Epitaxial substrate and method for manufacturing epitaxial substrate

A method for manufacturing an epitaxial substrate for use in a semiconductor device, said epitaxial substrate having a group of group-III nitride layers formed on a base substrate made of (111)-oriented single crystal silicon such that a (0001) crystal plane of said group of group-III nitride layers is substantially in parallel with a substrate surface of said base substrate, said method comprising:

• a buffer layer formation step for forming a buffer layer including a plurality of first lamination units each including a composition modulation layer and a first intermediate layer, said buffer layer formation step including a plurality of composition modulation layer formation steps and a plurality of first intermediate layer formation steps, said composition modulation layer formation step being a step for forming said composition modulation layer by alternately laminating a first composition layer made of AlN and a second composition layer made of a group-III nitride having a composition of AlxGa1-xN (0≦x<1), said first intermediate layer formation step being a step for forming said first intermediate layer made of a group-III nitride having a composition of AlyGa1-yN (0≦y<1);
• and
• a crystal layer formation step for forming a crystal layer above said buffer layer, said crystal layer being made of group-III nitride,
• wherein
• in said composition modulation layer formation step, said composition modulation layer is formed in such a manner that:
• the relationship of:
• x(1)≧x(2)≧ . . . ≧x(n−1)≧x(n);
• and
• x(1)>x(n),
• is satisfied, where n represents the number of laminations of each of said first composition layer and said second composition layer (n is a natural number equal to or greater than two), and x(i) represents the value of x in i-th one of said second composition layers as counted from said base substrate side;
• and
• each of said second composition layers is in a coherent state relative to said first composition layer,
• in said first intermediate layer formation step, said first intermediate layer is fog lied so as to be in a coherent state relative to said composition modulation layer.