We report a strong correlation between the location of Mn sites in ferromagnetic ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ measured by combined channeling Rutherford backscattering and by particle-induced x-ray emission experiments and its Curie temperature. The concentrations of free holes determined by electrochemical capacitance-voltage profiling and of uncompensated ${\mathrm{Mn}}^{2+}$ spins determined from superconducting quantum-interference device magnetization measurements are found to depend on the concentration of unstable defects involving highly mobile Mn interstitials. This leads to large variations in $T_C$ of ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ when it is annealed at different temperatures in a narrow temperature range. The fact that annealing under various conditions has failed to produce Curie temperatures above ∼110 K is attributed to the existence of an upper limit on the free hole concentration in low-temperature-grown ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}.$

Structural investigation of as-grown as well as annealed Ga1−xMnxAs epilayers was carried out using high-resolution x-ray diffraction (XRD) measurements for a wide range of Mn concentrations (0.027⩽x⩽0.083), with special attention on how the interstitial Mn atoms (MnI) influence the lattice parameter of this material. We observe a distinct decrease of the lattice parameter after low-temperature annealing of Ga1−xMnxAs, which is known to reduce the MnI concentration. The reciprocal space maps measured for all of the investigated samples showed that the Ga1−xMnxAs layers are fully strained—i.e., they remain pseudomorphic to the GaAs (001) substrate—for the entire thickness of the samples used (in the present case over 100 nm). In all cases studied, the XRD measurements revealed high crystalline perfection of both as-grown as well as annealed Ga1−xMnxAs epilayers.