Development of gallium nitride substrates

Large bulk gallium nitride (GaN) single crystal substrates with low dislocation density are the key material for the commercial production of violet lasers. Sumitomo Electric had developed a new process in order to obtain GaN substrate by means of vapor phase epitaxy. A thick GaN crystal layer is grown epitaxially on foreign substrate, followed by separation from the initial substrate, and then, GaN substrate crystal is obtained. However, a large number of crystal defects (dislocations) are generated at the interface of GaN and GaAs due to large mismatch in crystal lattice. Sumitomo Electric also had developed a new method for the dislocation reduction named DEEP (dislocation elimination by the epitaxialgrowth with inverse-pyramidal pits). The thick GaN layer grows with numerous large inverse-pyramidal pits maintained on the surface. As the growth proceeds, dislocations in the GaN crystal are concentrated to the center of the pit. As a result, a wide area with low dislocation density is formed within the pit except the center area. Furthermore, the improvement of the DEEP is described. The position of the pits is fixed at the pre-determined position by means of opposite polarity GaN. Thus, the total number of the dislocations is extremely reduced. This process was named as A-DEEP (advanced-DEEP). GaN substrates based on A-DEEP satisfy all the requirements for the violet laser diode.