A combined deep-level transient spectroscopy (DLTS) and electron spin resonance (ESR) study is performed to identify the electrically active defects at the AlN/Si (111) interface. It is shown that the density of deep-level states not only depends on the thermal budget of the epitaxial deposition but also on the strain built up during growth and upon cooling to room temperature (RT). At the same time, diffusion of Si into the 200 nm thick AlN layer produces a thin crystalline Si3N4 interfacial layer, identified by transmission electron microscopy (TEM). This gives rise to so-called dangling bond Pb centres at the Si3N4/Si (111) interface. In addition, a strong evolution of the electrically active defect clusters in the silicon substrate close to the interface has been observed both in DLTS and ESR.