Suppression of Polycrystal Nucleation by Methane Addition at Moderate Timing to Maintain GaN Crystal Growth on Point Seeds in the Na-flux Method

We have grown low-dislocation, large-diameter GaN substrates using small seed crystals, called as point seeds (PSs) in the Na-flux method. To further reduce dislocation density and improve wafer bowing, reducing the PS diameter is effective. However, it was found that crystals did not start to grow on a small diameter PS. In this study, we also found that a growth on a small diameter PS can be carried out in the condition without graphite addition. However, graphite is conventionally added to suppress polycrystals, so those could not be avoided at the latter process to planarize crystal surface by raising and lowering the crystal, called the flux film coated (FFC) process, in which polycrystals are likely generated. Thus, in this study, we newly proposed to suppress poly crystals by adding methane gas before the FFC process, instead of graphite which cannot be added during the middle of growth.