The effect of Si doping on defect density in AlN layers grown on sapphire was analysed. Si concentration in the range of 10(19) cm(-3) leads to dislocation line inclination in AlN layers with a threading dislocation density of 3x10(10) cm(-2). Overgrowth of Si doped AlN layers by non-intentionally doped AlN results in a reduction of threading dislocation density by a factor of two. In contrast, an increase of the Si concentration to an order of 10(20) cm(-3) leads to a structural degradation of the AlN layers. The degradation process takes place through transformation to columnar-like growth. In a second experiment the AlN/AlN:Si/AlN layers with a decreased defect density were trench-patterned and used for subsequent epitaxial lateral overgrowth. In comparison to the epitaxial lateral overgrowth of non-intentionally doped AlN templates, the use of the MN templates containing an AlN:Si interlayer allows to reduce the threading dislocation density in the defect-rich regions above the ridges in 6 mu m thick epitaxial laterally overgrown MN by a factor of 2.5.