2.3.4 The Effect of Strain

A general aim when growing a device is to avoid lattice mismatch between the substrate and the epitaxial layers. A main concern is that the strain originating from such mismatch can relax and lead to misfit dislocations and even to amorphous structures. Therefore, only materials which lattice constants match the ones of GaAs or InP, typical substrate materials, gained attention. Such materials are AlGaAs in the whole composition range and Ga$_{0.51}$In$_{0.49}$P grown on GaAs, and In$_{0.53}$Ga$_{0.47}$As and In$_{0.52}$Al$_{0.48}$As grown on InP. The quaternary GaInAsP can also match either GaAs or InP for certain mole fractions. However, if $x$ is sufficiently low or the layer is sufficiently thin, it can preserve the substrate lattice constant, respectively, the strain. The result can be alteration of the bandgap, which can be beneficial sometimes, e.g. for SiGe grown on Si.