Normal Incidence Mid-Infrared Photocurrent in AlGaN/GaN Quantum Well Infrared Photodetectors

We report the growth and characterization of AlGaN/GaN multiple quantum well structures designed to have intersubband transitions in the mid-infrared region of the spectrum. The samples were nominally undoped but were found to contain a high electron population in the wells induced by the local polarization fields. The sample was characterized by the use of the Raman spectroscopy and photocurrent spectroscopy. The Raman spectroscopy shows electronic Raman scattering from intersubband transitions in the AlGaN/GaN quantum wells. The e₁-e₂ and e₁-e₃ transitions of the confined 2d electron population in the wells can clearly be observed. A sample designed to absorb at 4 μm was fabricated into mesa structures and the vertical photocurrent measured under normal incidence illumination from the free-electron laser FELIX. A wavelength and bias dependent photocurrent was observed in the mid-IR region of spectrum. The peak responsivity was of the order of 50 μA/W at 4 K, the photocurrent still being measurable at room temperature.