We have studied high-k gate stacks since 2000. We have presented our results on high-k and also learned a lot at ECS symposium. We gave a talk concerning doped HfO₂ in order to achieve higher-k [1], and then also discussed a number of high-k properties including the interface dipole formation at high-k/SiO₂ [2], advanced ALD process [3], the interface dipole formation mechanism [4, 5], high-k/Ge gate stacks [6], non-equilibrium PDA-induced HfO₂ phase transformation [7], and the interlayer scavenging kinetics in HfO₂ gate stacks [8].

In this talk, we would like to look back on our way, and discuss the origin of versatile properties of HfO₂. The dielectric constant is important in high-k technology, and ferroelectric HfO₂ recently reported is also exciting for functional device applications [9]. Why does HfO₂ show such variety of properties? We would like to understand HfO₂ in more detail.

HfO₂ has already been used in advanced CMOS. It is technically important whether a new higher-k material should be searched for or HfO₂ should be further used. We have been interested in the structural phase control of HfO₂ in order to enhance the dielectric constant. The 1st-principles calculation predicted that high temperature phase HfO₂ such as cubic or tetragonal one had higher-k than amorphous or monoclinic one [10]. The monoclinic phase, however, is thermodynamically stable at Si processing temperatures, so usually cubic or tetragonal phase HfO₂ is not available. Nevertheless, the significant enhancement of the dielectric constant in HfO₂ was experimentally demonstrated through the stabilization of the cubic or tetragonal phase. The trick is the structural phase transformation by doping another metal-oxide to HfO₂ [1]. The typical example is Y₂O₃-doped HfO₂. In the ceramics community, it had been actually well recognized since long before that Y₂O₃ doping could induce the oxygen vacancy in ZrO₂ to maintain the charge neutrality, which stabilized the cubic or tetragonal phase [11]. Furthermore, another way to realize the symmetric phase HfO₂ was through a non-equilibrium thermal process of amorphous HfO₂ [7]. In particular, the fast ramping-up in the RTA process enabled us to achieve the symmetric phase HfO₂. It means that the nucleation rather than growth process is more important for implementing high symmetric phase HfO₂. In addition, the size effect has also been well investigated thermodynamically for characterizing the structural phase [12].

Very unfortunately for us, we did not notice that HfO₂ ferroelectricity came out by doping other oxides. An interesting point in the ferroelectric HfO₂ is that the ferroelectric orthorhombic phase is not in the conventional phase diagram. Very recently we found that ferroelectric HfO₂ was also made by anion doping into HfO₂ [13]. The key is how to realize non-centrosymmetric phase HfO₂ structure. Ferroelectric orthorhombic phase might be between monoclinic and tetragonal phases as a metastable state. This makes it more difficult to recognize such unexpected fabulous functionality easily.

Materials are so challenging but so exciting.

References

[1] A. Toriumi et al., ECS-Trans. 1(5), 185 (2006).

[2] A. Toriumi, ECS Trans. 11 (6) 3 (2007).

[3] A. Toriumi et al., ECS Trans. 16 (4) 69 (2008).

[4] A. Toriumi and K. Kita, ECS Trans. 19 (1) 243 (2009).

[5] A. Toriumi and T. Nabatame, ECS-Trans. 25 (6) 3 (2009).

[6] A. Toriumi et al., ECS-Trans. 33 (6) 33 (2010).

[7] A. Toriumi et al., ECS-Trans. 41 (7) 125 (2011).

[8] A. Toriumi and X. Li, ECS-Trans. 69 (10) 155 (2015).

[9] T. S. Bӧscke et al., Appl. Phys. Lett. 99, 102903 (2011).

[10] X. Zhao and D. Vanderbuild, Phys. Rev. B 65, 233106 (2002).

[11] I-W. Chen and Y-H. Chiao, Acta metall. 33, 1827 (1985).

[12] A. Navrotsky and S. V. Ushakov, “Materials Fundamnetals of Gate Dielectrics,” Chap. 3, (Springer, Dordrecht, 2005).

[13] L. Xu et al., Appl. Phys. Exp. 9, 091501 (2016).