Recently, an optical emission process, termed the H band, has been observed in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructures which has been related to the regions surrounding the heterointerfaces. We show here that the mechanism responsible for this H-band photoluminescence (PL) in our structures is the recombination of quasi-two-dimensional (2D) excitons--and is not due to either the recombination of carriers bound at impurities and/or defects, or the recombination of 2D carriers with 3D free carriers. We have used such PL in high-purity, virtually ``interface-free'' GaAs(${\mathit{n}}^{\mathrm{\ensuremath{-}}}$)/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As(p) double heterostructures to study H-band decay dynamics, and thus prove that these excitations in our structures are ``intrinsic'' and arise from quantum-mechanically bound quasi-2D excitons. Time-resolved PL spectra show the emission to be spectrally nonstationary, with lifetimes across the band varying from a few nanoseconds to more than 50 \ensuremath{\mu}s. Further, we find that large interfacial recombination velocities, in inferior samples, may mask the truly intrinsic H-band recombination dynamics. Our accompanying quantum-mechanical numerical modeling of such 2D excitons allows us to interpret and reproduce virtually all of our experimental observations. Indeed, they demonstrate that H-band PL cannot be impurity induced, but instead arises from the recombination of intrinsic excitonic states bound at both heterointerfaces. Hence we find that in thinner structures, these excitonic states may be simultaneously associated with both interfaces, with a critical GaAs thickness at which this ``exciton sharing'' between heterointerfaces becomes significant of 0.5 \ensuremath{\mu}m. We also discuss the time evolution of the initially photoexcited 3D bulk excitons as they acquire this subsequent 2D character, through a mechanism at the interfaces analogous to the quantum confined Stark effect in quantum wells.Our combined experimental and theoretical modeling, in these virtually interface-free samples, therefore, provide a direct measure and full quantum-mechanical explanation of the temporal evolution of these intrinsic 2D excitons--from 3D formation to 2D recombination--as undistorted by the deleterious influence of carrier trapping and nonradiative decay (in both the bulk and at interfaces), which may otherwise dominate the usually more-imperfect typical heterostructure. The results presented here are for specific, high-quality, interface-free GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As heterostructures; however, these measurement and analysis techniques may also be applicable to other types of structures.
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Jul 11, 1994
D Yan + 4
Open Access