Heteroepitaxial wafer and method for producing a heteroepitaxial wafer

A heteroepitaxial wafer and a method for producing a heteroepitaxial wafer. The heteroepitaxial wafer comprises in the following order: a silicon substrate having a diameter and a thickness; an AlN nucleation layer; a first strain building layer which is an AlzGa1-zN layer having a first average Al content z, wherein 0 < z; a first strain preserving block comprising not less than 5 and not more than 50 units of a first sequence of layers, the first sequence of layers comprising an AlN layer and at least two AlGaN layers, the first strain preserving block having a second average Al content y, wherein y > z; a second strain building layer which is an AlxGa1-xN layer having a third average Al content x, wherein 0 ≤ x < y; a second strain preserving block comprising not less than 5 and not more than 50 units of a second sequence of layers, the second sequence of layers comprising an AlN layer and at least one AlGaN layer, the second strain preserving block having a fourth average Al content w, wherein x < w < y,