Numerical Simulation On Residual Stress In Gan-on-Sapphire Heterofilm System

As a wide band-gap semiconductor material,GaN is widely used in high-brightness LED,semiconductor laser and high-power electronic devices.GaN is mainly grown on sapphire??-Al₂O₃?substrate.Due to the large lattice mismatch and the large difference in thermal expansion coefficient between GaN film and sapphire substrate,large stress occurs in the growth process,which often leads to warping and cracking of GaN film.The existence of these defects has great impact on the production of GaN materials to meet the device quality requirements.In this paper,the interface stress distribution,the influencing factors and the evolution of residual stresses of GaN-sapphire heterostructure film system during the cooling process are studied by finite element analysis with ANSYS APDL software.The main results are as follows:1.The interface stress of the GaN-sapphire heterostructure film system is uniformly distributed along the radial direction.Under the condition of constant thickness of sapphire substrate and neglecting the edge effect,there are maximum and minimum interface stresses with the increase of GaN film thickness;2.When the edge effect is ignored,changing the thickness of sapphire substrate and increasing the thickness of GaN film.It is worth noting that this minimum always occurs when the ratio of sapphire substrate to GaN film thickness is about 4.3.Therefore,the interface stress of the system is not only determined by the difference of thermal expansion coefficient between them,but also greatly influenced by the film thickness ratio;3.To investigate the effect of cooling rate on the thermal stress of the thin film system,a typical 4-inch sapphire substrate with a thickness of 650mm and a GaN film with a thickness of150mm was selected for the unsteady numerical simulation of the cooling process from 1373K to800K.It is found that the stress due to lattice mismatch is much greater than that due to thermal mismatch;4.The distribution of residual stress in the system is greatly influenced by the orthotropic materials.Because of the sudden change and edge effect of the interface material of the heterogeneous film,the stress at the edge of the interface of the film system is relatively concentrated,and the local stress is likely to exceed the strength of the material and fracture occurs,thus causing cracks or delamination;5.The results show that the AlN buffer layer can significantly reduce the interface stress of the heterofilm,and the optimal thickness of the AlN buffer layer is about 60 nm.