  The prospect of GeSn semiconductors for silicon-integrated infrared
optoelectronics brings new challenges related to the metastability of this
class of materials. As a matter of fact, maintaining a reduced thermal budget
throughout all processing steps of GeSn devices is essential to avoid possible
material degradation. This constraint is exacerbated by the need for higher Sn
contents along with an enhanced strain relaxation to achieve efficient
mid-infrared devices. Herein, as a low thermal budget solution for post-epitaxy
processing, we elucidate the effects of laser thermal annealing (LTA) on
strain-relaxed Ge$_{0.89}$Sn0$_{.11}$ layers and Ni-Ge$_{0.89}$Sn0$_{.11}$
contacts. Key diffusion and recrystallization processes are proposed and
discussed in the light of systematic microstructural studies. LTA treatment at
a fluence of 0.40 J/cm2 results in a 200-300 nm-thick layer where Sn atoms
segregate toward the surface and in the formation of Sn-rich columnar
structures in the LTA-affected region. These structures are reminiscent to
those observed in the dislocation-assisted pipe-diffusion mechanism, while the
buried GeSn layers remain intact. Moreover, by tailoring the LTA fluence, the
contact resistance can be reduced without triggering phase separation across
the whole GeSn multi-layer stacking. Indeed, a one order of magnitude decrease
in the Ni-based specific contact resistance was obtained at the highest LTA
fluence, thus confirming the potential of this method for the functionalization
of direct bandgap GeSn materials.
