  Strain engineering has been a ubiquitous paradigm to tailor the electronic
band structure and harness the associated new or enhanced fundamental
properties in semiconductors. In this regard, semiconductor membranes emerged
as a versatile class of nanoscale materials to control lattice strain and
engineer complex heterostructures leading to the development of a variety of
innovative applications. Herein we exploit this quasi-two-dimensional platform
to tune simultaneously the lattice parameter and bandgap energy in group IV
GeSn semiconductor alloys. As Sn content is increased to reach a direct band
gap, these semiconductors become metastable and typically compressively
strained. We show that the release and transfer of GeSn membranes lead to a
significant relaxation thus extending the absorption wavelength range deeper in
the mid-infrared. Fully released Ge$_{0.83}$Sn$_{0.17}$ membranes were
integrated on silicon and used in the fabrication of broadband photodetectors
operating at room temperature with a record wavelength cutoff of 4.6 $\mu$m,
without compromising the performance at shorter wavelengths down to 2.3 $\mu$m.
These membrane devices are characterized by two orders of magnitude reduction
in dark current as compared to devices processed from as-grown strained
epitaxial layers. The latter exhibit a content-dependent, shorter wavelength
cutoff in the 2.6-3.5 $\mu$m range, thus highlighting the role of lattice
strain relaxation in shaping the spectral response of membrane photodetectors.
This ability to engineer all-group IV transferable mid-infrared photodetectors
lays the groundwork to implement scalable and flexible sensing and imaging
technologies exploiting these integrative, silicon-compatible strained-relaxed
GeSn membranes.
