Thank you for your response Dr. Moussaab Belarb.

The previous discussion was focused toward, GaN on Si. Whereas I need help to grow GaN on Sapphire.

Usually GaN growth at comparatively low temperature at N2 environment produces N -face poor crystalline qaulity GaN

Thank you Dr. Anup Gorai, the articles you shared are helpful.

Thank you Dr. Bablu Ghosh for your response.

Here is the link to a very well written and in-depth review on this topic from UCSB called "N-polar GaN epitaxy and high electron mobility transistors".

http://iopscience.iop.org/article/10.1088/0268-1242/28/7/074009/meta