US Patent 7713847 Method for forming AlGaN crystal layer

A method for preparing an AlGaN crystal layer with good surface flatness is provided. A surface layer of AlN is epitaxially formed on a c-plane sapphire single crystal base material by MOCVD method, and the resulting laminated body is then heated at a temperature of 1300° C. or higher so that a template substrate applying in-plane compressive stress and having a surface layer flat at a substantially atomic level is obtained. An AlGaN layer is formed on the template substrate at a deposition temperature higher than 1000° C. by an MOCVD method that includes depositing alternating layers of a first unit layer including a Group III nitride represented by the composition formula AlxGa1-xN (0≦x≦1) and a second unit layer including a Group III nitride represented by the composition formula AlyGa1-yN (0≦y≦1 and y≠x) such that the AlGaN layer has a superlattice structure.