MOCVD growth of GaN islands by multistep nucleation layer technique

The widely adopted two-step growth process can reduce problems caused by lattice mismatch in growing gallium nitride (GaN) with metal-organic chemical vapor deposition on a sapphire substrate. The method involves a low-temperature GaN deposition step followed by a high-temperature one in which GaN island coalescence is followed by a quasi-two-dimensional growth mode. In this paper, a multistep growth method for controlling the size of hexagonal GaN islands on sapphire substrates is introduced. The method involves alternate low-temperature growth and recrystallization steps to increase the size of GaN clusters in a controlled manner. Hexagonal GaN islands with good crystalline quality are grown by metal-organic chemical vapor deposition in a vertical-flow close-coupled showerhead reactor. Reflectometry is used to monitor film growth in situ. The morphology of the resulting GaN films is determined using atomic force microscopy. Also, the crystalline structure and quality of the GaN islands are assessed by X-ray diffraction and transmission electron microscopy. Results clearly indicate gradual increase in island size after each recrystallization step without any observable change in island density relative to the first recrystallization step. Merging of GaN islands is evident after four cycles of the multistep growth process.