Optical Characteristics Of High K Materials Deposited By ALD

The demand for more integrated circuit functionality has driven the semiconductor industries to have increased circuit density which, in turn, have led to the rapid scaling down of the channel length and gate dielectric thickness of the transistors. However, when the thickness of the silicon dioxide layer, used as the gate dielectric, is less than 1.4nm, the leakage current due to direct tunneling becomes too large, thus increasing the power dissipation to an unacceptable value. Accordingly, to solve the leakage current problem, films of new materials of physically larger thickness with higher dielectric constant are required to replace such thin Si O₂ films as the gate dielectric. Among the methods for depositing thin films, atomic layer deposition（ALD） is unique for its capability of growing conformal thin films of compounds with a control of composition and thickness at the atomic level. In this dissertation, the optical properties of Nd2O₃, Hf O₂ and（Hf O₂/Al₂O₃） films deposited by ALD are described, and the author’s major contributions are outlined as follows:1. Optical characteristics of neodymium oxide. Firstly, optical properties of the Nd2O₃ films deposited by atomic layer deposition（ALD） at various process conditions are investigated. The obtained optimal ALD process condition for Nd2O₃ is determined as: Nd（thd）3 evaporation temperature: 185 oC, deposition temperature: 300-320 oC, saturation condition: Nd（thd）3 pulse time longer than 0.5s. Then, by analysing annealing dependence of optical properties of Nd2O₃ films, it is found that the refractive index and high frequency dielectric constant decrease with increasing annealing temperature. Moreover, effect of post-deposition annealing on the interfacial chemical bonding states and band alignment of neodymium oxide on silicon is studied. Through investigating the chemical shifts of Nd 3d, O 1s and Si 2p core level spectra, it is found that the interfacial Nd silicate like configuration is subject to a transformation from Nd2 Si O5 to Nd2Si2O7 with annealing temperature. Moreover, the direct optical band gap energy obtained from variable angle spectroscopic ellipsometry（VASE） data shifts to higher energy with increasing annealing temperature. Additionally, the obtained valence and conduction band offsets to the silicon substrate are all over 1e V. The suitable band gap and band offsets relative to Si make Nd2O₃ films one of the promising high-k dielectric candidates.2. Optical characteristics of Hf O₂ films. Firstly, the dependence of optical properties of Hf O₂ on film thickness is investigated. Analysis results suggest that real part of the dielectric constant（ε1） value of the as grown film increases with the increased deposition cycles when H2 O is used as oxidant. Besides, the optical band bap decrease with the increased deposition cycles. Whereas, no similar variation trend of the dielectric constant with film thickness is observed for Hf O₂ films grown by O₃. Due to the significant effect of oxidant on the optical properties of Hf O₂, selected samples（52 cycles） deposited by H2 O and O₃ separately are carefully studied in order to understand the dependence of the optical properties of Hf O₂ on the oxidant used. It is found that a higher ε1 value is observed for H2O-based film. Careful examination of imaginary part of the dielectric constant（ε2） leads to the conclusion that O₃-bsed gate stacks have less sub-band gap defect states besides the silicon’s critical features. Moreover, a larger high frequency dielectric constant, direct and indirect band gap values are obtained for O₃-bsed film. Meanwhile, it worth noting that suitable valence band offsets and conduction band offsets are obtained for H2 O and O₃ based Hf O₂ gate stacks respectively, indicating both type of dielectric films can provide sufficient tunneling barriers for both electrons and holes.3. Investigation of optical properties of（Hf O₂/Al₂O₃） nano-laminates. Three types of（Hf O₂/Al₂O₃） nano-laminates are focused. Firstly, for（Hf O₂/Al₂O₃）_x stacks, the influence of the layer thickness of Hf O₂/Al₂O₃ on the optical properties of（Hf O₂/Al₂O₃）_x stacks is studied. The obtained results indicate that the complex dielectric functions decrease with the decrease in layer thickness of Hf O₂/Al₂O₃. While the calculated optical band gap values display negligible change with the layer thickness of Hf O₂/Al₂O₃. After that, when thin Al₂O₃ layer locates in the different place（top, middle and bottom） of Hf O₂, the change in optical properties of the stacks is studied. It is found that the real part of the dielectric functions show the largest value when Al₂O₃ layer in the middle of the stacks. Furthermore, the calculated valence and conduction band offsets to the silicon substrate are both over 1e V for all the three stacks. At last, the effect of Al addition in Hf O₂ on the optical properties of the dielectrics is researched. The analysis results show that pure Hf O₂ film exhibits crystallization features after annealing at 500 oC. When Al incorporated in, Al-Hf O₂ films can maintain amorphous at temperatures up to 700 oC after rapid thermal annealing. Especially, as-deposition [1Al₂O₃+5Hf O₂] film shows the similar optical properties as that of pure Hf O₂. Moreover, least disorder in the amorphous film structure and excellent thermal stability of Eg（5.5±0.1e V） during annealing are obtained.