A GaN interlayer between low temperature (LT) AlN and high temperature (HT) AlN is introduced to combine HT AlN,LT AlN and composition-graded AlGaN as a novel buffer layer for GaN films grown on Si (111)substrates.The crystal quality,surface morphology and strain state of the GaN film with this new buffer are compared with those of GaN grown on a conventional buffer structure.By changing the thickness of LT AlN,the crystal quality is optimized and the crack-free GaN film is obtained.The in-plane strain in the GaN film can be changed from tensile to compressive strain with the increase in LT AlN thickness.