Semiconductor structures

A semiconductor structure, comprising:

• a nucleation layer, disposed on a substrate;
• an epitaxial growth layer, disposed above the nucleation layer;
• and
• a superlattice structure, disposed between the nucleation layer and the epitaxial growth layer, wherein the superlattice structure comprises a plurality of alternately stacked superlattice units, and adjacent two of the superlattice units comprises:
• a first superlattice unit, comprising a first superlattice layer and a second superlattice layer stacked on the first superlattice layer, wherein the first superlattice layer comprises a plurality of pairs of a (1-1)-th sublayer and a (1-2)-th sublayer, and the second superlattice layer comprises a plurality of pairs of a (2-1)-th sublayer and a (2-2)-th sublayer;
• and
• a second superlattice unit, disposed on the first superlattice unit and comprising a third superlattice layer and a fourth superlattice layer stacked on the third superlattice layer, wherein the third superlattice layer comprises a plurality of pairs of a (3-1)-th sublayer and a (3-2)-th sublayer, and the fourth superlattice layer comprises a plurality of pairs of a (4-1)-th sublayer and a (4-2)-th sublayer,
• wherein a composition of the first superlattice layer is the same as a composition of the third superlattice layer, a composition of the second superlattice layer is the same as a composition of the fourth superlattice layer, a thickness of the (2-1)-th sublayer is greater than a thickness of the (1-1)-th sublayer, and a thickness of the (2-2)-th sublayer is smaller than a thickness of the (1-2)-th sublayer.