STM and ARPES characterization of quality of GeSn grown on Ge(001) for atomic ordering investigations

GeSn has added interest due to the potential impact of short-range ordering between Ge and Sn on the electronic properties of GeSn. Scanning tunneling microscopy (STM) serves as an ideal tool for observing the quality of GeSn at the atomic scale and for directly observing short-range ordering on the sample surface by identifying the atomic sites using scanning tunneling spectroscopy. In this report, we discuss optimizing the growth conditions for a Ge buffer using molecular beam epitaxy, testing the surface quality at the atomic level using STM, utilizing that tested surface for subsequent growth of Sn and GeSn samples with 5% and 10% Sn content, and further studying GeSn at the atomic level using STM. Observed atomic-resolution STM images of Ge, Sn on Ge, and GeSn on Ge reveal well-defined surface reconstructions, confirming their crystallinity. Additionally, angle-resolved photoelectron spectroscopy (ARPES) studies reveal sharp and well-defined electronic bands, further confirming the excellent crystalline quality. We also demonstrate strategies to mitigate surface oxidation, a critical challenge even under ultrahigh vacuum (UHV) conditions due to residual oxygen. This report provides insight into the development of a molecular beam epitaxy, STM, and ARPES combination UHV systems as a powerful tool toward the direct exploration of short-range ordering in Group IV alloys.