  We report on detailed investigations of the electronic and magnetic
properties of ferromagnetic GaMnAs 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 GaMnAs/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 GaMnAs/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 GaMnAs
layers grown at low-temperature. Possible explanations are discussed.
