Multi-junction light-emitting diode and method for making the same

A method for making a multi-junction LED, comprising the steps of:

• (A) forming a first epitaxial structure,
• (B) forming a tunnel junction structure on the first epitaxial structure, wherein the tunnel junction structure includes:
• a highly doped p-type semiconductor layer that is made of a material represented by InzAlX1Ga1−X1As, z ranging from 0 to 0.05;
• a first composition graded layer that is disposed on the highly doped p-type semiconductor layer and that is made of a material represented by AlX2Ga1−X2As, X2 being greater than 0 and less than X1;
• a highly doped n-type semiconductor layer that is disposed on the first composition graded layer opposite to the highly doped p-type semiconductor layer and that is made of a material represented by GaYIn1−YP;
• and
• a second composition graded layer that is disposed on the highly doped n-type semiconductor layer opposite to the first composition graded layer and that is made of a material represented by AlX3Ga1−X3As, and
• (C) forming a second epitaxial structure on the tunnel junction structure opposite to the first epitaxial structure.