Intersubband transitions in InAs/AlSb quantum wells studied by resonant Raman scattering

We have used resonant Raman scattering to study intersubband transitions in InAs/AlSb quantum wells. For optical excitation in resonance with the ${\mathit{E}}_1$ band gap of InAs, both high- and low-frequency coupled longitudinal-optical phonon-intersubband plasmon modes are observed. From the measured energies of these plasmon modes the single-particle transition energies between the first and second confined electron subband were deduced as a function of the InAs well width. Good agreement was found with subband spacings predicted by theory including the effects of strain and nonparabolicity. InAs/AlSb surface quantum wells, where a pseudomorphically strained InAs quantum well is grown on an AlSb buffer layer without an AlSb top barrier, also show well-resolved intersubband plasmon modes, indicating higher-electron mobilities than those typically found in the surface inversion region of thick InAs layers.