Fabrication of a 1.5-inch freestanding GaN substrate by selective dissolution of sapphire using Li after the Na-flux growth

Abstract

Abstract Reducing the thermal stress induced in GaN crystals is key for the fabrication of large-diameter GaN crystals. In the Na-flux method, thermal stress is eliminated by sapphire dissolution at growth temperature after growth. We succeeded in fabricating a freestanding GaN substrate with this technique. However, the flux with Li used as a solvent to dissolve the sapphire can also dissolve the GaN. In this study, we demonstrate that increasing the amount of nitrogen can suppress GaN dissolution in the Li-added flux, without interfering in the dissolution of sapphire. This means that the sapphire and GaN have different dissolution mechanisms. By utilizing the different mechanisms, we succeeded in selectively dissolving sapphire and fabricating a 1.5-inch freestanding GaN substrate. It is revealed by secondary ion mass spectrometry (SIMS) that Li is incorporated in the crystal. The distribution of Li indicated that it was included in the crystal during sapphire dissolution.