  We report on a GaN/AlGaN quantum cascade detector operating in the terahertz
spectral range. The device was grown by metal organic chemical vapor deposition
on a c-sapphire substrate and relies on polar GaN/AlGaN step quantum wells. The
active region thickness is in micrometer range. The structural, electrical and
optical investigations attest of high structural quality of the synthetized
nitride material. The detector exhibits a peak photocurrent at 5.7 THz (23.6
meV) with a responsivity of 0.1 mA/W at 10 K under surface normal irradiation
through a 10 um period grating. The photocurrent persists up to 20 K.
