Fig.1ashows the XRD patterns of the four samples.All the peaks can be indexed to the hexagonal wurtzite structure AlN with lattice parameters of a¼3.111Åandc¼4.979Å(ICDDPDFno.25-1133).The strongest peak of (002)plane shows that all samples preferentially grow along the c axis and the growth temperature does not affect the preferred orientation.However,theintensity and the full wid that halfmaximum(FWHM)of(002)diffraction peak decrease with