Multilayer substrate having gallium nitride layer and method for forming the same

A method for forming a multilayer substrate, comprising the steps of:

• providing a substrate;
• forming a mesh layer having a plurality of openings on the substrate;
• forming a buffer layer essentially consisting of a plurality of nitride layers on the substrate in the openings;
• forming in sequence a lower aluminum gallium nitride layer, an intermediate aluminum gallium nitride layer and an upper aluminum gallium nitride layer with different aluminum concentrations on the buffer layer, wherein an aluminum concentration of the lower aluminum gallium nitride layer is higher than an aluminum concentration of the intermediate aluminum gallium nitride layer, the aluminum concentration of the intermediate aluminum gallium nitride layer is higher than an aluminum concentration of the upper aluminum gallium nitride layer, and while forming the intermediate aluminum gallium nitride layer, aluminum proportion is decreased along with an increase of a thickness of the intermediate aluminum gallium nitride layer;
• and
• forming a gallium nitride layer on the upper aluminum gallium nitride layer.