UBC Theses and Dissertations
Study of Si-Ge interdiffusion with highly n-type doping Cai, Feiyang
Silicon photonics has emerged as an effective solution to overcome the wiring limit imposed on electronic device (e.g. transistors) density and performance with continued scaling. In the past few decades, researchers all over the world have invested extensive effort on finding solutions to a Si-compatible lasing material system. Recently, Ge-on-Si lasers were demonstrated as promising candidates. Heavy n-type doping in Ge is the key technique to realize Ge lasing. However, Si-Ge interdiffusion during high-temperature growth or fabrication steps changes the distribution of Ge fraction and increases atomic intermixing, which degrades the device performance. Studies on the Si-Ge interdiffusion with high Ge fraction and P doping effects are not available.The subject is of technical significance for the structure, doping and process design of Ge-on-Si lasers and Ge based MOSFET. In this work, Si-Ge interdiffusion under high n-type doping was investigated both by experiments and by theoretical analysis: 1)Si–Ge interdiffusion with different P doping configurations was investigated. Significant interdiffusion happened when the Ge layer was doped with P at 10¹⁹ cm-³ after defect annealing, which resulted in a SiGe-alloy region at the Si-Ge interface. The thickness of this SiGe alloy was more than 150 nm. With high P-doped Ge, Si–Ge interdiffusivity is enhanced 10–20 times in the XGe > 0.7 region compared with the control sample without P doping. The phenomenon is attributed to the Fermi-level effect. Due to the high P concentration peak in the Si-Ge interdiffusion region, the concentration of negatively charged vacancy was greatly increased and thus the interdiffusivity of Si–Ge. Next, the impact of the Fermi-level effect on Si-Ge interdiffusion was further investigated by theoretical modeling. 2) Ge/Si0.25Ge0.75/Ge multilayered structures with no P doping and high P doping were investigated. A model of Si-Ge interdiffusion under high n-type doping was proposed to describe the impact of the Fermi-level effect. By fitting to the SIMS data from experiments with different anneal temperatures, it was found out that the Fermi-enhancement factor of Si-Ge interdiffusion was quadratically dependent on the ratio of electron concentration over intrinsic electron concentration (n/ni). This suggests that for Ge fractions from 0.75 to 1 under high n-type doping, Si-Ge interdiffusion is dominated by vacancies with double negative charge (V²-). This is the first work on the quantitative modeling of Si-Ge interdiffusion with high n-type doping.
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