Quantum wells with localized states at energies above the barrier height: A Fabry–Perot electron filter

The observation of electronic states localized in a quantum well at energies greater than the barrier height is reported. In our GaInAs wells quantum confinement is achieved at an energy corresponding to a continuum transmission resonance using as barriers high reflectivity AlInAs/GaInAs quarter-wave stacks. The latter strongly narrow the resonance, thus forming a high finesse Fabry–Perot electron filter. Intersubband infrared absorption measurements at 300 K reveal a strong narrowing of the transition to a localized state above the well as the number of periods in the λ/4 stacks is increased. The narrowest absorption peaks exhibit widths equal to those of bound-to-bound state transitions in GaInAs wells with thick AlInAs barriers.