Method of forming gallium nitride film over SOI substrate

A method of forming a GaN film, comprising:

• providing a silicon-on-insulator (SOI) substrate comprising a substrate, an insulator layer on the substrate and a silicon layer on the insulator layer;
• patterning the silicon layer into a patterned silicon layer comprising a plurality of recessed features, each recessed feature having a sidewall, wherein a process of patterning comprises:
• forming a patterned mask layer on the silicon layer;
• and
• etching a portion of the silicon layer not covered by the patterned mask layer using a wet etching process, wherein the wet etching process comprises applying a wet etchant comprising NaOH aqueous solution on the silicon layer, wherein a concentration of the NaOH aqueous solution ranges from about 15 wt % to about 50 wt %;
• epitaxially growing a plurality of GaN structures from the sidewalls, wherein the GaN structures are separated from each other;
• and
• continuously epitaxially growing the GaN structures vertically and horizontally to merge the GaN structures over top of the patterned silicon layer to form a GaN layer.