To mitigate the high operational cost caused by the liquid-helium cooling requirement for very-long-wave infrared quantum cascade detectors (VLW-QCDs), we propose and demonstrate a variable-barrier VLW-QCD structure. The active region adopts AlGaAs barriers with two different Al compositions: low-Al-content barriers are employed in the absorption region to increase the light absorption efficiency and achieve an optical absorption intensity of 1.55%, while high-Al-content barriers are introduced in the relaxation region to suppress thermally activated leakage of excited-state electrons and raise the device resistance. A coupled double-quantum-well structure is further integrated into the extraction region, raising the carrier extraction efficiency to 35%, which outperforms the 24% of traditional single-well devices.
Benefiting from the synergistic structural optimization, the fabricated 14 μm device achieves a peak responsivity of 32 mA/W and a detectivity of 1.7×10⁹ cm·W^-1·Hz^1/2 at liquid nitrogen temperature under zero bias, with the detectivity improved by two orders of magnitude compared with reported counterparts. When a forward bias of 1.75 V is applied, the responsivity is further enhanced to 121 mA/W, with the detectivity reaching 2.6×10⁹ cm·W^-1·Hz^1/2. This design successfully upgrades the operating temperature of 14 μm VLW-QCD from liquid-helium to liquid-nitrogen temperature, and the device still maintains a responsivity of 2 mA/W at 160 K.