Effects of HfO 2 buffer layer thickness on the properties of Pt/SrBi 2 Ta 2 O 9 /HfO 2 /Si structure
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Chen-Han Lin Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan, Republic of China
Chao-Hsin Chien Department of Electronics Engineering, National Chiao Tung University, Hsinchu 30050, Taiwan, Republic of China
Ming-Jui Yang National Nano Device Laboratories, Hsinchu 30043, Taiwan, Republic of China (Received 27 December 2007; accepted 17 April 2008)
We investigated structural and characteristic changes in thin HfO2 films ( monoclinic. Both its lower thickness (3.2 nm) and inferior structure (higher ratios of tetragonal/monoclinic and amorphous/ monoclinic phases) led the HfO2 to fail to resist against inner diffusion of the O atom. These results suggest that a thicker HfO2 layer (艌4 nm) is necessary to provide a sufficiently strong barrier for this MFIS process (750 °C, 3 min). Using the capacitances recorded at the accumulation region, the dielectric constants of the IL and HfO2 were extracted from the stack (Pt/HfO2/IL/Si). The capacitances of these specimens were plotted as a function of the HfO2 thickness (Fig. 9). Dielectric constants of the IL (∼5.7) and HfO2 (∼20.5) layers were extracted through linear fitting. The EOT is given as
J. Mater. Res., Vol. 23, No. 7, Jul 2008 Downloaded: 14 Mar 2015
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C-C. Leu et al.: Effects of HfO2 buffer layer thickness on the properties of Pt/SrBi2Ta2O9/HfO2/Si structure
FIG. 9. Scaling of equivalent oxide thickness (EOT) with HfO2 layer thickness.
EOT =
SiO2 HfO2
tHfO2 +
SiO2 IL
tIL
.
(1)
The linear dependence of EOT reveals that the dielectric constant for the HfO2 layer essentially remains constant for the HfO2 thickness between about 3 and 9 nm. This result suggests that the high volume fraction of amorphous and non-monoclinic HfO2 for the very thin films do little influence on their dielectric properties. This dielectric constant value (∼20.5) of the HfO2 layer was in good agreement with values reported elsewhere.28,29 Furthermore, the value of ∼5.7 for the IL layer implied that the IL was not simply a SiO2 but possibly a silicate.30 On the basis of these results, the dielectric constant of SBT (∼80–120) was calculated from the Pt/SBT/HfO2/ IL/Si structure. Therefore, we could obtain the individual effective voltages of the SBT, HfO2, and IL, which are governed by the following relationship31: Vi =
Ci共1 Ⲑ Cferro
Vg , + 1 Ⲑ CHfO2 + 1 Ⲑ CIL兲
(2)
where Vi and Ci are the effective voltage and capacitance of the ith layer, respectively, Vg is the voltage applied to the MFIS structure, and Cferro, CHfO2, and CIL are the capacitances of SBT, HfO2, and the IL, respectively. Using this equation, we roughly extracted the effective voltages of each layer as a function of the dielectric constant and the thickness. The estimated effective voltage applied to the SBT in AS-1m was as high as 0.76Vg; meanwhile, those across the HfO2 and IL were 0.05Vg and 0.19Vg, respectively. Therefore, the effective voltage across the SBT was 3 V when a potential of 4 V was applied; this value is two
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