Structure and Strain Properties of GaN Films Grown on Si (111) Substrates with Al_xGa_1?xN/Al_yGa_1?yN Superlattices

GaN films with an Al_xGa_1?xN/Al_yGa_1?yN superlattice (SL) buffer layer are grown on Si(111) substrates by metal-organic chemical vapor deposition (MOCVD). The structure and strain properties of the samples are studied by optical microscopy, Raman spectroscopy, x-ray diffractometry and atomic force microscopy. The results show that the strain status and crystalline quality of the GaN layers are strongly dependent on the difference of the Al composition between Al_xGa_1?xN barriers and Al_yGa_1?yN wells in the SLs. With a large Al composition difference, the GaN film tends to generate cracks on the surface due to the severe relaxation of the SLs. Otherwise, when using a small Al composition difference, the crystalline quality of the GaN layer degrades due to the poor function of the SLs in filtering dislocations. Under an optimized condition that the Al composition difference equals 0.1, the crack-free and compressive strained GaN film with an improved crystalline quality is achieved. Therefore, the Al_xGa_1?xN/Al_yGa_1?yN SL buffer layer is a promising buffer structure for growing thick GaN films on Si substrates without crack generation.