Curie temperature limit in ferromagnetic ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$

We provide experimental evidence that the upper limit of ∼110 K commonly observed for the Curie temperature $T_C$ of ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ thin films (thickness >50 nm) is caused by Fermi-level-induced hole saturation. Ion channeling, electrical, and magnetization measurements on a series of ${\mathrm{Ga}}_{1-x-y}{\mathrm{Mn}}_x{\mathrm{Be}}_y\mathrm{As}$ layers show 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 $\sim 5\times {10}^{20}{\mathrm{cm}}^{-3}.$ These results indicate that the concentrations of free holes and ferromagnetically active Mn spins are governed by the position of the Fermi level, which controls the formation energy of compensating interstitial Mn donors.