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Preconditioning of C-Plane Sapphire for GaN Epitaxy by Radio Frequency Plasma Nitridation

The crystalline quality of molecular beam epitaxy grown layers of GaN on sapphire is generally improved by nitridation of the substrate. We use x-ray photoelectron spectroscopy, low energy electron diffraction, and atomic force microscopy to examine the case for nitridation of c-plane sapphire upon exposure to rf plasma generated nitrogen radicals. We find that a monolayer of surface nitride is formed after ∼300 min exposure with the substrate at 400 °C. Extended exposure causes growth of protrusions from the c-plane sapphire and thus leads to a rough surface morphology. Moreover, we report removal of adventitious surface carbon upon heat treatment at 300 °C in nitrogen plasma, albeit with reduced efficiency compared with hydrogen plasma cleaning.

Selective growth of GaN was performed by low-pressure metalorganic vapor phase epitaxy using a mask-patterned GaN epitaxial layer on a sapphire substrate. GaN hexagonal microprisms of 5–16 μm in diameter, with smooth vertical facets and no ridge growth, were fabricated on a sapphire substrate. This vertical  facet of GaN was parallel to a face of the sapphire substrate. Both the use of an epitaxial GaN layer on the sapphire substrate and low working pressure contributed to obtaining smooth top and vertical facet surfaces. Also, the stripe structures of GaN were obtained that had a rectangular cross section. Moreover, the selection of the mask-patterning direction was found to be important because of the 30° rotation of the crystallographic orientation between the GaN layer and the sapphire substrate.


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