Up to date, different groups have reported ordering phenomena in nitride alloys like InGaN or AlGaN. These evidences of ordering during and after growth lead us to study the atomic arrangement of Al and Ga on site III in the AlGaN alloy using a thermodynamical approch. We have defined the enthalpy and the entropy of our system using the Keating model and the cluster variation method limited to the first coordination shell. This study gives us a good way to understand ordering, and moreover to see why at three special compositions (25%, 50% and 75%), the alloys have good chances to present a better crystalline quality than in other cases.
We have grown AlGaN samples increasing the Al composition by 10% steps, in the full range of composition from GaN to AlN. All the growth parameters were kept constant except for the alloy composition. X-Ray diffraction measurements performed on these samples indicate that FWHM increases linearly with Al composition, except for three compositions, where according to our thermodynamical study, we observe an improved crystalline quality.