Epitaxial wafer for heterojunction type field effect transistor
An epitaxial wafer for a heterojunction type FET includes an AlN primary layer, a stepwisely composition-graded buffer layer structure, a superlattice buffer layer structure, a GaN channel layer, and a nitride semiconductor electron supply layer, which are sequentially provided on a Si substrate, the stepwisely composition-graded buffer layer structure including a plurality of AlGaN buffer layers provided on each other such that an Al composition ratio is sequentially reduced, an uppermost layer thereof having a composition of AlxGa1−xN (0<x), a plurality of sets of an AlyGa1−yN (y≦1) superlattice constituting layer and an AlzGa1−zN (0<z<y) superlattice constituting layer being provided on each other alternately starting from one of the AlyGa1−yN superlattice constituting layer and the AlzGa1−zN superlattice constituting layer in the superlattice buffer layer structure, the AlxGa1−xN buffer layer and the AlzGa1−zN superlattice constituting layer satisfying x−0.05≦z≦x+0.05.
1. An epitaxial wafer for a heterojunction type field effect transistor, comprising an AlN primary layer, a stepwisely composition-graded buffer layer structure, a superlattice buffer layer structure, a GaN channel layer, and a nitride-based semiconductor electron supply layer, which are sequentially provided on a Si substrate,
said stepwisely composition-graded buffer layer structure including a plurality of AlGaN buffer layers provided on each other such that an Al composition ratio is stepwisely and sequentially reduced, an uppermost AlGaN buffer layer having a composition of AlxGa1−xN (0<x),
a plurality of sets of an AlyGa1−yN (y≦1) superlattice constituting layer and an AlzGa1−zN (0<z<y) superlattice constituting layer being provided on each other alternately starting from one of said AlyGa1−yN superlattice constituting layer and said AlzGa1−zN superlattice constituting layer in said superlattice buffer layer structure,
said AlxGa1−xN buffer layer and said AlzGa1−zN superlattice constituting layer having substantially the same Al composition ratio and satisfying x−0.05≦z≦x+0.05,
an Al composition difference of the uppermost AlxGa1−xN layer (0<x) in said stepwisely composition-graded buffer layer structure from the AlyGa1−yN (y≦1) superlattice constituting layer having a small band gap in said superlattice buffer layer structure being 0%.
2. The epitaxial wafer according to claim 1 , wherein a lowermost superlattice constituting layer in said superlattice buffer layer structure has a composition of AlN.