Light detecting device having a gradient layer with multiple sublayers

A light detecting device, comprising:

• a substrate having a lattice constant that is a reference value;
• a buffer layer disposed on said substrate;
• a gradient layer formed on said buffer layer opposite to said substrate, and including a plurality of sublayers that have respectively a plurality of lattice constants each of which is greater than said reference value of said lattice constant of said substrate, said sublayers being arranged in such a manner that said lattice constants of said sublayers undergo a gradual increase in lattice constant in a direction away from said substrate;
• a barrier layer formed on said gradient layer opposite to said buffer layer and having a predetermined lattice constant which is greater than said reference value of said substrate and no smaller than said lattice constants of said sublayers;
• and
• an absorption layer formed on said barrier layer opposite to said gradient layer,
• wherein said gradient layer, said barrier layer and said absorption layer are epitaxially grown in a direction that forms a growth angle with a line normal to said substrate, said growth angle ranging from 6 degrees to 14 degrees.