Spatially resolved experiments have been performed on the luminescence of bevelled AI,Gal -,As structures at different temperatures as a function of the alloy composition. The AlAs mole fraction ( x ) of the samples investigated ranges from 0.1 5 to 0.52. This was determined by measurements employing the electron microprobe analysis. At T= 85 and 300 K the band-gap energy of direct-gap material €,(x) has been evaluated from band-to-band optical transitions, whereas at lower temperatures (T=2 and 38 K ) the peak position of the luminescence line originating from the radiative decay of excitons bound to neutral donors served as a measure for the composition dependence of the fundamental energy gap in the direct-gap as well as in t h e indirect-gap region. For AlAs contents lower than 40 mol% the luminescence data show that € , ( x ) varies linearly with the composition. Our observations at different temperatures suggest that the temperature coefficient of the direct band gap increases with rising x. In contrast to Dingle's results for the crossover parameters we find x, = 0.374 and E, = 2.057 eV at T= 38 K and x, = 0.41 and E, = 1.98 eV at T= 300 K.
In contrast to usual quantum wells or barriers having a thickness of some lattice constants, spatially well-separated, electronically uncoupled monolayers of group-III or V elements are considered as isovalent δ doping or δ layers. Similar to the case of randomly distributed nitrogen dopants in GaP bulk material, it is shown that the two-dimensional arrangement of isovalent atoms brings forth a new quality of III-V semiconductor compounds: The optical emission and absorption properties near the fundamental band gap of indirect-gap, and even of direct-gap, host material, where the isovalent layers are incorporated, are drastically improved. Low-temperature luminescence and transmission experiments on metal-organic vapor-phase epitaxially grown InAs δ layers in GaAs, AlAs δ layers in GaAs, and GaAs monolayers in AlAs are dealt with.
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