TUNNELING AND RELAXATION IN SEMICONDUCTOR DOUBLE-QUANTUM WELLS

Double quantum wells are among the simplest semiconductor heterostruct ures exhibiting tunnel coupling. The existence of a quantum confinemen t effect for the energy levels of a narrow single quantum well has bee n largely studied. In double quantum wells, in addition to these confi nement effects which characterize the levels of the isolated wells, on e faces the problem of describing the eigenstates of systems interacti ng weakly through a potential barrier. In addition, the actual structu res differ from the ideal systems studied in the quantum mechanics tex tbooks in many aspects. The presence of defects leads, for instance, t o an irreversible time evolution for a population of photocreated carr iers, This irreversible transfer is now clearly established experiment ally. The resonant behaviour of the transfer has also been evidenced, from the study of biased structures. If the existence of an interwell transfer is now clearly established from the experimental point of vie w, its theoretical description, however, is not fully satisfactory. Th is review focuses on the theoretical description of the energy levels and of the interwell assisted transfer in double quantum wells. We sha ll firstly outline the problem of tunnel coupling in semiconductor het erostructures and then discuss the single particle and exciton eigenst ates in double quantum wells. In the remaining part of the review we s hall present and critically review a few theoretical models used to de scribe the assisted interwell transfer in these structures.