltd. Al 2 O 3 /SiO 2 interface model was constructed by sandwiching amorphous Al 2 O 3 blocks in between amorphous SiO 2 blocks as shown in Fig.1. The hetero-oxide structure was annealed by the isothermal-isobaric MD calculation for 100 ps, thermostated at 1000 K by a velocity scaling with keeping the pressure at atmospheric pressure. The Born-Mayer-Huggins potential is employed in the MD simulation. Figure 2(a) shows a charge density profile across the SiO 2 /Al 2 O 3 interface. Electric dipole layer appears at the interface, and it is directed from the SiO 2 side to Al 2 O 3 side. Figure 2(b) shows the electric potential profile around the interface, which is obtained by solving one-dimensional Poisson’s equation. The built-in potential at SiO 2 /Al 2 O 3 interface is about 0.5 V, which matches the experimental value of the flat-band voltage shift [1]. Thus the direction and magnitude of the dipole moment in our model agrees with the experimental result. Figure 3 shows the magnified image of the SiO 2 /Al 2 O 3 interface, in which the O ions originated from Al 2 O 3 are colored by green. It can be clearly seen that the O ion moves one-sidedly from Al 2 O 3 side to SiO 2 one. The simulation result indicates that the O ion is driven by some drift forces, but not by a simple diffusion due to the density difference at the interface. It is inferred that the O ions in Al 2 O 3 are attracted to the strong positive charges of Si ions. The positive charges of Si ions must be canceled by the negative charges of O ions in SiO 2 , but at very close distance, there is an appreciable octupole moment around the Si ion which is located at the center of SiO 4 tetrahedron (see Fig.4). On the other hand, there is an hexadecapole moment around the Al ion which located at the center of AlO 6 octahedron. The electrostatic field induced by the octupole moment around the Si ion is stronger than that of the hexadecapole moment around the Al ion, and thus O ions in the vicinity of the interface can be attracted to Si ions in specific directions. Opposite dipole formation at SiO 2 /Y 2 O 3 interface can also be explained by the difference in the multipole moments around Si and Y ions [4]. The multipole moment around an oxide cation is relevant to its electronegativity of the O density of the oxide. Therefore, our “multipole moment induced O migration model” can be regarded as the unified model of the O density [1] model and the electronegativity model [2] . Acknowledgement This work is supported by JST-CREST, and a Grant-in-Aid for Scientific Research (B) from the MEXT Japan. References [1] K. Kita, and A. Toriumi, Appl. Phys. Lett. 94, 132902 (2009). [2] K. Kakushima, K. Okamoto, M. Adachi, K. Tachi, P. Ahmet, K. Tsutsui, N. Sugii, T. Hattori, H. Iwai, Solid-State Electron. 52, 1280 (2008). [3] X. Wang, K. Han, W. Wang, S. Chen, X. Ma, D. Chen, J. Zhan, J. Du, Y. Xiong, and A. Huang, Appl. Phys. Lett. 96, 152907 (2010). [4] R. Kuriyama, M. Hashiguchi, R. Takahashi, A. Ogura, S. Satoh, T. Watanabe, JJAP, submitted.
Type of Medium:
Online Resource
ISSN:
2151-2043
DOI:
10.1149/MA2014-02/44/2084
Language:
Unknown
Publisher:
The Electrochemical Society
Publication Date:
2014
detail.hit.zdb_id:
2438749-6
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