Wavelength-Switchable Positive and Negative Photoconductivity in a Ge/Si Heterojunction Nanowire

The development of silicon-compatible, high-performance infrared photodetectors is crucial for advancing thermal imaging, security, and communication systems. While germanium is a promising near-infrared material, its behavior in nanostructured forms with silicon heterojunctions reveals complex photophysics. This work demonstrates a germanium nanowire photodetector grown on a silicon-on-insulator (SOI) platform that exhibits a striking, tunable coexistence of both positive photoconductivity (PPC) and negative photoconductivity (NPC). We show that the dominant photoresponse can be switched by the wavelength of incident light: NPC dominates at visible wavelengths (e.g., 532 nm), while PPC prevails in the near-infrared (e.g., 1310 nm). Through systematic experiments and FDTD and TCAD simulations, we elucidate that this phenomenon arises from the interplay of light absorption in the different layers of the heterostructure. At short wavelengths, strong absorption in the underlying Si layer forward-biases the heterojunction, injecting carriers that quench the Ge channel conductance (NPC). At long wavelengths, absorption is confined to the Ge layer, resulting in conventional PPC. Negative photoconductivity was consistently observed over the temperature range from 78 to 298 K. Notably, the maximum responsivity of the nanowire increased from −56.7 A/W at room temperature to −1421.5 A/W at 78 K. This is attributed to the suppression of surface recombination velocity, increasing the minority carrier lifetime by 2 orders of magnitude. The −3 dB bandwidth is 2.9 kHz under 532 nm light and 3.9 kHz under 1310 nm light. The minimum noise equivalent power is determined to be 5.3 × 10–14 W/Hz0.5, corresponding to a specific detectivity of 4.0 × 109 Jones at room temperature. Furthermore, we demonstrate that the crossover wavelength is intensity-dependent and that the photocurrent follows an established logarithmic model for nanowire photoconductors. This work provides a controllable model system for studying NPC and presents a novel device architecture with tunable, multifunctional photoresponse for advanced optoelectronic applications.