Observation of natural superlattice in Al_XGa_1-XAs layers grown by metalorganic vapor phase epitaxy

Atomic Cu-Pt type ordering in monolayer scale is well known in Ga_0.5 In_0.5 P epitaxial layers leading to change in its bandgap. Despite wide scale use of Al_xGa_1-xAs (0<x<0.7) in various optoelectronic devices and quantum well structures, spontaneous ordering in this system was not well studied beyond monolayer scale. In this work, we report observation of natural superlattice ordering due to compositional variation in MOVPE grown Al_xGa_1-xAs epitaxial layers. Clear evidence of such ordering was first noticed in the barrier layers of GaAs/Al_0.3Ga_0.7As quantum well structures. Further investigation on bulk epitaxial growth of Al_xGa_1-xAs layer with different composition (x=0.2) has also confirmed existence of such structures. We have probed the sample using high resolution Transmission Electron Microscopy coupled with x-ray rocking curve (XRC) and reflectivity (XRR) measurements. Sharp superlattice peaks around (004) substrate Bragg peak as well as around (002) forbidden peak in XRC have been observed. Similar peaks are present in XRR also. We have presented detailed analysis of X-ray diffraction data with the help of kinematical diffraction theory here. The stability of the superlattice structures has been further investigated by annealing the sample at different temperatures.