PHOTOCURRENT AND RAMAN-SPECTROSCOPY OF STARK LADDER SUPERLATTICES WITH SINGLE MONOLAYER ALAS BARRIERS

GaAs/AlAs superlattices with 1, 2, 3, and 4 monolayer (ML) wide AlAs b arriers have been studied by photocurrent and Raman spectroscopy in an electric field parallel to the growth direction. The sample with 1 ML AlAs barriers shows pronounced Franz-Keldysh (FK) oscillations at low fields and Wannier-Stark localization (WSL) at high fields, thus reve aling the field-induced transition from three-dimensional to two-dimen sional behavior. An increase in barrier width results in a reduced int er-well coupling, which manifests itself by a strong suppression of th e FK oscillations, and a clear modification of the oscillator strength s of the oblique transitions. However, as the electronic properties of these superlattices are of limited sensitivity to the precise spatial shape of the barriers, i.e., to a possible distribution of a single ' 'monolayer'' AlAs barrier over several monolayers of the crystal, we h ave also studied the confined phonons of the AlAs barriers by Raman sp ectroscopy. From these results we conclude that, e.g., a nominally 1 M L thick AlAs barrier layer is actually spread over at most 3 ML of mat erial. The high sensitivity of the phonon modes to changes in the barr ier composition is caused by the strongly localized nature of the long itudinal optical phonons in the AlAs barriers, which contrasts with th e delocalized nature of the electronic states.