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Photocurrent and Raman spectroscopy of Stark ladder superlattices with single monolayer AlAs barriers.

Photostrom und Raman-Spektroskopie von Stark-Leiter-Übergittern mit Einzel-Monolagen AlAs Barrieren


Journal of Crystal Growth 127 (1993), S.836-840
ISSN: 0022-0248
Fraunhofer IAF ()
Franz Keldysh-effect; Franz Keldysh-Effekt; raman spectroscopy; Ramanspektroskopie; Stark-localization; Stark-Lokalisierung; superlattice; Übergitter

GaAs/AlAs superlattices with 1, 2, 3, and 4 monolayer (ML) wide AlAs barriers 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 revealing the field-induced transition from three-dimensional to two-dimensional behavior. An increase in barrier width results in a reduced inter-well coupling, which manifests itself by a strong suppression of the FK oscillations, and a clear modification of the oscillator strengths 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 have also studied the confined phonons of the AlAs barriers by Raman spectroscopy. From these results we conclude that, e.g., a nominally 1 ML thick AlAs barrier layer is actually spread over at most 3 ML of material. The high sensitivity of the phonon modes to changes in the barrier composition is caused by the strongly localized nature of the longitudinal optical phonons in the AlAs barriers, which contrasts with the delocalized nature of the electronic states.