NEW OPTICAL-ABSORPTION AND PHOTOCURRENT REVERSAL PHENOMENA INDUCED BYLOCALIZED CONTINUUM RESONANCES IN QUANTUM-WELL HETEROSTRUCTURES

A new asymmetric quantum well heterostructure, characterized by strong spatial localization of continuum resonances, is investigated. The pr esence of these states and the asymmetry of the structure give rise to new striking absorption and photoconductivity phenomena. Our sample c onsists of doped GaInAs quantum wells cladded by AlInAs/GaInAs quarter wave electron stacks and by AlInAs barriers. In a strong electric fie ld of the appropriate polarity the first continuum resonance is strong ly confined above the wells, while the second continuum resonance beco mes localized in the latter. This manifests itself in a strong narrowi ng of the intersubband absorption spectrum as a function of the electr ic field as a greater fraction of the oscillator strength is concentra ted into a transition from the ground state of the well to a quasi-bou nd state in the continuum. The same quantum well heterostructure exhib its a new type of photoconductivity controlled by the wavefunction of the continuum states. For small applied bias this phenomenon exhibits directional charge transfer against the electric field as electrons in the wells are photoexcited to resonances localized above the barriers ; as the field is increased the photocurrent changes sign. A simple mo del to describe the essential features of this effect is presented.