Microstructural analysis of void formation due to a NH₄Cl layer for self-separation of GaN thick films

The role of a NH₄C1 layer grown between GaN and a sapphire substrate for self-separation of GaN films was investigated in detail. Microstructural analysis of the NH₄C1 layer reveals that self-separation of a GaN film was effectively assisted by the sub-micrometer-sized voids formed due to etching of GaN by evaporated NH ₄C1. An areal fraction of nonetched GaN in contact with sapphire substrate was found to be linearly dependent on the thickness of NH ₄C1 layer and to have two different proportionality constants below and above an NH₄C1 thickness at which GaN begins to be self-separated. This intriguing behavior was well explained by a simple model that takes account of void merging.