GeSn is expected as a light-emitting material in Si photonics because a direct-bandgap nature appears with Sn composition above 10%. However, the emission wavelength of GeSn is in the mid-IR range. Thus, GeSn nanodots (NDs) are needed for telecom-wavelength emission. In this paper, we propose a formation method for GeSn NDs with Sn mediation. This process consists of low-temperature deposition of Sn and GeSn by vacuum evaporation and subsequent low-temperature annealing. Crystalline GeSn NDs with high density and high Sn content (more than 10%) have been successfully formed without metallic Sn precipitation or segregation of Sn on the dot surface. The possible formation mechanism of Sn-mediated GeSn NDs is also discussed.

Germanium (Ge) nanodots are expected for various applications. We have proposed a unique fabrication method for them with bismuth (Bi) mediation, and we have successfully formed high-density crystalline Ge nanodots by vacuum evaporation followed by low-temperature annealing. In this paper, we show the experimental results, such as dependence of the process condition on the nanodot structure and some results of the analyses of the nanodot samples. Then we discuss and propose a schematic formation mechanism for the nanodots as follows. First, during the deposition process, sequentially deposited Bi and Ge layers are mixed by solid phase diffusion and form an amorphous-alloy layer. Next, during the annealing process, crystallization of the Ge nanodots and desorption of the Bi proceeds; during this time, Bi and Ge exist as a solid-liquid coexistence phase in the alloy system, and this condition may enhance the phenomena.