(650h) Understanding Growth and Segregation in Ge-Sn Surfaces through First-Principles Computations

Semiconductor materials comprising Group IV elements (Si, Ge, and/or Sn) have demonstrated promising optoelectronic properties, positioning them as candidates for next-generation low-temperature optical computing and communications technologies. However, the development of devices utilizing these elements has been limited by challenges in efficiently incorporating Sn, which is required to tune the band structure for direct transitions, in high-quality and uniform materials.

In this study, we present novel methodologies to develop favorable synthesis conditions. Using density functional theory (DFT), we propose GeSn growth mechanisms via chemical vapor deposition (CVD) reactions, considering the effect of surface composition, surface termination, and utilization of different precursors for Ge and Sn. Additionally, we investigate the migration of these adatoms under applied strain. The most favorable mechanisms exhibit relatively low activation barriers on hydrogen-passivated slabs, with an activation barrier of 0.85 eV when GeH₄ is used as the reactant. The reaction energy of SnD₄ is comparable with that of GeH₄. Interestingly, the energy required for sequential deposition in (1 0 0) direction is slightly lower compared to initial depositions, whereas it is not significantly different in (0 1 0) direction.

We additionally investigated CVD reactions on surfaces terminated by halogens. The reaction of GeH₄ on halogen-passivated surfaces require a significantly higher activation barrier due to the strong binding of halogen species to the slab surface, which eliminates dangling bonds necessary for the deposition of reactants onto the slabs. Although halogenated surfaces are suboptimal for deposition kinetics, our study showcases their ability to limit the migration of deposited Ge and Sn, thereby mitigating Sn surface segregation and enhancing the crystallinity of GeSn layers.