LUMINESCENCE KINETICS OF INTRINSIC EXCITONIC STATES QUANTUM-MECHANICALLY BOUND NEAR HIGH-QUALITY (N--TYPE GAAS) (P-TYPE ALXGA1-XAS) HETEROINTERFACES/

Recently, an optical emission process, termed the H band, has been obs erved in GaAs/AlxGa1-xAs heterostructures which has been related to th e regions surrounding the heterointerfaces. We show here that the mech anism responsible for this H-band photoluminescence (PL) in our struct ures is the recombination of quasi-two-dimensional (2D) excitons-and i s not due to either the recombination of carriers bound at impurities and/or defects, or the recombination of 2D carriers with 3D free carri ers. We have used such PL in high-purity, virtually ''interface-free'' GaAs(n-)/Al0.3Ga0.7As(p) double heterostructures to study H-band deca y dynamics, and thus prove that these excitations in our structures ar e ''intrinsic'' and arise from quantum-mechanically bound quasi-2D exc itons. Time-resolved PL spectra show the emission to be spectrally non stationary, with lifetimes across the band varying from a few nanoseco nds to more than 50 mus. Further, we find that large interfacial recom bination velocities, in inferior samples, may mask the truly intrinsic H-band recombination dynamics. Our accompanying quantum-mechanical nu merical modeling of such 2D excitons allows us to interpret and reprod uce virtually all of our experimental observations. Indeed, they demon strate that H-band PL cannot be impurity induced, but instead arises f rom the recombination of intrinsic excitonic states bound at both hete rointerfaces. Hence we find that in thinner structures, these excitoni c states may be simultaneously associated with both interfaces, with a critical GaAs thickness at which this ''exciton sharing'' between het erointerfaces becomes significant of < 0.5 mum. We also discuss the ti me evolution of the initially photoexcited 3D bulk excitons as they ac quire this subsequent 2D character, through a mechanism at the interfa ces analogous to the quantum confined Stark effect in quantum wells. O ur combined experimental and theoretical modeling, in these virtually interface-free samples, therefore, provide a direct measure and full q uantum-mechanical explanation of the temporal evolution of these intri nsic 2D excitons-from 3D formation to 2D recombination-as undistorted by the deleterious influence of carrier trapping and nonradiative deca y (in both the bulk and at interfaces), which may otherwise dominate t he usually more-imperfect typical heterostructure. The results present ed here are for specific, high-quality, interface-free GaAs/Al0.3Ga0.7 As heterostructures; however, these measurement and analysis technique s may also be applicable to other types of structures.