Sub-Band Gap Photodetection from the Titanium Nitride/Germanium Heterostructure

Photoexcited hot carriers through nonradiative decay offer new opportunities for harnessing longer wavelength light. Here, we have demonstrated a hot-carrier-mediated sub-band gap photodetection in germanium-based planar heterojunction devices. The planar samples that form in situ germanium/titanium nitride (Ge/TiN) interfaces are fabricated by the dc sputtering technique, and the generation of photocurrent by near-infrared (NIR) light illumination is confirmed up to 2600 nm, well exceeding the absorption limit of Ge. The photocurrent obtained with nickel contacts is 3 orders larger than that obtained without metal contacts or with gold contacts in similar structures. The specific detectivity ( D*) value for the TiN/Ge photodetector is obtained to be 6.32 × 10⁵ Jones at the sub-band gap excitation wavelength of 2000 nm without applying any bias. The superior performances of our device are attributed to the broad absorption of the TiN, the plasmonic hot carrier transfer from the TiN to Ge, and built-in potential of the TiN/Ge non-Ohmic junction, which allows efficient separation of photoexcited electron-hole pairs. Our results further support the use of TiN, which is robust and cost-effective, as an alternative to metals for NIR photodetection and photovoltaics when it forms a heterostructure with Ge.