  GeSn alloys are metastable semiconductors that have been proposed as building
blocks for silicon-integrated short-wave and mid-wave infrared photonic and
sensing platforms. Exploiting these semiconductors requires, however, the
control of their epitaxy and their surface chemistry to reduce non-radiative
recombination that hinders the efficiency of optoelectronic devices. Herein, we
demonstrate that a combined sulfur- and iodine-based treatments yields
effective passivation of Ge and Ge0.9Sn0.1 surfaces. X-ray photoemission
spectroscopy and in situ spectroscopic ellipsometry measurements were used to
investigate the dynamics of surface stability and track the reoxidation
mechanisms. Our analysis shows that the largest reduction in oxide after HI
treatment, while HF+(NH4)2S results in a lower re-oxidation rate. A combined
HI+(NH4)2S treatment preserves the lowest oxide ratio <10 % up to 1 hour of air
exposure, while less than half of the initial oxide coverage is reached after 4
hours. These results highlight the potential of S- and I-based treatments in
stabilizing the GeSn surface chemistry thus enabling a passivation method that
is compatible with materials and device processing.

Materials Science;Applied Physics