Direct-indirect GeSn band structure formation by laser Radiation: The enhancement of Sn solubility in Ge.

• Sn solubility can be enhanced by laser irradiation in GeSn alloy. • Graded bandgap GeSn structure can be formed by laser processing. • Direct bandgap GeSn is achieved by Sn enhancement from 4% to 14%. • Differential reflectivity signal & EDS confirm indirect-direct transition. • Carrier lifetime of ~25 ns verifies good quality of processed material. Low equilibrium solid solubility of Sn atoms in Ge (less than 1%) leads to limitations in application of this material for IR detectors and emitters. Providing of non-equilibrium conditions by powerful pulsed laser radiation can be successfully applied for enhancement of solubility of impurity atoms in the host material. Here we present laser-induced monotonous redistribution of Sn atoms in Ge, based on the thermogradient effect aiming overcoming equilibrium limitations in the solubility. We applied pulsed nanosecond laser radiation to epitaxial Ge 0.96 Sn 0.04 layer grown on Si substrate to increase Sn atomic concentration up to 14% at the surface layer. As a result, indirect-direct graded bandgap GeSn structure was formed. The TEM/EDS cross-section analysis, X-ray photoelectron spectroscopy, Raman and UV reflection spectra confirmed the increase of Sn atomic content at the surface by order of magnitude. SEM and AFM imaging provided evident microstructure changes, while carrier lifetime changes, determined by differential transmittivity, were not observed, indicating that laser irradiation does not generate defects which reduce electronic quality of the material. [ABSTRACT FROM AUTHOR]