Near-infrared photodetectors based on single germanium nanowires

Germanium (Ge) is a promising candidate for designing near-infrared photodetectors because of its bandgap (0.66 eV), which induces a large absorption coefficient at near-infrared wavelengths. Also, Ge has excellent compatibility of parallel processing with silicon technology [1,2]. Photodetectors based on Ge material have been fabricated with different structures such as metal-semiconductor-metal (MSM) and p−n junctions. On the other hand, the observation of high responsivity in semiconductor nanowires with a high surface-to-volume ratio has attracted growing interest in using nanowires in photodetectors. So far, significant efforts have been made to fabricate single nanowire-based photodetectors with different materials such as Si, Ge, and GaN to achieve miniaturized devices with high responsivity and short response time [3-5]. Hence, Ge nanowires are an excellent candidate to fabricate single nanowire-based near-infrared photodetectors. In this work, we report on the fabrication and characterization of an axial p−n junction along Ge nanowires. First, through a resist mask created by electron beam lithography (EBL), the top Ge layers of germanium-on-insulator (GeOI) substrates were locally doped with phosphorus ions using ion beam implantation followed by rear-side flash lamp annealing. Then, the single Ge nanowire-based photodetectors containing an axial p−n junction were fabricated using EBL and inductively coupled plasma reactive ion etching. The fabricated single Ge nanowire devices demonstrate the rectifying current−voltage characteristic of a p−n diode in dark conditions. Moreover, the photoresponse of the axial p−n junction-based photodetectors was investigated under light illumination with three different wavelengths: 637 nm, 785 nm, and 1550 nm. The measurements indicated that the fabricated photodetectors can be operated at zero bias and room temperature under ambient conditions. A high responsivity of 3.7×102 AW-1 and a detectivity of 1.9×1013 cmHz1/2W-1