  Selective sensing of terahertz (THz) radiation by beryllium and silicon
delta-doped GaAs/AlAs multiple quantum wells (MQWs) is demonstrated. A
sensitivity up to 0.3 V/W within 0.5{4.2 THz in silicon- and up to 1 V/W within
4.2-7.3 THz range in beryllium-doped MQWs at liquid helium temperatures is
shown. The built-in electric fields as estimated from the observed
Franz-Keldysh oscillations in photoreflectance spectra were found to be located
close to the cap and buffer layers of MQWs and vary from 18 kV/cm up to 49
kV/cm depending on the structure design.
