Epitaxial wafer, semiconductor device, method for producing epitaxial wafer, and method for producing semiconductor device

An epitaxial wafer comprising:

• a silicon-based substrate;
• a first buffer layer that is disposed on the silicon-based substrate and includes a first multilayer structure buffer region composed of AlxGa1-xN layers and AlyGa1-yN layers (x>y) which are alternately disposed and a first insertion layer which is composed of an AlzGa1-zN layer (x>z) and is thicker than the AlyGa1-yN layer, the second insertion layer being thinner than the first insertion layer, the first multilayer structure buffer regions and the first insertion layers being alternately disposed;
• a second buffer layer that is disposed on the first buffer layer and includes a second multilayer structure buffer region composed of AlαGa1-αN layers and AlβGa1-βN layers (α>β) which are alternately disposed and a second insertion layer which is composed of an AlγGa1-γN layer (α>γ) and is thicker than the AlβGa1-βN layer, the second multilayer structure buffer regions and the second insertion layers being alternately disposed;
• and
• a channel layer that is disposed on the second buffer layer and is thicker than the second insertion layer,
• wherein
• an average Al composition in the second buffer layer is higher than an average Al composition in the first buffer layer.

A method for producing an epitaxial wafer, comprising:

• preparing a silicon-based substrate;
• forming, on the silicon-based substrate by epitaxial growth, a first buffer layer including a first multilayer structure buffer region composed of AlxGa1-xN layers and AlyGa1-y N layers (x>y) which are alternately disposed and a first insertion layer which is composed of an AlzGa1-zN layer (x>z) and is thicker than the AlyGa1-yN layer, the first multilayer structure buffer regions and the first insertion layers being alternately disposed;
• forming, on the first buffer layer by epitaxial growth, a second buffer layer including a second multilayer structure buffer region composed of AlαGa1-αN layers and AlβGa1-βN layers (α>β) which are alternately disposed and a second insertion layer which is composed of an AlγGa1-γN layer (α>γ) and is thicker than the AlβGa1-βN layer, the second multilayer structure buffer regions and the second insertion layers being alternately disposed;
• and
• forming a channel layer that is thicker than the second insertion layer on the second buffer layer by epitaxial growth,
• wherein
• an average Al composition in the second buffer layer is made higher than an average Al composition in the first buffer layer, and
• wherein
• the second insertion layer is made thinner than the first insertion layer.