Curie temperature and carrier concentration gradients in epitaxy-grown Ga1−xMnxAs layers

We report on detailed investigations of the electronic and magnetic properties of ferromagnetic ${\mathrm{Ga}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{As}$ layers, which have been fabricated by low-temperature molecular-beam epitaxy. Superconducting quantum interference device measurements reveal a decrease of the Curie temperature from the surface to the ${\mathrm{Ga}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{As/GaAs}$ interface. While high-resolution x-ray diffraction clearly shows a homogeneous Mn distribution, a pronounced decrease of the carrier concentration from the surface towards the ${\mathrm{Ga}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{As/GaAs}$ interface has been found by Raman spectroscopy as well as electrochemical capacitance–voltage profiling. The gradient in Curie temperature seems to be a general feature of ${\mathrm{Ga}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{As}$ layers grown at low temperature. Possible explanations are discussed.