Correlation of Mn lattice location, free hole concentration, and Curie temperature in ferromagnetic GaMnAs

We provide experimental evidence that the electrical and magnetic characteristics of Ga_1-xMn_xAs for a given x depend primarily on the distribution of Mn atoms over their different possible locations in the crystal lattice. Using combined channeling Rutherford backscattering and particle-induced X-ray emission, we show that optimal postgrowth annealing-which leads to an increase of the Curie temperature T_C and is accompanied by an increase of free hole concentration and saturation magnetization-is caused by the reduction in the number of Mn atoms occupying interstitial positions. On the other hand, when Ga_1-xMn_xAs is additionally doped with Be, we observe that-while the hole concentration remains nearly constant-there occurs a strong decrease of T_C together with a dramatic increase in the concentration of Mn interstitials. These results indicate that there is a thermodynamic limit imposed on the maximum Curie temperature in Ga_1-xMn_xAs. © 2003 Plenum Publishing Corporation.