Effects of The Buffer Layer in Metalorganic Vapour Phase Epitaxy of GaN on Sapphire Substrate

High-quality GaN film with a smooth surface, free from cracks, can be grown on a sapphire substrate by metalorganic vapour phase epitaxy (MOVPE) using a thin AlN buffer layer. The most essential role of the buffer layer is found to be (i) the supply of the nucleation sites with the same crystal orientation as the substrate, and (ii) the promotion of the lateral growth of GaN due to the decrease in interfacial free energy between the substrate and the epitaxial GaN film.

GaN and Ga1−xAlxN (0 < x ≦ 0.4) films grown by MOVPE on sapphire substrate are found to consist of many mosaic crystallites with various orientations. By preceding deposition of a thin AIN buffer layer, the microscopic fluctuation in crystallite orientation can be considerably reduced and the crystalline quality of the film is remarkably improved. Both the thickness and the deposition temperature of the AIN layer are found to be optimal as a buffer layer to convey the information of the substrate such as the crystallographic orientation and to relax the strain in this heteroepitaxial growth. The essential role of the AIN buffer layer is thought to be the supply of nucleation centers having the same orientation as the substrate and the promotion of lateral growth of the film due to the decrease in interfacial free energy between the film and the substrate.


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