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RESEARCH PRODUCT

Photoluminescence from strained InAs monolayers in GaAs under pressure

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bulk GaAs. At pressures above the band crossover two emission bands are observed. These bands, characterized by having negative pressure coefBcients, are attributed to the type-I transition between conduction-band X „and heavy-hole states of the InAs monolayer and the type-II transition &om X states in GaAs to InAs heavy-hole states. The results are interpreted in terms of tight-binding band-structure calculations for the strained InAs-monolayer — bulk-GaAs system. I. INTRODUCTION Highly strained InAs jGaAs heterostructures have recently attracted interest due to their unusual electronic and optical properties. ~ 4 Epitaxial isomorphic growth of InAs on GaAs can be achieved only up to a small critical thickness of 2— 3 monolayers (ML) due to the very large mismatch in lattice constant of about 6.8'%%uo. s's There is a marked contrast in the electronic structure of the two materials: the lowest direct band gap of InAs (0.42 eV at 10 K) is about 1.1 eV smaller than that of GaAs. Thus, in spite of its small thickness, even a single monolayer of InAs has a strong inBuence on the optical properties in these heterostructures. As established recently in a series of optical studies of such structures at ambient pressure, ' the InAs monolayer acts as an attractive potential for electrons and holes. This gives rise to the formation of excitons localized at the two-dimensional sheet of In atoms, resulting in a redshift of the dominant photoluminescence (PL) band relative to that of bulk GaAs. A well established change is induced in the electronic properties of the GaAs host material under hydrostatic pressure. The energy of the direct I' point band gap increases with pressure while the energy of the X-like minimum is lowered, leading to a direct to indirect band gap crossover at about 4.2 GPa. As in the case of bulk GaAs, striking changes in the character and intensity of the optical emission are expected to occur under pressure in the GaAs/InAs monolayer system. An interesting aspect of technological importance is whether such structures show enhanced radiative recombination eKciency in the indirect case due to the breakdown of translational symmetry caused by the monolayer potential. A detailed understanding of the electronic structure at and above the crossover pressure can be obtained from a study of the excitonic luminescence for pressures covering the transition regime.