Compositional modulation and long-range ordering in GaP/InP short-period superlattices grown by gas source molecular beam epitaxy

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Abstract
Long-range ordering in a (GaP)2/(InP)2 short-period superlattice and a Ga0.525In0.475P buffer layer grown on a (001)GaAs substrate by gas source molecular beam epitaxy were studied. Transmission electron microscopy and low-temperature cathodoluminesence techniques were used to examine the microstructure of the short-period superlattice and to determine its band-gap energy. The superlattice layer was found to have a [001] long-range ordered structure with a band gap narrowing of about 130 meV, while the Ga0.525In0.475P layer had a 37 meV band-gap narrowing induced by spontaneous long-range ordering in the [111] direction. The ordered superlattice layer was found to have a growth-induced lateral periodic modulation of the composition along the [1̄10] direction. Within the modulating bands, which had a 200 Å periodicity, the In composition was found to vary from 42 to 56% while the Ga correspondingly varied between 58 and 44%.