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Reduction of Threading Dislocations in GaN on Sapphire by Buffer Layer Annealing in Low-Pressure Metalorganic Chemical Vapor Deposition

Threading dislocations caused by coalescence of GaN islands in the initial stage of low-pressure metalorganic chemical vapor deposition (MOCVD) growth is reduced by buffer layer annealing. Unlike the atmospheric pressure MOCVD, high nucleation density in low-pressure MOCVD makes it difﬁcult to form large isolated islands which enhance the lateral growth. Herein, it is demonstrated that a buffer layer is transformed into isolated islands by thermal annealing and the threading dislocations can be reduced by controlling the density of isolated islands. Nitridation of sapphire modiﬁes its surface energy and enhances the nucleation during buffer layer deposition resulting in a buffer layer with small grain size. Through thermal annealing, the as-deposited buffer layer with small grain size is transformed into isolated islands with (0001) facet on a ﬂat (0001) surface, which enhance the lateral growth at high temperature. Longer nitridation and shorter buffer layer deposition lead to lower density of islands resulting in lower dislocation density. The sample with low dislocation density showed strong band-edge PL emission with low yellow band emission.