Aluminum nitride films were prepared by mid-frequency magnetron sputtering on Si (111) substrate. The grown films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) to obtain the structural and the chemical information. The polycrystalline thin films were in a hexagonal wurtzite structure having a (002) preferred orientation, along which the columnar grain structure was found. XPS study revealed the presence of oxygen and carbon contaminations, as well as the Al-rich nature of the film. Anomalous C-V characteristics of Al/AlN/n-Si capacitors were studied. The measured C-V curves show rolloffs in the accumulation region and voltage stresses cause both horizontal and vertical shifts of the C-Vcurves. These anomalous behaviors are mainly due to the large current conduction and the charge trapping in the Al-rich AlN layer.