Electron minibands and Wannier-Stark quantization in anIn0.15Ga0.85

Abstract

We have investigated the electronic properties of an Ino. l&Gao. &As-GaAs strained-layer superlattice using photoluminescence excitation and photocurrent spectroscopies. Flatband spectra show transitions at the center and edge of the Brillouin minizone, and photocurrent spectra at finite bias show the effects of Wannier-Stark quantization. The heavy-hole transitions evidence the importance of the excitonic interaction between spatially separated carriers. The light-hole transitions show a qualitatively different behavior resulting from their weak confinement in the GaAs layers. Our data agree with a numerical calculation of the electro-optical absorption spectra.In the last few years, there has been a considerable interest in strained-layer superlattices (SL) and quantum wells (QW), because of their fundamental interest and their potential for devices. ' Although a number of studies have been devoted to the In"Gal-"As-GaAs heterostructures, the electronic properties of this system are still a matter of controversy. 2 More recently, novel electrooptical properties of semiconductor SL's have been discovered.The investigation of these Wannier-Stark effects in the In"Gal "As-GaAs system is interesting in many respects. In particular, the strain-induced enhancement of the heavy-hole to light-hole splitting allows a complete separation of the corresponding absorption bands, which makes the observation of the zone-edge or "saddle-point exciton" 5 transitions much clearer than in unstrained SL's. Also, at least for large enough indium concentrations, the light holes must be confined in the GaAs layers, which gives an opportunity to observe the Wannier-Stark effects in the type-II SL configuration, for which a qualitatively new behavior is predicted. In addition, short-period SL's can be accurately characterized, and the study of the electro-optical properties of such In, Gal "As-GaAs SL's is likely to bring a definitive answer to the controversy on the band offsets in this system. In this Rapid Communication, we report investigations by photoluminescence excitation (PLE) and photocurrent (PC) spectroscopies of the electronic structure and electro-optical properties of a Inp|5Gaps5As-GaAs SL in which the well and barrier thicknesses are small enough to ensure a strong coupling of the wells in the flatband conditions and, at least for the heavy-hole transitions, a negligible intrawell Stark eAect.Our sample was grown by low-pressure metal-organic vapor-phase epitaxy on a Si-doped GaAs substrate. It consists of a 10-period SL grown on top of a 1.8 pm thick buff'er layer of undoped GaAs. All the layers were undoped with a residual carrier concentration n = 5x10'cm . An important advantage of periodic structures, with respect to characterization, is the easy evaluation of their structural parameters by x-ray diff'raction. Here, the analysis of the high-quality x-ray rocking curve indicated that no plastic relaxation occurred. ' An In concentration of 15% and thicknesses of 31 and 90 A. for the In"-Ga|-"As and GaAs layers, res...

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Wannier–Stark resonances in optical and semiconductor superlattices

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