Evaluation of spontaneous superlattice ordering in MOCVD grown Al_xGa_1-xAs epilayer on GaAs (100) using X-ray reflectivity and rocking curve analysis

Spontaneous superlattice structure in few monolayer length scale has been observed very recently in Al_xGa_1-xAs epilayer on (001) GaAs substrate grown by Metal Organic Chemical Vapor Deposition. Here we have reported a detailed study of this structure using high resolution x-ray reflectivity (XRR) and rocking curve (XRC) analysis and tried to extract composition modulation in the superlattice. A model-independent Distorted Wave Born Approximation (DWBA) has been applied to the XRR results to successfully obtain the electron density profile of the structure from which the compositional information could be extracted. The XRR profile of the Al_xGa_1-xAs sample (x = 0.17) showed sharp equidistant peaks giving clear evidence of superlattice ordering in the epilayer. Simulation of the XRR results showed a bilayer segregation with alternating Al composition of 0.1 and 0.4 of thickness 20 and 8 Å, respectively with interfacial thicknesses of 12 Å on both sides, giving a periodicity of 52 Å. The cross-sectional transmission electron microscopic (XTEM) image also showed similar length scale. A kinematical model was then used successfully to fit the sharp peaks observed in the XRC using the characteristic length and compositional parameters obtained from the XRR simulation. These results also showed that the individual bilayers were of single crystalline zinc blende structures. Both the simulation techniques were first applied to a test structure of AlAs/GaAs multilayer of known thickness before applying the same to the spontaneous superlattice to validate the model.