Embedded clustering in Cr-doped AlN: Evidence for general behavior in dilute magnetic III-nitride semiconductors

We report a systematic density-functional theory investigation of the "structure-property relationship" of Cr:AlN by doping up to 5 Cr atoms in large supercells, for which exhaustive structural and magnetic configurations have been calculated-including full atomic relaxation. Our results demonstrate that the Cr atoms tend to segregate to form Cr-N-Cr bonded clusters, which are embedded in the AlN host wurtzite structure. Significantly, while the ferromagnetic state with a spin moment close to 3 μB Cr is the ground state for both isolated "single" and "pair" doping configurations, for larger cluster configurations states containing antiferromagnetic or ferrimagnetic coupling with net spin in the range of 0-1.53 μB Cr are found to be energetically more favorable. Electrical conductivity (half-metallic or insulating) is predicted to be sensitively dependent on the dopant concentration. We propose a picture that various sized Cr-N-Cr bonded clusters coexist and the statistical distribution and associated magnetic properties will depend sensitively on the growth conditions. Such a scenario is in agreement with recent experiments and can help understand a number of hitherto puzzling experimental observations, notably the low mean saturation magnetic moment, the contracted lattice constants, and the highly insulating behavior.