Photovoltaic intersubband detectors for 3-5 mu m using GaAs quantum wells sandwiched between AlAs tunnel barriers

The authors report on a new type of GaAs quantum well intersubband photodetector operating at wavelengths of 3-5 mu m. In these structures, the intersubband spacing is considerably enlarged by using ultrathin AlAs barriers on either side of the GaAs quantum wells followed by a thicker Al 0.3 Ga 0.7 As layer. These AlAs layers act as tunnel barriers allowing both photoconductive and photovoltaic detection of the intersubband excitation. Photovoltaic detector operation is possible because of an asymmetry between the confining AlAs layers, which is attributed to the epitaxial growth process. They also discuss the low-signal detector noise which is determined from the dark-current behaviour. The noise behaviour strongly favours photovoltaic detectors over photoconductive ones.