High-temperature AlN intermediate layers with different thicknesses were deposited during the growth of wurtzite GaN on (0001) sapphire substrates by plasma-assisted molecular beam epitaxy. When using a 3.5 nm ALN intermediate layer temperature-independent Van-der-Pauw Hall measurements revealed a mobility enhancement by a factor of 2.5 at room temperature and by a factor of 32 at 30 K. Transmission electron microscopy confirmed that the better material quality was due to a reduction of dislocation density by about one order of magnitude. Photoluminescence measurements indicate a decrease of full-width at half-maximum of the main emission peak for GaN samples with AlN intermediate layer. Photoluminescence peak position and Raman shift of the E-2 mode hint at increasing biaxial compressive strain with increasing AlN intermediate layer thickness.