Quantum-Confined Optical Interband Transitions in 5-Doped Doping Superlattices
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Quantum-Confined Optical Interband Transitions in 5-Doped Doping Superlattices
E. F. Schubert, T. D. Harris, and J. E. Cunningham* AT&T Bell Laboratories 600 Mountain Avenue Murray Hill, New Jersey 07974 *AT&T Bell Laboratories Crawfords Comer Road Holmdel, New Jersey, 07733 ABSTRACT Optical absorption and photoluminescence experiments are performed on GaAs doping superlattices, which have a 8-function-like doping profile of alternating n-type and p-type dopant sheets. Absorption and emission spectra reveal for the first time the clear signature of quantum-confined interband transitions. The peaks of the experimental absorption and luminescence spectra are assigned to calculated energies of quantum-confined transitions with very good agreement. It is shown that the employment of the &-doping technique results in improved optical properties of doping superlattices.
INTRODUCTION Doping superlattices were proposed approximately two decades ago by Esaki and Tsu [1] and consist of a semiconductor with thin alternating n- and p-type doping layers. The positive and negative charge of dopants result in a periodic potential which is called the superlattice potential. The electronic and optical properties of doping superlattices include (i) a reduced energy gap of the superlattice, as compared to the undoped host material, (ii) an enhanced lifetime of carriers due to a spatial separation of electron and hole wave functions, (iii) and a dependence of the energy-gap and lifetime on (optical) excitation density, i.e. the free carrier density [1-4]. Despite much experimental and theoretical work on doping superlattices, quantum-confined interband transitions were not observed in absorption or luminescence experiments [2-4]. In this publication we report the first observation of quantum-confined optical interband transitions in an improved doping superlattice structure. The structure consists of altematingly n- and p-type 8-doped sheets separated by intrinsic material. The sawtoothshaped band diagram and the doping profile of the superlattice are shown in Fig. 1. Inspection of the wave functions in Fig. 1 reveals that the maxima of the electron distribution and the maxima of the hole-distribution are spatially displaced by half a superlattice period. If this displacement is large, the overlap of electron and hole wave functions and thus the oscillator strength are strongly reduced. Fig. 1 also reveals that the energy-gap of the superlattice (i.e. the energy between the lowest electron state, Ee, and the highest heavy hole state, E~hh) is reduced as compared to the gap of the host material (i.e. Eg = Ec - E,,).
Mat. Res. Soc. Symp. Proc. Vol. 145. 01989 Materials Research Society
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