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Instability of Flatband Voltage in HfO2 Gate Stack Structures under Reducing/Oxidizing Annealing Conditions



K Ohmori, P Ahmet, M Yoshitake, T Chikyow, K Shiraishi, K Yamabe, H Watanabe, Y Akasaka, Kao-Shuo Chang, Martin L. Green, K Yamada


We have applied a combinatorial approach to fabricate work function (WF) tuned Pt-W alloy films and used the films as metal electrodes for HfO2/SiO2/Si capacitors. As the ratio RPt of Pt to W changes from 0 to 1, the WF value varies continuously from 4.7 to 5.5 eV. This tunability enables us to systematically investigate the effect of WF variation on electric properties. After a forming gas annealing process, the values of flatband voltage (Vfb) from capacitance-voltage (CV) properties are almost constant regardless of the WF variation because of a catalytic effect of Pt that results in Fermi level pinning. By additional oxidizing gas annealing (OGA), the effect of WF value on a CV curve becomes dominant. However, the difference in Vfb between W and Pt is 0.34 V which is much smaller than the observed WF difference of 0.8 eV. We attribute this phenomenon to the lowering of effective WF due to an electric dipole induced by oxygen vacancy formation at the metal/HfO2 interfaces. Moreover, a decrease of Vfb in W-rich regions was observed after the OGA suggesting that W-O bond formation at the interface. These results clearly indicate that controlling bonding states at the metal/HfO2 interfaces on an atomistic scale is essential for realizing a combination of metal and high-k dielectric films in upcoming Si-based microelectronic devices.
Journal of Applied Physics


gate stack, hafnium oxide, high-k gate dielectrics, Si microelectronics


Ohmori, K. , Ahmet, P. , Yoshitake, M. , Chikyow, T. , Shiraishi, K. , Yamabe, K. , Watanabe, H. , Akasaka, Y. , Chang, K. , Green, M. and Yamada, K. (2007), Instability of Flatband Voltage in HfO2 Gate Stack Structures under Reducing/Oxidizing Annealing Conditions, Journal of Applied Physics, [online], (Accessed July 18, 2024)


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Created August 12, 2007, Updated October 12, 2021