Nanosecond laser annealing of pseudomorphic GeSn layers: Impact of Sn content

Interactions between nanosecond laser pulses and various Sn content pseudomorphic GeSn layers were investigated. The aim was to evaluate the suitability of Ultraviolet Nanosecond Laser Annealing (UV-NLA), with an excimer laser emitting at 308 nm and a pulse duration of 160 ns, for the fabrication of performant contacts in GeSn devices such as electrically pumped lasers. Atomic Force Microscopy (AFM) and X-Ray Diffraction (XRD) revealed similar melt regimes for GeSn on Ge and SiGe on Si. At the melt threshold, surface structures formed where small areas melted. We were then in the so-called surface melt regime. The surface structures’ shape changed with the Sn content. A similar trend was recently evidenced when submitting SiGe layers on Si to UV-NLA. The shape change was more drastic in GeSn than in SiGe. A larger built-in compressive strain because of a much larger size difference between Ge and Sn than Si and Ge might be the reason why. Time Resolved Reflectivity maps showed a more reflective plateau after the melt peak at an Energy Density (ED) of around 1.00 Jcm−2, stemming from the presence of a smooth Sn rich surface layer, as revealed by AFM images at the same ED. Stacked XRD maps outlined that this ED corresponded to the formation of rather high Sn content layers with Sn contents of up to 6.3%, i.e., concentrations significantly above the solid solubility limit, which is below 1%, at variance with conventional annealing processes at 450 °C. UV-NLA has thus opened a new processing window that might be useful for contact formation.