Quantum well structure having differentially strained quantum well layers

A quantum well structure comprising:

• an n-type cladding layer;
• a p-type cladding layer;
• and
• a plurality of quantum well layers and one or more barrier layers each disposed between the n-type cladding layer and the p-type cladding layer, the quantum well layers and the barrier layers being laminated in an alternating manner;
• wherein the quantum well layers include at least two selected from the group consisting of a layer having tensile strain, a layer having no strain, and a layer having compressive strain, so that the quantum well layer that is disposed closest to the n-type cladding layer has the largest band offset (Ev) of the valence band among the quantum well layers, and the quantum well layer that is disposed closest to the p-type cladding layer has the largest band offset (Ec) of the conduction band among the quantum well layers;
• and
• wherein the thickness of each of the quantum well layers is selected so that the energy difference in each of the quantum well layers between the ground quantum state of an electron at the conduction band and the ground quantum state of a hole at the valence band is substantially the same.