The growth technology of AlGaN film with high Al composition and large thickness is investigated.The phase separation of AlGaN in AlGaN/GaN structure may be due to the too low growth of Ⅴ/Ⅲ and the tensile stress.Meanwhile,with ultra-high Ⅴ/Ⅲ,the Al incorporation rate can be saturated.By adopting AlN interlayer,no-crack and thick AlGaN materials with 45% Al composition are grown.The Al composition of this sample is the same as that in gas phase and the width of half height of the rocking curve of(0002) plane is 376".It is found that Al composition and crystal quality of the AlGaN film are influenced by the quality of AlN interlayer.It is also seen that as the Al composition increases,the morphology of AlGaN materials changes from micropit mode to microcrack mode,which can be retarded by the AlN interlayer.