Long-wavelength infrared spectroscopy of an asymmetrically structured ${\mathrm{Ga}}_{0.6}$${\mathrm{Al}}_{0.4}$As/GaAs superlattice

A long-wavelength infrared (LWIR) spectroscopic study of a doped multiple-quantum-well (MQW) structure, where each quantum well is clad by asymmetrically structured superlattices, is reported. The measured absorption and photocurrent spectra differ markedly from those of a MQW LWIR detector with conventional flat barriers. Different electron subband states are introduced, and the transition e1-e3, which in a flat-barrier MQW is normally forbidden, becomes the dominant transition; thus bringing about an extremely broad photoresponse band (Δλ/λ≊0.6), centered at 5 μm. This system also exhibits direct evidence of photon-assisted resonant tunneling. It is manifested by a distinct peak and negative differential photoconductance in the photocurrent-vs-bias-voltage characteristics when the structure is exposed to a ${\mathrm{CO}}_2$ 9.5-μm laser line. All these effects are explained by the dependence of the electronic eigenstate spectrum on the electric field, calculated by solving the Schrödinger equation using the transfer-matrix method.