High quality GaN epitaxy thin films have been desired for the energy-efficient,solid-state semiconductor illuminating devices.Silicon substrates offer high crystal quality,low wafer cost,large wafer size,and potential integration with the well-established silicon processing technologies.However,due to the large mismatch in lattice constants and thermal expansion coefficients,it is still challenging to grow high quality GaN on silicon substrates.In this study,high quality GaN epitaxy has been engineering designed to grow on Si(111)substrate using varying GaN/AlGaN composite buffer structures by an Axitron 200 metal-organic vapor phase epitaxy deposition system.A thin AlN seed layer of 25 nm was firstly grown at 720℃.AlGaN layer of different thickness was then grown at 1050℃with subsequent GaN thin film until the total thickness reached 500 nm.The thickness of the subsequent GaN thin film could be increased by reducing the AlGaN thickness in the composite buffer structures.The results have shown that the lower GaN/AlGaN thickness ratio would decrease the dislocation density and provide crack-free,mirror-like upper GaN crystal thin film.On the other hand,the GaN/AlGaN thickness ratio could be designed to be 2-6 to balance the processing cost and the thin film quality for engineering applications.The dislocation density has been about 2×10 9 cm-2.In addition,dislocation close loop was observed near the GaN/AlGaN interface.The annihilation mechanism could be depicted by the reduction in dislocation strain energy. High quality GaN epitaxy thin films have been desired for the energy-efficient, solid-state semiconductor illuminating devices. Silicon substrates offer high crystal quality, low wafer cost, large wafer size, and potential integration with the well-established silicon processing technologies. , due to the large mismatch in lattice constants and thermal expansion coefficients, it is still challenging to grow high quality GaN on silicon substrates. In this study, high quality GaN epitaxy has been engineering designed to grow on Si (111) substrate using varying GaN / AlGaN composite buffer structures by an Axitron 200 metal-organic vapor phase epitaxy deposition system. A thin AlN seed layer of 25 nm was grown at 720 ° C. AlGaN layer of different thickness was then grown at 1050 ° C. with subsequent GaN thin film until the total thickness reached 500 nm. The thickness of the subsequent GaN thin film could be increased by reducing the AlGaN thickness in the composite buffer structures. The result s have shown that the lower GaN / AlGaN thickness ratio would decrease the dislocation density and provide crack-free, mirror-like upper GaN crystal thin film. On the other hand, the GaN / AlGaN thickness ratio could be designed to be 2-6 to balance the processing cost and the thin film quality for engineering applications. The dislocation density has been about 2 × 10 9 cm -2. addition, dislocation close loop was observed near the GaN / AlGaN interface. annihilation mechanism could be drawn by the reduction in dislocation strain energy.