| Abstract |
Hafnia has attracted much attention because of the potential use for alternative dielectrics to SiO2. Several experiments reported that as-deposited HfO2/SiOx stacks exhibit negative fixed charges, which may be the origin of device degradation. During the post-annealing process, it was generally observed that the density of negative fixed charges decreases, while that of interface defects increases. Especially, for temperatures above 600oC, positive fixed charges appear in the SiOx stack. However, it is not clearly resolved what causes the fixed charge defects.
First-principles calculations are performed to study the role of hydrogen on defect properties in HfO2. A single O vacancy (VO) generate a deep level in the band gap, which is occupied by 2 electrons in the neutral charge state. When hydrogen forms a VO-H complex with VO, the H s and Hf d orbitals form the bonding and antibonding states, which lies inside the valence and conduction bands, respectively. The contour plots of their charge distributions are shown in Fig. 1. From the formation energies for various charge states in Fig. 2, the positive charge state of the VO-H complex is energetically most stable for the Fermi levels above 1.4 eV, which covers the band gap of Si substrates, independent of the gate voltages in MOS devices. Our finding of the positively charged VO-H complex suggests that the positive fixed charge defect is associated with hydrogen.
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