Preconditioning of c-plane sapphire for GaN molecular beam epitaxy by electron cyclotron resonance plasma nitridation

Abstract

Nitridation of c-plane sapphire is commonly employed in molecular beam epitaxy of GaN, in order to improve the crystalline quality of the deposited layers. In this study, we use x-ray photoelectron spectroscopy, Auger sputter profiling, reflection high energy electron diffraction, low energy electron diffraction, and atomic force microscopy to examine chemical and structural properties of sapphire (0001) substrate upon exposure to nitrogen activated by an electron cyclotron resonance plasma source. Incorporation of nitrogen into the sapphire surface was verified with x-ray photoelectron spectroscopy, and a monolayer of surface nitride is formed after approximately 60 min of nitridation at a substrate temperature of 620 °C. The thickness of the surface nitride layer is on the order of 4.5–6 Å for nitridation times ranging from 60 to 180 min, as estimated from the recorded nitrogen Auger sputter profiles. Reflection high energy electron diffraction gave clear evidence for formation of surface aluminum nitride, after prolonged (60 min) nitridation.