| Summary: | 碩士 === 國立交通大學 === 電子工程學系 電子研究所 === 102 === Recently, traditional Si-based MOSFETs are approaching its fundamental scaling limits, and then Ge has been comprehensively explored as a potential channel material to replace Si due to its high intrinsic carrier mobility for further performance enhancement. Nevertheless, the shallow junction depth is hard to form since the conventional n-type dopants such as phosphorous and arsenic have not only lower solid solubility but also faster diffusion rate in Ge substrate than in Si. Moreover, strong Fermi-level pinning near the valence band edge of Ge leads to high electron Schottky barrier height. Dopant segregation technique has been proposed to achieve shallower junction depth and heavier dopant concentration experimentally due to dopant segregated around the interface. However, the role of dopants at the NiGe/Ge interface is not clear.
In this thesis, we build the realistic polycrystalline phases NiGe/Ge contact by including NiGe (112) phase only, and then the first-principles calculations are employed to investigate the behaviors of the n-type dopant around the interface by LDA functional and whether the physical Schottky barrier height of the NiGe/Ge contact is reduced by dopant segregation or not is calculated by HSE06 hybrid functional.
For the conventional n-type dopant such as phosphorous and arsenic, our calculations show that those two elements may be segregated at the interface, but the preferred segregated site of phosphorous and arsenic are on the NiGe and Ge side, respectively. These results suggest that phosphorous would be a better choice for implantation before germanide process, while arsenic can migrate into the Ge layer and pile up at the interface in both implantation before and after germanide processes.
Then, we show that the physical Schottky barrier height of the NiGe/Ge contact modified by dopant segregation using those two elements on the Ge side is reduced by less than 0.1 eV. This small value is due to the strong Fermi-level pinning effect. By the way, there is no effect to modify the physical Schottky barrier height by doping phosphorous on the NiGe side. To sum up, the improvement of the NiGe/n-type Ge junction characteristics by dopant segregation using phosphorous and arsenic are mainly attributed to the increase of dopant concentration around the interface and partially attributed to the reduction of the physical Schottky barrier height.
For the specific case using nitrogen dopant, the calculated results show that it can be segregated around the interface but yield a large number interface states spreading the Ge bandgap. Although the effective conduction band edge is closer to the Femi level due to continuous interface states, the physical Schottky barrier is almost unchanged since the interface states disappear at about 17 Å away from the interface.
These first-principles calculations provide deep insight on the role of dopants near the NiGe/Ge interface and can explain the experimental observations very well. Further calculations can also help new process development.
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