Observation of novel step-like structure in the photocurrent and dark current of a superlattice: Charge collection by successive depletion of quantum wells.

We have observed novel manifestations of superlattice properties in a Ga(0.47)In(0.53)As/InP quantum well photodiode structure grown by gas source molecular beam epitaxy, evident as multiple steps in the reverse bias photocurrent and dark current. This striking effect is explained in terms of depletion of successive quantum wells as the reverse bias field is increased, while the diffusion of carriers generated in the undepleted region is suppressed by localization within the wells. As the electric field punches through into each well the carriers photogenerated and thermally generated in that well are collected by the field, while carriers generated in the undepleted wells remain localized and cannot contribute to the current. This causes a step-like increase in the photocurrent and the dark current.