Local constrained expansion induced surface tensile strain and optical enhancement in suspended GeSn microstructures

Abstract

Strain engineering is a key strategy for advancing GeSn in silicon optoelectronics, as it not only overcomes the compressive strain in epitaxial films but can also introduce additional tensile strain to facilitate a direct-bandgap transition. However, effectively applying and precisely controlling a high level of tensile strain remains a significant challenge. In this work, we systematically investigate the strain redistribution mechanisms in suspended microstructures of GeSn thin films grown by molecular beam epitaxy and their impact on the photoluminescence (PL) spectrum. We demonstrate that these microstructures effectively release the initial −1.1% compressive strain of the as-grown GeSn sample and introduce a substantial tensile strain within the top 20 nm surface layer of the suspended portion, reaching a maximum of 2.0%. A model based on the local constrained expansion effect was proposed to explain the origin of the enhanced tensile strain. Compared to the as-grown GeSn sample, the PL peak of Ge0.91Sn0.09 microdisks exhibits a redshift of 18.7 meV at 300 K and a narrower full width at half maximum. Temperature-dependent PL indicates that the PL signal originates from direct-bandgap transitions in the suspended Ge0.91Sn0.09 microdisks, which is consistent with the prediction by the eight-band k·p model. This work reveals a geometry-dependent strain redistribution mechanism, providing a design paradigm for high-performance, direct-bandgap GeSn light-emitting devices.