Spontaneous superlattice structures in Al_xGa_1−xAs/GaAs (1 0 0) grown by metalorganic vapor phase epitaxy

Spontaneous superlattice ordering has been observed in Al_xGa_1−xAs layers grown on (1 0 0) GaAs substrates by metalorganic vapor phase epitaxy. Transmission electron microscopic image clearly revealed superlattice structures and the selected area electron diffraction showed closely spaced superlattice spots around the main diffraction pattern. High resolution X-ray diffraction showed distinct and sharp superlattice peaks symmetrically positioned around the central (0 0 4) Bragg peak and the similar measurement for (0 0 2) planes, which is quasi-forbidden for Bragg reflections showed only superlattice peaks. A superlattice period of 5.35 nm was calculated with periodic Al variation in the two layers of the superlattice. Thermal annealing studies showed the superlattice structure was stable up to 800 °C and disappeared after annealing at 900 °C retaining the crystallinity of the epilayer. Photoluminescence at 4 K showed a distinct peak at 1.732 eV with full width at half maximum of 8 meV corresponding to the bandgap related excitonic transition for an average Al composition x = 0.17. A blue shift of 32 meV of the bandgap related excitonic peak for the 900 °C annealed sample is attributed to the loss of the superlattice ordering with the same average composition of the epilayer.