Epitaxial substrate and method for manufacturing epitaxial substrate

An epitaxial substrate in which a group of group-III nitride layers are 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 epitaxial substrate comprising:

• a buffer layer formed of a plurality of lamination units being continuously laminated;
• and
• a crystal layer made of a group-III nitride and formed on said buffer layer,
• said lamination unit including:
• a composition modulation layer formed of a first unit layer and a second unit layer made of group-III nitrides having different compositions being alternately and repeatedly laminated, in which a second group-III nitride of said second unit layer has an in-plane lattice constant, under a strain-free state, greater than that of a first group-III nitride of said first unit layer;
• a termination layer formed on an uppermost portion of said composition modulation layer, said termination layer being made of said first group-III nitride and formed with a thickness greater than that of said first unit layer;
• and
• an intermediate layer made of a third group-III nitride, wherein
• in said composition modulation layer, said second unit layer is formed so as to be in a coherent state relative to said first unit layer,
• said intermediate layer is formed so as to be in a coherent state relative to said termination layer,

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;
• and
• a crystal layer formation step for forming a crystal layer above said buffer layer, said crystal layer being made of a group-III nitride, wherein
• in said buffer layer formation step, a lamination unit formation step is continuously repeated a plurality of times, said lamination unit formation step including:
• a composition modulation layer formation step for forming a composition modulation layer by alternately and repeatedly laminating a first unit layer and a second unit layer made of group-Ill nitrides having different compositions;
• a termination layer formation step for forming a termination layer on an uppermost portion of said composition modulation layer;
• and
• an intermediate layer formation step for forming an intermediate layer on said termination layer,
• in said composition modulation layer formation step, said composition modulation layer is formed such that:
• a second group-III nitride of said second unit layer has an in-plane lattice constant, under a strain-free state, greater than that of a first group-III nitride of said first unit layer;
• and
• each of said second unit layers is in a coherent state relative to said first unit layer,
• in said termination layer formation step, said termination layer is made of said first group-III nitride and formed with a thickness greater than that of said first unit layer,
• in said intermediate layer formation step, said intermediate layer is made of a third group-III nitride and formed so as to be in a coherent state relative to said termination layer.