Ultrafast Carrier Dynamics and Enhanced Electroabsorption in (gallium, Indium)arsenide/(aluminum, Indium)arsenide Asymmetric Double Quantum Well Structures.

An experimental study, utilizing a novel nondegenerate transmission pump/probe technique, of ultrafast electron and hole tunneling in (Ga,In)As/Al,In)As asymmetric double quantum wells (ADQWs) is presented. A single time constant is observed at low carrier densities indicating the holes tunnel from the narrow well (NW) to the wide well (WW) at least as fast as electrons. At high carrier densities a two component decay is observed, consistent with phase -space filling and space-charge effects blocking tunneling carriers. The fast transfer of electrons was confirmed to be a LO-phonon assisted process. A detailed theoretical study of ultrafast hole tunneling at low carrier densities indicates that in ternary materials alloy disorder is responsible for fast hole transfer between the wells. Enhanced electroabsorption in selectively doped (Ga,In)As/(Al,In)As ADQWs by the use of real space electron transfer is demonstrated. The electron concentration in both the WW and NW is investigated by field-dependent absorption and photoluminescence spectroscopy. The results are compared to absorption changes in an undoped ADQW structure which utilizes the quantum confined Stark effect. The doped modulator exhibits a significantly larger red-shift with applied field than the undoped structure.