| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 97 === Due to the continuous down-scaling of the CMOS transistors, the conventional gate dielectric layers, SiO2, has become so thin that it may lead to large leakage current and thus degrade the reliability of devices. To solve this problem, the current trend is to replace SiO2 with a high dielectric constant material so that it can keep the same capacitance while decreasing the tunneling currents. To date, HfO2 and Hf1-xSixO2 are considered as the replacements for SiO2 as the gate dielectric materials. However, there remain several critical problems unresolved such as the low re-crystallization temperatures and high defect density at the HfO2/Si interface. Although many theoretical calculations have been done for high-k materials, most of them are focused on the electronic and dielectric properties of their crystalline phases. Little is known about the structures and properties of their amorphous counterpart, particularly for hafnium silicates.
In this study, we performed first principles molecular dynamics simulations to generate atomistic structure models for amorphous HfO2 and Hf1-xSixO2. According to our structure models, the density of amorphous HfO2 is predicted to be 8.63 ± 0.11g/cm3, which is about 83% of the monoclinic phase. The average formation energy of a neutral oxygen vacancy in the amorphous structure is found to be slightly higher than that in the cubic one, but still lower than that in the monoclinic phase by ~0.3 eV. Our calculations also show that the formation energy of a neutral oxygen vacancy in amorphous HfO2 is always higher than that in SiO2. Regarding the band gap calculations, our results show that the band gap of a-HfO2 is simply lower than that of the monoclinic phase by ~0.3eV, indicating that structural transformation may not have significant effect on the electronic properties of HfO2. Furthermore, our calculations show that the dielectric constant of amorphous HfO2 is found to be 22.7, which is in good agreement with a recent experimental measurement.
For hafnium silicates, we applied two different procedures, melt-and-quench and substitution-annealing, respectively, to generate the atomistic models of Hf1-xSixO2 with different compositions. We find that the structural models generated using these two approaches show pretty similar structural characteristics and structural evolution with compositions. The average coordination numbers of each kind of atoms are found to decrease with increasing the concentration of silicon (x), but the distributions of the bond lengths or the nearest neighbor distances remain unchanged. In addition, our results show that the densities of Hf1-xSixO2 decrease continuously with the Si concentration, but interestingly, their volumes shrink at the beginning but later expand as the concentration of silicon increases. Regarding the electronic properties of silicates, our calculations show that the band gaps of Hf1-xSixO2 do not change significantly as the concentration of Si increase from 0 to 0.5. For the analysis of the dielectric properties, their static dielectric constants are found to decrease nonlinearly with the Si content. Regarding the vacancy formation energy calculations, our results show that as an oxygen atom is just bonded with Hf atoms, the average formation energy of an oxygen vacancy is nearly the same as that in amorphous HfO2. Similarly, as an oxygen atom is only bonded with Si atoms, the average oxygen vacancy formation energy is nearly the same as that in amorphous SiO2. In general, the average formation energy of an oxygen vacancy in hafnium silicates is lower than that in hafnia, particularly as the oxygen atom is bonded to silicon atoms.
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