Photoresist‐Mediated Transfer Printing of Single‐Crystalline Germanium Membranes for Flexible Near‐Infrared Photodiodes

Abstract Flexible near‐infrared photodetectors are particularly promising for applications in wearable technology, non‐invasive biosensing and bioimaging, and environmental monitoring. Germanium (Ge) is widely used as an active material for high‐performance near‐infrared photodetectors, since its energy bandgap (0.67 eV) excellently matches the near‐infrared band. However, the fabrication of Ge‐based flexible near‐infrared photodetectors is strongly challenged by the difficulties in preparing flexible Ge membranes. Although various attempts have been developed, they suffer from one or more disadvantages, such as high cost, inconvenient transfer printing, and/or difficulty in processing. Herein, photoresist‐mediated transfer printing is proposed for preparation of Ge membranes, which are obtained by the wet thinning of a bulk wafer with vertical PN junction structures. The introduced photoresist medium layer could protect the doped surface of Ge against corrosion during the thinning process, and facilitate the complete transfer of the thinned Ge membranes onto flexible substrates. With the prepared Ge membranes, flexible near‐infrared photodiodes with good and robust optoelectronic performances against mechanical bending, including various bending radii and bending cycles, are successfully demonstrated. The proposed technique can be adapted for fabricating a broad range of flexible Ge‐based electronics.