Recently, HfO₂(hafnia) grown on Si substrates is widely studied as a potential candidate for replacing SiO₂ as the gate dielectrics in scaled CMOS. This thin film grows as an amorphous phase like a SiO₂ gate oxide. In order to increase the crystallization temperature and improve the device performance, several ingredients such as Si, N and Al atoms are incorporated in the high-k gate oxides. Although Zhao and Vanderbilt reported the amorphous phase of ZrO₂ using the ab-initio method, the amorphous HfO2 has not been studied theoretically. In this report, we present the amorphous HfO₂ and HfSiO₂ using the Projector Augmented Plane Wave method. We also analyzed the structure of the amorphous HfO₂ and the effects of the inclusion of Si atoms by changing the atomic concentration. Amorphous HfO₂ and HfSiO₂ are obtained by employing Zhao-Vanderbilt prescription. Our model employed 96 and 117 atoms in the cubic cell. After elevating the temperature of the system to 4000K and melting the system in 12ps, we gradually decreased the temperature in 200ps. In our amorphous structures there exist several pore channels with diameters 3-4 A like a SiO₂ case. If we increase the Si concentration, the size of the pore channels increases and lots of space appears. The oxygen atoms with the coordination number 2 increases linearly with the Si concentration. We found that the phase separation between HfO2 and SiO2 occurs at Hf_0.3Si_0.7O₂. By using our pure amorphous hafnia model, we analyze the problem that boron atoms doped in the poly-Si gate penetrates into the Si substrate through the amorphous hafnia. The estimated barrier height of boron diffusion using monoclinic crystal model was 2.5 eV and contradicts with the very fast diffusion. Our estimated barrier height is 1.2 eV and fairly explains the observed boron diffusion in amorphous HfO2. And thus our amorphous structure models seem to be useful in understanding the actual systems. This work was supported by NEDO.

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