| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 103 === With the continued down-scaling of the integrated circuits, the conventional dielectric oxide, SiO2, have been replaced by new dielectric materials, such as the high-κ oxides in CMOS transistors to avoid excess leakage current and the low-κ silicate insulating layers in-between copper wires to reduce the interconnect capacitances. To further control or improve their performance, a detailed understanding of the structural, electronic, dielectric, and the mechanical properties of these new dielectric materials are strongly in demand. In this thesis, we have employed first-principles density functional theory calculations to investigate the important properties of these new dielectric oxide materials in many fundamental aspects.
For gate dielectrics in CMOS transistors, we have investigated the he structural, electronic, and dielectric properties of the Si- and La-doped HfO2, as well as the O vacancy formation and the relevant induced defect states in band gaps over a wide range of chemical compositions. For Si-doped HfO2, i.e. Hf-silicates (a-Hf1-xSixO2), our results show that the Si-rich Hf-silicates possess distinct structural characteristics from the Hf-rich ones and the electronic band gaps were found to vary nonlinearly with the Si content. Different from the previous theoretical studies, our results support a nonlinear dependence of the dielectric constants on the composition of silicates, which is mainly attributed to the rapid reduction of the low-frequency vibrational motions of Hf atoms with increasing the Si content. Furthermore, based on the generated structure models, we have identified the most probable O coordination structures in amorphous HfO2 and in Hf-silicates, respectively. Our calculations showed that the formation energies of O vacancy and the positions of the induced defect states in the band gap are largely dependent on the local structures of the vacancy site rather than on the compositions of Hf-silicates. Considering the measured valence band offset between Si and Hf-silicates, a considerable amount of O vacancies are likely to stay in the charge neutral state in Hf-silicates when the Fermi level lies in the band gap region of Si. Furthermore, the concentration of O vacancy in Hf-silicates was found to be much lower than that in HfO2 when the Fermi level lies in/below the mid-gap region of Si. Consequently, the flat band voltage shift and the transient threshold voltage instability can be significantly reduced in Hf-silicates in comparison to that in HfO2, indicating that the presence of a Hf-silicate layer in between the gate oxides and the Si substrate could be beneficial to the performance of the CMOS device.
For the La-doped HfO2 (a-Hf1-xLaxO2-0.5x, x = 0 ~ 0.5), the mass density was found to decrease linearly with the La content, but the electronic band gap only reduces slightly by 0.29 eV as the La content increases from x = 0 to 0.5. Our Bader charge analysis shows that the average atomic charge on Hf (La) atoms can decrease from 2.31 (2.12) to 2.26 (2.06) as the La content increases from 0 to 0.5, which indicates that the ionicity of the Hf (La) atom tends to decrease as the La content increases. This results is consistent with our calculations for the Hf 4f core level energy, which reveal a binding energy shift of around 0.3 ~ 0.6 eV as the La content increases from 0 to 0.5, in good agreement with a recent experimental measurement. Our calculations also show that the dielectric constant of a-Hf1-xLaxO2-0.5x increases rapidly with the La content from x = 0 to 0.25 but it appears to be nearly unchanged as x ranges between 0.25 and 0.5, which is consistent with the recent experiments. Furthermore, based on the generated structure models, we have identified the most probable O coordination structures in a-Hf1-xLaxO2-0.5x. Similar to Hf-silicates, the formation energies are largely dependent on the local bonding configurations rather than on the compositions. However, O vacancy was found to be more likely to stay in positively charged states with lower formation energies than that in HfO2, implying that the flat band voltage shift and the threshold voltage instability could be enhanced due to La-doping. Furthermore, the trap energy level was found to decrease gradually with increasing the La contents, which indicates that the trap-assisted conduction can become more seriously in a-Hf1-xLaxO2-0.5x.
For the interlayer insulating dielectrics, we have investigated the structure and mechanical properties of the methyl- and ethyl-bridged organosilicate hybrid glasses (OSG) using first-principles calculations. Our results show that the mass density of OSG can gradually decrease with increasing the carbon content, but the porosity may not necessarily increase with carbon-bridging units in the amorphous bond network. Our results also reveal that the elastic moduli and fractured surface energies of OSG models tend to decrease with increasing the alkylene-bridged units in the silicate bond network, which imply that the crack-resistance of the alkylene-bridged OSG can be much worse than fused silica based on the Griffith theory of brittle fracture. According to the calculations of decohesive energy, the mechanical strength of a-SiO2 still outperforms the methyl-bridged OSG models, indicating that the crack-resistance of these two materials is largely determined by the chemical bond strength. However, the decohesion energy of the ethyl-bridged OSG models turns out to be higher than a-SiO2 though their critical stress for crack propagation predicted by Griffith theory is simply half of that in a-SiO2. Our results further show that the enhanced fracture toughness in the ethyl-bridged OSG model can be mainly attributed to the plastic deformation and bonding reconfiguration induced by the ethyl-bridged units during fracture, which was not observed for the methyl-bridged OSG models and a-SiO2.
|
|---|
