Semiconductor epitaxial wafer

A semiconductor epitaxial wafer, comprising:

• a substrate having a first lattice constant X1, wherein the substrate is a Ge substrate, a GaAs substrate or an InP substrate;
• a first epitaxial stack structure epitaxially grown on the substrate;
• a first ohmic contact layer epitaxially grown on the first epitaxial stack structure and having a second lattice constant X2, wherein the first ohmic contact layer includes a material selected from the group consisting of InxGa1-xAsyN1-y, InxGa1-xAsyNzSb1-y-z, InxGa1-xAsyNzBi1-y-z and InxGa1-xAsyNzSbwBi1-y-z-w, wherein x is 0˜1 and y is 0.001˜0.2, z is 0.001˜0.2 and w is 0.001˜0.2;
• and
• a second epitaxial stack structure epitaxially grown on the first ohmic contact layer,
• wherein a difference between the first lattice constant X1 and the second lattice constant X2 is less than or equal to ±10000 ppm.