Transfer-Printed Multiple GeSn Membrane Mid-Infrared Photodetectors

Due to their narrow band gap and compatibility with silicon processing, germanium-tin (Ge$_{1-x}$Sn$_{x}$) alloys are a versatile platform for scalable integrated mid-infrared photonics. These semiconductors are typically grown on silicon wafers using Ge as an interlayer. However, the large lattice mismatch in this heteroepitaxy protocol leads to the build-up of compressive strain in the grown layers. This compressive strain limits the material quality and its thermal stability besides expanding the band gap, thereby increasing the Sn content needed to cover a broader range in the mid-infrared. Released Ge$_{1-x}$Sn$_{x}$ membranes provide an effective way to mitigate these harmful effects of the epitaxial strain and control the band gap energy while enabling the hybrid integration onto different substrates. Nevertheless, the epitaxial strain is also known to affect the fabrication of membrane devices due to a significant bowing upon release from the growth substrate, especially in high Sn content structures. With this perspective, herein these limitations are discussed and addressed by introducing bow-free, strain-relaxed Ge$_{1-x}$Sn$_{x}$ membranes in the fabrication of mid-infrared devices. These devices are transfer-printed with metal contacts to create multiple photodetectors in a single transfer step. The resulting photodetectors exhibit an extended photodetection cutoff reaching a wavelength of $3.1 \,\mu$m for a Sn content of ${x=0.11}$ compared to as-grown photoconductive devices. The latter yields a reduced cutoff of $2.8 \,\mu$m due to the inherent compressive strain. Additionally, a significant reduction in the dark current of two orders of magnitude is observed, which could be related to the formation of a Schottky barrier or to a change in the contact resistivity during the processing steps of the membranes. Furthermore, the impact of chemical treatment and annealing on the device performance was also investigated showing a further reduction in the dark current. The demonstrated transfer printing, along with the use of an adhesive layer, allows the transfer of multiple GeSn membranes onto virtually any substrate. This approach paves the way for scalable fabrication of hybrid optoelectronic devices leveraging the tunable band gap of Ge$_{1-x}$Sn$_{x}$ in the mid-infrared range.