Reduction of structural defects in a-plane GaN epitaxy by use of periodic hemispherical patterns in r-plane sapphire substrates

Abstract

Using hemisphere-shaped patterned r-plane sapphire substrates, high quality nonpolar (112¯0) a-plane gallium nitride (GaN) films have been obtained with superior structural characteristics to films grown on conventional (unpatterned) r-plane sapphire. This is evidenced by reduced x-ray rocking curve widths, smaller in-plane crystallographic anisotropy, and smoother surface morphology. Observations by transmission electron microscopy and cathodoluminescence reveal that the defect density is remarkably reduced in regions above the patterned hemispheres. The growth of a-plane GaN on the patterned substrates proceeds first by a relatively fast growth of the film on the flat surface region, and then a gradual lateral overgrowth over the protruding hemispheres, where the direct epitaxial growth is severely retarded.