First-principles study on the mechanism for boron diffusion at SiGe/SiO$_2$ interfaces.

Abstract

The semiconductor technology roadmap shows Si-based complementary metal-oxide semiconductor technology will reach absolute limits on its performance by around 2020. To enhance the performance of metal-oxide-semiconductor field-effect transistors without changing the scaling trend, much attention has been paid to devices utilizing SiGe alloys due to high hole mobility. While B is commonly used as a p-type dopant, it easily segregates to the oxide in Si/SiO$_2$ interface during ion implantation followed by thermal annealing. On the other hand, there is a lack of studies for the segregation behavior of B dopants in SiGe/SiO$_2$ interface. In this work, we generate the atomic model for SiGe/SiO$_2$ interface, in which $\alpha$-quartz SiO$_2$ is placed on the (100) surface of Si$_{0.75}$Ge$_{0.25}$. The SiGe alloy is generated by using the special quasi-random structure approach. We examine the stability of various B-related defects in the SiGe/SiO$_2$ interface. Similar to the results in Si/SiO$_2$ interface, an interstitial B in SiO$_2$ is more stable than a defect complex consisting of a substitutional B and Si self-interstitial in SiGe. However, in contrast to Si/SiO$_2$ interface, interface Ge atoms significantly enhance the stability of B-related defects in the interface region and thereby increase the migration barrier for B diffusion from SiGe to SiO$_2$. The calculated migration barrier is about 3.6 eV, which is much higher than the result for Si/SiO$_2$ interface, indicating that the B diffusion is suppressed in the presence of Ge at the interface.