Recently, conventional wisdom has reversed its view on the disadvantages of strain in active semiconductor devices. Now lattice-mismatch-induced biaxial strain is being actively studied as a technique for optimizing bulk and quantum well (QW) device performance. It has been realized that, in addition to permitting once inaccessible energy ranges to be reached, strain has the potential of altering the properties of energy bands in a precise and advantageous manner provided that care is taken that the strain remain pseudomorphic (i.e., that the critical thickness, beyond which the layer relaxes with detrimental effects on quality, not be exceeded).
This paper reviews progress in this field including techniques for determining the magnitude of pseudomorphic strain in single QWs, and recent theory and experiments detailing the advantages which strain, both compressive and tensile, can bring to QW laser diodes. The use of strain to balance the polarization anisotropy inherent in QW structures is also discussed. Finally, a novel use of biaxial tensile strain in the barriers of QW devices to minimize polarization sensitivity is introduced. The idea is to create a structure in which the light and heavy hole valence bands are degenerate, as in bulk material, but which still retains the advantages of QWs (higher quantum and differential quantum efficiency, etc.).