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Fundamental Curie Temperature Limit in Ferromagnetic Ga 1-x Mn x As

We provide unambiguous experimental evidence that the upper limit of ~ 110 K commonly observed for the Curie temperature T C of Ga 1-x Mn x As is caused by the Fermi-level-induced hole saturation. This conclusion is based on parallel studies of the location of Mn in the lattice, the effectiveness of acceptor center, and ferromagnetism on a series of Ga 1-x-y Mn x Be y As layers, in which the concentration of magnetic moments and of free holes can be independently controlled by the Mn and Be contents. Ion channeling and magnetization measurements show a dramatic increase of the concentration of Mn interstitials accompanied by a reduction of T C with increasing Be concentration. At the same time the free hole concentration remains relatively constant at ~5x10 20 cm -3 . These results indicate that the concentrations of free holes as well as of ferromagnetically active Mn spins are governed by the position of the Fermi level, which controls the formation energy of compensating interstitial Mn donors. Based on these results, we propose to use heavy n-type counter-doping of Ga 1-x Mn x As (by, e.g., Te) to suppress the formation of Mn interstitials at high x, and thus improve the T C of the alloy system.