Application of Flexible NIR Photodetectors
Flexible photonic devices have drawn extensive attention as a promising candidate toward various flexible and wearable applications. Low-cost flexible near infrared (NIR) photodetectors (PDs) have great potential in defense and civilian systems for night-vision applications. Among various materials for flexible PDs, Germanium (Ge) has been one of the most attractive candidates for NIR applications due to its high absorption coefficient, high mobility, and inherent compatibility with CMOS technology. The additional Tin (Sn) and external tensile strain could reduce the bandgap of Ge, leading to efficient transition from indirect to direct band-structure and therefore broader detection wavelength range and higher photo response. 
NIR PD can be used in night vision goggle for defense systems
Thorny Problem
It has been proved that rigid GeSn PDs with 8% Sn composition could achieve a light absorption cut-off wavelength beyond 2 µm, improving light absorption in NIR region while extending its applications toward mid-IR region.[1] However, degradations on surface roughness and crystalline quality resulted from severe Sn segregation limit the fabrication of flexible GeSn NMs. Though strain-free GeSn nanomembranes (NMs) was fabricated via undercut and transfer printing technique,[2] study on the effect of mechanical bending on GeSn has not been demonstrated at device level yet due to the lack of device quality flexible GeSn materials. 
Fig.1: Comparison of the measured dark current density of our devices with that of other published results
Research in Singapore
In Singapore, some research groups have been devoted to the study of GeSn related optoelectronic devices. However, their research area is limited to rigid devices. The Nanyang Technological University (NTU)’s Electrical and Electronic Engineering (EEE) research team, Nano Engineering Device Laboratory, led by Prof. Kim Munho expands its application in flexible devices based on his abundant experience in flexible devices. The team has demonstrated flexible GeSn metal-semiconductor-metal (MSM) PDs by exploring the effect of mechanical strain on its optoelectronic properties. The PDs were fabricated from transfer-printed GeSn NMs on polyethylene terephthalate (PET) substrates.[3] Strain was introduced into the GeSn PDs under bend-down (uniaxial tensile strain) and bend-up (uniaxial compressive strain) conditions. Applied strain can affect band-structure of GeSn alloys, leading to a modulation of electrical and optical characteristics of the PDs. Accordingly, dark current increases from 8.1 to 10.3 µA in the bend-down condition and decreases to 7.2 µA in the bend-up condition, respectively. Responsivity at the wavelength of 2 µm was increased from 1.03 to 3.68 mA/W by 151% under bend-down condition while it decreases to 0.45 mA/W by 35% under bend-up condition. The study indicates that GeSn NMs can be used to realize flexible NIR PDs with an absorption wavelength approaching 2 µm. In addition, such flexible GeSn PDs with the capabilities of excellent mechanical durability represent significant advances in the field of group IV NIR optoelectronic devices. This work has been published in Journal of Material Chemistry C and selected as the front outside cover. 
Image of flexible GeSn PDs
Front outside cover image selected for Journal of Materials Chemistry C
About The Author
Nanyang Technical University, School of Electrical & Electronic Engineering
Asst Prof Kim Munho Assistant Professor, School of Electrical & Electronic Engineering
An Shu, PhD candidate, School of Electrical & Electronic Engineering
References:
- W. Wang, D. Lei, Y.-C. Huang, K. H. Lee, W.-K. Loke, Y. Dong, S. Xu, C. S. Tan, H. Wang, and S.-F. Yoon, “High-performance GeSn photodetector and fin field-effect transistor (FinFET) on an advanced GeSn-on-insulator platform,” Opt. Express 26, 10305-10314 (2018).
- Y.-C. Tai, P.-L. Yeh, S. An, H.-H. Cheng, M. Kim, and G.-E. Chang, “Strain-free GeSn nanomembranes enabled by transfer-printing techniques for advanced optoelectronic applications,” Nanotechnology 31, 445301 (2020).
- S. An, S. Wu, C. S. Tan, G.-E. Chang, X. Gong, and M. Kim, “Modulation of light absorption in flexible GeSn metal–semiconductor–metal photodetectors by mechanical bending,” Journal of Materials Chemistry C 8, 13557-13562 (2020).
