Cubic rock salt structure Al0.60Cr0.40N and Al0.68Cr0.32N films of different thicknesses were grown epitaxially onto MgO(111) substrates by reactive unbalanced magnetron sputtering at 500 degrees C. Rutherford backscattering spectroscopy reveals stoichiometric nitrides with Al/Cr ratios close to the ones of the used compound targets of 60/40 and 70/30. High resolution X-ray diffraction proves epitaxial growth over the whole film thickness up to thicknesses of similar to 1.8 mu m. Reciprocal space maps and selected area electron diffraction show that the AlxCr1-xN films grow fully relaxed. Scanning and transmission electron microscopy imaging reveals columnar microstructures with column widths between 12-16 nm and {001} surface faceting on individual columns. The fully relaxed growth and the columnar structure can be attributed to limited ad-atom mobility on the initial AlxCr1-xN(111) growth surface. (c) 2008 Elsevier B.V. All rights reserved.