Lattice location of Mn and fundamental Curie temperature limit in ferromagnetic Ga1-xMnxAs

Using ion channeling techniques, we find that a substantial fraction (up to 15%) of the Mn atoms reside in interstitial sites in Ga1-xMnxAs alloys. The increase of saturation magnetization, hole concentration and Curie temperature T-C of Ga1-xMnxAs after low temperature annealing is found to be related to the relocation of Mn atoms from interstitial sites to form random clusters. We believe that the redistribution of the Mn atoms between different sites is controlled by the location of the Fermi energy. This conjecture is investigated using Ga1-x-yMnxBeyAs samples in which the concentration of Mn moments and the Fermi level position can be determined separately by Mn and Be doping. We find a dramatic increase of the concentration of Mn interstitials accompanied by a reduction of T-C with increasing Be concentration, while the free hole concentration remains relatively constant at similar to5 x 10(20) cm(-3). These results confirm that the maximum T-C near 110 K in Ga1-xMnxAs is achieved for the Fermi energy corresponding to a saturation hole concentration in the range of 5 x 10(20)-1 x 10(21) cm(-3). (C) 2004 Elsevier B.V. All rights reserved.