GaMnAs layers with Mn content varying from 0.25 to 3.5% and thickness from 0.2 to 1.0 mu m have been grown by molecular beam epitaxy (MBE) on GaAs (100) substrates. The Mn content was estimated both from in situ reflection high energy electron diffraction (RHEED) and ex situ high resolution X-ray diffraction (XRD) measurements. The samples have been grown in an MBE system connected to a photoelectron spectrometer system at the MAX-lab storage ring. Our, photoemission, XRD and RHEED studies show that GaMnAs form uniform, ternary compound without any signs of MnAs precipitations. The samples with Mn content of 1.5-3.5% showed the ferromagnetic phase transition in the temperatures 15-50 K depending on composition. We have done precise measurements of the lattice parameter both in surface plane and perpendicular directions. The reciprocal space maps show that GaMnAs layers are fully strained to the GaAs (100) substrates and their structural perfection is comparable to that of the low temperature GaAs buffer layer.