Intersubband transitions to minigap-confined states in doped quantum wells

We have demonstrated a new type of infrared photodetector based on infrared intersubband transitions in doped quantum wells in which the excited state is confined by the minigap between two minibands in surrounding superlatti ce barrier layers. Sharp peaks in the infrared photocurrent spectra were ob served at 78 K in the 2.7-3.6 mum wavelength region corresponding to transi tions in which the excited state energy is substantially above the conducti on band edge of the higher-bandgap component of the superlattice. This indi cates that the excited state is strongly confined within the well layer by the forbidden-energy minigap region between two allowed minibands. Broader, longer-wavelength photocurrent features corresponding to transitions into the lower-lying miniband states were also observed. The spectral features i n the photocurrent spectra are well described by calculations of the optica l absorption cross-section, and the relative magnitudes of these features a re shown to be strongly affected by bias-dependent photocarrier transport. (C) 2001 Published by Elsevier Science B.V.