Method for heteroepitaxial growth using tensioning layer on rear substrate surface

A method, and products formed by such method, of providing a substantially planar surface to a layer of semiconducting material (24) formed on a first surface of a substrate (20), the substrate having a second surface opposite the first surface. The method comprising forming a layer (22) of a first material on the second surface of the substrate; forming a layer of the semiconducting material (24) on the first surface of the substrate; whereby said layer of said first material exerts a tensioning force on said second surface of the substrate (20) which countereffects a tensioning force exerted on said first surface of said substrate by said layer of semiconductor material (24) so that said first surface of said substrate has a substantially planar form. In some embodiments tensioning forces arise due to differential thermal expansion of said first material and said substrate and said semiconductor material and said substrate.